Analysis of engine air handling systems for light-duty compression ignition engines using 1-D cycle simulation: Achieving high dilution levels for advanced combustion

Previous research studies have shown that low temperature combustion (LTC) strategies are capable of achieving very low NO_x and soot emissions while maintaining high thermal efficiency. To achieve LTC, there has to be sufficient mixing time between the fuel and air in a dilute, yet overall lean, environment. Dilution with a combination of fresh air and exhaust gas recirculation (EGR) is typically used to achieve longer mixing times and reduce the peak combustion temperatures. However, there are challenges associated with today's engine air handling systems' ability to move large combinations of EGR and air simultaneously. As the EGR demand is increased to reduce NO_x emissions or retard combustion phasing, the global equivalence ratio tends to increase because of the boosting systems' limited ability to supply fresh air. In this study, a one-dimensional engine modeling approach was used to analyze the behavior of a production light duty diesel engine equipped with a variable geometry turbocharger and a high-pressure loop EGR system under LTC conditions. The model is used to predict the global equivalence ratio as a function of the EGR level at a variety of operating conditions. The EGR level was varied from 0 to 50% at speeds ranging from 1,500 to 2,500 rpm and loads from 2 to 10 bar brake mean effective pressure. The objective of this study is to evaluate the air handling system's capability of driving high amounts of EGR and air simultaneously for light duty engines to successfully achieve LTC operation over a large portion of the operating space. The results of the simulations show that at light loads, large amounts of EGR can be used while maintaining globally lean operation. However, as the engine load increases, a globally stoichiometric condition is reached relatively quickly, and high engine loads with greater than 30% EGR and overall lean conditions were achievable.