Combustion phasing model for control of a gasoline-ethanol fueled SI engine with variable valve timing

Concern over the availability of fossil fuels and energy usage have produced an interest in both alternative fuels and new engine technologies such as variable valve timing to improve engine efficiency. Fuel-flexible engines permit the increased use of ethanol-gasoline blends. Ethanol is a renewable fuel which has the added advantage of improving performance in typically knock-limited operating regions due to the higher octane rating of the fuel. Furthermore, many modern engines are also being equipped with variable valve timing (VVT), a technology which allows increased control of the quantity of burned gas in-cylinder and can increase engine efficiency by reducing the need for throttling. The burned gas fraction as well as the blend ratio of ethanol impact the combustion timing and capturing these effects is essential if the combustion phasing is to be properly controlled. Combustion efficiency is typically tied to an optimal CA50 (crankangle when 50% of fuel is burned) for an engine. This paper proposes a physically-based model which captures combustion phasing and is designed to provide accurate estimates of CA50 for real-time control efforts allowing the CA50 to be adjusted to its optimal value despite changes in fuel and valve overlap. This control-oriented model was extensively validated at over 500 points across the engine operating range for four blends of gasoline and ethanol.