Effect of Charge Non-uniformity on Heat Release and Emissions in PCCI Engine Combustion

Homogeneous Charge Compression Ignition (HCCI) engines are currently of great interest as a future alternative to Diesel and Spark Ignition engines because of HCCI's potential to achieve high efficiency with very low NO_x emissions. However, significant technical barriers remain to practical implementation of HCCI engines: difficult-to-control combustion, low power density, rapid pressure rise, and high hydrocarbon and carbon monoxide emissions. To overcome some of these barriers, operational strategies that involve relaxing the constraint of truly “homogeneous” HCCI combustion have been studied. The phrase “Premixed Charge Compression Ignition” or “PCCI” combustion can be used to describe this class of combustion processes, in which combustion occurs similarly to HCCI engines as a non-mixing controlled, chemical kinetics dominated, auto-ignition process, but the fuel, air, and residual gas mixture need not be homogeneous.

This paper presents analysis of the effect of charge non-uniformity upon combustion in PCCI engines. The analysis is conducted using a numerical approach for simulation of PCCI that utilizes a fully-coupled combination of multi-dimensional computational fluid mechanics with a multi-zone detailed chemical kinetics solver. Baseline simulations have been validated against HCCI experiments with isooctane fuel. Further simulations have been conducted to assess the effect of charge non-uniformity on the heat release and emissions for PCCI operation. The results show that charge non-uniformity used in the PCCI strategy may sometimes have a beneficial effect of reducing carbon monoxide and hydrocarbon emissions. However, charge non-uniformity within the combustion chamber has the potential to increase NO_x emissions.