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Potential of Negative Valve Overlap for Part-Load Efficiency Improvement in Gasoline Engines
ISSN: 1946-3936, e-ISSN: 1946-3944
Published April 03, 2018 by SAE International in United States
Citation: Rodriguez, J. and Cheng, W., "Potential of Negative Valve Overlap for Part-Load Efficiency Improvement in Gasoline Engines," SAE Int. J. Engines 11(6):657-668, 2018, https://doi.org/10.4271/2018-01-0377.
This article reports on the potential of negative valve overlap (NVO) for improving the net indicated thermal efficiency (η NIMEP) 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, 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 (xr). As the load increases, achieving the wide-open throttle limit, the benefits of NVO for reducing the pumping losses diminish, while the blowdown losses from early exhaust valve opening (EVO) 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 EVO timing can be optimized to minimize the blowdown losses.