The Effect of Fuel Characteristics on Combustion in a Spark-Ignited Direct-Injection Engine

An experimental study was conducted on a spark-ignited direct-injection engine burning fuels with different evaporation and autoignition characteristics. The test engine was a single-cylinder Direct-Injection Stratified-Charge (DISC) engine incorporating a combustion process similar to the Texaco Controlled Combustion System. Two fuels were tested and compared with a baseline gasoline fuel: diesel fuel, and gasoline mixed with an ignition improver. The tests were done at low to medium engine loads. Diesel fuel was found to have similar levels of hydrocarbon (HC) emissions as gasoline but had different characteristics. The optimum timing for diesel fuel was retarded from that for gasoline and combustion variability was much less with diesel than with gasoline. Gasoline with a commercial ignition improver normally used to increase the cetane number of diesel fuel was also tested. The effect of changing the autoignition quality of the fuel depended on the injector used. Two geometrically similar injectors that had substantially different emissions levels when run on pure gasoline were compared with gasoline mixed with ignition improver. The injector with the higher emissions was found to have a significant decrease in HC emissions when ignition improver was added to the gasoline. In conjunction with the drop in emissions, heat release analysis showed a secondary combustion burst retarded from the primary combustion event. From these experiments it was postulated that the spark-ignited direct-injection engines prefer volatile fuels with low octane (high cetane) ratings for optimal low load operation. However, with appropriate injection system controls, these engines will accept fuels with a range of characteristics.