GB2355943A - Control of diesel particulate pollution - Google Patents

Abstract

The combustion of PM from a diesel engine, trapped on a diesel particulate filter or trap, using the reaction with NO<SB>2</SB>, is promoted by providing a source of potassium and/or caesium for example, the source of K and or Cs may be a compound dissolved or dispersed in diesel fuel.

Description

2355943 IMPROVEMENTS IN POLLUTION CONTROL This invention concerns

improvements in emissions control, and more especially is concerned with the improved low temperature combustion of sooty particulate matter ("PM") resulting from certain combustion processes. There is widespread concern over the health and environmental impact of PM emitted from internal combustion engines.

PM from heavy duty diesel engines operating under certain conditions such as start-up, high speed and/or heavy load, are particularly visible as "black smoke". Despite the PM emissions of diesel engines of all sizes, their inherent fuel economy and, in turn, low C02 emissions, means that diesel engines are increasingly used in car and light truck applications.

The PM from heavy duty diesel trucks and buses can be removed effectively, in combination with low sulphur fuel, by Johnson Matthey's Continuous Regenerating Technology ("CRT TM,,). This device uses a catalyst to convert NO in the exhaust gases into N02, and a trap for PM downstream of the catalyst. PM trapped may then be combusted using the reaction with N02, which proceeds at a significantly lower temperature than combustion of PM by reaction with 02. Reference is made to USP 4,902,487, which describes the CRTTm.

It has been found, however, that current and future light duty diesel engines, and even heavy duty engines operating at low speed for a large proportion of time, e.g. city buses, exhibit relatively low exhaust gas temperatures. All chemical reactions slow as temperatures are decreased, and even the CRTTm can accumulate PM in the trap until a period of higher gas temperatures.

The use of additives in fuels has been suggested. For example, Platinum Plus Inc.

have described a system involving a platinum additive, preferably in admixture with cerium, and in combination with a PM trap, in USP 5,749,928. Recently, PSA Peugot-Citron have published on their world-wide web site, details of a PM control system for a light duty diesel engine which involves passing exhaust gases through a treatment unit incorporating a catalyst and a trap. A fuel additive based on cerium is 2 dosed continuously into the fuel tank. However, it appears that high temperature trap regeneration mode may still be required, which is achieved by dosing extra fuel into the exhaust gases. There remains a need for other strategies for applying the clear benefits of the CRT TM to engines with low exhaust gas temperatures.

Accordingly, the present invention provides a PM control system for diesel engines comprising means to supply N02 and a trap for PM, characterised in that PM combustion promotion means comprising a source of an element which promotes the reaction between N02 and soot, preferably a source of potassium and/or cesium is provided to the trap.

The invention further provides a method of conibusting trapped PM using reaction with N02, comprising supplying a source of an element which promotes the combustion reaction between N02 and soot, preferably selected from potassium and/or cesium to the trap.

The trap may comprise a K and/or Cs-containing coating, for example by impregnating the trap directly with a K and/or Cs salt solution, followed by firing or by coating the trap with a high surface area slurTy, e.g. comprising alumina, followed by firing and impregnation with a K and/or Cs salt solution. It is, however, preferred to include a source of K and/or Cs in an oil-soluble or oil- miscible form in the fuel used for the diesel engine. Such compounds may be included in a mixture or solution added to each tankful of diesel fuel or in a more sophisticated and reliable method, dosed from an on-board reservoir into the fuel tank or into the fuel line. It is envisaged that suitable concentration of K and/or Cs compound is such to provide up to 200ppm in the fuel, preferably from about 0.25 to 10ppm. Suitable dosing and related technology is available in the art and plays no part in the present invention. However, one embodiment of the invention incorporates a form of temperature sensor, or other measurand sensor, capable of detecting exhaust gas conditions which are not conducive to combustion of PM using N02, and either initiating dosing or increasing the dose, of the K and/or Cs source. Our tests indicate that increasing the dose of the K or Cs source reduces the ignition temperature for the PM into the range that is commonplace in light duty diesels. It is envisaged, although 3 we do not wish to be restricted by any theory, that it is more beneficial to include the K and/or Cs source in the fuel so that the active catalytic ingredients are distributed throughout the PM, rather than relying on simply a surface reaction. Of course, the trap may be also catalysed.

Therefore, the invention in a further aspect comprises a composition for dosing into diesel fuel, comprising an oil-soluble or oil-miscible source of K and/or Cs. The invention also includes a fuel for diesel engines, comprising an oil-soluble or oil-miscible source of K and/or Cs.

The invention will be further described with reference to the three accompanying drawings showing C02 intensity (representing combustion of PM) plotted against temperature, for the combustion of PM collected from diesel engine exhaust some of which are physically mixed with amounts of K and/or Cs nitrates at the defined loadings.

Referring first to Fig 1, the combustion of PM using either oxygen (5% by vol in He) or N02 (1% by vol in He) clearly shows the advantage of using the N02 reaction, which shows peaks almost 3000C apart.

In Fig 2, a similar plot, under identical conditions, and using N02, compares the effect of mixing PM with a promoter in 1: 1 weight ratio. One test was carried out with no promoter; Promoter A is 10% by wt K on alumina; Promoter B is 10% by wt Cs on alumina and Promoter C is 10% by wt Cs on a typical alumina/ceria/zirconia washcoat material. It can readily be seen that there are significant reductions in combustion temperatures upon the addition of these K or Cs promoters.

In Fig 3, the reduction of combustion temperatures with increasing loading of Cs from 1% to 32% by wt on alumina, admixed with soot on a 1:1 weight ratio, is demonstrated.

It will be appreciated that the present invention may be carried out in a number of ways by the skilled person, without departing from the basic inventive concept.

4

Claims (8)

1. A PM control system for diesel engines comprises a means to supply N02 and a trap for PM, on which the PM can be combusted by reaction with N02, characterised in that PM promotion means comprising a source of an element which promotes the reaction between N02 and PM, preferably a source of potassium and/or cesium, is provided to the trap- io

2. A system according to claim 1, comprising means for dosing said element into the diesel fuel for combustion in the engine.

3. A system according to claim 2, comprising an on-load reservoir of an oil-soluble or oil-miscible compound of potassium and/or cesium, optionally dissolved or admixed with diesel ftiel, and means to dose said compound(s) into the fuel tank or the fuel line.

4. A system according to claim3, wherein the dosing means is programmed to provide up to 200ppm of compound in the diesel fuel combusted.

5. A method of combusting trapped PM using reactions with N02, comprising supplying to the trap, a source of an element which promotes the reaction between N02 and PM, preferably a source of potassium and/or cesium.

6. A method according to claim 5, wherein said promotion means is applied to the trap in a quantity determined by the PM-generating conditions experience by the engine.

7. A composition of dosing into a diesel fuel, comprising an oil-soluble or oil-miscible compound of potassium and/or cesium.

8. A ftiel for diesel engines, comprising an oil-soluble or oil-miscible potassium and/or cesium compound in admixture with or in solution in diesel fuel.