Limitation of Charge Stratification for High Load Extension of HCCI Combustion in a GDI Engine with NVO 2009-01-1343
The effects of charge stratification on homogenous charge compression ignition (HCCI) combustion high load extension were studied in a gasoline direct injection engine with negative vale overlap. The stratified charge was created by a first injection during the middle of intake stroke and a second injection during the compression stroke. 3D simulation results showed that a stratified mixture can be formed by the dual injections and stratification degree can be adjusted by changing the second injection timing and ratio. Experimental results show that slight charge stratification will lead to advancement of ignition and increase of the maximum rate of pressure rise (MRPR) and NOx emissions. The earliest ignition, maximum MRPR, lowest cycle variation occurs under the conditions of rich zone is around stoichiometric. As the amount of stratification further increases, over-stoichiometric zones exist, the MRPR and NOx emissions decrease, THC and CO emissions increase because of combustion deterioration.
These results indicate that, in order to lower MRPR and NOx emissions, which is good for high load extension, the concentration of rich zone should be over-stoichiometric. But this will lead to combustion deterioration and significant increase of CO emissions. MRPR, NOx and CO emissions are three limitations of HCCI combustion with charge stratification for high load extension.