Combined Benefits of Variable Valve Actuation and Low-Pressure EGR on SI Engine Efficiency Part 2: High Load

The abnormal autoignition of the unburned gas, namely knock, at high loads is a major challenge for modern spark ignited engines. Knock prevents the application of high compression ratios due to the increased unburned gas temperature, and it becomes increasingly severe for downsized engines with high specific powers. The current paper reports on the potential of utilizing continuously variable valve actuation (VVA) and low-pressure exhaust gas recirculation (EGR) to reduce knock tendency at high loads. Five speed / load points were investigated on a 1.6 L turbocharged gasoline direct injection engine. The brake specific fuel consumption (BSFC) response to the valve phasing and the intake valve lift was investigated with the design of experiment (DoE) approach. The DoE was conducted using a Box-Behnken surface response model. The results exhibited insensitive response of BSFC to intake valve lift and overlap. Following the DoE analysis, additional engine testing was performed at each speed / load point to confirm the engine efficiency and combustion performance when targeting different valvetrain controls and EGR strategies. The results indicated that the reduced intake valve lift and early intake valve closing reduced unburned gas temperature near top dead center (TDC). However, the benefit of reduced unburned gas temperature was offset by the degraded combustion duration, thus similar or even higher amount of unburned fuel was present at the knock onset. At the investigated operating conditions, EGR was a more effective method to reduce knock tendency.