Potential of Negative Valve Overlap for Part Load Efficiency Improvement in Gasoline Engines 2018-01-0377
This paper reports on the potential of negative valve overlap for improving the net indicated thermal efficiency of gasoline engines during part load. Three fixed fuel flow rates, resulting in indicated mean effective pressures of up to 6 bar, were investigated. At low load, negative valve overlap (NVO) significantly reduces the pumping loses during the gas exchange loop, achieving up to 7% improvement in indicated efficiency compared to the baseline. Similar efficiency improvements are achieved by positive valve overlap (PVO), with the disadvantage of worse combustion stability from a higher residual gas fraction. As the load increases, achieving the wide-open throttle limit, the benefits of NVO for reducing the pumping losses diminish, while the blow-down losses from early exhaust valve opening increase. However, a symmetric NVO strategy combined with a shorter exhaust duration has a higher potential for reduction in part-load fuel consumption, as the exhaust valve opening timing can be optimized to minimize the blow-down losses.