Gasoline partially premixed combustion in a light duty engine at low load and idle operating conditions

Partially premixed combustion (PPC) has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions, even at higher loads. The problem is the ignitability at low load and idle operating conditions. The objective of this study is investigation of the low load limitations with gasoline fuels with octane numbers RON 69 and 87. Measurements with diesel fuel were also taken as reference. The experimental engine is a light duty diesel engine equipped with a fully flexible valve train system. Trapped hot residual gases using negative valve overlap (NVO) is the main parameter of interest to potentially increase the attainable operating region of high octane number gasoline fuels. Much lower soot is emitted with 69 and 87 RON gasoline compared to diesel at engine loads 1 bar IMEPgross to 3 bar IMEPgross but the combustion efficiency is significantly lower with gasoline at low load compared to diesel. Combustion efficiency increases with NVO for both diesel and gasoline. The 69 RON gasoline fuel can be run at idle (1 bar IMEPgross) operating conditions without a significant fraction of trapped hot residual gases. The 87 RON gasoline fuel could be run at 2 bar IMEPgross but with a high setting of NVO. There is a clear decrease of net indicated efficiency with NVO because of the decrease in gas-exchange efficiency. To achieve highest possible efficiency for a given fuel, at low load, as low as possible NVO should be used.