Analysis of mixture structure and of its influence on combustion in a CAI engine

Abstract

Homogeneous charge compression ignition combustion mode theoretically relies on a perfectly homogeneous air/fuel mixture simultaneously releasing heat across the whole combustion chamber. However, practical implementation always leads to a staged combustion process that is still significantly quicker than conventional spark ignition or compression ignition combustion modes. The degree of combustion staging is central to the limitation of combustion-induced noise, to cycle efficiency, as well as to combustion stability and completion. The present study numerically explored on the whole operating map the mixture structure at ignition for a specific type of homogeneous charge compression ignition combustion, namely the controlled auto-ignition combustion mode attained through burnt gas trapping with negative valve overlap. The analysis of the complete controlled auto-ignition operating map outlined the similarity of mixture structure for any operating condition and the mixture/chemistry interactions depending on the end-of-compression temperature domain. Temperature stratification appeared as the key input defining combustion-induced noise and combustion stability but mean in-cylinder temperature and dilution combine with temperature stratification to define operating map limitations.