Valve Train Variabilities on Gasoline Engines between Dethrottling and Turbulence Generation

Homogeneous stoichiometric SI engine combustion is influenced most persistently by global charge motion and the turbulence it produces around the spark plug. This is particularly true when the natural in-cylinder flow field is adversely affected by shifts in timing for intake-valve closing. This article examines these phenomena and their influence on combustion on the basis of engine tests and CFD simulations. Also discussed are measures employing optimized valvetrain strategies to mitigate the tradeoff between achieving good gas exchange behavior while still maintaining fast combustion. In the context of combustion stability while running a very early intake valve closing strategy, it is found that poor mixture preparation and as a consequence stratification of the mixture is bound to the injection timing. As a generic mixture stratification is known to being one contributor for provoking cycle to cycle variations of IMEP (in addition to the complex nature of flow stability), a strategy for injection timing is fundamentally discussed allowing to clearly improve running smoothness.