Cycle by cycle variations in combustion are investigated as a means to characterize the lean misfire limit of a spark ignited, internal combustion engine. The cyclic variations are represented as parameters of the cylinder pressure versus crankangle curves. The specific parameters investigated are peak pressure, maximum rate of pressure rise, crankangle of maximum pressure, crankangle of maximum rate of rise, indicated mean effective pressure, and area of the pressure curve. The variations are statistically analyzed on a digital computer. Exhaust gas analysis is performed to determine the correlation between the misfire limit and emissions of hydrocarbons, carbon dioxide, carbon monoxide, nitric oxide, and formaldehyde.
Imep and area variations are found to be the most promising as an indicator of the lean misfire limit. As the equivalence ratio decreases from stoichiometric, the variations stay constant, until a sharp linear increase is achieved. Correlation with hydrocarbon emissions shows a strong combustion change occurs at this point of rapid variation increase. This point is adopted as the definition of the lean misfire limit.