Effects of ethanol on combustion and emissions of a gasoline engine operating with different combustion modes

The introduction of fuel economy and CO₂ emission legislations for passenger cars in many countries and regions has spurred the research and development of more efficient gasoline engines. The pumping loss at part-load operations is a major factor for the higher fuel consumption of spark-ignition gasoline engines than the diesel engines. Various approaches have been identified to reduce the pumping loss at part-load operations, leading to improved fuel economy, including early intake-valve closing, positive valve overlap and controlled auto-ignition combustion. On the other hand, in order to reduce the CO₂ emissions from the fossil fuel, ethanol produced from renewable resources is becoming widely used in the gasoline engine. In this article, the performance, combustion and emissions were measured, analysed and compared between gasoline and its mixture with ethanol (E15 and E85) at a typical part-load condition when a direct-injection gasoline engine was operated with the controlled auto-ignition combustion by means of the negative valve overlap and spark-ignition combustion by means of the intake throttled, early intake-valve closing and positive valve overlap. An electro-hydraulic actuated camless system enabled the engine to be operated with controlled auto-ignition combustion and spark-ignition combustion of different valve timings and durations at the same load. The results showed that the controlled auto-ignition combustion reduces nitrogen oxide emissions by more than 90%. The positive valve overlap results in better mixture preparations and improved combustion efficiency and best fuel economy compared to all the other modes. The early intake-valve closing operation led to a moderate improvement in the fuel conversion efficiency over the throttled spark-ignition operation, but it was characterised by the slowest combustion and worst hydrocarbon and carbon monoxide emissions. Fewer and smaller particle numbers were detected in early intake-valve closing using E0 and E15 fuel blends. Using ethanol blends reduces the knocking combustion in controlled auto-ignition modes by about 50%. The use of E85 resulted in an increased number of particulate emissions in early intake-valve closing but increased indicated specific fuel consumption in all the modes. The particulate emission results showed that soot is the dominant particle in the exhaust.