Evaluation of the combustion process in a spark-ignition engine based on the unrepeatability of the maximum pressure

The article presents the results of an empirical study of unrepeatability of the spark-ignition engine BMW N43 B20 AY, which uses a spray-guided gasoline direct injection system to create the fuel mixture. The values of the so-called unrepeatability index of maximum indicated pressure were analysed for two different engine operating conditions: under maximum load (absolute throttle at 91%) and under partial load (absolute throttle at 33%). The values of the index were calculated on the basis of values of indicated pressure recorded over 100-cycles at five predefined rotational speeds of the engine crankshaft. It was found that the unrepeatability of maximum indicated pressure during operation of the test engine was lower under partial load than under maximum load.