In the normal operation of a diesel engine atmospheric air is first compressed in the combustion chamber of the engine to a pressure of about 500 PSI. Compression of the air raises its temperature to about 1,000.degree. F. Diesel fuiel is then injected to the compressed hot air through a fuel injection nozzle. The fuel is atomized in the combustion chamber where it rises to its auto ignition temperature, resulting in the spontaneous ignition, burning, and expansion of the gases in the chamber. The expansion of the combustion products drives the cylinder downwardly thereby providing the power stroke of the engine.
In order for a diesel engine to operate efficiently, i.e., with minimum fuel consumption at maximum power, it typically is operated under air to fuel ratios which produce exhaust gases that contain large amounts of oxygen and usually only minimal amounts of unburned hydrocarbons. Unfortunately, operating a diesel engine for maximum power and efficiency also results in conditions that raise the peak operating temperatures and therefore NO.sub.x emissions. One method for lowering the NO.sub.x emissions is, of course, to bring the exhaust gas into contact with a catalyst capable of reducing the NO.sub.x species in the gas stream. However, for catalysts known to be effective in the diesel exhaust environment, catalytic de-NO.sub.x is usually more effective when reducing species are present in the exhaust gas. In order to generate these species in the engine one normally has to operate at conditions of low peak temperature which are conditions that are directly opposed to what is desired from the standpoint of overall efficient engine operation.
One method for providing reducing species at the catalyst is secondary injection, wherein a hydrocarbon is injected into a diesel engine's cylinder at a fixed crank angle ner the end of the expansion stroke. One problem associated with this method is that the quantity of various reductant molecules required for complete reduction of the NO.sub.x species depends upon engine operating parameters such as engine speed, engine load, and inlet gas (air and exhaust gas recirculation, "EGR") pressure when a compressor is present, and secondary injection at a fixed crank angle makes no provision for adjusting the quantity of injected hydrocarbon in response to changes in engine operating parameters. A second problem associated with secondary injection at a fixed crank angle is that the most effective reductants, i.e. olefins and oxygenates, are not the engine's primary fuel source. A secondary source of these compounds must be provided for the engine for injection of these compounds into the exhaust stream. Still another problem associated with secondary injection at a fixed crank angle is the introduction of high boiling point aromatic molecules into the exhaust. This places an extra burden on the exhaust treatment system because such molecules will contribute to PNA emissions unless they are oxidized to CO.sub.2 and H.sub.2 O.
Accordingly, it is an object of the present invention to provide an approach to operating a diesel engine so as to produce effective NO.sub.x reducing species in a diesel engine's combustion chamber in quantities sufficient for catalytically converting the NO.sub.x present in the engine's exhaust gas. It is further an object of the invention to convert NO.sub.x in the exhaust gas in a way that avoids the constraints normally imposed if these species are to be formed as by-products of normal combustion.
Stated differently, it is an object of the present invention to operate a diesel engine efficiently while generating sufficient organic cracked products, i.e. reducing species, in the combustion chamber for the catalytic reduction of NO.sub.x in the exhaust gas.