The present invention relates generally to compression-ignition engines and, more particularly to, techniques for reducing undesirable pollutants in emissions from diesel engines.
Compression-ignition engines, such as diesel engines, operate by directly injecting a fuel (e.g., diesel fuel) into compressed air in a combustion chamber of one or more piston-cylinder assemblies, such that the heat of the compressed air lights the fuel-air mixture. Compression-ignition engines in some embodiments include a glow plug to provide heat that ensures ignition in the combustion chamber. The direct fuel injection atomizes the fuel into droplets, which evaporate and mix with the compressed air in the combustion chambers of the piston-cylinder assemblies. Typically, in a turbo-charged diesel engine, air is drawn from the atmosphere and compressed in a compressor. The compressed air is cooled in an intercooler. The compressed and cooled air is then introduced into the combustion chamber. Upon igniting, the fuel-air mixture causes the combustion chamber to expand by moving the piston along the cylinder, thereby producing output power for the particular application. The combustion exhaust gases also power a turbine coupled to the air compressor. A variety of operating parameters affect the engine performance, efficiency, exhaust pollutants, and other engine characteristics. For example, these operating parameters include compression ratio, fuel-air ratio, and fuel injection timing. Exhaust emissions generally include pollutants such as carbon oxides (e.g., carbon monoxide), nitrogen oxides (NOx), sulfur oxides (SOx), hydrocarbons (HC), particulate matter (PM), and so forth.
A variety of techniques may be used to reduce pollutants in the emissions from compression-ignition engines. One technique is to reduce the temperature of the compressed air before introducing it into the combustion chamber. Disadvantageously, this method requires an additional cooling system, which is often expensive to implement. Another technique of reducing the emissions of NOx is to pass the exhaust gases through a NOx catalyst system. Disadvantageously, the NOx catalyst system operates only between certain ranges of temperature and requires additional heating and control systems for effective operation. A further technique involves injecting hydrocarbon into the cylinder of the diesel engine. This technique also requires an additional hydrocarbon injection system and is usually expensive to implement.
Accordingly, a cost effective technique is needed for reducing pollutants in emissions from compression-ignition engines.