The Prospects of Using Alcohol-Based Fuels in Stratified-Charge Spark-Ignition Engines 2007-01-4034

Near-term energy policy for ground transportation is likely to have a strong focus on both gains in efficiency as well as the use of alternate fuels; as both can reduce crude oil dependence and carbon loading on the environment. Stratified-charge spark-ignition direct-injection (SIDI) engines are capable of achieving significant gains in efficiency. In addition, these engines are likely to be run on alternative fuels. Specifically, lower alcohols such as ethanol and iso-butanol, which can be produced from renewable sources.

SIDI engines, particularly the spray-guided variant, tend to be very sensitive to mixture preparation since fuel injection and ignition occur within a short time of each other. This close spacing is necessary to form a flammable mixture near the spark plug while maintaining an overall lean state in the combustion chamber. As a result, the physical properties of the fuel have a large effect on this process. Since alcohols tend to have higher heats of vaporization & lower energy densities compared to gasoline (which requires a greater fuel mass to be injected), the local conditions at the time of ignition can be very different from more conventional fuels.

In this study, an optically accessible engine is used along with high-speed image and pressure data, to study the operating characteristics when fuelled with ethanol & iso-butanol. These results are compared to those of iso-octane, a pure chemical commonly substituted for gasoline. First, a mapping study is completed to demonstrate that comparable performance can be achieved between the three fuels. Next, ignition and burning characteristics are analyzed and any differences between the fuels are highlighted. Finally, high-speed laser-induced fluorescence is used to study the evaporation and mixing process of the three fuels around the time of ignition. Through this study, a better understanding of SIDI engines run on alternative fuels can be achieved.