Cyclic variability has long been recognized as limiting the range of operating conditions of spark ignition engines, in particular, under loan and highly diluted operation conditions. Previous studies have shown that if cyclic variability could have been eliminated, there would be a 10% increase in the power output for the same fuel consumption. The cyclic variability results also in high levels of variation in the engine speed that is interpreted as poor driveability. At full load, some of the cycles tend to knock, while some others may not have complete combustion by the time the exhaust valve opens.
The cyclic variability is usually attributed to the result of random fluctuations in equivalence ratio and flow field due to the turbulent nature of the flow in the cylinder. These spatial fluctuations that are also time-dependent, contribute to an imperfect mixing of the cylinder content, partial stratification, random convection of the spark kernel away from the electrodes, random heat transfer from the burning kernel to the spark electrodes, etc.
During the last decade, extensive researches have enriched our knowledge in this subject. It is the purpose of the present paper to review and discuss the different indicators of cyclic variability used in the literature, to evaluate the relative contribution of the various factors by means of a literature survey, and to designate the best operation conditions to minimize the cycle-by-cycle variations.