A single-cylinder OKP (optimized kinetic process) engine, which uses homogeneous-charge compression-ignition (HCCI) technology, was tested, following a previous study, to evaluate the combustion system robustness and to improve the engine performance near the boundaries of the HCCI operating regime at light loads, high loads and high speed.
To evaluate the robustness of HCCI combustion control, gasoline fuels with different RON were used, and the engine was tested at different coolant temperatures. It was demonstrated that the proposed HCCI control approaches could control the OKP engine system to operate robustly using different fuels and at different coolant temperatures.
The effects of fuel injection timing and residual gas fraction on HCCI combustion and emissions, especially CO emissions and combustion efficiency, were tested at light loads; and the mechanisms were analyzed. At high loads, the effects of intake pressure and residual gas fraction on the combustion rate, knocking tendency, NOx emissions and the sensitivity of HCCI combustion timing control were investigated. The HCCI combustion-timing-control sensitivity at different engine speeds was also evaluated. Knowledge from this study may help in improving the HCCI engine system control and in expanding the HCCI operating regime.