Experimental Investigation of Cyclic Variability on Combustion and Emissions of a High-Speed SI Engine

Cyclic combustion variability (CCV) is an undesirable characteristic of spark ignition (SI) engines, and originates from variations in gas motion and turbulence, as well as from differences in mixture composition and homogeneity in each cycle. In this work, the cycle to cycle variability on combustion and emissions is experimentally investigated on a high-speed, port fuel injected, spark ignition engine. Fast response analyzers were placed at the exhaust manifold, directly downstream of the exhaust valve of one cylinder, for the determination of the cycle-resolved carbon monoxide (CO) and nitric oxide (NO) emissions. A piezoelectric transducer, integrated in the spark-plug, was also used for cylinder pressure measurement. The impact of engine operating parameters, namely engine speed, load, equivalence ratio and ignition timing on combustion and emissions variability, was evaluated. The variations in mixture stoichiometry were found to have a strong effect on engine combustion variability. Rich cyclic mixture compositions exhibit lower coefficient of variation (COV) for the indicated mean effective pressure (IMEP) and NO emissions (COV_NO) compared with lean mixtures. The mean value of CO emission was found to be mainly affected by stoichiometry while COV_CO is affected by lambda fluctuations. At higher engine loads, maximum cylinder pressure and IMEP are increased, while COV_IMEP decreased. Furthermore, ignition timing was found to strongly affect combustion and NO emissions, as it is related with early flame kernel development and thereby with flame propagation. Maximum braking torque (MBT) operation exhibits maximum IMEP and minimum COV_IMEP. Compared to MBT operating conditions, advanced ignition timing leads to higher maximum cylinder pressure, higher NO and COV_NO, while retarded ignition timings lead to lower maximum cylinder pressure, lower NO concentration and higher NO variability (COV_NO).