Use of dilution with exhaust gas recirculation (EGR) offers substantial efficiency gains in spark ignition (SI) engines, especially when boosting and downsizing are employed. However, the onset of instabilities in engine operation, due to misfires and partial burns, limits the dilution levels. Active controls can be employed to improve engine stability during high dilution operation, with spark and fueling being the main control parameters available for cycle-to-cycle control implementation. This paper aims to understand the sensitivity of the combustion process to changes in fueling under dilute operation achieved with both excess air (lean operation) and EGR. Sinusoidal perturbations were introduced into the injected fuel quantity, and the sensitivity to these perturbations was characterized using a fast Fourier transform (FFT) analysis of the cycle cumulative heat release data. Furthermore, the minimum fueling change required to produce a measurable effect on engine operation was quantified at various levels of dilution. For fixed spark timing (similar ignition conditions), sensitivity to fuel perturbation increases nearly exponentially as EGR dilution is increased, due to the highly nonlinear relationship between combustion quality and in-cylinder charge properties at the dilution limit. However, when spark timing is advanced to compensate for the lower flame speeds of dilute combustion and maintain constant combustion phasing, this effect is suppressed and there is an initial decrease in sensitivity to fuel perturbations as EGR is increased, until the dilute limit is reached, at which point sensitivity again increases. For lean operation, response to very small perturbations was detected at all dilution levels.
