Characteristics of Ethanol, Butanol, Iso-Octane and Gasoline Sprays and Combustion from a Multi-Hole Injector in a DISI Engine

Recent pressures on vehicle manufacturers to reduce their average fleet levels of CO₂ emissions have resulted in an increased drive to improve fuel economy and enable use of fuels developed from renewable sources that can achieve a net reduction in the CO₂ output of each vehicle. The most popular choice for spark-ignition engines has been the blending of ethanol with gasoline, where the ethanol is derived either from agricultural or cellulosic sources such as sugar cane, corn or decomposed plant matter. However, other fuels, such as butanol, have also arisen as potential candidates due to their similarities to gasoline, e.g. higher energy density than ethanol. To extract the maximum benefits from these new fuels through optimized engine design and calibration, an understanding of the behaviour of these fuels in modern engines is necessary. In particular, the use of direct injection spark-ignition technology requires spray formation and combustion characteristics to be quantified in order to improve both injector design and operating strategies. To this end an optical investigation of spray development and combustion was undertaken in a single-cylinder direct-injection spark-ignition engine with a centrally mounted multi-hole injector. Specifically, crank-angle resolved imaging studies were performed and batches of images from 100 consecutive cycles were acquired with synchronised in-cylinder pressure logging. The engine was motored and fired at 1500 RPM stoichiometrically under part load (0.5 bar intake pressure), with injection timing set early in the intake stroke to promote homogeneous mixture formation. The effects were investigated at engine coolant temperatures of 20 °C and 90 °C using gasoline, iso-octane, ethanol and butanol. Projected spray areas as seen through the piston crown were calculated to reveal information about the atomization and evaporation processes for each fuel. Additionally, flame areas and centroids were calculated to analyse the combustion process relative to measured in-cylinder pressure histories.