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

Paper #:
  • 2018-01-0377

Published:
  • 2018-04-03
Abstract:
The net indicated fuel efficiency of a in a gasoline engine during part load was studied for a wide set of valve timings through cycle simulation and engine experiments. Three different load levels were investigated corresponding to gross indicated mean effective pressures of 4, 5 and 6 bar respectively. For the lower level of the loads considered, symmetric negative valve overlap (NVO) reduces the pumping loses during the gas exchange loop, achieving a 7% improvement compared to the baseline. Similar efficiency improvement are achieved by positive valve overlap (PVO), with the disadvantage of reducing the combustion stability due to the higher residual gas fraction. A symmetric NVO combined with shorter exhaust duration has a higher potential for reduction in part-load fuel consumption, as the disadvantages of early exhaust valve open on the expansion stroke are mitigated. As the load increases, the aggressive NVO strategy reaches the wide open throttle limit and cannot further reduce the pumping losses.
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