Modelling Combustion Variability in LPG Injected Engines for Improved Engine Performance at Idle

The variability of in-cylinder combustion of gasoline at idle has been investigated previously, culminating in the development of a model relating the past and future indicated torque deviations from the mean at given engine operating conditions of intake manifold pressure, engine speed and spark advance. The developed model has the potential to be used in an idle speed control algorithm to improve vehicle noise vibration and harshness (NVH) at low engine speeds and loads. While environmental considerations have spawned the development of liquefied petroleum gas (LPG) as a viable alternative fuel, adaptation of the variability model to multipoint LPG injected automotive engines is complicated by the fact that the fuel mixture concentrations of propane and butane are subject to wide variations depending on a variety of factors including geographic location and local market pricing. Furthermore, evidence on one engine family suggests that the variability of torque production using LPG injection under cold start conditions is significantly higher than that observed with gasoline injection, a condition that is improved through enhanced modeling of the cyclic torque production process, and subsequent compensation. This paper investigates the development of a model relating past and present indicated torque deviations from the mean at a given engine operating condition in LPG, and explores the variability inherent in the model as a function of temperature. The implication is the model may be incorporated into an idle speed control algorithm, which will provide improved idle speed regulation in multipoint injected LPG vehicles.