Measurement of Residual Gas Fraction Using IR Absorption 2006-01-3337

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. As a consequence, the bandwidth and the central wavelength of the filters significantly affect the way pressure and temperature alter the CO₂ concentration measurements. In order to optimize the optical filters' shape, a simulation model using the HITRAN [8, 9] database was developed. This model was also used to understand pressure and temperature influences on the CO₂ absorption spectra. Finally, the model was used to estimate the measurement uncertainties and the optimum pressure and temperature ranges at which the measurements should be performed.

Eventually the technique was applied to measure the RBG rate of a firing SI single cylinder, optically accessible engine. An RBG rate of 6.5 +/- 0.4% was measured (i.e. 6% relative accuracy).