Effects of different fuels on a turbocharged, direct injection, spark ignition engine

Abstract

The following pages describe the experimentation and analysis of two different fuels in GM's high compression ratio, turbocharged direct injection (TDI) engine. The focus is on a burn rate analysis for the fuels - gasoline and E85 - at varying intake air temperatures. The results are aimed at aiding in a subsequent study that will look at the benefits of direct injection in turbocharged engines, ethanol's knock suppression properties, and the effects of ethanol concentration in gasoline/ethanol blends. Spark sweeps were performed for each fuel/temperature combination to find the knock limit and to assess each fuels' sensitivity to spark timing and temperature. The findings were that E85 has lower sensitivity to spark timing in terms of NIMEP loss for deviation from MBT timing. A 5% loss in NIMEP was seen at 3° of spark advance or retard for gasoline, whereas E85 took 5' to realize the same drop in NIMEP. Gasoline was also much more sensitive to intake air temperature changes than E85. Increasing the intake air temperature for gasoline decreased the peak pressure, however, knock onset began earlier for the higher temperatures, indicating that end-gas autoignition is more dependent on temperature than pressure. E85's peak pressure sensitivity to spark timing was found to be about 50% lower than that of gasoline and it displayed much higher knock resistance, not knocking until the intake air temperature was 130°C with spark timing of 30° bTDC. These results give some insight into the effectiveness of ethanol to improve gasoline's anti-knock index. Future experiments will aim to quantify charge cooling and anti-knock properties, and determine how ethanol concentration in gasoline/ethanol blends effects this knock suppression ability.