Injection strategies for improving emissions characteristics

Premixed charge compression ignition reduces NOx and soot emissions, but requires the optimization of the injection timing and pressure, fuel mass in pilot injection, and EGR rate.

Traditional diesel engine combustion controlled by fuel injection and fuel-air mixture is typically diffusion combustion. There is a trade-off between NOx emissions and soot emissions in the combustion process. NOx emissions from a diesel engine are greatly affected by the nonhomogeneous nature of diesel combustion resulting from the different equivalence ratios of the air-fuel mixture.

Either excessively rich or excessively lean combustion may result in reduced NOx. To reduce NOx, attempts have been made to ensure a premixed condition in which the mixture is distributed homogenously and, at the same time, set at a lean ratio prior to ignition. Therefore, research has been carried out extensively on new combustion modes, including homogenous charge compression ignition (HCCI) and premixed charge compression ignition (PCCI), which are effective measures to reduce NOx and soot emissions and improve the fuel economy of diesel engines.