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A Comprehensive Analysis of Characteristics of Hydrogen Operation as a Preparation for Retrofitting a Compression Ignition Engine to a Hydrogen Engine

Innovative Vehicle Technology Competence Center, Department of Automotive Technologies, Faculty of Transportation Engineering and Vehicle Engineering, Budapest University of Technology and Economics, Stoczek József u. 6, H-1111 Budapest, Hungary

Department of Energy Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3, H-1111 Budapest, Hungary

Department of Mechatronics, Optics and Mechanical Engineering Informatics, Faculty of Mechanical Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3, H-1111 Budapest, Hungary

Author to whom correspondence should be addressed.

Processes 2025, 13(3), 718; https://doi.org/10.3390/pr13030718

Submission received: 19 December 2024 / Revised: 16 January 2025 / Accepted: 28 February 2025 / Published: 2 March 2025

(This article belongs to the Section Environmental and Green Processes)

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Abstract

Hydrogen is a carbon-neutral fuel, so in theory it holds enormous potential. The use of hydrogen as a fuel for traditional internal combustion engines is becoming increasingly prominent. The authors now have the opportunity to retrofit a single-cylinder diesel research engine to an engine with hydrogen operation. For this reason, before that conversion, they prepared a comprehensive review study regarding hydrogen. Firstly, the study analyzes the most essential properties of hydrogen in terms of mixture formation and combustion compared to diesel. After that, it deals with indirect and direct injection, and what kind of combustion processes can occur. Since there is a possibility of pre-ignition, backfire, and knocking, the process can be dangerous in the case of indirect mixture formation, and so a short subsection is devoted to these uncontrolled combustion phenomena. The next subsection shows how important, in many ways, a special spark plug and ignition system are for hydrogen operation. The next part of the study provides a detailed presentation of the possible combustion chamber design for operation with hydrogen fuel. The last section reveals how many parameters can be focused on analyzing the hydrogen’s combustion process. The authors conclude that intake manifold injection and a Heron-like combustion chamber design, with a special spark plug with an ignition system, would be an appropriate solution.

Keywords:

hydrogen; diesel; engine conversation; mixture formation; combustion concepts; combustion chamber; ignition system

1. Introduction

Hydrogen as an energy carrier has much potential for use in meeting future global goals. However, significant efforts must be made to make physical progress toward this goal, and there are many challenges. Since a great opportunity can be seen in the use of hydrogen as an energy carrier, many different approaches can be taken. One approach could be sustainability. Ref. [1] calls for a significant change regarding the development of a new global cognitive-based system, because what we have done in the past few decades is damaging the Earth and humanity. Our actions and habits need a completely new approach and rethinking. The above can be achieved in the framework of cognitive sustainability. Among several sectors, cognitive sustainability also affects mobility. If we examine mobility from a cognitive point of view, we have to delve deeper into the field of mobility and rethink things. Since mobility is one of the foundations of modern life, cognitive changes are necessary for its sustainability. In this sense, mobility does not only mean transport [2]. Cognitive transport is only one part of cognitive sustainability and cognitive mobility. Much effort is being made towards sustainable transport worldwide, but many challenges exist [3]. If hydrogen production occurs on a renewable basis, it is possible to achieve a carbon-neutral goal with even more significant steps [4]. Among the relevant literature, fairly specific case studies can be found that carry out specific calculations regarding converting a city’s vehicle fleet to hydrogen [5]. This study performs predictive calculations for a medium-sized Spanish city, and finds that hydrogen technology (especially green technology) can significantly benefit the automotive industry.

Based on forecasts in the report [6] made according to different methodologies, the global total energy consumption may increase or decrease slightly up to 2025. The transport sector’s energy consumption is only a part of the total energy consumption. Transport energy consumption also shows an increasing trend; a significant part of it is still based on fossil petroleum. The use of zero-carbon hydrogen and hydrogen-based fuels is constantly increasing. In the scenarios forecast up to 2050, hydrogen-based fuel and hydrogen fuel appear prominently among the transport energy sources. According to a report by one of the world’s largest fuel companies, the world will need crude oil and natural gas for many years. Hydrogen is mentioned in the long-term forecasts, according to which electricity or hydrogen will be the main decarbonization path, depending on technology development, government policy and customer preferences. Much is invested in researching and developing new hydrogen-based technologies [7].

There is great interest in alternative fuels and drive modes in all transport sectors. Political support is also needed for the large-scale application of hydrogen-powered vehicle fleets, which will help financially with implement infrastructural investments. The wide availability of green fuels (green hydrogen and its derivatives) is essential in decarbonizing mobility [8].

Due to increasing greenhouse gas emissions, environmental pollution, and oil supply issues, the search for renewable and clean alternative energy sources is receiving more and more attention. Due to its many beneficial properties, hydrogen energy is now seen as the energy source of the future. The hydrogen internal combustion engine combines the technical advantages of conventional internal combustion engines in terms of application, and comprehensive additional advantages can make it competitive, such as production costs, applicability and reliability. Thus, the use of hydrogen in internal combustion engines can represent one of the segments of hydrogen energy utilization [9].

According to the research in [10], vehicle and engine manufacturers can be divided into three groups according to “where they are” in terms of hydrogen internal combustion engine developments. All three groups contain on-road and off-road vehicle manufacturers. Vehicle manufacturers have already announced the start of series production. Many manufacturers are developing hydrogen internal combustion engines. Among them is a motorcycle manufacturer. The third group features manufacturers who have announced that they will start development. Hydrogen has the potential to be a sustainable fuel in the future, so as to reduce dependence on fossil fuels and emissions from carbon-based vehicles. Non-road manufacturers prefer hydrogen because hydrogen operation is very cost-effective in applications involving many steady-state and minor transient operations. SI (spark ignition) and CI (compression ignition) engines can be adapted to hydrogen operation. Since the electric drive was created to replace coal-based fuels completely, running on hydrogen can be an alternative to electric vehicles.

Due to the growing demand for environmentally friendly vehicle propulsion designs, hydrogen is increasingly coming to the fore as an alternative and clean fuel. The hydrogen engine can, therefore, play an essential role in decarbonizing the non-road segment. Like traditional internal combustion engines, the efficiency of hydrogen engines largely depends on their design and construction. Among the requirements for using hydrogen as a fuel, the engine manufacturer must declare the hydrogen content of the gas. It must be 98% (minimum mole fraction) [11], or 99.97% in the ISO 14687 standard, which outlines the quality requirements for hydrogen as a fuel [12].

A meta-study [13] has evaluated the possibility of using hydrogen combustion engines as an alternative mode of drive. According to the study, the state of the art shows that many things have already been developed, and their operation ensured. The chemical and thermodynamic principles of hydrogen combustion are well known, but further research is needed regarding the special conditions of internal combustion engines. In terms of technical implementation, an engine with a port fuel injector proved simple and easy to implement. The emissions and efficiency benefits are only apparent in the lean and very lean modes. The disadvantages are the low power density, tendency to backfire, and uncontrolled pre-ignition. The direct injection engine eliminates the above disadvantages and achieves a significantly higher power density, while ensuring low emissions. However, direct injection requires considerable effort due to combustion control, and places significantly higher demands on the injection or intake valves. In the future, experts expect the development of mixed engines that use an intake pipe and direct injection simultaneously, both in the automotive industry and in the case of vehicles operating in the industrial sector. The results of the application analysis also show that the hydrogen engine is particularly suitable for areas with high power requirements and limited installation space, and for engines operating under extreme environmental conditions. The hydrogen engine can be used without restrictions at temperatures below the extreme freezing point or above 40 °C. In summary, the hydrogen engine is a workable and feasible propulsion technology that can potentially be an alternative propulsion method.

One of the best-known experiments on hydrogen internal combustion motor vehicles focused on BMW, with a fleet of 100 vehicles of BMW7, approximately 15 years ago. However, this experiment was stopped for two reasons. (i) The twelve-cylinder engine was bi-fuel, so it ran on petrol and hydrogen separately. It was not designed to run on hydrogen alone. On the other hand, (ii) using liquid hydrogen requires much effort. Burning hydrogen in an internal combustion engine can be considered a clean energy conversion mode, which is a strong argument for hydrogen. In theory, only water and heat are produced, but NO_x (oxides of nitrogen) and particulate emissions are also produced. At present, regarding hydrogen internal combustion engine drive upgrades, employing battery electric drive is out of the question. Such areas can—in addition to road vehicles—include agricultural and construction machinery, or some logistics areas, as well as military and marine applications [14].

Hydrogen internal combustion engines developed by one of the world’s largest TIER I (direct suppliers of the final product) automotive suppliers will be on the market in 2024. The engine’s construction is almost identical to a gasoline or diesel piston engine. It also saves weight because, in addition to the fuel cell, a heavy traction battery is not needed, and is replaced with a liquid hydrogen tank. End-of-pipe emissions include water vapor and nitrogen oxides. However, some technical challenges related to the hydrogen engine have not yet been definitively overcome. Examples of these include the following: H₂ (hydrogen) comes into contact with the lubricating oil and can eliminate carbon macromolecules during combustion. In addition, hydrogen, as a very reactive element, can therefore combine with substances found in the engine. According to the company, the engine it has created is ready for series production. The hydrogen engine is part of the global corporate strategy for sustainable mobility, promoting electric and hydrogen propulsion [15].

A major European engine and vehicle manufacturer has introduced a hydrogen-fuelled internal combustion engine for off-road applications. This engine is based on a diesel engine. The hydrogen engine is a sustainable alternative to the diesel engine. The essential components of the new hydrogen engine, such as the crankcase, crankshaft, connecting rods, and cooling and oil circuits, including pumps, sump and filters, are approximately 80% identical to those of the original diesel engine. The two internal combustion engines have almost identical dimensions, which make it easier for machine manufacturers to integrate them into existing vehicle concepts. However, significant modifications have been made to the hydrogen supply and combustion components, the engine control system and the exhaust gas regulation. A particular ignition system must be designed for a hydrogen-powered engine, enabling the reliable external ignition of the hydrogen mixture. The electronic engine control unit controls the hydrogen and air supply, controls the ignition, and continuously adjusts the engine parameters depending on the current operating state to achieve safe and efficient combustion. The new hydrogen fuel engine can be operated as a dual-fuel engine in combination with conventional diesel or alternative fuels [16].

European manufacturers are just some of the ones dealing with the hydrogen internal combustion engine issue. The Corolla Cross Hydrogen Concept prototype was tested in 2022. The manufacturer believes that it is too early to focus on a single zero-emission solution on a global scale. One of the reasons for this relates to the rapidly changing customer needs and market environment. According to their approach to carbon neutrality, the solution lies in diversification, so they offer more technologies with zero carbon emissions. The engine of the concept vehicle is equipped with a 1.6 L, three-cylinder, turbocharged and high-pressure direct hydrogen injection engine technology, and Toyota also included tank packaging knowledge derived from the Toyota Mirai Hydrogen. Hydrogen combustion offers key advantages, such as (i) the ability to exploit existing internal combustion engine technologies and (ii) fast refuelling times [17].

Component manufacturers also deal with internal combustion engines made from hydrogen. Together with OEMs (original equipment manufacturers), they have developed a degree of proper cooperation between the turbocharger and the engine using hydrogen fuel. In this way, they want to exploit the advantages of zero-emission fuel on turbocharged internal combustion engines. The company develops customized solutions to meet the challenging growth needs of the H₂ICE (hydrogen internal combustion engine) [18].

German universities and research institutes are leading the way in research related to hydrogen-powered internal combustion engines. One of these big names is the Technical University of Braunschweig, who say that the hydrogen combustion engine is still an outsider in the context of future mobility, but could appear as an alternative drive mode, for example, in heavy commercial vehicles such as trucks, agricultural and construction machinery (well-known technologies) without complicated conversion. Researchers see hydrogen engine technology less as a competitor with electric cars and fuel cell vehicles, and more as a complement to alternative forms of propulsion [19].

Several fuels were investigated in diesel engine in research [20]. First, a so-called waste tire pyrolysis oil was compared with standard diesel oil. Then, the effect of hydrogen was also investigated by injecting it into the intake pipe separately for both tested oils. In both cases, adding hydrogen improves the mixture’s calorific value, thus improving the engine’s torque, power and specific fuel consumption. Waste tire pyrolysis oil and hydrogen are considered a promising combination to replace standard fossil-based diesel. The research published in [21] also assessed a diesel engine by adding hydrogen and methane separately to the intake pipe as a second fuel. The combustion-accelerating and combustion-improving effects of both gas fuels have been reported.

The researchers who worked on [22] achieved significant results with hydrogen experiments on bus drivetrains. The tests were carried out in several areas; the combustion process was optimized with the injection application and injection valve designs. WHR (waste heat recovery) tests were also carried out, and further efficiency improvements were achieved. An SRC (selective catalytic reduction) system was tested with the after-treatment of the exhaust gas. In addition to these, successful experiments were also carried out on engine control, where further consumption reduction results have been achieved with systems based on ANN (Artificial Neutral Network), EMS (Energy Management System) and ECMS (Minimization of Equivalent Fuel Consumption).

Also, a hydrogen-relevant scientific publication [23] analyzed well-to-wheel energy consumption and equivalent GHG emissions using real-world simulations. The diesel engine drives with two fuel types and Otto engine drives with hydrogen fuel were the driving resources being compared. During the simulation, a realistic real-life route was developed, which was 100 km long. An important result from the point of view of hydrogen is that CO₂ emissions are reduced significantly (by about 30%) compared to standard diesel at the WtW level. Based on the results, the HVO (hydrotreated vegetable oil) fuel is rated even better than H₂ regarding energy consumption and CO₂ emissions.

According to research on hydrogen developed in shipping [24], comprehensive theoretical research was carried out for a service vessel that could be equipped with four different types of propulsion. These include internal combustion engines with MDO (marine diesel oil), LNG (liquified natural gas), MeOh (20% methanol-fueled), and H₂ fuel. It was investigated whether, on such a ship, taking into account the characteristics of hydrogen as a fuel, it is possible to create enough fuel tank storage space for an average 14-day operation at sea, as is used for these service ships. The results show that the weight and volume of the tanks used to store the required amount of hydrogen would significantly reduce (more than 50%) the weight and volume of the ship’s payload. In addition, hydrogen costs were assessed as being higher than those of other fuels.

This paper [25] presents simulation work and results related to various powertrains, mainly passenger car-sized powertrains. The drivetrains employ a traditional gasoline engine, which is retrofitted to hydrogen and applied to a hybrid drive with different electric outputs and BEV drive. The results regarding engine power, vehicle energy consumption and vehicle range parameters are evaluated. Six different drive cycles were used. In terms of performance, the hydrogen engine approaches the performance level of the gasoline engine by adding electric motor power. The energy consumption of the gasoline and hydrogen engines is higher than that of the hybrids in each cycle, and the two are almost the same. As for the range of the vehicles, the hydrogen-powered one is comparable to the range of the BEV, which can be improved with the hybrid electric motor.

1.1. Engines That Appeared on the Market Earlier in Small Series

In the same way as for traditional fuel engines, we distinguish between piston and rotary piston engines. One of the oldest German on-road and non-road diesel engine manufacturers, MAN, produced a small series of hydrogen bus engines installed in buses, comprising a naturally aspirated engine and a supercharger [26].

They were named Mazda H₂-Wankel, known as hydrogen propellant engines, and were made only with a rotary piston design. Wankel’s rotary-piston construction method and the related combustion space form provide favorable conditions for hydrogen’s material properties, such as its high combustion speed. Mazda have undertaken many experiments with hydrogen internal combustion rotary piston engines, more than most other road vehicle manufacturers [26,27,28].

For over three decades, BMW has been dealing with hydrogen in internal combustion engines. In May 2000, a hydrogen fleet consisting of 15 BMW 750 hl vehicles with a tank containing liquid hydrogen and a fuel cell was presented at the EXPO 2000 world exhibition. In 2007, BMW presented the Hydrogen 7, the first passenger car powered by a hydrogen internal combustion engine, which was already presented in a state intended for series development. The BMW Hydrogen 7 implements a bi-fuel mode of engine operation, which can run on hydrogen and gasoline. This allows for an unnoticeable changeover from hydrogen to gasoline, which can be performed automatically on the fly. The extremely low-temperature liquid hydrogen tank system is located in the luggage compartment behind the rear seats [29].

A new type of hydrogen fuel engine has been developed by Volkswagen AG, which is suitable for use as an industrial engine for driving industrial vehicles. Direct injection has been the company’s speciality in gasoline and diesel engines for years. Accordingly, in the planned H₂ engine, the fuel is introduced directly into the combustion chamber. The question of cryogenic or gaseous injection remains open. Very high specific engine power and torque could be elegantly achieved with cryogenic hydrogen injection. In the forklifts for which the engine was designed, hydrogen storage on the vehicle was to be solved with replaceable bottles. For this reason, they decided to use gas injection for quick refuelling on-site. Due to the truck’s hydrostatic drive, the usable speed of the engine is limited to 3000 rpm, which precludes increasing the power by increasing the speed. The motor must also generate maximum torque in less than 150 ms at idle. These requirements eventually led to the adaptation of mechanical filling [30].

Regulations have already been developed and are in force for the type-testing of hydrogen-fuelled passenger cars or trucks with internal combustion engines [31,32]. Therefore, no legal obstacle exists to the serial production of road vehicles powered by hydrogen fuel internal combustion engines.

It is unrelated to automobiles, but we would like to mention that hydrogen-related internal combustion engine research is also intensive in other areas, especially in the context of stationary and industrial engines [30,33,34,35,36,37].

1.2. The Aim of the Study

The authors now have the opportunity to retrofit a research diesel engine to operate with hydrogen, and to experiment on it. The authors have prepared this study to derive a comprehensive picture of what aspects should be considered before retrofitting an experimental diesel engine, and what paths should be taken to retrofit this engine to a hydrogen engine. Due to the limited scope, we will not detail this conversion from all aspects. Therefore, we only present an analysis from the following aspects, because we consider these the most important in the first instance. There are many research reports in the literature in which an engine is tested with the addition of hydrogen, or with different combustion chamber designs. There are also studies wherein a minor conversion of the hydrogen plant is carried out, and the effect of the hydrogen is examined after the conversion. However, only some studies in the literature provide an overview of the principles and logic of a significant transformation.

2. Physical and Chemical Properties of Diesel and Hydrogen Related to Mixture Formation and Combustion

The properties of hydrogen fundamentally differ from the properties of materials used as fuel for internal combustion engines. Compared to diesel, the gas state at ambient temperature is the most obvious, but by no means the main, difference. Figure 1 shows the relevant properties of hydrogen for use (mainly regarding mixture formation and combustion) in an internal combustion engine with the same properties as diesel. Based on this kind of comparison of fuels, hydrogen can be evaluated based on the various requirements for traditional use. The data in the table were taken from the indicated sources, but the characterization of each property is described in our own words in the following. Density is an essential fundamental characteristic that denotes the mass of a unit volume of material. This is very important because, even though refuelling is performed on a volumetric basis, the mixture is created using the mass data of air and fuel. The density of hydrogen in the liquid state is approximately one-tenth of the density of diesel. The table also shows the density of hydrogen in its gaseous state, but this parameter cannot be applied for diesel. The density of gaseous hydrogen is approximately one-ten-thousandth of the density of liquid diesel. The molar mass determines the theoretical air requirement. The molar mass of hydrogen is significantly smaller, which is why the need for theoretical air is much greater, as can be seen in the next row of the table. The calorific value of hydrogen based on mass is approximately three times that of diesel. This tendency changes significantly when the density parameter comes into the picture and the energy densities are compared. The calorific value of the mixture (air and fuel mixture) can only be interpreted for a diesel engine if the engine takes in air, so in this case, it can be compared with hydrogen. The heating value of the mixture formed with hydrogen is approximately 30% higher than the calorific value of the mixture formed with diesel oil. That is a crucial aspect of internal combustion engine performance. The ignition range of hydrogen is much wider than that of diesel, which can be both an advantage and a disadvantage. The wide ignition limits of hydrogen enable quality control over the engine’s entire operating range. One of the significant differences compared to conventional fuels is that the hydrogen–air homogeneous mixture can theoretically be burned up to an air-to-fuel excess ratio of λ = 10 using conventional ignition technology. The auto-ignition temperature of hydrogen is higher, from which more efficiency can be obtained, so this can represent an advantage in applications operating according to the Otto process from the point of view of knocking. The minimum ignition energy of hydrogen is more than one order of magnitude lower than that of conversional hydrocarbon fuels, which shows that, from this point of view, hydrogen does not necessarily need a different ignition system than the conventional one. However, other considerations (knocking combustion, pre-ignition) may justify the use of a special ignition system. The higher flame speed of hydrogen enables a faster and more efficient combustion process than a diesel engine’s prolonged combustion. However, the engine is more loaded and excited due to the rapid combustion and high rate of pressure increase, which results in noisier combustion. In the case of hydrogen, carbon is omitted from the elemental composition of the fuel, which makes operation with hydrogen carbon-neutral, as is also crucial in the search for zero-carbon fuel. Hydrogen motor combustion is theoretically possible without generating CO₂, CO, and HC. However, in real engine operation, carbon monoxide and carbon–hydrogen are present due to the lubricating oil present in the combustion chamber. Only nitrogen oxides should be considered a relevant emission component when operating with hydrogen.

3. Possibilities for Formatting Mixture

3.1. Injectors to Be Placed and Timing of Injections

In the following section, we provide an overview of the possibilities of indirect and direct injection, and their advantages and disadvantages. A division can be made according to the place and time of the mixture formation. Contrary to external mixture formation (H₂-EMF, hydrogen–external mixture formation), in which hydrogen is fed into the intake manifold, with internal mixture formation (H₂-IMF, hydrogen–internal mixture formation), blowing or injection takes place directly into the combustion chamber of the engine. A combined mixture formation procedure consists of a combination of the previously mentioned versions [26]. Possible divisions according to the place and time of mixture formation are given in Table 1. The authors know that there is a difference in meaning between the words injection and blowing, depending on the physical state of the fluids. Therefore, in the case of a gaseous substance, the word “blowing” would always be correct, but in order to avoid many repetitions, the words “injection” and “blowing” are used interchangeably [26].

3.2. Additional Considerations of Mixture Formation and Injection Valves

In addition to the distinction based on the location of the mixture formation process and the time of fuel injection, further divisions can be made based on the following criteria, according to the reviewed study [26]:

The temperature of the introduced hydrogen (ambient temperature/extremely low temperature);
Method of ignition introduction (external ignition/compression ignition);
Partial load control (throttling → quantity control/without throttling → quality control);
According to the state of cylinder filling (naturally aspirated/turbocharged);
According to the design of the mixture (homogeneous/stratified).

In order to achieve variable and dynamic operation in the vehicle within wide limits, it is often advisable to combine the procedures distinguished according to the above classification criteria.

According to the ignition method, hydrogen engines can also be categorized, in the same way as usual, as spark ignition or compression ignition. Due to the high auto-ignition temperature of hydrogen compared to diesel fuel (approximately 585 °C), a stable self-ignition process can only be achieved with a high compression ratio and with additional partial air preheating.

The division of the engine concepts can finally be carried out based on the design of the cylinder filling. In principle, hydrogen engines are suitable for both naturally aspirated and supercharged operation [26].

Compared to the previous table, wherein the basic concepts are presented, Table 2, taken from [10], gives a more up-to-date and detailed picture of hydrogen operation. The concepts can be divided into spark ignition–homogeneous premixed charge and compression ignition non-premixed mixture combustion. With the spark ignition, premixed homogeneous air–fuel mixture, 100% CO₂ savings are possible if only the carbon dioxide from the fuel is considered. The main disadvantages of the homogeneous mixture solution are the lower power density, higher specific fuel consumption, and transient operation. Multi-point injection (MPI) combined with spark ignition is the most cost-effective solution for low-pressure mixture-formation concepts [10].

MPI-type indirect injection requires a higher boost pressure to maintain the same air excess ratio as direct injection. On the other hand, high-pressure fuel system designs create a hydrogen injection pressure of 250–300 bar. In order to achieve the highest specific power, the lowest specific fuel consumption and the highest stability in the transient operating conditions of the engine, diffusion combustion (similar to diesel fuel) is the most suitable. To initiate stable hydrogen ignition, it is advantageous to use pilot diesel fuel injection (torch injection) [10].

Advanced technologies include injection technologies that create a stratified mixture and control the entire combustion process, with multiple injections per cycle, despite the low density and high diffusion coefficient of gaseous fuels, primarily hydrogen [38]. All other mixture formation concepts, seen in Table 3, are simpler than the ones mentioned above. Both the structurally simpler version with external cryogenic mixture formation (CPI, cryogenic port injection) and a more complex version with internal mixture formation (DI, direct injection), offering a greater degree of freedom in the development of the combustion process, seem promising [38].

Injection valves are critical components in gas engines with internal combustion. The determination of not only the location of the nozzle, but also the geometric design of its part in the combustion chamber, is important. In addition, the actuation of the nozzle needle must also be carefully selected [38]. Table 4 shows an overview of valve concepts already researched and tested in practice by the authors of [38].

So far, many injectors have been tested and compared for use in hydrogen operation. Initially, inward nozzle needles and blind hole nozzles with symmetrical or asymmetrical hole patterns were used, and injectors with entirely newly developed slot nozzles came later. There are also injectors with an outwardly opening nozzle (Table 5) [32].

The scientific publication [39] analyzes the injection process in detail. The design of the valve is not here presented in detail, and only the effects of the injection pressures are dealt with. They found that the injection pressure significantly affects the penetration depth and the size of the jet cross-section. In research on hydrogen injection valves [40], solutions using one and two injection valves were investigated for a single-cylinder spark-ignition hydrogen engine. The valves were purchased commercially for the PFI engine application. The geometry of the valves was not presented, only their flow characteristics. The results show that single-valve delivery was insufficient to generate adequate engine power. Perfecting the two-valves mode could make the mixture sufficiently rich (λ = 1.49). Another advantage of the two-valve solution is that it is possible to shorten the injection times under this approach, thereby reducing the waiting time for hydrogen in the intake pipe. The two-valve solution resulted in higher combustion pressure and maximum heat release.

3.3. Mixture Formation and Combustion Process

The starting point for developing all combustion processes is the concept of mixture formation. On the one hand, several promising combustion processes can be investigated between the limit cases of the complete homogenization of the fuel–air mixture in the Otto engine process and the pure diffusion combustion occurring in the diesel engine process [38]. Figure 2 presents concepts regarding hydrogen mixture formation and combustion between the two extreme solutions mentioned above. Of these, (i) late single injection with mixture stratification and (ii) multiple injection for combustion control have proven to be particularly beneficial in previous works, and have already been examined in detail by the authors in [38].

The project aimed to achieve a specific 100 kW/liter output with high-pressure direct injection. With advanced electromagnetic and state-of-the-art piezoelectric actuated injectors, significant efficiency and raw emissions have been achieved—far beyond the levels shown by internal combustion engines powered by conventional propellants. This was achieved through optimized injection strategies, in which the timing of the injections can be varied freely. The total amount of hydrogen can be distributed between several injection pulses to inject directly into the flame, and predominantly diffusion combustion takes place. This makes it possible to combine the performance advantages of a gasoline engine with the efficiency and torque advantages of a diesel engine, potentially exceeding both, in a single internal combustion engine [38].

4. Pre-Ignition, Backfiring, Knocking

In the hydrogen-powered internal combustion engine, we can cite three abnormal combustion types—pre-ignition, backfiring, and knocking. Their formation can be traced back to the properties of hydrogen used as a fuel, such as its wide flammability range, low ignition energy and high flame speed. The processes of development of the three phenomena are also related to each other [41]. Many factors from the engine design side can influence and induce the above abnormal combustion processes. These can be, for example, improper valve timing, fuel injection timing, the air-to-fuel excess ratio, an improper compression ratio, improper ignition timing or hot spots in the cylinder [42].

Hydrogen can also ignite by self-ignition if the first ignition is caused by an external energy source, i.e., a spark. During the subsequent serial injection, self-ignition can occur with the help of the previously generated temperature and the remaining oxygen. In this way, multiple injections can be used to control combustion [43].

4.1. Pre-Ignition

The AHP–entropy weighting method (Analytic Hierarchy Process) was used to investigate the abnormal combustion index layer of an intake pipe injection hydrogen internal combustion engine under different hydrogen injection angles and hydrogen injection flow rates (mass flow). In this research, a modified Jialing JH600 engine was used, which was a single-cylinder water-cooled four-valve engine. A model was created based on this engine for simulations. The relevant engine parameters are as follows: (i) compression ratio of 9.7:1, (ii) maximum power output of 30 kW at 6000 rpm, and (iii) maximum torque of 51 Nm at 4500 rpm. The research results show that with the AHP–entropy method, all indexes could be brought to a reasonable level, and pre-ignition or backfiring caused by unjustified index selection could be avoided in the optimization process. When the hydrogen injection angle and rate were too large or too small, which was not conducive to the uniform mixing of hydrogen and air, there was a greater danger of abnormal combustion. The calculation results show that with a fuel-to-air equivalence ratio of 0.5 and 0.67, a hydrogen injection angle of 45 °CA and a hydrogen injection flow rate of 4.96 kg/h, it was possible to ensure that all parameters of the index layer were at the desired level. The optimization goal was achieved numerically; the relative maximum abnormal combustion risk factors decreased by 5.4% and 5.5% for the two air condition factors examined [44].

Dual injection modes, especially spaced dual injection, can increase the spatio-temporal distribution of hydrogen, and significantly improve in-cylinder mixture uniformity at different engine speeds, mixing ratios, and nozzle diameters. The improvement between symmetrical and intermittent dual injection increases with increasing engine speeds and nozzle diameters. Regardless of the injection method, the mixture uniformity coefficient decreases as the nozzle diameter increases. Compared to distributed dual injection, the uniformity of symmetric dual injection fluctuates significantly when varying the nozzle diameter. The diameter of the nozzle affects the uniformity of the mixture in the cylinder and the mass of the mixture that flows back into the intake port. Combining these two factors results in peaks in indicated power and indicated thermal efficiency in either of the dual injection modes when the nozzle diameter is 4 mm [45]. Not only are cylinder temperatures the highest when using distributed dual injection modes, but the high-temperature regions are also more widely distributed, significantly increasing the likelihood of uncontrolled pre-ignition as the equivalence ratio and engine speed increase. After closing the intake valve, the mass of hydrogen remaining in the intake port can be used in the HICE (hydrogen internal combustion engine) to evaluate its backfiring. Inlet clogging is alleviated by dual injection modes, and mainly distributed dual injection, which improves charging efficiency and reduces the occurrence of backburn. However, backburn is more likely to occur if the equivalence ratio is higher (e.g., 0.67), since the intake port contains more residual hydrogen mass. The residual hydrogen mass in the intake port decreases with increasing engine speed and nozzle diameter, and reduces the probability of backfiring [45]. Figure 3 shows a typical pressure function for pre-ignition. The curves are for an engine speed of 3500 rpm and an air-to-fuel ratio of 1.5. The thick continuous curve indicates pre-ignition, both in the cylinder chamber and the intake space. You can see where the intake valve closes as a result of the pressure change in the intake space between −180 and −90 CA. After this point, the phenomenon of pre-combustion begins at approximately −90 CA, with a sharp pressure rise of approximately 10 bar, after which the slope falls back to the value associated with normal combustion and continues along this line. In case of abnormal combustion, the maximum value of the pressure is ca. 15% higher than the normal pressure peak, and it peaks earlier, close to the top and right in the center. The pre-ignition, therefore, has no direct effect on the pressure of the medium in the intake port, but its presence may facilitate the process of a subsequent back-ignition [41].

4.2. Backfiring

In this subsection, to avoid repetition, the words backfiring, backburn and back-ignition are used for the same phenomenon.

Hydrogen fuel applications in internal combustion engines have received increasing attention due to the absence of carbon dioxide emissions and excellent combustion characteristics in terms of thermal efficiency. The brake thermal efficiency of hydrogen-powered internal combustion engines is higher than that of other fossil fuels. However, abnormal combustion, such as backfiring in indirect hydrogen injection engines, limits the improvement in internal combustion engine performance due to hydrogen’s low ignition energy and high flame propagation speed. The volumetric energy content is significantly reduced if the engine backfires. In addition, it causes damage to the intake system and the fuel injection system. High-temperature residual exhaust gas, hot spots, and abnormal spark plug discharge cause backburn. All the factors that cause the pre-ignition of the hydrogen–air mixture also promote its backfiring [42].

Backfire describes the combustion of the fresh hydrogen–air charge during the intake stroke in the engine’s combustion chamber and intake manifold. The engine sucks the fresh hydrogen–air mixture into the cylinder space by opening the intake valves. When the fresh charge ignites, for example, in the hot spots of the combustion chamber due to the hot residual gas or particles, or the remaining charge in the ignition system, it ignites in a manner similar to pre-ignition. However, backfiring occurs, since the intake valve is still open at that time. The main difference between back-ignition and pre-ignition is the time at which it occurs. Pre-ignition occurs during the compression stroke, with the intake valves already closed, while back-ignition occurs with the intake valves open. This results in combustion and increased pressure in the intake manifold, which is not only audible, but can also damage or destroy the intake system. Due to hydrogen’s lower ignition energy requirement, backfiring is more likely to occur when the fuel–air mixture approaches the stoichiometric mixture. Since most direct hydrogen injection strategies start injection after the intake valves are closed, the occurrence of backfiring is generally limited to external mixture formation concepts. As soon as the intake valves open, the fresh charge ignites in the cylinder and the intake manifold, causing the intake pressure to increase by up to 3 bar [41]. An example of this process is shown in Figure 4. Similar to the previous (pre-ignition) figure, the engine operates at high speed, while the value of the air condition factor is the same as the previous one.

After the fresh charge is burned, the intake manifold pressure drops, and the cylinder pressure increases compared to the average pressure drop when the intake valve closes. The peak cylinder pressure for this re-ignition cycle is only about 30 bar, and the indicated mean effective pressure is negative. A further correlation inducing backfiring may be related to the fact that uncontrolled pre-ignition and backfiring are closely related to pre-ignition as a precursor to the occurrence of backburn. Uncontrolled pre-ignition heats the combustion chamber, which can eventually lead to backburn in a subsequent cycle. Consequently, any measure that helps to avoid unwanted pre-ignition also reduces the risk of back-ignition [41].

Significant results were achieved by the authors, who converted a 2.0 L naturally aspirated four-cylinder gasoline engine into an inlet port fuel injection HICE. The engine stroke and cylinder diameter were the same, and the compression ratio was 10.0:1. Frequent backburning can occur in the intake manifold in hydrogen indirect fuel injection internal combustion engines when the fuel-to-air equivalence factor is greater than 0.56, thus limiting further increases in engine power. Thus, to control backburn, an in-rail fuel injection test system and a computational fluid dynamics model were established to explore the factors that cause backburn under high loads. The temperature and concentration of the gas mixture near the intake valves are among the most critical factors that result in backburn. Optimizing the timing and pressure of hydrogen injection reduces the concentration distribution of the intake mixture and the temperature of the highly concentrated mixture through the intake valve, thus enabling backburn control. Backburn control allows HICE to function normally with fuel-rich mixtures (Φ above 1.0). HICE with optimized hydrogen injection timing and pressure causes a significant increase in performance [46]. In another study, CFD (computational fluid dynamics) simulation was used to investigate the effect of injection timing on mixture formation in an intake pipe (indirect) injection H₂ICE at different engine speeds and air-to-fuel ratios. It was concluded that the intake manifold injection H₂ICE limits injection timing so as to prevent backburn in the intake manifold. Finally, engine experiments have proposed and validated a limit value for H₂ICE injection timing. The subject of the experiments was a four-cylinder engine with a total displacement of 1998 cm³ and a compression ratio of 10.5, which had a nominal output power of 60 kW at 5500 rpm and an output torque of 111 Nm at 4000 rpm [47]. A study on the exhaust gas after-treatment system of a hydrogen engine [48] also stated that, in the case of an internal mixture-forming system, i.e., a direct injection engine, the higher tank pressure reduces the effective tank capacity, which can only be used up to the pipe pressure level. This leads to relatively early injection, possibly even with open intake valves, highlighting indirect injection’s disadvantages. In the worst case, due to hydrogen’s wide ignition limit, backburn occurs when the hydrogen/air mixture ignites in the intake manifold. This poses a very high risk of engine damage [48]. Research was carried out in [49], with the change of the combustion cycle covariance of the indicated mean pressure used to characterize said risk. The test series was conducted on a four-cylinder hydrogen internal combustion engine with port fuel injection. The engine had a cylinder diameter and stroke of the same scale (86 mm), a displacement of 1.998 dm³ and a compression ratio of 10.0:1. During the research, the backburn phenomenon was also observed. At idle, the covariance increased, since the turbulence of the mixture was very weak, and hydrogen injection significantly affects fresh air mass flow and cooling. Backburn was probably not observed because no hot spot was formed in the cylinder. Therefore, when idle, the combustion process was stable and abnormalities-free. According to research in [50], backburn is a known risk factor for intake pipe injection and hydrogen engines. The study dealt with direct mixture formation systems of hydrogen engines. From this point of view, its considered the direct injection of H₂ICEs more advantageous. The design of the injection valves is especially important for hydrogen, because the desired cylinder performance depends on the flow rates available from the injection valve. The flow rate depends, of course, on the pressure of the hydrogen system. The article mentions a thermal efficiency of 25–45% in relation to the hydrogen engine. Adaptation experiments are dealt with in [51]. A diesel engine was here converted into an H₂ HP-EGR (high-pressure–exhaust gas recirculation) lean combustion concept. The basic engine was a make of Deutz diesel engine, with the most important parameters set as follows: (i) six cylinders, (ii) 7.8 L displacement, (iii) 210 kW power and 1000 Nm torque output, and (iv) a compression ratio of 12:1. In that paper, as in the above, the low ignition energy of hydrogen and the limits of a high flammability mixture are also mentioned as disadvantages, which can lead to uncontrolled backburning. To eliminate this, they laid down the fundamental principle that the injection and air intake must work in sync with the highest possible airflow speed. In [52], it is also established that when designing the operation of truck engines with hydrogen fuel, due to the risk of backburn, the port fuel injection concept should no longer be considered. In addition, they presented valuable information about the efficiency of hydrogen engines. According to this, an engine of the model year 2025 with H₂ LP-DI reached the maximum efficiency value of 42%, and an engine of the 2030 model year with H₂ HP-DI can reach the maximum efficiency value of 46.5%. In [53], unlike the previous study, the goal was explicitly open-valve injection and, with it, the avoidance of backburn and minimization of losses due to unburned fuel. However, we are here discussing an engine concept for passenger cars and light commercial vehicles. As a result of the experiments, a brake thermal efficiency of 37% was achieved at a given speed and brake mean effective pressure. In this research, the hydrogen concept engine was a hybrid engine from the mixture formation point of view, because it had an indirect injection valve and a direct injection valve. Otherwise, the engine was a four-cylinder, turbocharged engine, and its compression ratio was 9.8:1. Ref. [54] also deals with direct injection hydrogen internal combustion engines. It states that the disadvantage of indirect injection solutions is that hydrogen injection outside the combustion chamber carries the risk of backburning, especially in the high load range. Direct injection systems can only achieve the high specific power required for heavy-duty, long-distance transport applications. Ref. [55] examines the characteristics of hydrogen combustion in detail. Also, one of the disadvantages of hydrogen engines with indirect injection spark ignition is the low energy of hydrogen ignition; here, not only can the ignition spark cause a flame, but it can also generate a hot surface and a hot material part, which is not controlled. This has several adverse effects. In addition to the sudden heat and mechanical load, the effective torque of the motor decreases, and the engine’s power must be limited. If the engine’s full output is to be used, solutions must be found to eliminate abnormal ignition phenomena.

The factors leading to backburn mainly include the high residual exhaust gas, extremely slow combustion, and inadequate hydrogen distribution around the intake valve seats. Backfire control strategies ensure their effectiveness under specific application conditions, beyond which they harm the effectiveness of backfire control. When re-ignition control strategies are used, power loss is almost inevitable for naturally aspirated engines. Several control strategies are proposed to mitigate engine power loss caused by backfiring; meanwhile, multi-objective optimization is proposed to achieve optimal global performance [42].

In reducing the residual hydrogen in the intake pipe, hydrogen injection should be enacted early enough. According to the change laws of turbulent kinetic energy, to improve the uniformity of the gas mixture in the cylinder, hydrogen injection should be neither too early nor too late. The relative retention time of the highly concentrated gas mixture in the intake manifold is longer when the injection occurs early, quickly leading to back-ignition. If hydrogen injection is delayed, the increase in cylinder pressure prevents hydrogen from entering the cylinder. This increases the concentration of hydrogen at the intake valve in the intake manifold after closing. At this point, the degree of mixture uniformity is poor, and the gas mixture with a high fuel concentration collects on the intake valve. This can easily lead to pre-ignition or back-burning if the flames penetrate the intake manifold. The optimal hydrogen injection starting point should be set between 390 and 430° CA (crankshaft angle) based on the research results. If the hydrogen injection time is set early, the remaining hydrogen in the inlet for the different nozzle positions will be almost the same; no difference can be seen. When the hydrogen injection timing is delayed, it becomes more and more evident that hydrogen does not enter the cylinder due to the remote position of the nozzle, and more residual hydrogen is generated at the cylinder inlet when the position of the nozzle is further away. At the end of the compression stroke, the spatial distribution of the equivalence ratio will be similar for the different nozzle positions examined. However, the distance between the nozzle and the intake valve will be greater. In that case, the more residual gas mixture near to the valve opening, the greater the probability of backfire. Therefore, it is advisable to choose a nozzle position that is close to the intake valve. It is not favorable for the gas mixture to enter the cylinder “quickly” if the diameter of the nozzle is small and the duration of hydrogen injection increases significantly. The longer the residence time of the gas mixture in the inlet, the greater the possibility of back-ignition. If the diameter of the nozzle is too large, a more significant difference in gas mixture concentration will develop in the cylinder, which does not favor the uniformity of the mixture. However, the hydrogen enters the cylinder early, so the gas mixture is formed over a long time, promoting the formation of a more homogeneous gas mixture. In the overall analysis, the diameter of the nozzle should be adequate, with 4 mm being the most appropriate [56].

In order to determine the possibilities of the direct cylinder injection of hydrogen-powered engines, an experimental study was carried out with an ASTM-CFR (American Society for Testing and Material–Cylinder Firing Rate) engine. The standard Otto cylinder head and the standard diesel cylinder head were used for the experiments. Performance, thermal efficiency and emissions of nitrogen oxides were measured. The feasibility of employing a scheme in which hydrogen gas injection starts late in the compression stroke, ignition occurs as early as possible, and the injection rate determines the burning rate has been investigated. This scheme prevents undesirable combustion phenomena such as pre-ignition, significant increases in cylinder pressure and high-amplitude pressure waves in the cylinder. It also avoids backburn in the mixer [57].

4.3. Knocking

A summary study dealing with the short-term perspectives of hydrogen internal combustion engines also calls direct injection strategies advantageous because they eliminate the disadvantages of indirect injection systems, such as the abnormal combustion patterns of knock, backburn and low power density [58]. Intake port fuel injection is a fuel-flexible, durable, inexpensive mixture formation system. Combustion knocking, as an abnormal combustion phenomenon, not only limits the torque delivered by the engine and the thermal efficiency, but also loads the piston and the engine, both thermally and mechanically. This study uses a 2.0 L PFI (port fuel injection) hydrogen internal combustion engine and a calculated model to study the contributing factors and frequencies of combustion knock. The results show that combustion knocking occurs at a relatively higher engine speed (over 3000 rpm), while gasoline engine knocking is more characteristic of lower engine speeds. The average end-of-combustion temperature calculated using a two-zone thermodynamic model is 1000–1100 K for hydrogen engines, which is higher than that of gasoline engines. Knocking and other abnormal combustion phenomena (uncontrolled flashback and pre-ignition) mutually influence each other. When backburn occurs, the components in the cylinder heat up. In the next cycle, the cylinder components emit heat into the intake port, which can increase the initial temperature of the charge at the time of ignition. A high initial temperature leads to combustion knocking. However, combustion knocking further induces this process because, as a result, the temperature of the cylinder elements continues to rise, generating hot spots and ultimately leading to another uncontrolled pre-ignition and re-ignition. An analysis of the pressure indication results obtained from knocking combustion using Fast Fourier Transformation (FFT) showed that the frequency of knocking combustion occurring in hydrogen engines is higher than that of gasoline engines in all operating modes. The pressure waves of the combustion knock propagate radially in the case of mild combustion knock, and in the circumferential direction in the case of strong combustion knock [59]. As an abnormal combustion phenomenon, knocking in hydrogen engines differs from that in gasoline engines. The gaseous properties of hydrogen, such as low ignition energy and short extinguishing distance, contribute to combustion knocking at high engine speeds (>3000 rpm), contrary to gasoline engines. Ignition timing and backfire are the two critical factors contributing to combustion knocking at high engine speeds. Backfiring can increase the potential for combustion knocking because backfiring increases the temperature of the charge at the time of ignition. Combustion knocking that occurs in series can increase the temperature of the cylinder components, thereby generating uncontrolled pre-ignition and, ultimately, backburning. The knocking frequency of hydrogen engines is higher than that of gasoline engines in all operating modes. The pressure wave of a strong knock propagates mainly in the circumferential direction, while the pressure wave of a light knock propagates in the radial direction [59]. Figure 5 shows the knocking frequency spectra for knocking combustion at different knocking intensities. The definition of knocking intensity can be found in the source document [59]. Processes with knocking intensity have different characteristic frequencies, and their extents are also different.

In hydrogen engines, increasing the fuel-to-air excess factor reduces in-cylinder pressure and pressure fluctuation, resulting in a significant reduction in knock intensity and the probability of knock occurrence. Engine speed and ignition timing are important factors contributing to knocking in a hydrogen internal combustion engine. The engine is more likely to knock at low revs, while the ignition delay amplifies and then delays the tendency to knock. For example, delaying injection timing and reducing injection pressure can affect combustion characteristics, pressure drop, heat generation and flame speed, thus suppressing the tendency to knock. Based on the analysis of the Taguchi–Grey method, the engine speed is the most significant factor in knocking. Based on the simulation results, there is a slight knock tendency under the following conditions: speed—1300 rpm, ignition timing—15 °CA ATDC (after top dead center), injection timing—120 °CA ATDC, and injection pressure—2.5 MPa [60]. The effects of the Miller cycle and the oxygenated strategy on knocking are opposite. The former is a flame-retardant, while the latter is a fire-enhancer. Overall, both reduced λ and increased oxygen ratio increase knock intensity, while using the Miller cycle suppresses combustion knock to some extent. A deep Miller cycle can be achieved in a direct injection hydrogen engine by advancing the IVC (intake valve control) timing, and thereby effectively suppressing knock. Timing can affect the exhaust process and slightly improve the degree of combustion knock, which can be used as an additional optimization strategy. By analyzing the knocking characteristics at a high oxygen ratio, it can be seen that the knocking duration is mainly concentrated in the interval between 5 and 18 °CA at 30.6% O₂, while the exact value is at 7–25 °CA at 32.0% O₂. Increased O₂ makes the engine more prone to a more drastic heat release process and higher knock intensity at short crank angles, while extending the knock duration [61]. Comparing the knock peak pressure with the crankshaft angle, the knock intensity and the knock peak pressure combination can be used to identify the knock type using the Gaussian mixture model. This work compared the performances of Multiple Linear Regression Support Vector Machines with different kernel functions and Backpropagation Neural Networks in hydrogen-powered knock prediction. Based on 50 dataset partitions, the multiple linear regression model was found to have a better global predictive performance than other algorithms. In addition, due to the different generation mechanisms, it is necessary to set up separate predictive models. A strong knock can also be accurately predicted by the multiple linear regression model, but a weak knock cannot, and requires more advanced machine learning algorithms to be discovered, or an algorithm trained with more parameters to predict. In addition to knock prediction, the Support Vector Machine can also be applied to knock classification [62].

Research explicitly dealing with the knocking combustion of hydrogen [63] measured constant knocking at λ = 1.25, while increasing the excess air factor showed a decreasing trend and stopped at λ = 2. PPmx (peak pressure maximum) and Mahle KI (knocking intensity) indicators were used to detect knocking.

5. Spark Plug and Ignition System

Although the auto-ignition temperature of hydrogen is significantly higher than that of gasoline or diesel [26,41], this justifies the need for an external source for the ignition process. Despite this, experiments were also carried out with hydrogen compression ignition engines, namely, car engines and large stationary engines. However, efforts have also been made to experiment with hydrogen in an engine operating as an HCCI.

In most of the relevant literature, including [41], it is recommended to use the cold-rated spark plug in a hydrogen-powered internal combustion engine because the electrodes of this plug do not overheat, and thus do not cause auto-ignition or backburning. Another reason could be that deposits of this type hardly form on the spark plug. Platinum should be avoided as an electrode material because it acts as a catalyst for hydrogen oxidation. Platinum is used as a catalyst material in three-way catalysts to oxidize unburned hydrocarbons. It is used as a catalyst to reduce the activation energy and temperature of the process.

A research report [53] investigated and evaluated the mixture formation, combustion process and emissions of a hydrogen engine. The research framework adapted a turbocharged spark ignition engine for hydrogen operation. Their results show significant opportunities to use hydrogen even with a relatively small transformation and adaptation effort. Among the areas to be adapted were the injection, ignition, and charging systems. Only the use of a cold spark plug was marked as an adaptation related to the ignition system. The direct injection of hydrogen, with internal mixture formation, leads to a significant improvement, especially in the case of short injection times, when the injection starts after the intake valves are closed. In addition to hydrogen’s high auto-ignition temperature and low ignition energy, the combustion process of a spark ignition engine using a cold rate spark plug is ensured in the case of a homogeneous mixture. The combination of wide ignition limits and a very high laminar velocity enables efficient, homogeneous fuel-lean mixture operation with the lowest possible emissions. A properly selected spark plug is, from a technical point of view, crucial. Hot spots in the combustion chamber represent difficult conditions from the point of view of combustion control. Such hot places can be particle deposits on the surface of the spark plug, which can be created due to the pyrolysis of the lubricating oil. Deposits, especially deposits from lube oil additives, act as catalysts during ignition and combustion, further increasing uncontrolled pre-ignition events early in the intake process. However, hot spots are hot floating particles and hot metal surfaces. The spark plug is also a critical component in terms of knocking combustion. Spontaneous ignition and detonation combustion are also considered abnormal, and can cause knocking. Based on the detonation theory, engine knocking can be caused by the flame accelerating from the spark plug into the other ranges. Self-ignition can also be caused by the ignition of unburned tail gas parts at hot spots. Better efficiency, lower specific fuel consumption and higher power density can be achieved primarily with a higher compression ratio. However, this makes the engine prone to knocking, especially at high load and low rpm [42]. During the ignition of hydrogen fuel, the ignition arc, as an ion current, is less likely to occur than, for example, in the case of gasoline fuel. For this reason, the energy stored in the coil is not used up with the help of the ignition arc, but residual energy accumulates, which is only slowly used up. There may also be an operation mode wherein the hot exhaust gas remaining in certain places of the cylinders from the previous cycle provides an opportunity to ignite the fresh hydrogen–air mixture. This abnormal discharge could be due to the energy remaining in the controlled ignition because the ignition spark could not create an adequate ion current [54]. Some of the ignition energy may have remained in the ignition systems even after the normal formation of the ignition arc, which may have abnormally ignited the fresh hydrogen–air mixture. The residual energy release capacity of the ignition systems determines the re-ignition possibilities. A modified-capacity spark plug and ignition cable were used to release the ignition systems’ residual energy quickly. An additional measure in the experiment series was the water-cooled spark plug, which significantly reduced the temperature of the spark plug; in this way, the electrodes did not represent a hot spot, which would be an ignition source for hydrogen. The disadvantage of using an ignition system with a water-cooled spark plug is the system’s complexity [64].

In order to avoid uncontrolled ignition due to residual energy in the ignition system, the ignition system must be grounded properly, or the resistance of the high-voltage cable must be changed. Coil-on-plug ignition systems, now almost exclusive to passenger car engines, offer both advantages and disadvantages to the hydrogen engine in terms of application. The higher secondary voltage produced is an advantage in terms of the ignition of hydrogen because, in the case of hydrogen, the ion concentration is lower than in the ignition of hydrocarbons. On the other hand, the phenomenon of unwanted induced induction in the adjacent coil can be cited as a disadvantage. The spark plug gap can be reduced to lower the breakdown voltage. This is not a problem for hydrogen-powered engines, as no deposits would further affect this gap. An electrode gap of 0.25 mm can be used [41].

Reference [65] concerns a patent submitted to the European Patent Office by German passenger car manufacturing company employees. It suggests the geometric design of spark plug electrodes for hydrogen engines. The proposed spark plug is conventional in that it has a central electrode and at least one body electrode attached to the screw thread. In order to make the known spark plug more usable for hydrogen-fuelled internal combustion engines, it is proposed that at least the edges and ribs of the body electrode toward the centre electrode should be rounded (Figure 6).

Another patent is related to ignition, and is recommended for hydrogen-powered internal combustion engines. The fuel supply, mixing and ignition are located in that unit in one plug. The plug can be placed in the cylinder head, and belongs to an engine cylinder. The ignition spark is generated in the part above the combustion chamber of the plug, where the fuel–air mixture enters the combustion chamber. The electrodes are made of platinum, which improves the catalytic conversion of intermediate products generated during combustion [66]. There is a historic Swedish ignition system manufacturer that also produces ignition systems for heavy-duty hydrogen engines. They produce a robust ignition system that meets the challenges of hydrogen regarding pre-ignition and wear resistance. The ignition system has capacitive energy storage. It works with FlexiSpark^® technology, which means that it can generate variable outputs according to the needs of the essential ignition parameters, such as ignition voltage, ignition time and ignition current. It also has the feature of spark monitoring. It has two designs; one is the coil on plug, and the other is the pencil coil [67]. In [68], a pre-chamber spark plug is presented, which is proposed for gas engines in which the hydrogen and natural gas mixture is at its maximum of 20 v/v%. The company recommends this solution, which can be retrofitted later, for its engines with different cylinder numbers, which operate as stationary engines in power plants. With this type of spark plug, the component’s service life increases due to the lower ignition energy. An additional advantage is the better attainable combustion quality. The effects of spark plugs of different materials on engine performance were investigated on a single-cylinder, four-stroke, water-cooled, low-power engine. The engine was initially powered with gasoline, but a small amount of hydrogen was added to the original fuel during the experiments. Replacing copper spark plugs with iridium and platinum spark plugs accelerated combustion and increased engine power at all compression ratios, engine loads, and hydrogen blend ratios. The results were more obvious for platinum spark plugs than iridium plugs [69]. This work [70] tested a single-cylinder engine designed by a research institute with a displacement corresponding to a passenger car engine, developed for extreme lean burn and passive pre-chamber ignition, but adapted for hydrogen fuel operation. With hydrogen combustion, a customized cooling system resulting in low metal temperatures was simulated and optimized to avoid hot spots in the combustion chamber. The single-cylinder engine tested had a compression ratio of 12.2, and featured indirect fuel injection and conventional spark plugs. During the research, the engine was operated with a passive pre-chamber to examine the effect of the hydrogen mixture on ignition. The results show the advantages of pre-chamber combustion, short burn duration and high knock resistance, especially at full load. In a relevant study [71], an indirect injection hydrogen engine was investigated using different operating strategies, such as a base spark plug and a cold spark plug, and, as the last parameter, changing the ignition energy. The total displacement of the tested engine was 2 dm³. The injection valve was originally used for natural gas, but was here used to run hydrogen by being integrated into the engine’s intake manifold. The examined parameters were the engine’s NO_x emissions, the length of the flame spread, and the engine’s torque and efficiency. Among the results, it was observed that the phenomenon of backfire at the maximum torque limited the value of the air condition factor. Similar to the previous case, here, as the engine speed increased, the backburn tendency and the excess air ratio related to maximum torque also increased. Even if the ignition energy was reduced, this did not eliminate the occurrence of backfiring; therefore, the excess air ratio and maximum torque did not change significantly with the decrease in ignition energy. Back-ignition was effectively suppressed when using a cold-type spark plug, and the engine’s maximum torque increased significantly. However, the efficiency—due to the increase in combustion temperature and the consequent increase in heat loss—decreased. Ref. [72] offers a comprehensive study of hydrogen internal combustion engines, and touches on the spark plug issue. Its most important finding, which other studies still need to address, is that with a very lean air/fuel ratio (from 130:1 to 180:1), the flame speed decreases so much that using a dual spark plug system is beneficial. According to the study, using the waste spark ignition system is not recommended because, in this case, the waste sparks are a pre-ignition source. Spark plugs for hydrogen engines must be cold-rated, and platinum electrodes must not be used. Warm-rated spark plugs are designed to maintain a certain amount of heat to prevent carbon deposits from building up in the plug’s combustion chamber. Since hydrogen does not contain carbon, hot spark plugs are useless.

Here, on the subject of ignition, it is worth mentioning [73] that the introduction of external energy is possible not only with a traditional ignition spark, but also with, e.g., a barrier-discharged ignition system, which was also experimented with under hydrogen operation. In addition to its ignition stability, it accelerates the first oxidation stage, reducing the risk of backburn compared to a conventional candle ignition plant.

6. Designing the Combustion Chamber of the Engine

6.1. Pure Hydrogen Engines

The work cycle of the hydrogen internal combustion engine is based on the process of traditional internal combustion engines; however, the mixture formation system and the combustion process can be modified to operate exclusively with hydrogen or in dual operation, and thus they can operate with hydrogen or a fuel mixture containing hydrogen (hydrogen–natural gas). Using hydrogen as a fuel for piston engines is by no means new; in the 20th century, in the 1930s, researchers experimented, with partial success, on the conversion of piston engines to hydrogen operation, mixing hydrogen with fuel to increase the efficiency of a conventionally operating engine [26]. The left part of Figure 7 shows a section of an Erren hydrogen engine from the 1930s. The right part of Figure 7 also shows a section of a MAN hydrogen bus engine produced in a small series. The Erren engine has a slightly curved piston crown and a combustion chamber in the cylinder head, while the MAN engine has a piston chamber. In this cross-section, the piston chamber appears tub-like, with a truncated cone, the lower part of which is flat, and the mantle expands towards the top of the piston.

The following presents the results of experimental work on hydrogen engines. The main subjects addressed by the experiments are the following: mixture formation, combustion of air–hydrogen mixtures with different air/fuel ratios, NO_x formation, and performance parameters. The R&D work was performed on a single-cylinder test engine (aspirated and supercharged) and a turbocharged six-cylinder test engine. The engine under study is a supercharged spark ignition flat six-cylinder engine that uses gas fuel, and here, the gas is injected directly into the cylinder [74]. Figure 8 shows the combustion chamber of the engine. The combustion chamber here is also a piston chamber. The lower half of the chamber is flat, and part of the side mantle has a bay design.

In another relevant study [75], the performance of a hydrogen-powered ORP (open rotary piston) engine at different compression ratios was investigated using numerical simulation. As a fundamental parameter, the compression ratio provides the basis for the design and structural optimization of hydrogen ORP engines. The engine’s compression ratio was changed, in response to which the maximum pressure inside the cylinder reacted sensitively. The pressure increased from 3.5 MPa to 4.0 MPa when ε was increased from 8.90 to 9.66; however, the increase reached an additional 1.0 MPa when it was increased from 9.66 to 10.55. The burning rate of hydrogen before the top dead center was almost the same in the three cases, and the changes in the burning rate under the high compression ratios after TDC (top dead center) were greater than those of the low-compression-ratio counterparts. Burn durations decreased almost linearly with increasing compression ratios; however, the combustion phase was the earliest at a compression ratio 9.66.

Another scientific study has aimed to present and investigate, for the first time, the possibility of using hydrogen in a particular internal combustion engine—a free-piston engine generator. The work involved the development of a full-cycle zero-dimensional dynamic simulation model, and presented an extensive coupled research method using a zero-dimensional dynamic model of the combustion process. The iterative simulation of a multidimensional computational fluid dynamic engine model provides insight into the operational characteristics and performance of the new hydrogen-powered engine. The effects of piston movement on the combustion process were analyzed. The performance characteristics of the new hydrogen engine were found to be significantly different from those of the corresponding conventional hydrogen engines and free-piston gasoline engines. The results show that, compared to the conventional hydrogen engine mentioned above, the free-piston hydrogen engine shows higher piston acceleration around the top dead center, and displays an advantage in terms of emissions due to faster stroke length expansion, but its indicated efficiency is slightly lower due to the longer and later combustion process, according to [76]. The examined combustion chamber can be seen in Figure 9, with the piston moving from the bottom dead center (left side) to the top dead center (right side). The piston chamber is a flat circle-based chamber with bay side walls.

Another relevant paper presents a new type of hydrogen fuel engine from Volkswagen AG, which is suitable for use as an industrial engine for driving forklifts thanks to its combination of hydrogen direct injection and mechanical charging technology, used for the first time in this form [30]. At the beginning of the development of the H₂ internal combustion engine, the first question is how the mixture is created. Direct injection has been the company’s core speciality in the context of gasoline and diesel engines for years. Accordingly, the fuel was introduced directly into the combustion chamber in this hydrogen engine. The base engine was an EA113 2.0-l TFSI (turbo fuel stratified injection) engine [30]. Figure 10 depicts a section of the cylinder head and highlights the injector. However, the combustion chamber is also partially visible. Based on this, this engine does not have a piston combustion chamber, and the combustion chamber is in the cylinder head.

During the development of the engine, attention was paid to wide usability, with only minor changes made to the unit. However, both the stoichiometric and lean combustion processes were successfully performed in bench tests of the engine. In addition to the version presented for the λ = 1 mode, the motor can also be operated in other applications by running the appropriate software. For hydrogen operation, the compression was reduced to ε = 9 to avoid straying too far from the optimal high-efficiency operating conditions as a result of knocking. However, operating the engine with natural gas, which is less sensitive to knocking, makes it possible to maintain the compression of the series engine (ε = 10.5) [30].

The retrofit solution for internal combustion engines developed by the FEV and NGV powertrain offers an economical and robust solution. It is an exciting alternative to fuel cell or battery–electric drive solutions. The fact that there is no carbon in the hydrogen molecules helps to avoid CO₂, and significantly reduce NO_x, emissions. The innovative technology can be adapted to vehicles with gasoline or diesel engines, and can also be used on roads, in agriculture, and in shipping. An additional benefit is that retrofitting can be done at a fraction of the cost of replacing vehicles. The 8.7-L engine works with hydrogen in the same way as it works with conventional fuel, even when using hydrogen with two injection modes. With indirect injection, the fuel enters the engine in gaseous form, while with direct injection, it enters the combustion chamber in a liquid state—similar to modern direct injection systems [77].

In addition to the fuel supply system, the optimized pre-chamber spark plug is one of the critical technologies enabling the gas engine to run on hydrogen and natural gas with the highest possible efficiency and the lowest emissions. The pre-chamber ignition is similar to a conventional spark plug, with the difference that the electrode is in the pre-chamber, and the spark is created there. The connection to the main combustion chamber is through openings in the top of the main chamber. During the compression stroke, thanks to the movement of the piston, the pre-chamber is filled with a gas mixture enriched with fuel (Figure 11). Combustion is initiated in the pre-chamber, and the resulting flame flows from the antechamber to the main chamber. The chamber design is crucial to the combustion properties, and must be optimized for the characteristics of the combustion chamber—that is, for each engine. The relationship between the fuel content and the flow dynamics at the electrode gap is regulated by the shape of the pre-chamber, and the two are always the same during ignition. Thus, the pre-chamber spark plug, together with the multiple ignition of the main chamber, makes it possible to reduce the fluctuation of the combustion cycle compared to conventional spark plugs. In addition, flame jets reduce combustion time, increasing engine efficiency [78]. Figure 11 shows the combustion chamber design. Similar to the previous ones, where only hydrogen gas was used as the fuel, in this case, it is also a combustion chamber with a piston chamber. The lower part of the chamber is flat, and its cross-section becomes more narrow towards the top, contrasting the previous designs.

6.2. Hydrogen Is Part of the Fuel Mix

6.2.1. Diesel–Hydrogen and Combustion Chamber

In compression ignition engines, it is difficult to reach the auto-ignition temperature of hydrogen by compression alone. Therefore, in the framework of this research, an attempt is made to enrich the air taken in by the engine with hydrogen, but the primary fuel of the engine remained diesel oil. Experiments have been conducted to study the effect of piston geometry on the emission characteristics of hydrogen-enriched diesel engines at different hydrogen addition and flow rates, such as 2 L per minute (lpm), 4 lpm, and 6 lpm, on a four-stroke, single-cylinder diesel engine at 1500 rpm constant speed and under different loads. Furthermore, the experiments were also performed with a toroidal piston chamber geometry for different hydrogen flow rates without changing the compression ratio. A knocking tendency was observed at flow rates above 6 L/min at all loads due to increased temperature and peak pressures with hydrogen addition [79].

This study investigated the effects of combustion chamber geometry and alternative fuel (hydrogen–diesel) on combustion characteristics and emissions. This study was conducted for experimental and modeling purposes. The modeling was performed in the ESE diesel section of the AVL-FIRE software. The standard combustion chamber (SCC) geometry and the modified combustion chamber (MCC) geometry were compared during the modeling. Additionally, hydrogen–diesel (dual fuel) fuels were tested in this study, using 5 wt. % hydrogen for diesel fuel, one of the available fuels in the AVL FIRE program. The pilot study used the Antor 3 LD 510 direct injection diesel engine. In order to fully understand the effect of chamber geometry on engine parameters, all properties except geometry were held constant. Due to the cyclic design of the MCC chamber geometry, compared to the SCC chamber geometry, the evaporation rate of the mixture increased. Due to the geometry of the MCC, the flame propagation process in the chamber also increased. The fire extinguishing zone decreased. In this way, the combustion became more homogeneous in the chamber. Adding hydrogen to diesel fuel increased the flame speed and shortened the burning time. Because of this, the combustion chamber pressure and the maximum peak pressure increased. The combustion chamber temperature also increased with the addition of hydrogen fuel [80].

The study in [81] provides a literature review on the effects of hydrogen ratio and combustion chamber geometry on engine performance and emissions in compression ignition engines operating in hydrogen–diesel dual-fuel mode. As a result of the study, they concluded that the hydrogen–energy ratio should be between 5 and 20%, and the combustion area should be designed taking into account the combustion characteristics.

Especially after the impact of the fuel jet in the piston chamber, the spatial distribution of the droplet diameter is one of the most critical factors in terms of combustion quality and exhaust gas emissions. However, this property is complicated to study due to the difficulties of measuring the droplet diameter. The simulations were performed on different hemispherical, toroidal, and re-entrant toroidal piston chambers (combustion chambers), without hydrogen as supplemental fuel and with hydrogen as supplemental fuel. The minimum diesel droplet diameter in the case of toroidal piston geometry barely exceeds the values obtained with a hemispherical shape. Furthermore, it can be reduced to the desired level, approximately 6%, by adding hydrogen. This is due to the reduction in carbon content caused by adding hydrogen. Hydrogen promotes the combustion process by increasing the temperature of the cylinder. The results of the three different geometries with the hydrogen-containing test fuel mixture show a 3% reduction in the minimum diameter compared to the pure diesel operation. The maximum fuel droplet diameter for the re-entry toroid chamber improved by 9.5% compared to that of the pure diesel operation. This is due to the better vortex movement of the air and the high turbulence during combustion, which affects the diameter of the droplets [82].

6.2.2. Methane-Hydrogen and Combustion Chamber

In this research, the natural gas/hydrogen hybrid fuel (NHHF) and the triangle rotor piston (TRP) engine with turbulent jet ignition (TJI) and a combustion booster were investigated. The effects were investigated with different ignition angles (ignition advanced angle, IAA). The research results show that the TRP structure changed the combustion process by affecting the ignition area in the ignition stage and the kinetic energy of the turbulence during the combustion stroke. In addition, the SC (symmetrical convex) shaped cylinder showed the highest peak pressure for any fixed pre-ignition angle [83]. Regarding flow field and mixture formation in a rotary piston engine, the shape of the triangular rotary piston had no significant effect on the fuel distribution and flow field from the intake stroke to the initial stage of the compression stroke. However, in the later stages of the compression stroke, compared to the FP-shaped cylinder, the high fuel concentration zones in the AC-shaped cylinder, the PC-shaped cylinder, the MC-shaped cylinder and the SC-shaped cylinder were all closer to the position of the jet nozzle [83]. A relevant technical article has contributed to the CFD modeling of reactive flows in natural gas-fired internal combustion engines. Even though natural gas has been extensively researched in the past few decades, the literature still needs more reliable models and correlations that can be used to support the design of internal combustion engines effectively. The article deals with developing an accurate CFD model that captures the effect of engine operating conditions and mixture composition on combustion [84]. The combustion chamber model of the examined engine is shown in Figure 12. Its design is analogous to the previous ones, meaning the combustion chamber is a piston chamber. The bottom of the chamber is flat, and the mantle is curved, with a certain radius rising from the bottom.

As a result of adding hydrogen to the fuel, the laminar flame speed of the mixture increases, and pre-ignition must be reduced to maintain the optimal combustion phase. The effect of hydrogen on the duration of turbulent combustion is slightly noticeable [84]. The research report investigated the effect of piston shape using CFD, supplemented with a chemical reaction mechanism, to investigate what happens when a diesel engine piston is converted to fit a synthetic gas HCCI engine. In addition, the compression ratio has also been optimized for the HCCI engine. The essence of this synthetic gas HCCI engine is that synthetic gas has a low calorific value and also consists of an inert component, CO₂, which burns under excessively lean conditions. The piston was redesigned by reducing the piston chamber depth and including an indentation area ratio from 34% to 5% of the piston’s baseline shape. The results show that the modified piston exhibited high peak pressure and heat release due to the fast burning rate. In addition, the modified piston had a high burn rate due to the weak counter-compression flow and high turbulent kinetic energy under all load conditions. The results also show that the modified piston has high combustion and thermal efficiency. For gas/synth gas HCCI engines, the shape of the piston must be compatible with the compression ratio, which reduces heat transfer losses. Reducing the compression ratio by 40% for the HCCI engine improves combustion and thermal efficiency [85].

6.3. The Heron Combustion Chamber

The combustion chambers presented above, for those engines that use only gaseous fuel, all resemble an older combustion chamber construction, the Heron combustion chamber. The Heron cylinder head is suitable for gasoline and diesel engines, OHV (Over Head Valve) and OHC (Over Head Cam) valve timing, and small and large displacement engines. It was also used in racing engines but is no longer used today.

The Heron combustion chamber is a type of combustion chamber designed for an internal combustion piston engine named after engine designer S. D. Heron. The cylinder head is flat and contains the intake and exhaust valves, the spark plug and the injectors. The combustion chamber is located in the piston, so the engine is a piston chamber engine. While a flat cylinder head can be combined with simple flat-top pistons, this option ignores the reasons for the top part of each piston being recessed, namely, (i) it provides a compact space for combustion to start, allowing for optimal flame, and (ii) it creates significant swirl, when the piston reaches the top dead center. This creates turbulence, which is desirable because it promotes more extensive mixing of the fuel/air mixture [86]. The advantages of the Heron combustion chamber include (i) ease of manufacture, (ii) compact dimensions, (iii) precision of flat machined surface, (iv) simplified valve control and (v) efficient combustion with low fuel consumption. However, it also has disadvantages because (i) the size and weight of the pistons are more significant, and (ii) the volume efficiency is lower than that of conventional cylinder heads with non-parallel valves [86]. This construction is old, and it is hardly used today. German, Italian, English and Swedish manufacturers also used it in their engines earlier [86,87]. Figure 13 shows the combustion chamber of the Volvo 900/700 Heron.

There are few research results related to the Heron combustion chamber. The study in [88] aims to compare the effects of different chamber geometries on spark ignition engine combustion parameters in lean and homogeneous mixtures. The most crucial variable of the combustion parameters is the cyclic change, which determines the stable operation of the engine. Cycle-to-cycle variations in spark ignition engines increase as the mixture leans. The newly designed and manufactured combustion chamber (MR-type chamber) geometry reduces the cyclic deviations compared to other geometries even when testing homogeneous and lean mixtures. The lowest COV (coefficient of variation) values were obtained for the MR-type geometry in lean mixtures under all load conditions. Figure 14 shows the tested piston designs.

7. Examined Combustion-Related Characteristics in Simulation Works

In the following, we would like to reveal what kinds of simulations are typically carried out in connection with hydrogen research and what parameters are examined.

The computer-aided design of new, efficient and clean hydrogen engines requires mathematical tools for the computer modeling of the mixing of hydrogen–oxygen components and the dynamics of the combustion gas. The thesis presents the results of the development, verification and validation of a mathematical model and a numerical tool that enables the simulation of unstable ignition and combustion processes of different types of engines, and the study of their characteristics. It is a predictive, modeling, 3D code that allows the simulation of chemically reacting turbulent premixed and non-premixed flows. A modified k-epsilon model was used as the turbulence model. The modeling of the temperature fluctuation was solved with an additional equation. In this case, the turbulent heat flux equation is a two-term sum. The eddy kinematic viscosity is also part of the k-epsilon model. Studies have shown that the dependence of the ignition delay on pressure in the case of hydrogen–oxygen mixtures is not monotonic and has three characteristic ranges. It generally increases for both fuel-rich and fuel-lean mixtures compared to stoichiometric mixtures. A decrease in water vapor concentration indicates thermal dissociation caused by increased temperature. During the detonation, the wave reflected from the wall as a shock wave causes an increase in temperature, which is associated with a decrease in water vapor concentration due to dissociation [89].

The effects of hydrogen addition and exhaust gas recirculation (EGR) strategies on engine performance, combustion process, emissions, energy and power balance were investigated. To this end, by creating a lean mixture combustion, a high-compression-ratio (CR) (13.6) engine was used and tested on an engine brake bench, the primary fuel of which is natural gas (with 99% methane content), and the engine uses spark ignition (natural gas spark ignition, NGSI)). The corresponding 1D GT-Power simulation model was built and validated based on the experimental data. In this simulation, a SITurb (spark ignition turbulence flame combustion) model was applied to the 1D cylinder model, which determines the in-cylinder combustion rate and knock appearance. The model can also handle two types of mixtures: the homogeneous mixture and the layered mixture. In the first stage, the peak cylinder pressure (PCP) and the maximum heat release rate (HRR) increase with the addition of hydrogen, but decrease with the increase in the EGR ratio. In addition, adding EGR slows the effects of various hydrogen contents on engine performance and combustion [90].

One paper presents an experimental and numerical study of the combustion, autoignition and octane number of hydrogen. The tests therein were performed in a standard engine that complies with the American Society for Testing and Materials (ASTM) Research Octane Number (RON) method for testing liquid fuels. Minor modifications were made to the engine to enable hydrogen fuelling. The numerical analysis was performed using a combination of calibrated two-zone combustion and kinetic modeling of the tail gas. The software’s proprietary SI turbulent flame module was used to simulate flame propagation during combustion in the two-zone combustion model. For the subsequent correct MFB (mass fuel burnt) profile results, it was first necessary to calibrate the initial flame kernel size, the turbulent flame speed multiplier, the Taylor length scale multiplier and the flame kernel growth multiplier. Applying the standard RON method first showed that the RON value of hydrogen is between 62 and 64, which is significantly lower than that of conventional motor gasoline. However, this standard RON method requires conditions that are irrelevant for practical hydrogen-powered engines and do not appear to be typical of autoignition and knocking. Therefore, modified RON tests had to be performed under more practical conditions. These indicate that hydrogen with average knock intensity and air factor λ = 1, 1.5, and 2 has RON values of 93.7, 117, and greater than 120, respectively, spanning the range corresponding to conventional spark ignition in terms of the energy supplied by engine fuels. Together, these results show that hydrogen has significantly higher compression tolerances than conventional gasoline when providing similar energy to the premix. Based on these reuslts, hydrogen should have a knock tolerance similar to high-octane fuels. Based on the simulation results, hydrogen has a significantly higher laminar flame speed than iso-octane, which results in a higher temperature and pressure for a given crankshaft angle position [91].

This work aims to demonstrate the feasibility of using an H2 engine with FC-like 60%+ Brake Thermal Efficiency (BTE) level for output brake power using the Double Compression–Expansion Engine (DCEE) concept, combined with high-pressure direct injection (high-pressure direct injection, HPDI) in a non-premixed H2 combustion engine. Experimentally validated 3D CFD simulations were combined with 1D GT-Power simulations to produce predictions. An RNG (Re-Normalization Group) k-epsilon model with standard settings was used for turbulence modeling. Numerous modifications to the system’s design and operating conditions have been systematically implemented, and their effects are being investigated. Adding a catalytic burner to the combustion exhaust, insulating the expander, dehumidifying the EGR, and removing the intercooler resulted in BTE improvements of 1.5, 1.3, 0.8, and 0.5% points, respectively. Increasing the peak pressure to 300 bar with a larger compressor further improved BTE by 1.8%, but a higher injector-to-cylinder pressure differential must accompany this. A λ of ~1.4 provided the optimal compromise between mechanical and combustion efficiency. A peak BTE of 60.3% was achieved for H₂DCEE, ~5% higher than the best diesel DCEE alternative [92].

The paper presents two concepts being developed to improve the fuel conversion efficiency of internal combustion engines. The concept works on combustion development to increase the amount of fuel energy available for in-cylinder piston work. The concept works with hydrogen (in this case, water injector is also required) and short carbon chain alkanes (methane, propane, butane). These results were obtained using GT-SUITE models with the ICE system. The Wiebe functions used to model the combustion evolution were first calculated from the diesel-only experiments, and then corrected based on the CFD results. The heat release profile was used as the input of the GT-SUITE model, as that software cannot cope with modeling the complex phenomena of new combustion systems. Preliminary simulation results show that fuel conversion efficiency was improved to over 50% in high-power-density operation. Faster warm-up during a cold start also reduces the cold start fuel loss of driving cycles [93].

Adding hydrogen to the original fuel of diesel engines offers a short-term solution to reducing the greenhouse gas emissions of heavy commercial vehicles. Variations in combustion behavior between cylinders can limit performance and H₂ consumption in these engines. This research evaluates the effects of varying H₂ concentration between cylinders due to the phasing of the pulsed fuel injection in the intake manifold and the possibility of optimizing the diesel injection strategy of each cylinder to compensate for these effects. The study was conducted using a predictive phenomenological combustion model for hydrogen–diesel combustion in the GT-SUITE™ software environment, which was calibrated and validated with experimental data from a four-cylinder, 5.2-L engine at three loads and with two different H₂ concentrations per load. Since this is a dual-fuel engine, the combustion model contains two combustion sub-models. One is for non-premixed diesel spray, and the other is for premixed flame spread. The premixed flame model for H₂ uses a two-zone model, one of which is the laminar flame speed, and the other is the estimated bulk turbulence. Together, the two can model turbulent flame combustion. The engine and combustion performance predictions were within 5% of the experimental results for the main parameters. The validated model was then used to quantify the effects of varying inter-cylinder hydrogen distribution on combustion performance and structural constraints, including cylinder peak pressure, pressure rise rate and exhaust gas temperature. Using these results, the diesel injection parameters, including the timing and quantity of the pre- and main injections, are optimized for each cylinder to minimize variations in engine performance. The results show that the cylinder-to-cylinder variation of H₂ concentration increased with faster H₂ injection due to the limited dispersion of hydrogen between cylinders. The variation in indicated power was close to 13.5% between cylinders, and the high H₂ content raised a significant risk of knocking and noise [94].

This study uses an artificial neural network (ANN) to investigate combustion process prediction, and presents an effective prediction method. In particular, the conditions for using hydrogen as an additive for turbo-gasoline direct injection (T-GDI) engines are analyzed. However, many data must be collected under different conditions to make these predictions. In addition, the procedure requires data collection under different conditions to predict complex phenomena such as engine combustion processes. To implement this method, the target engine was modeled using the commercial GT-Power 1D engine simulation software under specific conditions based on the experimental results. An SI turbulence model was used within the engine model, as part of GT-Power. It is a burn model that can show the burn rate. To investigate the combustion of the two different fuels, it was necessary to modify the parameters of the laminar flame speed [84]. Based on the experimental data, the engine was modeled using GT-Power. The engine model is shown in Figure 15. The main purpose of this engine model was to obtain combustion data quickly under as many conditions as possible. Therefore, the engine model was designed according to the cylinder and the inlet mixture conditions. To minimize the computation time, the engine was modeled from the front of the intake pipe to the rear of the exhaust port [95]. There are many ways to describe the combustion process in an engine cylinder. The Wiebe function is one of the best-known methods for describing this process. This function describes the profile of the fraction of fuel burned (mass fraction burnt, MFB) as a function of the crank angle. With this MFB profile, we can analyze the combustion process and calculate the pressure drop inside the cylinder. The data for the ANN training under extended conditions were created using the GT-Power engine model. A reasonable agreement was observed between the compared results, proving the prediction model’s validity and reliability [96].

Brown gas, also known as hydrogen–hydrogen–oxygen gas, is a hydrogen fuel in which oxygen is present. The effect of adding hydrogen–hydrogen–oxygen gas in a direct injection diesel engine was investigated using GT-POWER software with two selected parameters, pressure rise rate and heat release rate. The engine was modeled in the GT-POWER environment. The single-zone combustion model was adapted to the Woshni heat transfer model. In this model, the effect of the induction of hydrogen–hydrogen–oxygen gas was subsequently incorporated and analyzed for 1, 3 and 5 volume per cent hydrogen–hydrogen–oxygen gas. The injection rate was modified to use the hydrogen–hydrogen–oxygen gas in the model. The results show a higher heat release rate with a shorter burning time. A higher level of hydrogen–hydrogen–oxygen gas indicates an advanced initiation of combustion and a reduction in the duration of combustion [96].

The Wankel engine is one of the best internal combustion engines for converting hydrogen. The paper presents some details of an initial CAD and CAE modeling study of a new design wherein two jet ignition devices per rotor replace the traditional two spark plugs for faster and more complete combustion. The CAE model (using the GT-SUITE software) was first developed to describe the wide-open throttle behavior of a two-rotor dual spark plug per rotor gasoline-injected high-performance Mazda Renexis engine at baseline. The model was then modified to describe the operation of gasoline or hydrogen-powered versions with two jet igniters replacing the spark plugs [97]. Direct injection is mandatory for hydrogen, while intake manifold or direct fuel injection can be used for gasoline. Fuel conversion efficiency and torque output are drastically increased over the direct injection and spark ignition of gasoline compared to conventional port injection spark ignition gasoline engines. For hydrogen, the λ = 1.2 torque and efficiency are similar to gasoline direct injection jet ignition. The hydrogen stoichiometric torque continues to increase. Hydrogen is the better fuel for the Wankel application, as its combustion occurs faster and more completely at higher pressures and temperatures [97].

Significant findings were made regarding the laminar flame speed in [98]. In other words, hydrogen significantly increases the flame speed of methane, which is also a gas. However, it increases not only the flame speed but also the flammable areas, and it also has a flame-stabilizing effect on the methane flame.

It is increasingly common to find research examining the effects of several fuel components (typically three). These are multiple comparative tests because, first, a comparative test is performed with the base fuel (the original fuel of the engine) and the first mixing component; the effect of the first mixing component is checked. On the occasion of the second comparative test, the tested base values will be taken as the values of the first mixture in terms of any of the tested parameters. Then, the effect of the fuel mixed into the fuel mixture is assessed in the third test. Some of these kinds of multiple investigations have also been performed in relation to hydrogen and diesel engines [99,100,101] and HCCI engines [102]. The mentioned literature mainly examines the combustion process, engine efficiency, and pollutant emissions with a ternary mixture. Regardless of the engine and the two other fuels used in these studies, better efficiency and lower emissions are reported using hydrogen.

8. Some Other Aspects of Such a Retrofitting

There are many other technical points of view from which such a conversion can be analyzed. We have selected the exhaust gas after-treatment system, turbocharging, and crankcase ventilation. This chapter discusses these topics based on some of the selected literature.

The first topic concerns exhaust gas after-treatment systems. A scientific publication relevant to the issue of emissions [39] deals explicitly with the exhaust gas after-treatment systems of hydrogen engines. Tests were performed with a hydrogen engine converted from a gasoline engine. According to the results, the stoichiometric operation showed the highest specific power, but high NO_x emissions were als produced. TWCs (three-way catalysts) or TWNSCs (three-way nitrogen storage catalysts) are recommended. SCR is recommended for low loads, where an even higher efficiency has been measured with lean operation, with TWC or TWNSC as the precatalyst. In the case of ultra-lean operation, NO_x emissions are below the detectability limit. However, in this case, the mixture is so lean that the exhaust gas temperature does not reach the light-off temperature of the catalyst, so it cannot operate continuously, depending only on the load. In the case of a 2.3 lambda operation, the NO_x emissions are still low, but the exhaust gas temperature is still so high that the catalyst can operate continuously. In [103], tests were carried out with a spark ignition gasoline engine with the addition of hydrogen. They found that adding hydrogen significantly increases NO_x emissions, so one of the NO_x after-treatment systems may be necessary. In terms of CO₂, they obtained reduced emissions compared to operations without hydrogen, which was attributed to the fact that hydrogen leads to cleaner combustion. In the scientific publication cited as [104] in the reference list, the authors showed that, among other things, NO_x emissions were measured on a hydrogen engine with EGR (exhaust gas recirculation). They found that the recirculated exhaust gas slows the combustion, lowering the temperature and lowering thermal NO emissions. With 40% EGR recirculation, the engine worked stably. As published in [105], NO_x emissions were investigated in an SI (spark ignition) engine in hydrogen mode, designed and optimized for gasoline mode. Hydrogen improves combustion stability and reduces cycle-to-cycle variation. This also applies to NO_x emissions. A series of tests over a wide lambda range gave almost zero NO_x emissions at a lambda of 2.75. NO_x measurements were also performed in the case of DI and PFI hydrogen dosing, and the result was that less NO_x was formed with the PFI system. Ref. [106] also states that only NO_x reduction should be addressed in the case of a hydrogen engine. The urea SCR exhaust gas after-treatment system is also suitable for hydrogen engines, which have already been proven suitable for diesel engines. However, another after-treatment solution, the H₂ SCR solution, looks like a promising future solution based on their results.

Compared to a naturally aspirated hydrogen engine, a very significant (>100%) increase in power and torque can be achieved with a supercharged hydrogen engine [107]. In the study [108], the investigation of different turbocharging systems aimed to assess the reduction in NO_x emissions of a hydrogen engine. Full-load results were achieved with each charging concept. Low NO_x emissions were achieved with a CMR value 2.4 (CMR = cylinder mass ratio). Regarding efficiency, the VGT (variable-geometry turbo) concept achieved the highest break thermal efficiency. A lower efficiency can be achieved with the two-stage turbocharging concept, and the lowest efficiency with the FGT (fixed-geometry turbo) concept. Based on their results, they suggested what kind of charging concept should be used for the engines of different vehicles. Based on these, FGT is supposed to be used for Genset applications—VGT two-stage turbocharging for long-haul vehicle types, and two-stage turbocharging for construction vehicle engines. Further, FGT and VGT should be used for marine engines, and VGT and the two-stage method for off-road applications. In this study [109], the effect of turbocharging compared to a naturally aspirated engine was directly investigated on a pure hydrogen engine. Using a turbocharger increases maximum power and torque in line with previous knowledge, and this is valid for any fuel. Stable combustion was obtained up to 5000 rpm. By applying turbocharging, BTE also increased. As for the mixture composition, the naturally aspirated engine operated with a richer mixture while being supercharged with a leaner mixture in the high rpm range. Using the leaner mixture in this speed range (>5000 rpm) led to lower engine power and worse efficiency. At the maximum speed (6000 rpm), the turbocharged turbine was choking so much that there was a lot of residual exhaust gas in the cylinder, which led to an uncertain combustion process. Turbocharging is good for increasing engine torque. However, problems with valve timing and back pressure appear at high revs. Extensive simulation analyses have been performed [110] on how hydrogen affects engine parameters compared to results for the original engine fuel, natural gas. The ideal turbocharging level with natural gas had to be reduced by 15% for the hydrogen-powered engine. The forced depletion of the mixture is the effect of hydrogen, which further reduces the previously mentioned reducing effect of filling. Thus, these two have an additional negative effect on the BMEP when acquiring the full load curve. This negative effect can be reduced by setting the combustion phase, which positively affects the specific fuel consumption. Reducing the size of the supercharger turbine at low rpm proved helpful for the hydrogen engine because it increased engine power and gave an acceptable BMEP at 2000 rpm. However, the same turbine size at high speed (4000 rpm) slightly reduced the performance achieved with the original turbine size in the simulation.

In the case of a hydrogen-powered engine, gap loss [111] is an important aspect, but not for the same reason as in the case of conventional fuels. Gap loss refers to hydrogen escaping into the crankcase through the gap between the piston and the cylinder. The piston crown, the compression ring, and the ring groove are all highest-temperature zones in the cylinder space. They thus represent a potential ignition opportunity for hydrogen. On the other hand, hydrogen can also ignite in the crankcase due to its low ignition energy and wide ignition limits.

9. Conclusions

The above study aimed to give an overview of the areas considered very important in connection with a planned engine conversion. Topics have been taken from areas wherein others have already achieved results—not necessarily because of a conversion, but rather because of the investigation of hydrogen engines. Its secondary purpose was to review the main topics, and not to delve deeply into individual topics. In the main sections of the review, the most important conclusions are the following:

The ICE-relevant properties of hydrogen can be advantageous if handled well from the engine side. A low ignition temperature, wide flammability limit, high flame speed, and high energy content can all be beneficial, but only if properly managed. If the control of these properties is not preserved, they only cause disadvantages;
The best performance, efficiency and controllability can be obtained with diffuse combustion, which can be induced by the direct high-pressure injection of hydrogen. That is the most complicated and expensive system. The financial and infrastructural possibilities available to us allow the opposite of the most straightforward intake port injection mode by installing a spark plug in the cylinder head;
Pre-ignition, backfiring, and knocking are all types of abnormal combustion, which must be eliminated in the case of hydrogen, especially during indirect injection. The three processes are related, and as such they can be the root causes of each other. The distance from the injection valve to the intake valve, the diameter of the needle, the operating pressure of the valve, the injection strategy, the intake valve control strategy, the speed of the incoming air, and the thermal condition of the metal parts are all influencing factors affecting abnormal processes;
The ignition system to be applied must undoubtedly be a quick discharge condenser energy storage ignition system. The essential requirements for the spark plug are a cold start and the appropriate geometrical design of the electrodes;
Regarding the design of the combustion chamber, the experiments can be divided into two parts. In experiments containing pure hydrogen or natural gas, the piston’s combustion space (chamber) is such that its lower part is a flat surface connected to the piston roof, with some contours. These are Heron-like combustion chambers. We want to convert the original piston of the experimental engine into this type of combustion chamber. We thus need more simulation results to determine its exact ideal geometric properties;
The simulation of hydrogen combustion has been carried out in many studies in the GT-Suite environment, but scientific publications in which a pure hydrogen engine is investigated are different. The typical research (simulation and experimental) focuses on adding hydrogen to the traditional fuel of the given engines in small amounts, and they try to simulate and validate this change. The most critical parameter examined is cylinder pressure, from which many other parameters and phenomena can be derived. Examples include heat release and abnormal combustion phenomena.

Author Contributions

Conceptualization, M.Z., M.V., K.L. and G.S.; methodology, M.Z., M.V., K.L. and G.S.; resources, M.Z.; writing—original draft preparation, K.L. and G.S.; writing—review and editing, M.Z. and M.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Main characteristics of compared fuels (on the basis of [26]) ((1) at pressure 1.013 bar; (2) at temperature 0 °C; (3) at temperature 25 °C; (4) at λ = 1; (5) in air; (6) at pressure 250 bar and at temperature 280 K).

Figure 2. Combustion processes can be implemented with hydrogen (on the basis of [38]) (HCCI stands for homogenaous charge compression ignition).

Figure 3. A typical pressure function during the pre-ignition process [41] (black line—backfire, dash line—normal process) (reproduced with permission from Verhelst, S., and Wallner, T., Progress in Energy and Combustion Science, Elsevier, 2009).

Figure 4. Pressure functions over the crankshaft angle in the cylinder and in the intake manifold in case of a backfire (black line—backfire, dash line—normal process) [41] (reproduced with permission from Verhelst, S., and Wallner, T., Progress in Energy and Combustion Science, Elsevier, 2009).

Figure 5. Frequency curves of low- and high-intensity knocking [59] (reproduced with permission from Luo, Q. H., and Sun, B. G., International Journal of Hydrogen Energy, Elsevier, 2016).

Figure 6. Drawing of a spark plug registered at the Deutsches Patent und Markenamt (Patent number: DE 10 2006 041 161 A1) [65].

Figure 7. Combustion chambers of the Erren hydrogen engine (left) and the MAN small-series hydrogen engine (right) (on the basis of [26]).

Figure 8. Cross-section of a hydrogen-fuelled research engine [74].

Figure 9. Meshed model of the combustion chamber [76] (Blue color lines form the mesh) (reproduced with permission from Yuan, C. et al., International Journal of Hydrogen Energy, Elsevier, 2016).

Figure 10. Combustion chamber of the VW hydrogen engine (on the basis of [30]).

Figure 11. Counter drawing of the main combustion chamber (on the basis of [78]).

Figure 12. CAD model of engine combustion chamber including intake and exhaust ducts, valves and spark plug [84].

Figure 13. The Heron combustion chamber of Volvo 900/700 [87] (yellow—combustion chamber; blue—other engine parts).

Figure 14. Chamber geometries: simple design (top left), Heron-type geometry (top right), MR-type geometry (bottom left), plate-type geometry (bottom right) [88].

Figure 15. Engine model in GT-Power (left side) and cylinder pressure functions at different hydrogen mixing ratios (right side) [95] (explanation of model color marking: blue arrows—fluid flow between engine components; yellow—yellow—physical spaces, spatial connections; green—model inputs and outputs; red—notation of the author of the original article about each component) (reproduced with permission from Cho, J., and Song, S., Applied Thermal Engineering, Elsevier, 2020).

Table 1. Mixture formation concepts for hydrogen operation (on the basis of [26]).

Mixture formation concepts	External mixture formation	Continuous injection
		Sequential injection
	Internal mixture formation	Early injection
		Late injection
		Combustion control
	Combined mixture formation

Table 2. Various concepts of mixture formation ignition and combustion concepts for hydrogen ICE [10].

Conception	Homogenous Combustion/Spark Ignition			Diffusion Combustion/Compressed Ignition
	Multi-Point Injection (MPI)	Low- and Mid-Pressure Direct Injection (LP-DI and MP-DI)		Multi-Point Injection (MPI)	High-Pressure Direct Injection (HPDI)
	Homogenous Lean Pressure Injection	Homogeneous Lean Pressure Injection	Stratified Lean Pressure Injection	Lean Pre-Mixed + Diesel Diffusion Combustion	Diffusion Combustion Lean (Diesel Like)	Diffusion Combustion Lean (Diesel Like)
Mixture formation	Swirl	Swirl	Tumble	Swirl	Swirl/Tumble	Swirl/Tumble
Ignition	Spark Plug			Compression ignition	Diesel pilot injection	Glow plug or spark plug (with pre-injection)
Combustion	Stoich./Lean	Lean	Lean	Stoich./Lean	Lean—Diffusive
Advantages	Low conversation effort	No risk of backfire	Good efficiency Low-NO_x raw emission	Low conversation effort	Diesel-like efficiency Low-NO_x raw emission	Diesel-like efficiency Low-NO_x raw emission
	Easy to integrate	Robust against backfire	Robust against backfire	Easy to integrate	Same as LP-DI	Same as LD-PI
	Hardware available	Power density	Power density	Hardware available	Diffusive combustion possible	Diffusive combustion possible
	Low failure risk	Transient response	Transient response	Low failure risk	Diffusive combustion possible	Diffusive combustion possible
Disadvantages	Transient performance challenging; risk of backfire	Conversion effort w/o benefits in terms of efficiency and power density	Dedicated cylinder head engine required	CO₂ emission existing due to diesel	High pressure fuel supply	Very high injection pressure
CO₂ reduction compared to diesel	−100%	−100%	−100%	−30%~−70%	−95%	−100%

Table 3. Processes of mixture formation for H₂ICEs (on the basis of [38]).

Mixture Formation (MF) Conceptions
External MF		Combined MF	Internal MF
Continuous	Sequential	-	Simple	Multiple	Combustion control

Table 4. Variations of actuation of DI injection valves (on the basis of [38]).

Direct Injection Systems
Servo-hydraulic	Electromagnetic		Piezoelectric
-	Passive closing	Active closing	Stroke increase		Direct acting
			Mechanic	Hydraulic

Table 5. Nozzle types of DI injection valves (on the basis of [38]).

Direct Injection Systems
Inward opening		Outward opening
Hole pattern	Orifice geometry	Cone Angle

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Zöldy, M.; Virt, M.; Lukács, K.; Szabados, G. A Comprehensive Analysis of Characteristics of Hydrogen Operation as a Preparation for Retrofitting a Compression Ignition Engine to a Hydrogen Engine. Processes 2025, 13, 718. https://doi.org/10.3390/pr13030718

AMA Style

Zöldy M, Virt M, Lukács K, Szabados G. A Comprehensive Analysis of Characteristics of Hydrogen Operation as a Preparation for Retrofitting a Compression Ignition Engine to a Hydrogen Engine. Processes. 2025; 13(3):718. https://doi.org/10.3390/pr13030718

Chicago/Turabian Style

Zöldy, Máté, Márton Virt, Kristóf Lukács, and György Szabados. 2025. "A Comprehensive Analysis of Characteristics of Hydrogen Operation as a Preparation for Retrofitting a Compression Ignition Engine to a Hydrogen Engine" Processes 13, no. 3: 718. https://doi.org/10.3390/pr13030718

APA Style

Zöldy, M., Virt, M., Lukács, K., & Szabados, G. (2025). A Comprehensive Analysis of Characteristics of Hydrogen Operation as a Preparation for Retrofitting a Compression Ignition Engine to a Hydrogen Engine. Processes, 13(3), 718. https://doi.org/10.3390/pr13030718

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