Abstract
The extent of the cycle-to-cycle variations of charge composition and their contributions to the variations in Gross Indicated Mean Effective Pressure (GIMEP) in a spark ignition (SI) engine at idle are assessed. The methodology is to use a set of charge perturbation experiments to determine the sensitivity of the engine output parameters to the charge composition variations. Then through this sensitivity matrix, the observed engine output variations at idle are inverted to obtain the charge variations. At idle, the air, residual and fuel mass variations are 5.5, 4.4 and 0.64% respectively. The residual gas fluctuation contributes to ∼1/3 (in terms of the square of the coefficient of variations) of the GIMEP variations. The air and fuel fluctuations contribute much less: 7.5 and 4.6%, respectively. The remaining ∼54% is attributed to the variations in flowfield inhomogeneity.
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