Cyclic Variability has long been recognized as limiting the range of operating conditions of spark ignition engines, in particular, under lean 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 level of variations in the engine speed which is interpreted as poor driveability. At full load, some of cycles tend to knock, while other may not have complete combustion by the time the exhaust valve opens.
An experimental study has been performed in order to evaluate the relative contribution of several relevant parameters on the cyclic variability in spark ignition engines. The cyclic variability has been examined via five major different pressure-related identifier, i.e. Pmax, θPmax, IMEP, (dp/dθ)max and θ(dp/dθ)max. Due to their high sensitivity to small fluctuation in the operation conditions, the two latter were found to be problematic in the characterization of the cyclic variability. The identifier (dp/dθ)max was found to be highly correlated to Pmax.
MBT ignition resulted in minimal cyclic variability. Variations in the ignition timing were found, however, to be less important under early ignition condition than under retarded ignition condition. The standard deviation of the spark jitter was found to be around 1.7 degrees and therefore cannot be ignored. Noticeable cyclic variations were observed also under motoring conditions which indicated that the role of the valves and rings leakage cannot be neglected.
The spark energy and spark duration were found to be less important than suggested in the literature. On the Contrary, the spark plug kind and its orientation were found to be very significant in determining the cyclic variability. This supports the idea that random fluctuations in the flow field due to the turbulence of the flow in the cylinder are important. These spatial fluctuations, that are also time-dependent, contribute to the 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.