A comparison of burning characteristics of iso-octane and ethanol fuels in an optical SI engine

Over the next few decades the automotive industry will be faced with a number of challenging decisions as the world’s supply of oil reduces and the global population increases beyond 7 billion. These factors have driven some researchers to look at blending fossil fuels with alternatives such as crop-produced alcohols (or so-called biofuels). The currently reported study was concerned with the combustion characteristics of ethanol-isooctane blended fuels in a specially designed, single cylinder, spark ignition research engine equipped with full bore overhead optical access. The testing was undertaken using port fuel injection (PFI) and was focused on the behaviour of differing ethanol concentrations under varied internal exhaust gas re-circulation levels (IEGR). Simultaneous high speed imaging and in-cylinder pressure data analysis was used to understand the fundamental influence of varying ethanol content on turbulent flame propagation and subsequent mass burning. The resulting images were analysed looking at the speed of the advancing flame and the shape factor of the burning velocity. The initial evidence suggested that under the moderate speeds and loads tested, poor charge mixture preparation associated with ethanol was leading to fast but unstable burn rates. This could be avoided and combustion improved by using increased valve overlap settings, with the hot residuals entering the intake port aiding the full evaporation of the fuel.