Study of the Effect of the Residual Gas Fraction on Combustion in a S.I. Engine Using Simultaneous CARS Measurements of Temperature and CO2 Concentration

The aim of the work was to study the important issue of the residual gas mixing variations, using CO₂ as a tracer for the residual gas fraction. Therefore simultaneous measurements of CO₂ concentration and temperature have been performed inside a fired IC engine. Temperature is deduced from a broadband N₂ CARS signal and the CO2 concentration from a narrowband CO₂ CARS signal. To overcome the main difficulties encountered when making measurements in an engine (i.e. cycle-to-cycle variations and beam steering by density gradients), the shot-by-shot referencing technique and the in-situ referencing technique using the N₂ CARS signal intensity to determine CO₂ concentration were used.

All the measurements were performed at the center of the combustion chamber, using different spark plug locations. Three locations were measured: 4cm, 0.5cm and 0.1cm from the plug. The first measurements were performed with three conventional spark plugs, regularly spaced around the combustion chamber. The CARS probe volume was about 4 cm away from the ignition kernels. Using a spark plug with extended electrodes, measurements were then performed at 1 mm and 5mm from the spark location, respectively.

Various correlations were studied, the measured correlations range from 0.486 to - 0.272. A positive correlation was found between CO₂ concentration and temperature which indicates a nonuniformity in the mixture between the fresh gas and the residual gas. The correlations between CO₂ concentration prior to ignition and cycle-to-cycle variations of pressure were studied. Correlations found between the concentration and the maximum pressure, and those between CO₂ concentration and the crank angle of maximum pressure, indicate that the residual gas fraction influences the combustion processes during ignition and flame development.