EP2143778B1 - Low-soot diesel fuel containing a fuel additive, and its use in the reduction of soot in diesel fuel combustion - Google Patents

Description

The invention relates to low odor burning diesel fuels containing at least one fuel additive. Furthermore, the invention relates to their use for reducing the formation of soot in the combustion of diesel fuels.

Due to the rapidly increasing demand for oil in recent years and the associated strong price increase, synthetic fuels and fuels from renewable raw materials have become increasingly important. Such fuels are usually either hydrocarbons, e.g. produced by Fischer-Tropsch synthesis from raw materials such as gas, coal and plant residues, or from alcohols, e.g. can be obtained from sugar cane by fermentation. Hydrocarbon fuels are typically produced using XTL technology, where X is a placeholder for gas (G), coal (C) or biomass (B). The letters T and L in this context mean that the starting material is converted into a liquid (to liquids). The XTL technology is often based on the Fischer-Tropsch synthesis at low temperature and provides a liquid fuel as a product.

Synthetically produced XTL diesel fuels lead to an approximately 20% less soot-burning compared to mineral oil-based fuels ( J. Krahl et.al., 5th Internat. Colloquium Fuels, Jan.12-13,2005, Ed .: WJ Bartz, TAE Ostfildern, p.207 / 212 ). Such less sooty combustion is particularly desirable in urban transport and is currently and in the near future by the relevant emission standards such as the commercial vehicle engines according to EU V (EU Directive 2006/51 / EC) and EU VI (EU Directive will be published in 2009) by law limited. An analogous legislation exists in USA and Japan. Furthermore, such diesel fuels make it possible to significantly increase the EGR (EGR) rates of the diesel engines so as to lower NO _x emissions without increasing soot emission and fuel consumption. XTL fuels consist mainly of aromatics-free alkanes due to the production technique of the Fischer-Tropsch synthesis, the density at 0.735 g / ml (manufacturer: Shell) and 0.765 (manufacturer: Sasol), however, by 7.6 to 13% lower than the standard for diesel fuels (DIN EN 590, issue 3.2004 requires a density of 0.820 - 0.845 g / ml). The boiling point of such a fuel is about 170 ° C, the boiling end at about 330 ° C. A disadvantage in the production of such fuels is also that in the Fischer-Tropsch synthesis predominantly a crude product of straight-chain n-alkanes having up to 60 carbon atoms is formed, which partially reacted in a subsequent cracking and isomerization step to short-chain n-alkanes and isoalkanes Need to become. This further step is necessary because long-chain n-alkanes generally have high benchmarks, which adversely affects the fluidity of the fuels, and can lead to clogging of the fuel filter at cold temperatures, such as in winter. By the admixture of isoalkanes to the fraction of n-alkanes, a filtration capacity can be ensured even in the cold (Cold Filter Plugging Point, CFPP according to DIN EN 590 in winter: -20 ° C), because isoalkanes usually have lower fixed points and so the flowability of the mixture is improved. However, besides the necessary isomerization step, this procedure has a further disadvantage. The addition of isoalkanes to the n-alkanes also leads to a decrease in the cetane number of the fuel, so that the ignitability of the Fischer-Tropsch fuel also decreases.

Even with XTL diesel fuels, the development of soot can be halved at best. As an alternative to their use has been repeatedly demonstrated that a significant reduction in soot formation can also be achieved by oxygen-containing fuel additives. Such additives have the advantage that they lead to a reduction in the average combustion temperature in the cylinder and increase the proportion of premixed combustion. Premixed combustion is understood to mean the combustion of a homogenized mixture of vaporized fuel and air as opposed to diffusion combustion (inhomogeneous combustion of fuel droplets with soot formation). So far, predominantly oxygenates have been tested and used as fuel additives in which the oxidation number of the carbon atoms containing bound oxygen atoms is greater than +1. In particular, carbonates such as dimethyl carbonate (DMC) and esters such as dibutyl maleate (DBM) and fatty acid methyl ester (FAME) containing carbonyl groups have been used. Furthermore, acetals and polyacetals such as butylal and methylal were used with a -COCOC skeleton, in which the central carbon atom is bound to two adjacent O atoms

(A. Bertola, K. Boulouchos, SAE Paper 2000-01-2885). In these cases, the oxidation states of the oxygen-carrying carbon atom with ≥ + 2 are already very high, which leads to a corresponding reduction in the calorific value. For esters with -C = O (-OR) groups, their combustion leads to a decarboxylation with local CO ₂ cleavage, so that the oxygen contained can contribute little to reducing soot formation. The use of such fatty acid methyl esters (FAME according to EN 14214: 2003) as fuel, which contain about 11% oxygen, leads to the reduction of particle emissions compared to diesel fuel from 40 to 60% ( J. Krahl et.al., 5th Internat. Colloquium Fuels, Jan. 12-13, 2005, Ed .: WJ Bartz, TAE Ostfildern, p.207 / 212 ).

In addition, dialkyl ethers and other monoethers having 2-24 C atoms ( WO 1995/025153 such as US 5 520 710 ), and ethylene and diethylene glycol dimethyl ethers (SAE Paper 2000-01-2886) as fuel additives for diesel fuels. However, the described ether oxygenates generally have either a low oxygen content at acceptable boiling points, or a very low boiling point at high oxygen content, such as dimethyl ether or diethyl ether, which makes their use in fuels problematic. In addition, many low molecular weight ethers have a pronounced tendency to form peroxides, so that their use in practice is not effective. The diethylene glycol dialkyl ether described so far, in particular Cetaner ^®, a mixture of 20% 1,2-dimethoxyethane and diethylene glycol dimethyl ether 80%, have a highly rußsenkende effect. However, both components of Cetaner ^® are extremely toxic substances whose general use as a fuel or fuel additive is not approvable. Another disadvantage is that both substances are relatively volatile, which is additionally problematic in connection with their toxicity.

Alcohols are also less suitable than fuel packs, either because they have very low cetane numbers (such as ethanol or methanol), or at acceptable cetane numbers already too high benchmarks and have only a low oxygen content. In the case of low molecular weight alcohols (1-4 carbon atoms), they are poorly or not miscible with diesel fuels ( Nylund et al. "Alcohols / Ethers as Oxygenates in Diesel Fuel", Report TEC 3/2005 ).

The GB 565 465 A discloses a diesel fuel containing polyethylene glycol dialkyl ethers and a diesel fuel containing a mixture of tetraethylene glycol dialkyl ethers and diethylene glycol diethyl ether having a high ignitability. The EP 0 014 992 A1 describes diesel fuels containing a polyether, ethanol and / or methanol, and mineral based diesel fuels which burn with very little soot formation. The GB 1 246 853 A discloses a fuel having reduced smoke properties, comprising a diesel fuel mixed with an ether. In the EP 0 903 395 A1 discloses a cetane number increase diesel fuel composition containing dimethoxypropane and optionally higher dialkoxyalkanes. The EP 1 178 101 A2 discloses a fuel based on an ethylene glycol compound mixed with a liquid hydrocarbon. The US Pat. No. 3,594,138 discloses a fuel composition comprising a liquid hydrocarbon, a Group II metal salt, an alkanoic acid and an alkyl ether. The JP 59 232176 A describes fuels for diesel engines containing polyethers which burn with reduced soot formation. In the EP 0 861 882 A1 discloses fuel compositions for diesel engines comprising as a major component a mineral oil and also dialkyl phthalate compounds and glycol ethers. The US 5,425,790 A discloses fuel compositions consisting of hydrocarbons, aromatics and polyethers. DE 697 33 363 T2 describes diesel fuel additives which comprise a polyether derivative and serve to reduce emissions of white smoke.

The invention is therefore based on the object to develop the diesel fuels described above so that the disadvantages of known diesel fuels are largely excluded. In particular, the diesel fuels should have a reduced formation of soot and lead to a reduction in the combustion temperature and thus the formation of NO _x and be substantially free of toxic components. Furthermore, the additives contained in the diesel fuel for soot reduction during combustion should have the highest possible oxygen content. On the other hand, the oxygen content of the fuel should be kept just as low as the soot-reducing effect requires in order to allow the lowest possible volumetric calorific value loss compared to diesel fuel according to DIN EN 590. The boiling points of the additives used should preferably be such that they meet the requirements of international standardization for diesel fuels (eg according to DIN EN 590 only 5% of the components may boil higher than 360 ° C). Furthermore, the diesel fuel should have a good low-temperature behavior ( CFPP according to the test method EN 116 in DIN EN 590) and thus allow a good filtration capacity in the cold. The diesel fuel should preferably approach the density requirements of the EN 590 standard. Finally, the ignition properties of the diesel fuels should remain at a high level (cetane number> 50). The flash point should be <55 ° C as with diesel fuel.

According to the invention this object is achieved in that a diesel fuel containing a fuel additive comprising at least one poly-oxa-alkane of the general formula (I):

R ¹ (-O-CH ₂ -CHR ² ) _m -OR ³ (I),

where R ^{1 is} a straight-chain or branched alkyl radical, R ^{2 is} a straight-chain or branched alkyl radical or H, R ^{3 is} a straight-chain or branched alkyl radical, and wherein the fuel additive is free of toxic constituents, toxic being poisonous according to the Ordinance on Hazardous Substances (GefStoffV 23.12 2004, BGBI., I p. 3758), and wherein the diesel fuel contains 5 to 20% by volume of fatty acid methyl ester (FAME), and where m ≥ 4, and wherein the diesel fuel contains up to 10% by volume of polyoxa contains alkane.

The term "at least one poly-oxa-alkane" expressly includes mixtures of poly-oxa-alkanes.

Toxic is understood as toxic according to the Ordinance on Hazardous Substances (GefStoffV 23.12.2004, BGBI. IS 3758). For example, the diesel fuel according to the invention contains no polyethylene glycol dimethyl ether of the formula CH ₃ O (C ₂ H ₄ O) _n CH ₃ where n = 1-3. Toxic components should also include teratogenic components.

The diesel fuels according to the invention have a significantly reduced soot formation.

Furthermore, the addition of fuel in combination with a diesel fuel, in particular a hydrogenated vegetable oil, preferably NexBTL or an XTL diesel fuel, leads to an increase in the density and thus a compensation of the volumetric calorific value reduction which occurs when replacing CH ₂ groups in long-chain alkanes, O groups enters.

Furthermore, the air requirement of the engine is lowered by up to one third. This has a number of advantages: The cylinder filling ratio (quotient of boost pressure and air pressure) can be reduced by up to one third compared to an engine operation with XTL fuel. Accordingly, the cost of charging (compression) of the charge air decreases, which means that can be dispensed with an expensive two-stage charging and a cheaper one-stage Aufladegruppe can be used. Furthermore, the amount of exhaust gas produced is also about one third lower when the air demand for diesel engine combustion is one third lower. The catalytic exhaust aftertreatment then saves almost a third of catalyst volume and can be intensified and simplified.

The poly-oxa-alkanes are derived from alkanes in which more CH ₂ groups are replaced by oxygen atoms.

The radical R ¹ preferably denotes an alkyl radical having a chain length of 1 to 4 carbon atoms, more preferably a straight-chain alkyl radical, since such increase the cetane numbers of the oxygenates. Methyl, ethyl and n-butyl radicals can also be produced biogenically. This is a possibility to supply the bioalcohols methanol, ethanol and n-butanol to a refinement, since their direct use as diesel fuels is made very difficult especially by their low ignitability. The cetane numbers of methanol, ethanol and n-butanol are 3, 8 and 17, respectively.

The radical R ² preferably denotes a straight-chain or branched alkyl radical having 1 to 4 carbon atoms or H, more preferably a straight-chain alkyl radical or H. Der

R ² contains, more preferably 0 to 2 carbon atoms and most preferably 0 to 1 carbon atoms. When the radical R ² contains no carbon atoms, R ² = H.

For cost reasons, R ^{2 is} preferably H.

The radical R ³ likewise preferably denotes a straight-chain or branched alkyl radical having 1 to 4 carbon atoms, more preferably a straight-chain alkyl radical or H.

In a further preferred embodiment, R ¹ and R ^{3 are} a methyl, an ethyl and / or a butyl radical. The use of such compounds leads to an improved solubility of the poly-oxa-alkane in diesel fuel, ie, the miscibility is increased.

As mentioned, m is ≥ 4, in particular 4-16 and particularly preferably 4 to 12. Such compounds are particularly cost-effective.

The polyoxaalkane is preferably free of carbon atoms having an oxidation number> +1, more preferably free of carbon atoms having an oxidation number> 0.

In a preferred embodiment, the at least one poly-oxa-alkane is a Polyalkylenglykoldiethylether, in particular a polyethylene glycol dimethyl ether of the general formula CH ₃ O (C ₂ H ₄ O) _m CH ₃ , where m = 4 or a mixture of m = 4 to m = 12, preferably from m = 4 to m = 8.

In a further preferred preparation form, the polyalkylene glycol dialkyl ether is selected from the group consisting of tetraethylene glycol ethyl methyl ether, tetraethylene glycol butyl methyl ether, dipropylene glycol dimethyl ether, and polypropylene glycol dimethyl ether with 4-5 propylene units.

The polyoxaalkane is particularly preferably a polyethylene glycol dibutyl ether, in particular a polyethylene glycol dibutyl ether having an average molecular weight of about 300, such as polyglycol BB 300 (available from Clariant Produkte GmbH). With this poly-oxa-alkane, the formulation of diesel fuels is possible, which meet the density requirements of DIN EN 590 with a correspondingly high content.

The boiling point of the poly-oxa-alkane is preferably between about 100 ° C and about 450 ° C, more preferably between about 150 and about 375 ° C, and most preferably between about 170 and about 330 ° C. When the boiling point of the polyoxaalkane is the same or less than about 330 ° C, this increases the proportion of premixed, low-carbon combustion and at the same time reduces the proportion of soot-forming diffusion combustion of the non-evaporable fuel droplets.

In a further preferred embodiment, the polyoxaalkane has a melting point of less than about -10 ° C, preferably less than about -20 ° C. This has the advantage that the CFFP value required in the standard DIN EN 590 can be achieved when mixing with n-alkanes (high fixed point).

In a further preferred embodiment, the polyoxaalkane has a density of from about 0.8 to about 1.1 g / ml, preferably from about 0.85 to about 1.0 g / ml. A high density has the advantage that it can compensate for the possibly lower density of a diesel fuel, so that the diesel fuel according to the invention fulfills the EN 590 standard or approaches this standard.

In a further preferred embodiment, the flash point of the poly-oxa-alkane is greater than about 55 ° C. This has the advantage that the DIN EN 590 is met.

As starting material for the diesel fuel according to the invention, any diesel fuel can be used. However, the diesel fuel used is preferably substantially free of aromatic constituents, since such constituents are generally responsible for increased soot formation. Hydrogenated vegetable oil, in particular NexBTL, or XTL diesel fuels are particularly preferably used as diesel fuels. XTL as used herein is intended to encompass fuels that have been prepared by a gas to liquids (GTL) coal to liquids (CTL) or biomass to liquids (BTL) process. Most preferred is the use of GTL diesel fuel.

The diesel fuel used preferably has a density of about 0.7 to about 0.8 g / ml, in order not to allow the density to drop too much below the standardized value of 0.82 g / ml and thus also the volumetric calorific value drops only slightly ,

The diesel fuels, ie fuel additives, according to the invention preferably have a density of from about 0.8 to about 0.845 g / ml in order to approximately meet the standard (EN 590).

The diesel fuel of the present invention is preferably substantially free of components having a boiling point greater than about 450 ° C to meet diesel fuel standardization in boiling behavior.

Preferably, the diesel fuel of the invention contains less than 5% by volume of components having a boiling point greater than about 360 ° C (EN590).

The proportion of poly-oxa-alkane in the diesel fuel according to the invention is up to 10% by volume, and more preferably up to 5% by volume. The lower the proportion of the additive, the more cost-effective is the diesel fuel according to the invention. In addition, the calorific value reduction of the diesel fuel according to the invention is even lower.

The diesel fuel according to the invention contains 5 to 20% by volume of fatty acid methyl ester (FAME). This has the advantage that the biogenic components of the diesel fuel are increased, which is otherwise required by law in many countries. In addition, FAME has the property of serving as a solubilizer. This has the advantage that more poly-oxa-alkane of the general formula (I) can be brought into solution in the diesel fuel. The diesel fuel according to the invention preferably contains 5 to 10% by volume of FAME. This has the advantage that up to 10 vol.% Poly-oxa-alkane, in particular polyethylene glycol dimethyl ether of the general formula CH ₃ O (C ₂ H ₄ O) _m CH ₃ , where m = 4 to m = 8, in Solution can be brought.

The present invention also relates to the use of a diesel fuel to reduce soot formation in the combustion of diesel fuels.

In use, in particular, any of the aforementioned diesel fuels can be used. The used special also each of the aforementioned diesel fuels are used. However, the diesel fuel used is preferably a fuel that is free of aromatic constituents, and more preferably a hydrogenated vegetable oil, especially NexBTL, or an XTL diesel fuel.

Also disclosed is a process for homogenizing diesel fuel / alkanol mixtures which comprises admixing polyoxaalkanes of general formula (I) as solubilizers and cetane improvers, the same restrictions being imposed on R ¹ , R ² and R ³ , m and on Fuel additive should apply as described above for diesel fuels. In the application of the method, any of the aforementioned diesel fuels can be used.

"Homogenization" as used herein describes the conversion of a multiphase mixture to a single phase mixture. The alcohol used herein is preferably a primary alcohol, more preferably a primary alcohol having 1 to 6 carbon atoms, and most preferably ethanol or n-butanol. Ethanol is an alternative fuel to mineral oil-based fuels that can be produced inexpensively from organic sources by fermentation. n-Butanol has the advantage that in the future it can be produced from cellulose by a biochemical process and thus does not compete with food production.

Diesel fuel / alkanol mixtures are of interest in view of the increasing scarcity of mineral oil-based fuels because, in particular, ethanol can be obtained by fermentation from renewable raw materials. Such bio-ethanol can be produced very cheaply, for example, in Brazil and increase the total amount of fuel by admixture with conventional diesel fuel.

As a two-component mixture, however, diesel fuel / alkanol mixtures can generally not be used as fuel because the two components are immiscible or only in small proportions. This is especially true for the low molecular weight alkanols methanol and ethanol. By admixing polyoxaalkanes of the general formula (I), however, diesel fuel / alkanol mixtures can be prepared which are single-phase at the temperature of use and which at the same time permit the admixture of large quantities of alkanol with the diesel fuel. In such three component blends of diesel fuel / alkanol and additive, the amount of adjuvant added is preferably at least about 4%, more preferably at least about 5 to 11%. The amount of alkanol component is usually about ≥ 10%, preferably about ≥ 20%, and most preferably about ≥ 30%.

In the following, the invention will be further illustrated by way of examples. However, the examples are not intended to be limiting or limiting in any way for the present invention.

example 1

Generation of a stable solution of GTL diesel fuel and oxygenate.

As GTL fuel, GTL from Sasol / FM / No. 243 with a density at 20 ° C of 0.7656 g / cm ² used. The oxygen content in the final mixture was 11% by weight. The densities of the respective mixtures are given in Table 1. Table 1 Diesel fuel (% m / m) Glyme (% m / m) Oxygen (% m / m) Density at 20 ° C (g / cm ³ ) Diethylenglykolethylmethylether 66.46 33.54 11.0 .8071 diethylene glycol diethyl ether 62.84 37.16 11.0 0.8083 dipropylene 62.84 37.16 11.0 .8078 tetraethylene 69.44 30.56 Maximum solubility 6% Polyglycol BB 300 55.47 44.53 11.0 .8253 diethylene glycol dibutyl ether 50,00 50,00 11.0 .8176 Polyglycol DME 250 / DME 500 (1: 4) 69.53 30.47 Maximum solubility 2%

Polyglycol BB 300 is a polyethylene glycol dibutyl ether with an average molecular weight of about 300 (manufacturer Clariant Products GmbH, 84504 Burgkirchen).
Polyglycol DME 250 or DME 500 are polyethylene glycol dimethyl ether having an average molecular weight of about 250 or 500 (manufacturer Clariant products GmbH, 84504 Burgkirchen).

From Table 1 it can be seen that can be produced by the addition of poly-oxa-alkanes to GTL diesel fuels diesel fuels, which have a significantly higher density than pure GTL diesel fuels. In the case of Polyglycol BB 300, the diesel fuel / glyme mixture also meets the density criteria of the EN 590 standard. It also shows that tetraethylene glycol dimethyl ether and polyglycol DME 250 / DME 500 mixtures are not suitable for the formulation of diesel fuels with an oxygen content of 11%. Both additives have oxygen contents of about 36%, which results in reduced miscibility with nonpolar GTL diesel fuel.

Example 2

On a MAN single-cylinder research engine with a displacement of 1.75 l, an engine output of 55 kW, a common rail injection system (rail pressure 1800 bar), a compression of 20.5, an injection start before the upper dead center of - 8 ° crank angle and an EGR rate of 20%, a fuel mixture of 95 vol.% Diesel fuel according to EN 590 and 5 vol.% Tetraethylene glycol dimethyl ether was tested.

The results are shown in Table 2 below. As a comparison served pure diesel fuel according to EN 590. Table 2 operating point BP 1 BP 2 BP 3 BP 4 Soot reduction (NOx emission level constant)% -41.2 -53.1 -52.9 -27.4 Speed rpm 914 1542 1542 1800 Torque Nm 75 200 270 140 Medium pressure bar 6.3 15.7 20.7 11.7 air ratio 3.1 1.35 1.35 1.9

This experiment shows that the addition of tetraethylene glycol dimethyl ether leads to a significant reduction in soot, which is the higher the lower the excess air in the combustion (see air ratio in Table 2).

Example 3

Several diesel fuel / ethanol blends with poly-oxa-alkanes have been prepared as solubilizers. For this purpose, diesel / ethanol mixtures were prepared with the highest possible ethanol content of ≥ 30 wt .-%. As diesel fuel GTL were from Sasol / FM no. 243 with a density at 20 ° C of 0.7656 g / cm ² and Aral Ultimate with a density at 20 ° of 0.8236 g / cm ² and ethanol with a density at 20 ° of 0.7893 g / cm ² used , The densities and the corresponding oxygen contents of the corresponding mixtures are given in Table 2. Table 3 Diesel (% m / m) Glyme (% m / m) Ethanol (% m / m) Oxygen (% m / m) Density at 20 ° C (g / cm ³ GTL diesel diethylene glycol dibutyl ether 68.3 0.7 31.0 immiscible 65.6 3.5 31.0 immiscible 62.1 6.9 31.0 12.3 .7776 Polyglycol BB 300 65.6 3.5 31.0 immiscible 62.1 6.9 31.0 12.5 .7797 Aral UltiMate Diesel diethylene glycol dibutyl ether 65.5 3.5 31.0 immiscible 62.1 6.9 31.0 12.3 .8147 Polyglycol BB 300 65.6 3.5 31.0 immiscible 62.1 6.9 31.0 12.5 .8170 - 83.3 - 16.7 immiscible

From Table 3 it can be seen that stable diesel fuel / ethanol mixtures can be prepared using relatively small amounts of poly-oxa-alkane.

Claims (11)

1. A diesel fuel containing a fuel additive, comprising at least one polyoxaalkane of the general formula (I):
   
           R¹(-O-CH₂-CHR²)_mO-R³     (I),
   
   where R¹ is a straight-chain or branched alkyl group, R² is a straight-chain or branched alkyl group or H, R³ is a straight-chain or branched alkyl group, and wherein the fuel additive is free of toxic constituents, wherein "toxic" is understood as being toxic according to the German Dangerous Substances Act (GefstoffV 23.12.2004, BGBI. I p. 3758), and wherein the diesel fuel contains 5-20% by volume of fatty acid methyl esters (FAME), and wherein m ≥ 4, and wherein the diesel fuel contains up to 10% by volume of polyoxaalkane.
2. A diesel fuel according to claim 1, wherein the diesel fuel contains up to 5% by volume of polyoxaalkane.
3. A diesel fuel according to any one of the preceding claims, wherein m is 4 -16 and is more preferably 4 -12.
4. A diesel fuel according to claim 3, wherein R² is H.
5. A diesel fuel according to any one of the preceding claims, wherein R¹ and R³ are a methyl, ethyl and/or a butyl group.
6. A diesel fuel according to any one of the preceding claims, wherein the diesel fuel is a hydrogenated vegetable oil, especially NexBTL, or an XTL diesel fuel.
7. A diesel fuel according to any one of the preceding claims, wherein the polyoxaalkane is free of carbon atoms with an oxidation number of > +1.
8. A diesel fuel according to any one of the preceding claims, wherein the at least one polyoxaalkane is a polyalkylene glycol dialkyl ether.
9. A diesel fuel according to claim 8, wherein the polyalkylene glycol dialkyl ether is a polyethylene glycol dimethyl ether of the general formula CH₃O(C₂H₄O)_mCH₃, where m=4 or a mixture of m=4 to m=12, preferably of m=4 to m=8.
10. A diesel fuel according to claim 8, wherein the polyalkylene glycol dialkyl ether is selected from the group consisting of tetraethylene glycol ethyl methyl ether, tetraethylene glycol butyl methyl ether, and polypropylene glycol dimethyl ether having 4-5 propylene units.
11. The use of a diesel fuel according to any one of the preceding claims for reducing the formation of soot during the combustion of diesel fuels.