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An experimental study was performed to set up a new non intrusive technique to measure the rate of RBG (Residual Burnt Gases) in the cylinder of a firing SI (Spark Ignition) engine. The method is based on time-resolved in-cylinder CO2 concentration measurements; this molecule being used as an RBG tracer. The measurement technique that has been developed is based on the absorption of an IR (infrared) beam (around 4.3μm) that crosses the combustion chamber. The method was first set up in a heated and pressurized constant volume vessel that could be filled with controlled N2/CO2 mixture. Optical filters are used to select the useful part of the IR spectrum (the one which interacts with the CO2 absorption spectrum). It has been pointed out that the CO2 absorption coefficient is temperature and pressure dependent. Moreover, these dependencies vary with the absorption wavelength.

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.