Flame chemiluminescence studies of cyclic combustion variations and air-to-fuel ratio of the reacting mixture in a lean-burn stratified-charge spark-ignition engine

• Published in: Combustion and flame 2004-01, Vol.136 (1-2), p.72-90
• Main Authors: Aleiferis, P.G., Hardalupas, Y., Taylor, A.M.K.P., Ishii, K., Urata, Y.
• Format: Article
• Language: English
• Subjects: Applied sciences; Cyclic variations; Energy; Energy. Thermal use of fuels; Engines and turbines; Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc; Exact sciences and technology; Flame chemiluminescence; Lean-burn engines
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2003.09.004

The operating range of lean-burn spark-ignition engines is limited by the level of cyclic variability in the early flame development that typically corresponds to the 0–5% mass fraction burned duration. An experimental investigation was undertaken to study the levels of flame chemiluminescence in an optical stratified-charge spark-ignition engine, using a Cassegrain optical system with high spatial resolution. Measurements of OH and CH-radical intensities were simultaneously acquired with double flame images per cycle for a range of air-to-fuel ratios (A/F=12–22). These signals of chemiluminescence were used to evaluate the in-cylinder equivalence ratio of the reacting mixture and to further examine its contribution to the flame growth speed and the cyclic variability in the crank angle by which 5% mass fraction was burned (θXb5%). Specifically, the ratio of the OH/CH chemiluminescence signals was calibrated in the engine and tested extensively for different injection strategies and spark advances, to measure the “global” and “local” in-cylinder A/F ratio around the spark plug. The complications encountered towards this goal are discussed in detail. The results showed that the equivalence ratio exhibited large variations on a cycle-by-cycle basis and consistently produced negative correlation coefficients with θXb5%, especially for lean-set operating conditions (A/F=20–22). Particularly, for open-valve injection strategy that yielded a stratified mixture, the degree of this correlation lied in the range ≈−0.4 to −0.8, being lower for the locally measured A/F ratio and higher for the globally evaluated one. Some issues related to the opposite gradients of the calibration curves deduced for the measurement of the global and local in-cylinder A/F ratios need to be examined outside the engine using a combustion facility with controlled conditions of pressure, temperature, turbulence intensity, and dilution by combustion residuals.