Combustion and emission characteristics of alcohol fuels in a CAI engine

Abstract

The combustion and emission characteristics of alcohol fuels in a controlled auto-ignition (CAI) combustion engine have been investigated. Combustion characteristics were studied by heat releases analysis through in-cylinder pressure measurement and auto-ignition reactions through high-speed chemiluminescence imaging. By means of the negative valve overlap, varied amount of residual gas was trapped in the cylinder to achieve the controlled auto-ignition with/without spark-assisted ignition. Total chemiluminescence images were captured with a high speed camera equipped with a high repetition image intensifier. Methanol, ethanol and their blends with gasoline were tested and their combustion and emission characteristics were compared with the results of gasoline.Alcohol fuels, in particular methanol, resulted in advanced auto-ignition and faster combustion than that of gasoline. In addition, their use could lead to substantially lower HC, NOX and CO exhaust emissions. Spark-assisted ignition was most effective on gasoline combustion by advancing ignition timing and initiating flame kernel at the centre of combustion chamber but it had marginal effect on alcohol fuels. Auto-ignition always took place at the perimeter of the chamber and occurred earlier with alcohol fuels because of their greater heat release and fuel reforming reactions during the negative valve overlap period. Chemiluminescence of the gasoline fuel was most visible and it decreased with increasing percentage of oxygenated fuels.