With its high octane number and potentially favorable greenhouse gas and energy balance characteristics, ethanol offers potential to replace portions of gasoline as a transportation fuel. System optimization to utilize the increased knock resistance and evaporative cooling effect of ethanol can increase the performance and efficiency of spark-ignition engines. Though basic engine emissions and performance effects of ethanol fuel blends have been widely reported, limited studies have examined the details of combustion behavior and the interplay between fuel ethanol content and the behavior of the engine control system. This paper quantifies the response of a production engine control unit to ethanol fuel blends, along with the subsequent combustion behavior and resulting engine performance at high-load operating conditions. Steady-state testing is conducted on a modern direct-injection, spark-ignition, four-cylinder engine using a base engine calibration at full-load (wide-open throttle) conditions across a range of engine speeds from 1500 to 4000 rpm. Test fuels include gasoline, neat ethanol, and an intermediate blend of gasoline and ethanol. A combination of low-speed engine measurements and crank angle based cylinder pressure measurements are used to demonstrate the impact of increasing fuel ethanol content on engine control parameters. Ethanol’s increased knock resistance, demonstrated by its higher octane number, compared to gasoline makes combustion less susceptible to knock as ethanol fraction in the fuel increases. Accordingly, less spark retard is required to avoid knock at high engine load, translating to higher fuel conversion efficiency and increased specific power output. This effect is explored within the framework of a production engine calibration which uses active knock-avoidance feedback control. The relative contribution between a more aggressive engine calibration and increased fuel-evaporation charge-cooling to the increased efficiency and power resulting from increasing fuel ethanol percentage is also characterized.

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