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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.