1992-02-01

Variable Gas Exchange Systems for S.I. Engines - Layout and Experimental Data 920296

Load control by means of early intake valve closing (EIVC) permits brake mean effective pressure (BMEP) to be improved by as much as 14 % at full load and pumping losses in part load to be reduced comparable to the unthrottled engine. Concomitant to this, though, the marginal conditions for good mixture formation and part load combustion optimized for efficiency are greatly impaired. With ideal mixture formation, improvements in specific part load consumption (BSFC) of the order of 8 to 12 % are achievable.
The mixture formation which occurs at low part load in the combustion chamber itself is not effective as the charge motion induced by the inflow process with EIVC dies away rapidly and at the same time fuel still condenses. The inhomogeneities to which this gives rise impair ignition conditions and the combustion pattern, which greatly limits the actual useful work of the theoretical charge cycle benefit.
Significant increases in BMEP of 8 - 12 % are achievable likewise with variable intake systems with cross-section variation. However, this is feasible only within a limited engine speed range, in contrast to systems with freely selectable valve timing. An improvement in torque patterns can be achieved in particular by the combination of port deactivation with tuned intake pressure charging.

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