Cycle-by-Cycle Variations of a Spark Ignition Engine - A Statistical Analysis

The cycle-by-cycle combustion variation as reflected by the pressure development of a single cylinder, spark ignition, internal combustion engine is discussed. The data originated from a CFR-RDH engine which was instrumented with a pressure transducer whose signal was modified and digitized for statistical analysis of peak pressure, peak rate of pressure change, and crank angles at which peak pressure and peak rate of pressure changes occurred. These parameters were analyzed for averages, standard deviations, and cross-correlations. In addition, auto-correlations were made of peak pressure. A study was conducted on the effects of charge homogeneity using the F-test, and Bartlett's test was used to determine if significant changes in the standard deviations occurred with changes in engine conditions.

The results indicate that significant differences in the standard deviations of the four pressure development parameters exist for different engine operating conditions. Suggested variations in the combustion process are then inferred from these significant differences The inferences made are with respect to the average flame speed, the variations in flame speed and the phasing of the combustion process. While it is shown how engine operating variables affect cycle-by-cycle variations, evidence is also presented to establish that these variations are random.