Influence of the Local Mixture Characteristics on the
Combustion Process in a CAI™ Engine 2008-01-1671
Among the existing concepts to help improve the efficiency of spark ignition engines on low load operating points, Controlled Auto-Ignition™ (CAI™) is an effective way to lower both fuel consumption and pollutant emissions at part load without major modifications of the engine design. The CAI™ concept is founded on the auto-ignition of a highly diluted gasoline-based mixture in order to reach high indicated efficiency and low pollutant emissions through a low temperature combustion.
Previous research works have demonstrated that the valve strategy is an efficient way to control the CAI™ combustion mode. Not only the valve strategy has an impact on the amount of trapped burnt gases and their temperature, but also different valve strategies can lead to equivalent mean in-cylinder conditions but clearly differentiated combustion timing or location. This is thought to be the consequence of local mixture variations acting in turn on the chemical kinetics.
In the present study, 3D CFD computations and optical diagnostics are combined to explore the local mixture differences due to the choice of valve strategy. The mixture characteristics are then linked to the experimentally observed differences in terms of autoignition location and timing, combustion duration and pressure rise.
The results provide an insight into the effects explaining the suitability of the Negative Valve Overlap (NVO) strategy for low CAI™ loads and the capability of the Exhaust Gas Re-Breathing (EGRB) strategy to reach higher loads.