Power plants – Internal combustion engine with treatment or handling of... – Methods
Patent
1997-11-19
1999-08-31
Denion, Thomas E.
Power plants
Internal combustion engine with treatment or handling of...
Methods
60297, 60301, F01N 300
Patent
active
059438576
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention concerns improvements in emission control, more especially, it concerns improvements in the control of regulated emissions from diesel engines.
Light duty diesels, for example those of up to about 2.5 liter capacity used in private cars and light vans, show much better fuel economy and hydrocarbon emissions than gasoline engines and are increasing in number. Emissions from diesel engines are now being regulated by legislation, and whilst hydrocarbon emissions do not present a problem in meeting emission limits, NOx is problematical. Since the exhaust gases from lean-burn engines such as diesels are high in oxygen content throughout the cycle of operation, it is more difficult to reduce NOx to N.sub.2. A contribution to this problem is the low exhaust gas temperature in comparison to gasoline-fuelled engines, which can also vary quite dramatically according to operating conditions. Thus, for a state of the art diesel exhaust catalyst system, 50% of hydrocarbons are oxidised at a temperature in the range 190 to 250.degree. C., and this inclines to 100% at higher temperatures. The conversion of NOx, however, shows very different characteristics, with a marked peak, creating a "window" of conversion at the 30 to 60% level, at a range of temperatures depending upon the catalyst system and engine system. This is illustrated in accompanying FIG. 1. As mentioned, the exhaust gas temperature will vary over a test cycle, so that the catalyst is seldom at a suitable temperature for optimum conversion. Over a test cycle therefore, the cumulative conversion efficiency for diesel NOx may only be about 10%. In fact, as the catalyst temperature drops below the peak conversion temperature for NOx, the temperature is also below "light off" for HC conversion. Accordingly, most of the unburnt hydrocarbons go out the tail pipe unreacted, along with unconverted NOx. It is to be noted that the exhaust temperature with heavy duty diesels is much higher and therefore these do not face quite the same problems. Nonetheless, the present invention offers improvements in emission control particularly during start-up of heavy duty diesel engines, and offers the potential to control emissions to meet expected future regulations.
SUMMARY OF THE INVENTION
It is an aim of the present invention to provide a diesel engine system capable of a substantial reduction in emissions of NOx, especially but not exclusively from light duty diesel engines.
Herein, diesel engines are those fuelled by fuel oil of the type often known as "gas-oil". The invention may also be applied to diesel compression ignition engines fuelled by other fuels including oxygenated fuels.
We would mention GB Patent 1407772 (Nissan), which concerns an internal combustion engine fitted with a three-way catalyst, and teaches a complicated system which includes supplying additional fuel into the exhaust upstream of the catalyst, in order to assist in the reduction of NOx to N.sub.2. The skilled person considers that the teaching of this patent is only relevant to gasoline-fuelled engines. There is no suggestion that unburnt fuel may be collected from the exhaust gas instead of the supply of additional fuel upstream of the catalyst.
WO 94/22564 (Engelhard) describes a catalyst composition for treating diesel engine exhausts, comprising a ceria component and a catalytically active amount of a zeolite. The purpose of this catalyst composition is stated to be the oxidation of volatile fractions, and the catalyst is operated under oxidising conditions, and is completely unconcerned with levels of NOx.
The present invention provides a diesel engine system which exhibits a higher exhaust gas temperature and a lower exhaust gas temperature, said lower temperature being insufficient for significant catalytic conversion of NOx to N.sub.2, and a lean NOx catalyst system comprising lean NOx catalyst and an adsorbent for unburnt fuel, arranged and constructed so that during the lower exhaust gas temperature parts of the engine operating c
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Ansell Graham Paul
Fisher Janet Mary
Gray Peter Geoffrey
Hamilton Hugh Gavin Charles
Rajaram Raj Rao
Denion Thomas E.
Johnson Matthey Public Limited Company
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