Investigation of Mixture Quality Effect on CAI Combustion 2005-01-0141

The purpose of this study was to gain a better understanding of the fundamental aspects of the CAI combustion process in order to assess the possibilities of controlling CAI combustion through mixture quality. The experimental work was conducted on a single-cylinder gasoline engine equipped with optical access and was completed with 3D simulation of the gas exchange and compression strokes. Two engine configurations (negative valve overlap and burned gas re-breathing) were tested.

In CAI combustion, controlling the heat release rate implies limiting the volume of the reaction zone by charge stratification, or decreasing the reaction rate by increasing the dilution. Consequently, a significant part of the work was focused on the investigation of the correlation between the mixture quality and the CAI combustion process. It was found that a low level of fuel stratification obtained with asymmetric port fuel injection does not significantly affect the combustion phasing even though it directly influences the location of the auto-ignition zones. Indeed, single cycle observations of the fuel distribution and the combustion process showed that the location of the first auto-ignition zones is strongly correlated with the position of the fuel rich areas.

Experimental observations of the residual gas distribution and 3D simulation showed that residual gas stratification is extremely difficult to obtain with a pentroof 4-valve combustion chamber. Only the burned gas re-breathing strategy allowed the creation of a slightly burned gas / fresh mixture stratification along the vertical axis of the cylinder liner. As a consequence the effect of residual gas stratification could not be directly studied.

Finally, direct fuel injection was found to provide a potential means of controlling auto-ignition timing and to some extent the heat release rate. Combining multiple injections during the negative overlap period, the intake stroke and late in the compression stroke, the auto-ignition delay may be accurately adjusted to optimize the heat release phasing. It is believed that the control capability offered by multiple direct fuel injections can be used to extend the CAI operating range and further improve the fuel efficiency benefits.