A Thermodynamic Comparison of SI-Engine Combustion Systems

Due to optimization with respect to friction reduction, as well as electrification of auxiliary devices, internal combustion engines are typically operated at even lower load conditions than already observed. However, low load conditions result in significant “pumping losses” for SI-engines. To address this problem, different approaches are employed.
First, variabilities within the valve train are used to overcome these effects, secondly combustion modes like stratified combustion are means to reduce the pumping losses.
This work investigates these approaches for a single cylinder Si-engine using variable valve lift (intake Cam) and –phase as well as stratified combustion with inlet cam phasing only.
Due to the fact, that inlet valve throttling is limited by the minimum opening of the valve and the stability of combustion, the benefit (6-8%) of inlet valve variabilities is significantly less than the effect of stratified combustion (15-35%) for the investigated engine.