EP0071134B1 - Ignition improver for combustion engines - Google Patents

Abstract

Nitric acid esters of monosaccharides and/or polysaccharides can be used as ignition improvers for alcoholic fuels for internal combustion engines.

Description

The invention relates to ignition improvers in alcoholic fuels for engines which function on the principle of self-igniting internal combustion engines.

Methanol and ethanol can be used as fuels for petrol engines instead of the usual petroleum products. However, this is not possible with conventional diesel engines, since the cetane number of ethanol with about 8 and methanol with about 3 is too low. For trouble-free operation, diesel engines require a fuel with a cetane number of at least 45 (DIN 51601; Winnacker-Küchler, Chemische Technologie, Vol. 3/1, 326 (1971)).

Ignition improvers are required to increase the cetane number. Alkyl and cycloalkyl nitrates are known as ignition improvers for fuels made from methanol or ethanol (DE-A-2701 588, DE-A-2 039 609, Mineralöl Technik 80, 25 (4), 1 to 12), but they are expensive to produce and hydrolyze in the presence of water. This creates nitric acid, which destroys the motors through corrosion.

Nitric acid salts of primary, secondary and tertiary amines such as mono-, di- and triethylammonium nitrate are also known as ignition improvers for methanol and ethanol (DE-A-2 909 565); they also have corrosive properties.

FR-A-719 708 describes the use of nitrated sugars as an additive to alcoholic fuels. From DE-C-1646635 it is known to use alcohol with about 0.5% dinitrocellulose as a propellant for engines. These known blowing agents cannot be used for Dielsel engines.

The use of ignition improvers for alcoholic fuels for diesel engines based on nitric acid esters of mono- and / or polysaccharides with a degree of nitration in the range from 1 to 3 has been found, characterized in that they are present in an amount of 2% by weight to 10% by weight .-% are contained in the alcoholic fuel.

The ignition improvers according to the invention are outstandingly suitable as additives for alcoholic fuels. The fuels produced in this way are equivalent or superior to commercially available petroleum-based fuels.

The alcohol-based fuels are particularly suitable in combination with the ignition improvers for the operation of diesel engines. This is possible because with the help of the ignition improvers, the ignitability of alcoholic fuels can be increased to the level of commercially available diesel fuels (cetane number at least 45).

Nitric acid esters of mono- and / or polysaccharides are known per se (K. Fabel, nitrocellulose production and properties, Enke-Verlag, Stuttgart (1950)).

Alcoholic solutions of the nitric acid esters of polysaccharides are known from US 1 995 911, US 2 001 070 and US 2046 101.

Preferred monosaccharides as starting products for the nitric acid esters according to the invention are pentoses and hexoses. Examples include the following monosaccharides: ribose, glucose, mannose and galactose.

Polysaccharides for the nitric acid esters according to the invention are all linear, branched or cyclic polymers from monosaccharide basic building blocks. Polysaccharides for the nitric acid esters according to the invention are also di- or oligosaccharides which give at least two monosaccharide building blocks in the hydrolysis. Examples include the following polysaccharides: sucrose, lactose, maltose, raffinose, cellulose, hemicellulose, and starches such as amylose, amylopectin and dextrin.

It is of course also possible to use degradation products of polysaccharides obtained hydrolytically or oxidatively for the nitric acid esters according to the invention. Such degradation products are known per se (Houben-Weyl, Volume XIV / 2, pages 866 to 868 (1963)). Preferred saccharides for the nitric acid esters according to the invention are degradation products of the polysaccharides. Such degradation reactions can also on the finished nitric acid esters of the polysaccharides in a conventional manner, for. B. by pressure cooking (Winnacker-Küchler. Chemische Technologie, Vol. 4/11, 737 to 738 (1960)).

The nitric acid esters of the mono- and polysaccharides can be prepared by known processes (Houben-Weyl, Volume VI / 2, 356 (1963) and Volume XIV / 2, 868-871 and 603 (1969)).

The preparation can be carried out, for example, in a homogeneous and / or heterogeneous phase with nitric acid or its anhydride in the presence of dehydrating agents such as sulfuric acid, phosphoric acid, phosphorus pentoxide or acetic anhydride. The degree of nitration can be controlled as desired by the amount and concentration of the nitric acid or the dehydrating additives.

The degree of nitration affects both the alcohol solubility and the effectiveness of the claimed ignition improvers. The nitric acid esters of the mono- and polysaccharides according to the invention preferably have a degree of nitration of at least one nitro group per monosaccharide unit. The complete esterification of all hydroxyl groups of the corresponding carbohydrates is to be regarded as the upper limit of the degree of nitration; in these cases, however, the solubility of the nitric acid esters in the alcohol is reduced.

It is advantageous to use nitric acid esters of mono- and / or polysaccharides a degree of nitration of 1.5 to 2.7, particularly preferably of 2 to 2.5 as ignition improver.

In order to improve alcohol solubility, other substituents can be bonded to the saccharides in addition to the nitro group, such as. B. methoxyl, ethoxyl, carboxymethyl, hydroxyethyl, longer fatty acid residues, nitrite groups or sulfuric acid ester groups.

Alcoholic fuels to which the ignition improvers according to the invention are added are generally alcohols having 1 to 4 carbon atoms. Examples include methanol, ethanol, propanol, butanol and their isomers. Mixtures of these alcohols can of course also be used. Higher alcohols can also be contained in the alcoholic fuels.

In order to increase the solubility of the ignition improvers according to the invention in the alcoholic fuels, it is possible to add solubilizers. As such z. B. low molecular weight ketones and esters such as acetone, methyl ethyl ketone, diethyl ketone, methyl, ethyl and butyl acetate, ethers such as diethyl, diisopropyl and dibutyl ether, tetrahydrofuran, furfurol and dioxane, ethylene glycol mono- and diethyl ether, triethylene glycol mono- and diethyl ether.

The claimed fuel mixtures can also contain small amounts of water, generally up to 20%. A water content in the fuel mixtures according to the invention of 0.5 to 6% is particularly preferred.

The fuel mixtures according to the invention have proven themselves for the operation of conventional diesel engines and are characterized by high operational reliability, no misfires and excellent running properties, especially in continuous operation. Of the ignition improvers according to the invention, preference is given to using those nitric acid esters of mono- and / or polysaccharides which have sufficient stability at room temperature. This is especially the case with nitric acid esters of polysaccharides, e.g. B. were stabilized by pressure cooking (Houben-Weyl, Volume XIV / 2, page 870 (1963)) or by adding suitable stabilizers. Stabilizers of this type are known. In US-A-2404887 nitrocellulose is stabilized by stirring with dilute, aqueous ammonia solution. A number of other stabilizers such as ethyl acetate, amylacetate, acetone, aniline, diphenylamine or diethyldiphenylurea are described by H. Brunswig in "The Explosives VIII", "The Smokeless Powder" on pages 368 to 378 (1926).

Nitric acid esters of mono- and / or disaccharides can be stabilized like those of the polysaccharides if their stability is not ensured by adequate purification such as recrystallization.

The hybrid mixtures according to the invention also have the advantage that they do not destroy the engine through corrosion. They are technically easy to manufacture and can be used without further precautionary measures.

Examples

The effect of the ignition enhancers and the motor properties of the alcoholic fuel mixtures according to the invention were determined on a test engine. An air-cooled 1-cylinder 4-stroke diesel engine with a displacement of V _h = 1.6 dm ³ and a compression ratio of _E = 18.2 was used at an operating pressure of 7 bar effective mean pressure and an average engine speed of n = 1,600 min- ¹ .

At this operating pressure and with an engine setting corresponding to series operation, as well as corresponding component and oil temperatures, there are auto-ignition conditions as are common with standard direct injection engines in full-load operation and are therefore representative. FIGS. 1, 3, 5 and 7 show, by way of example, the cylinder pressure p [bar] as a function of the respective position of the reciprocating piston, indicated in a = degree crank angle (also referred to below as ° KW). A crank angle of 180 ° corresponds to the top dead center of the reciprocating piston.

Figures 2, 4, 6 and 8 show an example of the standardized fuel conversion (dx) / (da) [rad- '] as a function of a = degree crank angle [° KW].

The start of injection is about 164 ° crank angle, the injection duration E is about 22 ° crank angle.

The cylinder pressure curve, the ignition delay and the combustion curve when using a commercially available diesel fuel DK according to DIN 51 601 and when using a mixture of 20% cetane (hexadecane) and 80% a-methylnaphthalene, hereinafter served as a basis for comparison for assessing the ignitability of the fuel mixtures according to the invention Base fuel called GK.

When using GK with a cetane number of 20, the single-cylinder diesel engine used under full load operation and correspondingly good ignition conditions still shows sufficient ignition reliability, the start of combustion, characterized by the first significant increase in dx / da, for example at the top dead center of the work cycle, as ignition delay the difference between the start of combustion and the start of injection, specified in degrees crank angle. The following% figures are to be understood as% by weight.

example 1 a) Manufacture of the ignition improver

• 79,6 % Schwefelsäure
• 11,4% Salpetersäure
• 9,0 % Wasser

behandelt. Nach 2 Stunden Nitrierzeit trennt man das entstandene Cellulosenitrat von der Säure ab, wäscht mit destilliertem Wasser neutral und kocht das Produkt 3 Stunden in der 20-fachen Menge 3 %iger wässriger NaHCO_a-Lösung zur Stabilisierung. Das Cellulosenitrat trennt man wieder von der Flüssigkeit ab, wäscht mit Wasser neutral, und trocknet das faserige Produkt bei 30 °C im Vakuum.Bleached cotton sinters are made in a manner known per se at 20 ° C. while stirring with 50 times the amount of a nitrating acid

• 79.6% sulfuric acid
• 11.4% nitric acid
• 9.0% water

treated. After 2 hours of nitriding, separate the resulting cellulose nitrate is washed off from the acid, washed neutral with distilled water and boiled for 3 hours in 20 times the amount of 3% aqueous NaHCO _a solution for stabilization. The cellulose nitrate is separated from the liquid again, washed neutral with water, and the fibrous product is dried at 30 ° C. in vacuo.

The elemental analysis of this cellulose nitrate shows

b) formulation of the fuel mixture

The cellulose nitrate described in a) is 4% or 8% dissolved in an azeotropic mixture of 96% ethanol and 4% water. To remove suspended matter, the two solutions are filtered with a pressure filter.

c) Motor test

The test engine described is operated with the fuel solutions according to the invention described in b). For comparison, test runs are carried out with base fuel GK (cetane number 20) and diesel fuel DK (cetane number 55).

Figures 1 and 2 show the motor properties of DK (curve 1), GK (curve 4), the ethanolic solution with 4% (curve 3) and the ethanolic solution with 8% cellulose nitrate according to Example 1a (curve 2).

Pressure curve (Figure 1)

When using DK, the pressure rises steeply after the start of injection and begins to drop again at around 1900 kW. The pressure curve is almost identical when the engine is operated with ethanol and 8% of the cellulose nitrate. When operating with base fuel, the pressure rise takes place much later, while 4% of the cellulose nitrate after 1a in ethanol already brings about a significant improvement.

Fuel turnover (Figure 2)

The ignition delay of the 8% ethanolic cellulose nitrate solution is almost the same as for diesel fuel. The 5% cellulose nitrate solution has a considerably lower ignition delay than the base fuel, although pure ethanol only has a cetane number of 8.

The burning process of ethanol is almost identical to that of DK due to an 8% addition of cellulose nitrate and a 4% addition of cellulose nitrate already results in a significant improvement over the base fuel.

Example 2 a) Manufacture of the ignition improver

A cellulose nitrate is produced as in Example 1a).

b) formulation of the fuel mixture

The cellulose nitrate described in a) is used to prepare a 4% by weight and an 8% by weight solution in technical grade methanol and the solution is filtered using a suction filter.

c) Motor test

The test engine is operated with the methanolic solutions of cellulose nitrate as in Example 1) and the characteristic quantities are compared with diesel fuel DK and basic fuel GK. FIG. 3 describes the pressure curve and FIG. 4 the fuel conversion when using DK (curve 1), GK (curve 4), 4% (curve 3) and 8% (curve 2) methanolic cellulose nitrate solution.

Although the ignitability of methanol (cetane number = 3) is even worse than that of ethanol (cetane number = 8), the 8% solution achieves the same ignition delay as with diesel fuel. 4% by weight of cellulose nitrate in methanol already results in a considerable improvement in ignitability, as can be seen from the low ignition delay of only 11 ° KW.

Example 3 a) Manufacture of the ignition improver

• 200 g of a commercially available nitrocellulose with a K value according to Fikent'scher (Cellulosechemie 13. [1932] 58) of K = 730 and a nitrogen content of 11.2% are in
• Distributed 2 l of water and, after adding 5 ml of pyridine, heated to 90 ° C. with stirring. After 15 hours at 90 ° C, the yellowish collodion wool is sucked off and boiled for 1 hour
• 2 I 2% aqueous formaldehyde solution.

It is then suctioned off again, washed neutral with water and the fine, fibrous product is dried at room temperature to 10% residual moisture.

b) formulation of the fuel mixture

4.4 parts of the wet nitrocellulose are dissolved in 92.2 parts of ethanol and 3.4 parts of water. A 4% solution of the ignition improver is obtained in the ethanol-water azeotrope.

c) Motor test

The test engine is operated with the ethanolic solution of this ignition improver and the combustion behavior compared to DK is determined. Figure 5 shows the pressure curve of DK (curve 1) and the test solution (curve 2).

Figure 6 shows the fuel conversion of DK (curve 1) and the test solution (curve 2). The ignition delay of the test solution is somewhat larger than with DK, but the pressure curve is very similar.

Example 4 a) Manufacture of the ignition improver

Arabinosetetranitrate is produced by adding 10 g arabinose in 100 cm ³ nitric acid (98%) at 0 ° C. 300 cm ^{3 of} sulfuric acid (98% strength) are added dropwise to this solution with cooling, so that the temperature does not exceed 2 ° C.

The reaction mixture is then poured onto 2,000 g of ice, a powdery solid substance separating out. This is filtered off, washed with water and dissolved in 50 ml of absolute ethanol at 50 ° C. When this solution cools down to -10 ° C, colorless crystals separate, which are filtered off and dried at 20 ° C in a vacuum.

b) formulation of the fuel mixture

The arabinosetetranitrate is dissolved in 4% by weight or 8% by weight in azeotropic ethanol-water (96: 4) with gentle heating.

c) Motor test

The test engine is operated for comparison with DK, GK and the two test solutions.

Figure 7 describes the cylinder pressure curve when using DK (curve 1), GK (curve 4), the 4% ethanolic solution of arabinosetetranitrate (curve 3) and the 8% ethanolic solution of arabinosetetranitrate (curve 2). With a 4% addition of arabinosetetranitrate to the ethanol, an ignitable fuel mixture is already obtained, while the 8% solution gives almost the same pressure curve as diesel fuel.

FIG. 8 shows the fuel turnover, which increases with the 8% alcoholic solution (curve 2) as steeply as with DK (curve 1). With 4% arabinosetetranitrate in ethanol (curve 3) you get faster fuel conversion than with GK (curve 4).

The ignition delay is almost as small with (curve 3) as with DK (curve 1).

Claims (1)

1. Use of ignition improvers for alcoholic fuels for diesel engines, based on nitric acid ester of mono- and/or polysaccharides having a degree of nitration in the range of 1 to 3, characterised in that they are contained in the alcoholic fuel in a quantity of 2 % by weight to 10 % by weight.

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