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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.

Reliable initiation of flame kernel development by an electrical spark in IC engines operating at ultra-lean air-fuel mixtures lays down specific demands to spark discharge variables. The demands become especially strict under conditions of intensive mixture motion through the spark gap. The main goal of this study was to determine experimentally the minimum values of spark duration and spark current in the glow phase of the discharge necessary for reliable ignition of methane-air mixtures with equivalence ratios of 0.7 down to 0.6. Different combinations of flow velocity and spark plug orientation with respect to mean flow vector have been considered. A special experimental ignition system was employed to set up spark discharge variables. Besides, a numerical study of early flame kernel development under conditions used in the tests has been carried out.