Relating Subjective Idle Quality to Engine Combustion

Engine designers need an objective measurement which can be tested on the engine to indicate acceptable idle quality. An experiment was performed to select objective measures based on cylinder pressure data, and two measures were selected. Standard deviation of indicated mean effective pressure (SDimep) is a measure of the statistical instability of combustion. Lowest normalized value (LNV) is a measure of the tendency toward misfire.

These two measures are shown for a set of typical engines. The body of data shows the relation of SDimep and LNV to burn duration and timing.