Technological levers for the enhancement of mixture dilution for spark ignited engines

In addition to downsizing, mixture dilution through lean burn or EGR diluted mixture is emerging as a promising strategy to address the increasing demand for fuel economy. The present study was performed on a GDI single cylinder engine. The engine is equipped with a LP EGR circuit and with a variable tumble motion capacity. Up to 4 injections per cycle could be used. Ignition energy could be varied from 50 to 200 mJ. Several parameter variations were run to push further the air or EGR dilution limits. The increase of tumble motion level and ignition energy enabled to reach leaner combustions. However, the modification of the injection strategy (late 2nd injection) doesn’t permit to reach leaner combustions.
A detailed cycle by cycle analysis was performed for in-depth understanding of the mechanisms leading to combustion instabilities. In the reference configuration, at low load and low engine speed, combustion dilution is limited by propagation of the end of combustion. Whatever the engine speed, for operating points at high loads the combustion initiation is limiting the mixture dilution. When tumble motion level is increased, the limiting factor of dilution is the combustion initiation for all the studied operating points. Increasing the ignition energy enables to improve the combustion initiation. The modification of the injection strategy (late 2nd injection) doesn’t permit to improve the global combustion behaviour. When air dilution is limited by propagation of the end of combustion, the EGR dilution becomes limited by the combustion initiation.