US20140366507A1

US20140366507A1 - Diesel Engine With In-Cylinder Soot and NOx Control Using Water or Aqueous Injection

Info

Publication number: US20140366507A1
Application number: US13/919,098
Authority: US
Inventors: Stefan Simescu; Vlad L. C. Ulmet
Original assignee: Southwest Research Institute SwRI
Current assignee: Southwest Research Institute SwRI
Priority date: 2013-06-17
Filing date: 2013-06-17
Publication date: 2014-12-18

Abstract

A method of controlling emissions in the exhaust gas of a diesel engine, the engine having one or more combustion cylinders that each generates a combustion jet having a soot region. A water injector is operable to inject a spray of water or aqueous solution into the combustion jet, specifically toward the area of highest soot concentration. This water injection is timed to occur at or immediately before the combustion phase in which the soot reaches its greatest concentration. The size and shape of the spray are such that the majority of the spray reaches the area of greatest soot concentration.

Description

TECHNICAL FIELD OF THE INVENTION

This invention relates to diesel-fueled internal combustion engines, and more particularly to methods of controlling undesired emissions from such engines.

BACKGROUND OF THE INVENTION

Internal combustion engines are subject to strict emission limits. Approaches to reducing emissions include improved combustion designs and fuel modifications, but these improvements have fallen short of meeting emissions limits. Other approaches involve the use of exhaust aftertreatment devices, which have achieved significant emissions reductions.
For diesel engines, which are conventionally run at a lean air-fuel ratio, the main pollutants of concern are oxides of nitrogen (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by sulfur in fuel, and components of unburned fuel and oil.
In today's diesel combustion systems, exhaust gas recirculation (EGR) is frequently used to lower combustion temperatures and thereby lower NOx formation rates during combustion. However, the displacement of oxygen (O2) by EGR typically results in further increased soot emissions.
In the past, various methods, such as small holes and high injection pressures, and post injections have been used to mitigate the soot emissions increase. Water has also been used for NOX and soot control in a diesel engine. In these methods, the water injection occurred either as an emulsion of diesel fuel and water, or as separate slugs of diesel fuel and water, originating from the same location as the diesel fuel injection.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:

FIG. 1 illustrates a diesel-fueled engine system having in-cylinder water injectors in accordance with the invention.

FIG. 2 illustrates a combustion jet at a mixing-controlled burn phase of combustion.

FIG. 3 illustrates the soot intensities within the soot region of FIG. 2.

FIG. 4 illustrates injection of water at the combustion phase illustrated in FIG. 2.

FIG. 5 illustrates a method of injecting water into the combustion jet of a diesel engine.

DETAILED DESCRIPTION OF THE INVENTION

The following description is directed to injecting water (or an aqueous solution) for in-cylinder control of soot and NOx exhausted from a diesel engine. As explained below, the water injection is provided by separate water injectors, each associated with a different cylinder and directing its spray into the combustion chamber. The spray is targeted at the soot region of the diffusion flame of each diesel jet.
For simplicity, the term “water” as used herein includes water equivalents. For example, various aqueous solutions could be used instead of water. Experimentation can be used to discern appropriate aqueous solutions.
FIG. 1 illustrates a diesel engine system suitable for operation in accordance with the invention. In the illustrative embodiment, the system has a diesel engine 12, an exhaust gas recirculation (EGR) loop 13, and is an air-boosted system having a turbocharger 26.
In the example of FIG. 1, engine 12 has four combustion cylinders 12 a. Although only one in-cylinder water injector 31 is explicitly shown, each cylinder 12 a has a water injector 31 in accordance with the invention. Each cylinder further has other elements of a conventional direct injection diesel engine cylinder, such as a fuel injector, combustion chamber, piston, and the like.
Further, in the example of FIG. 1, engine 12 is a direct injection diesel engine, with its fuel injectors mounted at the tops of the combustion chambers. However, the exact type of fuel injection can vary, with the fuel injection feature relevant to this description being the formation of a diffusion flame and soot region in the combustion chamber as described below.
The direction of flow of exhaust gas through the EGR loop 13 is indicated by directional arrows. Exhaust gas discharged from the engine's exhaust manifold 14 is directed through the EGR loop 13, which may include a filter and/or heat exchanger (not shown). The recirculated exhaust gas flows to an EGR valve 18, and then to the engine's intake manifold 22 where it is mixed with fresh intake air.
The engine's intake air is compressed by the turbocharger's compressor 26 a, which is mechanically driven by its turbine 26 b. Desirably, the compressed air discharged from the compressor 26 a is cooled through an intercooler 30 positioned between the compressor 26 a and the intake manifold 22.
The exhaust aftertreatment system comprises a diesel oxidation catalyst (DOC) 21 and a catalyzed diesel particulate filter (DPF) 22. Oxidation catalyst 21 and DPF 22 are typically installed in-line on an under-floor exhaust line. The treated exhaust exits the DPF 22 into the atmosphere via the tailpipe.
Oxidation catalyst 21 may be a flow-through device with a platinum or other precious metal formulation, designed to either reduce the concentration of or to oxidize hydrocarbon and CO, and for conversion of NO to NO2. Alternatively, oxidation catalyst 21 may be a flow-through device with O2 storage capability as well as a precious metal formulation.
DPF 22 reduces PM emissions in the exhaust by trapping soot particles. The catalyzation of DPF 22 improves its ability to passively regenerate by additional NO to NO2 conversion. Soot oxidation by NO2 occurs at lower temperatures than soot oxidation by O2. Forced or active regeneration may still be required periodically, during which fueling actions (and/or other events) are performed to raise the temperature within the DPF 22 high enough to oxidize the collected PM. An example of a suitable DPF 22 is a ceramic wall flow filter with a platinum catalyst coating on the walls of the DPF 22. Additional catalyst formulations may be used, such as rhodium, for NO and NO2 conversion.
Control unit 11 may be processor-based, programmed to control various aspects of engine operation as described herein. In general, control unit 11 may be implemented with various controller devices known or to be developed. Further, control unit 11 may be part of a more comprehensive engine control unit that controls various other engine and/or emissions devices.
Control unit 11 is programmed to receive input signals and provide control output signals, in the manner described below. In addition to conventional combustion control, such as the amount and timing of fuel injection, control unit 11 is programmed to control the timing and amount of water injection for implementing the method described herein. Only those control inputs and outputs relevant to this description are explicitly shown; control unit 11 may perform various other engine control functions.
More specifically to this description, control unit 11 receives data representing the combustion phase and fuel amount so that it can use this input data in appropriate processing to control the water injection. Control unit 11 may also receive engine load data, which may optionally be used to determine if water is to be injected during a particular engine cycle.
FIG. 2 illustrates a model of a diesel combustion fuel jet, the diffusion flame, and the soot region during a “quasi-steady” period of combustion inside the combustion chamber of a cylinder. The model represents a cross sectional slice through the mid-plane of the fuel jet in a single combustion cycle.
The model of FIG. 2 is described in further detail in a paper entitled “A Conceptual Model of DI Diesel Combustion Based on Laser-Sheet Imaging” by John E. Dec, published by SAE International as SAE paper No. 970873, 1997. This model is for purposes of illustration herein, and other models or combustion flame descriptions may be suitable, with common features being an area of high soot concentration at the end of the diffusion flame distal to the point of fuel injection.
The phase of the fuel jet shown in FIG. 2 is after initial fuel injection and after a premixed burn to the mixing-controlled phase of the burn. This phase of combustion is referred to herein as the “end phase of the premixed burn” or the “mixing-controlled phase” of the combustion period. It occurs just prior to the end of fuel injection and until the end of fuel injection. In terms of crank angle degrees, this phase occurs at least after 10 degrees after start of fuel injection.
Fuel injector 20 is seen as injecting liquid fuel into the combustion chamber. The liquid fuel region 21 is the maximum extent of the liquid fuel droplets. A vapor fuel/air mixture region 22 develops along the sides and beyond the liquid fuel jet 21. A fuel-rich premixed flame 23 forms along the periphery of the vapor region 22. A region of initial soot 24 forms just past the flame 23.
As indicated by the arrow, soot occurs as particles throughout the cross section of the downstream portion of the fuel jet. These soot particles arise from the premixed flame region 23, and occur at varying intensities within the soot region.
At the burn phase illustrated in FIG. 2, the soot has continued to increase, and the soot region 27 now extends outwardly from the fuel injection point. The leading edge of the soot region 27 is where the largest concentrations of soot collect.
FIG. 3 illustrates the soot region 27 of FIG. 2 and the varying soot intensities at this phase of combustion. The soot region 27 can be described as being generally conical with an apex at the end proximate the fuel injection and being generally spherical at the end distal to fuel injection. The highest soot concentrations occur in this spherical portion. These high concentrations occur in concentric layers of decreasing concentrations.
Referring again to FIG. 2, a diffusion flame 25 initially forms at the periphery between the premixed burn and the surrounding air and eventually encircles the soot region 27 of the jet. A thermal NO production zone 28 forms around the perimeter of the diffusion flame 25.
The boundary between the diffusion flame 25 and the soot region is a soot oxidation zone. In other words, soot formed within the diesel jet is oxidized primarily at the boundary between the diffusion flame and the interior soot.
The hydroxyl radical (OH) that naturally occurs within the hot diffusion flame is one of the key species for soot oxidation, along with the O2 present in the combustion chamber. The soot emissions of the engine are the net effect of the competing soot formation and soot oxidation processes. The soot oxidation suffers from the lack of mixing between the soot and the oxidizing species.
FIG. 4 illustrates the use of a water injector 31 to inject water or an aqueous solution (both referred to as “water” herein) targeted into the combustion chamber at a specific region of each diesel jet. Only a single water injector 31 is shown; as stated above in connection with FIG. 1, in practice each cylinder 12 a would be equipped with a water injector 31.
The water injection originates from a different location than the fuel injection. In the example of FIG. 4, the fuel injector 20 and water injector 31 are oriented so that their respective fuel and water jets enter the combustion chamber 32 in perpendicular directions. Thus, the axis of the water spray injection is generally perpendicular to the axis of the diffusion flame.
In other embodiments, the fuel jet and the water spray could originate from different directions, not necessarily perpendicular. By “direction” is meant the axis of the fuel jet or the spray. The actual angle of the water spray relative to the fuel jet could depend on experimentation and modeling and physical constraints of the engine.
The water jet is targeted at the high soot concentration volume of the diesel jet. The dimensions of the spray are designed so that the maximum spray coverage coincides with the sphere of the most concentrated soot (at the distal end of the soot region).
Generally, the method described herein occurs at or immediately prior to the combustion jet phase at which the soot region reaches its largest extent and highest concentration. The start of injection of water is timed to correspond with the mixing-controlled phase of the combustion period. This is probably around the end of the main fuel injection.
The amount of water injected will vary depending on engine operating conditions, but is expected to range from 0 to 50% of the amount of fuel injected. The duration of the water injection may depend on the amount of water injected, the water pressure, and the aperture size of the water injector, as determined by experimentation and/or modeling. It is expected that the duration will be in the range of 200-2000 microseconds.
In operation, the water spray provides enhanced mixing in the region where needed for improving soot oxidation within the diesel jet. More specifically, the water spray entrains O2 from outside the diffusion flame toward the inside the soot oxidation zone. In addition, the water spray dissociates inside the diffusion flame to form hydroxyl (OH) and hydrogen (H), thus providing additional OH in the same region.
The combination of enhanced mixing and presence of the additional O2 and OH effectively oxidizes the soot formed within the diesel jet. The result is lower soot emissions of the diesel engine.
The water spray will also provide some cooling of the diffusion flame 25 surrounding the diesel jet (the thermal NO production zone) as the water will vaporize and dissociate while traveling through this zone. This will result in lower NOx emissions of the diesel engine.
FIG. 5 illustrates a method of using in-cylinder water injection to decrease undesired emissions (soot and NOx) from a diesel engine.
Step 51 is modeling or otherwise characterizing the combustion jet of the engine within the combustion chamber. The location of the soot region is identified, as is its area of greatest concentration. The time during the combustion cycle of this maximum concentration is also identified. The time is related to crank angle degrees or to some other parameter that can be measured and monitored.
Step 52 is installing a water injector so that it sprays water (or other aqueous solution) into the combustion chamber. The water injector is oriented so that the area of maximum spray coincides with the area of the soot region having the highest soot concentration. Typically, this means that the water injection will spray into the combustion chamber perpendicularly to the direction of fuel injection. The spray will be directed to the portion of the diffusion flame farther away from the fuel jet origin.
Step 53 is receiving data representing the phase of combustion. Using this data, it can be determined when the maximum soot formation occurs and when water should be injected. For example, crank angle data may be used to determine when water injection should occur. Further data can be received representing the amount of fuel injected, which can be used to determine the amount of water injected.
In Step 54, at the appropriate time, water is injected. The injection is at or before the time of greatest soot concentration, which is typically during the mix-controlled burn phase of combustion and near the end of fuel injection. The amount of injected water typically varies from between 0% to 50% of the amount of injected fuel, depending on the engine and its operating conditions. Various other injection parameters, such as the duration, pressure, and aperture size, may vary depending on the engine and its operating conditions.
Step 53 is repeated throughout engine operation, and Step 54 is performed on a cycle-by-cycle basis, as appropriate for the engine and its operating conditions. Under some operating conditions, such as during engine load conditions when soot and NOx formation is higher, water injection may be more desired than under others.

Claims

What is claimed is:

1. A method of controlling emissions in the exhaust gas of a diesel engine, the engine having one or more combustion cylinders that each generates a combustion jet having a soot region, comprising:

modeling the combustion jet to determine a soot region within the combustion jet and an area of highest soot concentration in the combustion jet;

installing a water injector operable to inject a spray of water or aqueous solution into the combustion chamber,

wherein the direction of the spray is different from the direction of the combustion jet;

during engine operation, receiving data representing the combustion phase of the engine's combustion cycles;

injecting the spray into the combustion jet in response to the data;

wherein the injecting step is performed by timing injection of the spray so that it occurs at or immediately before the combustion phase in which the soot reaches its greatest concentration, and by directing the size and shape of the spray so that the majority of the spray reaches the area of greatest soot concentration.

2. The method of claim 1, wherein the soot region is generally conical, and wherein the area of highest soot concentration is at the end of the soot region distal to the fuel injection.

3. The method of claim 1, wherein the timing step is further performed to correspond with a mixing-controlled phase of combustion.

4. The method of claim 1, wherein the installing step is performed such that the water spray enters the combustion chamber in a direction perpendicular to the direction of fuel injection.

5. The method of claim 1, further comprising receiving data representing the amount of fuel injected during the engine's combustion cycles, and further comprising determining an amount of spray to inject based on the amount of fuel injected.

6. The method of claim 1, wherein the amount of water injected is in a range of 0% to 50% of the amount of injected fuel.

7. The method of claim 1, wherein the installing, receiving, timing and directing steps are performed for each cylinder of the engine.

8. The method of claim 1, wherein the timing step is based on crank angle data.

9. The method of claim 1, further comprising receiving engine load data, and wherein the injecting step is performed or not performed based on the load data.

10. A system for controlling emissions in the exhaust gas of a diesel engine, the engine having one or more combustion cylinders that each generates a combustion jet in a combustion chamber, comprising:

a water injector operable to inject a spray of water or aqueous solution into the combustion chamber,

wherein the water injector is positioned so that the direction of the spray is different from the direction of the combustion jet;

a control unit programmed to perform the following tasks during engine operation: receiving data representing the combustion phase of the engine's combustion cycles, and delivering control signals for injecting the spray into the combustion jet in response to the data;

wherein the injecting is performed by timing injection of the spray so that it occurs at or immediately before the combustion phase in which the soot reaches its greatest concentration, and by directing the size and shape of the spray so that the majority of the spray reaches the area of greatest soot concentration.

11. The system of claim 10, wherein the timing step is further performed to correspond with a mixing-controlled phase of combustion.

12. The system of claim 10, wherein the water spray enters the combustion chamber in a direction perpendicular to the direction of fuel injection.

13. The system of claim 10, wherein the control unit is further programmed for receiving data representing the amount of fuel injected during the engine's combustion cycles, and for determining an amount of spray to inject based on the amount of fuel injected.

14. The system of claim 10, wherein the amount of water injected is in a range of 0% to 50% of the amount of injected fuel.

15. The system of claim 10, wherein the injection timing is based on crank angle data.

16. The system of claim 10, wherein the control unit is further programmed for receiving engine load data, and wherein the injecting step is performed or not performed based on the load data.