Potential of Negative Valve Overlap for Part Load Efficiency Improvement in Gasoline Engines

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. Introduction Improving the thermodynamic efficiency of spark ignition (SI) engines operating in part load one of the key levers for improving the in-use fuel consumption of passenger vehicles. The locus of engine operating points over different driving cycles depends on the full load engine curve, and the aerodynamic, rolling resistance, gravitational and inertial forces resulting from vehicle operation over the driving cycle. As an example, Figure 1 shows the locus of engine operating points resulting from the vehicle simulation of a typical 2.5 liter, naturally aspirated (NA) mid-size sedan over the Urban Dynamometer Driving Schedule (UDDS). Over the UDDS, this representative US mid-size sedan mostly operates in the low engine load region, where the brake thermal efficiency is below 30%. The average operating point of this representative example has an approximate brake mean effective pressure (BMEP) of 3.3 bar. The departure from the ideal thermodynamic efficiency of (SI) engines is due to several factors: limited flame speed, deviations from the ideal isentropic compression and expansion processes, blow-down losses at exhaust valve opening (EVO), and pumping losses associated with intake throttling in stoichiometric engines. Figure 2 shows a pressurevolume diagram of a SI engine during part load. Figure 1. Operating points on the engine map (Mid-size sedan vehicle, 2.5L NA engine with compression ratio of 11.5:1) determined using Autonomie over the UDDS cycle. Spark timings at MBT timing. Adapted from: Jo, 2016 [1] Figure 2. Pressure – volume diagram for a part load operation in a SI engines (logarithmic scales) at 3.75 bar NIMEP At low load, the efficiency losses are dominated by the pump mean effective pressure (PMEP); the latter represents a significant share of the net indicated mean effective pressure (NIMEP). In the operating point shown in Figure 2, which is one of the baseline operating points B M EP /b ar Engine speed / rpm Operation points Full load curve Average operation point Efficiency iso-lines 0 2 4 6 8 10 12