EP0492101B1 - Use of organometallic additives for improving soot burning in diesel fuels - Google Patents

Abstract

A diesel fuel containing an additive, which improves soot burning, for reducing the pollutant emissions of the combustion exhaust gases of diesel engines by discontinuous burning-off of soot which has been deposited on the exhaust gas filter is described. For this purpose, a lithium salt, sodium salt or potassium salt of an aliphatic or aromatic alcohol, of a phenol, of an aliphatic acid or a naphthoic acid, of phenylacetic acid or of cinnamic acid, individually or as a mixture, is added to the diesel fuel before the latter is burned in the internal combustion engine. As a result of the addition of the alkali metal salts, the ignition temperature of the soot deposited in the particle filter is reduced and the soot is oxidised at a temperature which is considerably lower than the normal ignition temperature. The regeneration range for the particle filter is thereby much more frequently reached in real driving practice. As a result, a critical filter loading with soot, which can cause filter damage on burning-off, is avoided. A further advantage of the process described is that, according to present knowledge, no additional substances injurious to health are emitted during running as a result of the addition of these alkali metal salts to the diesel fuel. <IMAGE>

Description

The invention relates to the use of organometallic additives to improve the combustion of soot in diesel fuels.

In addition to the pollutants that also occur in Otto engines, diesel engines emit soot particles, which have been assessed very critically for several years. Animal studies have shown that diesel exhaust has a carcinogenic potential. That is why diesel exhaust was included in the list for maximum workplace concentrations in 1987 as a carcinogenic agent.

In order to reduce the particle emission in the exhaust gases of diesel engines, it has become part of the state of the art to separate the particles formed during the combustion process in a downstream filter device and to oxidize them there. Monolithic, honeycomb-shaped ceramic bodies or, for example, ceramic wound filters, in which a yarn made of ceramic fibers is applied to perforated steel tubes, are predominantly used as such filter devices. With such filter bodies, a very good separation of the soot particles can be achieved. The absolutely necessary regeneration of the particle filter has not yet been satisfactorily resolved. Without additional measures, the soot separated in the particle filter is oxidized sufficiently quickly only at temperatures above 600 ° C. In normal driving operation of the Such high exhaust gas temperatures are only very rarely reached in a motor vehicle. With an increasing filter load, the exhaust gas back pressure rises sharply and has a significant impact on the combustion behavior and the performance of the engine. Above all, however, there is the risk that a filter body that is too heavily loaded with soot particles will be subjected to excessive stress during regeneration due to the heat released during the exothermic soot oxidation and thus be damaged.

Various measures are known from the prior art which are intended to enable regeneration of the particle filter even at lower temperatures. For this purpose, it has been proposed to coat the ceramic carrier material of the particle filter with a catalytically active substance (DE-OS 32 32 729). However, the coatings used to date have not proven to be sufficiently effective. In addition, there are concerns about certain proposed coating substances, for example the vanadium oxide according to the abovementioned document, for toxicological reasons. It is also already known to arrange an additional burner in the particle filter in order to achieve a targeted free burning of the particle filter from the deposited soot. Direct heating of the particle filter is already part of the prior art (DE-OS 35 38 155). It has also already been described to add a catalytically active substance in a metered amount to the exhaust gas stream for burning the soot (DE-OS 33 25 391). To reduce the soot content in the exhaust gases of diesel engines, organic boron compounds (DE-OS 23 40 522) which have been mixed with the diesel fuel or copper and ammonium salts (DE-OS 33 25 391) or perchlorates (DE-OS 34 36 351) recommended, which were metered into the exhaust gas upstream of the soot filter. However, the results achieved were not convincing and some of the proposed compounds are an additional health hazard Environmental pollution cannot be ruled out. Thus, "Automobiltechnische Zeitschrift" 86 (1984) 2, page 76, left column, shows that fuel additives for preventing soot emission have been developed which consist of organometallic compounds of alkaline earth metals or alkaline earth sulfonates. However, it is expressly pointed out that metal oxides formed during combustion, which caused increased engine wear and an increase in the toxicity of the exhaust gases could not be excluded.

From EP application 0 216 635 it is also known to add fat-soluble organometallic compounds, namely lithium, sodium or potassium salts of saturated or unsaturated aliphatic carboxylic acids having 2 to 32 carbon atoms, to a diesel fuel to improve performance. FR-PS 1 263 322 states that lithium, sodium or potassium salts of branched, aliphatic, unsaturated carboxylic acids having up to 36 carbon atoms are soluble in hydrocarbons. In these two documents, too, no solutions are given as to how the particle filter can be regenerated.

The object of the invention is therefore to achieve regeneration of the particle filter, arranged in the exhaust pipe of a diesel engine, even at low temperatures.

The object is achieved according to the invention by using organometallic additives that are added to the diesel fuel.

Surprisingly, it has been found that an improvement in the soot combustion in the diesel fuel is achieved by jointly burning the added compounds an extraordinarily reactive soot is formed with the diesel fuel in the diesel engine. After being deposited in a soot filter, the soot particles are Oxidize quickly even at very low temperatures. The investigations have shown that the regeneration temperatures depend very much on the speeds and loads at which the engine is operated. The motor conditions have a very strong influence on the morphology of the soot and thus also its reactivity. Under certain engine conditions, good regeneration of the soot filter is possible even at exhaust gas temperatures below 200 ° C. In contrast to the additives added to the diesel fuel according to the prior art, there are no health concerns regarding the use of the alkali metal salts added according to the invention. No MAK values can be found in the literature and there are no indications of a possible carcinogenicity or co-carcinogenicity.

Of the alkali metal salts added according to the invention, there are indications, especially for lithium compounds, that the combustion process is influenced favorably and the emission is already reduced within the engine. Above all, however, a soot is formed during the combustion process in the presence of the additives under investigation, which is much easier to oxidize after being deposited in a particle filter. This means that much lower exhaust gas temperatures are sufficient for particle filter regeneration and thus a critical soot mass occupancy in the filter is avoided.

In the experiments described below, lithium or sodium salts of tertiary butyl alcohol were added as additives to the diesel fuel in various concentrations. Concentrations were each based on the amount of alkali metal of the salt, expressed in millimoles of metal, added to one liter of diesel fuel. The tests were carried out with a stationary prechamber diesel engine (DB OM 616 series). There was a honeycomb in the exhaust pipe Ceramic monolith arranged from cordierite. The additives were mixed directly into the diesel fuel before the combustion process.

The effectiveness of the additives was tested in 4 different stationary and unsteady engine operating modes.

1st attempt

In test 1, no additive was added to the diesel fuel.

The test was carried out with a stationary engine operating mode. The particle filter was loaded at an engine speed of 4,000 rpm and an average pressure in the combustion chamber of approx. 1.0 bar. The exhaust gas temperature upstream of the soot filter is approx. 350 ° C at this engine point. The particle filter was loaded until the pressure in front of the filter had risen to 500 mbar. Fig. 1 shows the very steep pressure rise within a short time (phase 1). Following the loading phase, the temperature was increased by increasing the load (phase 2). The equilibrium temperature (TGL) is reached at about 560 ° C in this experiment. At the equilibrium temperature, the pressure upstream of the filter remains constant. The proportion of the newly deposited soot corresponds to the proportion that is already oxidized at this exhaust gas temperature. The filter is then slowly regenerated by increasing the temperature to 600 ° C (phase 3). However, the filter is only completely burned free at 700 ° C.

Second attempt:

The following test was carried out with an engine operating mode as described in test 1. The tertiary butylate of lithium, dissolved in cyclohexane, was added. The solution prepared was metered into the fuel in a ratio such that 1.2 millimoles of lithium were added with the metal salt per 1 liter of fuel.
As can be seen from FIG. 2, the sooting time, ie the time until a pressure of 550 mbar is built up in front of the particle filter, is significantly extended. The equilibrium temperature is now reached at 450 ° C. The temperature increase to 600 ° C leads to a very quick and complete regeneration of the particle filter. The less steep pressure increase in test 2 compared to test 1 and the much lower blackening number could be an indication that the addition of the fuel additive has already reduced the particle emission within the engine. In addition, it is also possible for soot already deposited in the filter to be oxidized continuously without complete regeneration occurring.

3. Attempt:

The tertiary butylate of lithium, dissolved in cyclohexane, was added to the diesel fuel. This time, 3.4 millimoles of lithium were used per 1 liter of fuel with the metal salt. The engine was run transiently at different speeds and loads with exhaust gas temperatures between 120 and 180 ° C. 3 clearly shows that regeneration of the particle filter at this additive concentration already takes place below 200 ° C. at a maximum pressure upstream of the filter of approximately 130 mbar. This experiment also shows that the regeneration temperatures depend on the mode of operation of the engine. Exhaust gas composition and morphological particle properties have a very strong influence on regeneration.

4th attempt:

This time, the tertiary butoxide of sodium, dissolved in isopropanol, was added to the diesel fuel as an additive. The Solution was dosed so that 1.2 millimoles of sodium was present per 1 liter of fuel. In this test, the engine was operated with a constant exhaust gas temperature upstream of the filter of 200 ° C. It can be clearly seen from the pressure curve in FIG. 4 that the particle filter regenerates several times at 200 ° C. In addition to a few smaller regenerations in the initial phase of the test, two very strong regeneration processes can be seen in the further test process, in which the particle filter is almost completely burned free of the deposited soot. The exhaust gas back pressure upstream of the particle filter rose only slightly above 250 mbar in this test.

5th attempt:

The tertiary butylate of sodium, dissolved in isopropanol, was added to the diesel fuel. The metal salt added 1.2 mmol sodium per 1 liter of fuel. The engine was operated transiently at different speeds and loads with exhaust gas temperatures between 200 ° C and 400 ° C. Numerous partly very strong regenerations take place. The maximum pressure upstream of the filter is approximately 400 mbar (Fig. 5).

6th attempt:

The lithium salt of palmitic acid, dissolved in cyclohexane, was added to the diesel fuel. The lithium palmitate used 1.2 or 3.4 mmol lithium per 1 liter of fuel. The equilibrium temperatures associated with the different lithium concentrations were determined in accordance with tests 1 and 2 with stationary engine operation. The equilibrium temperature for 1.2 mmol lithium is approx. 520 ° C, for 3.4 mmol Li approx. 500 ° C. The filter regenerations carried out at 600 ° C are comparable to lithium tertiary butylate.

7th attempt:

3.4 mmol of lithium was added to the diesel fuel with the lithium palmitate. The effectiveness of the additive was carried out in unsteady-state engine operation with exhaust gas temperatures between 200 ° C and 400 ° C according to test 5. Numerous z. Sometimes there are also vigorous regenerations.

5, the results of the experiments 1 to 4 given above are entered again. Likewise, from this FIG. 5, the results of further tests carried out, in which different amounts of tertiary butoxide of lithium or sodium were added to the diesel fuel, and the regeneration of the filter in the four different engine operating modes described above can be seen.

8th attempt:

1.2 mmol sodium was added to the diesel fuel with the sodium phenylethanolate dissolved in butanol. The test was then carried out with the engine operating mode specified in test 1, the equilibrium temperature being reached at 480 ° C. The filter regeneration at 600 ° C is significantly faster compared to the diesel fuel, to which no additive has been added, and only slightly slower compared to the sodium tert-butoxide as an additive.

9th attempt:

1.2 mmol of sodium was again added to the diesel fuel with the sodium salt of para-cresol dissolved in butanol. The effectiveness was also tested using the procedure described in Experiment 1. The equilibrium temperature was approximately 480 ° C. There was no difference in the reaction rate compared to the sodium tert-butoxide.

10th attempt:

The lithium salt of phenylacetic acid was added to the diesel fuel. Since this compound has a much poorer solubility in diesel fuel than the other additives, only the lowest additive concentration of 0.24 mmol lithium / liter diesel fuel was tested; the experiment is again carried out as in experiment 1. The equilibrium temperature was approximately 520 ° C. There was no difference in the regeneration rate compared to the lithium tertiary butanolate, which had also been tested in the lowest concentration of 0.24 mmol lithium liter diesel fuel.

11th attempt:

This test was carried out with the engine operating mode described in test 1 with an engine of the type OM 603 (Mercedes 300 D). A sodium tertiary butoxide dissolved in butanol was added to the diesel fuel in an amount such that 0.1 mmol of sodium was added per liter of diesel fuel. Compared to carrying out the test without adding an additive, the equilibrium temperature was lowered by about 30 ° C. The filter regeneration was significantly accelerated compared to carrying out the test without an additive. The experiment showed that an addition of 0.1 mmol sodium in the OM 603 engine was just as effective as an addition of 0.24 mmol sodium in the OM 616 engine. One reason could be the much lower carbon-containing particle emissions of the (more modern) engine type OM 603.

From the tests carried out, it is clear how the Amount of additives added affects the equilibrium temperature TGL (p before filter = constant) with stationary engine operation. For example, adding 3.4 mmol of lithium per 1 diesel fuel reduces the equilibrium temperature from 560 ° C to below 350 ° C. For driving practice, this means that the filter regeneration area is reached much more often and critical filter loading can be avoided.

At the same time, in practical unsteady-state engine operation, a very good regeneration of the particle filter is made possible with a much lower exhaust gas back pressure than without the addition of the additive.

The advantages of the described method are, in particular, that with the metal salts of the specified organic compounds added according to the invention to the diesel fuel before combustion, the soot separated in the particle filter oxidizes at a temperature which is significantly lower than the normal ignition temperature and thus the particle filter can be regenerated more easily. Compared to other additives for diesel fuel known from the prior art, the additional emission which can occur as a result of the addition of the diesel fuel with the alkali metal compounds proposed here can be classified as harmless from an environmental and health point of view.

Claims (1)

1. Use of 0.1 to 50 millimole, per 1 litre of diesel fuel, of lithium, sodium or potassium salts

   a) of an aliphatic alcohol of the formula CH₃-X-OH, X being an alkylene group having 1 to 8 carbon atoms, or of a compound isomeric with such an alcohol, or

   b) of an aromatic alcohol of the formula

   

   X being an alkylene group having 1 to 8 carbon atoms, or

   c) of a phenol of the formula

   

   X being an alkyl group having 1 to 8 carbon atoms, or

   d) of an aliphatic carboxylic acid of the formula CH₃-X-COOH, X being an alkylene group having 3 to 16 carbon atoms, or of a compound isomeric with such a carboxylic acid, or

   e) of 1-naphthoic acid, 2-naphthoic acid, phenylacetic acid or cinnamic acid, singly or as a mixture, as the additive in diesel fuels which improves the combustion of soot.

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