Generic Misfire Processing

The acceleration that a piston undergoes during a normal firing event is directly related to the amount of torque that cylinder produces. The calculated piston acceleration values are compared to a misfire threshold that is continuously adjusted based on inferred engine torque. Deviant accelerations exceeding the threshold are conditionally labeled as misfires. The calculated deviant acceleration values are also evaluated for noise. Normally, misfire results in a non symmetrical loss of cylinder acceleration. Mechanical noise, such as rough roads or high RPM or light load conditions, produce symmetrical acceleration variations. A noise limit is calculated by applying a negative multiplier to the misfire threshold. If the noise limit is exceeded, a noisy signal condition is inferred and the misfire monitor is suspended for a brief interval.

Noise free deviant acceleration exceeding a given threshold is labeled a misfire.

The number of misfires are counted over a continuous 200 revolution and 1, 000 revolution period. The revolution counters are not reset if the misfire monitor is temporarily disabled, such as for negative torque mode. At the end of the evaluation period, the total misfire rate and the misfire rate for each individual cylinder is computed. The misfire rate evaluated every 200 revolution period (type A) and compared to a threshold value obtained from an engine speed and load table. This misfire threshold is designed to prevent damage to the catalyst due to sustained excessive temperature 871°C (1, 600°F). If the misfire threshold is exceeded and the catalyst temperature model calculates a catalyst mid-bed temperature that exceeds the catalyst damage threshold, the malfunction indicator lamp (MIL) blinks at a 1 Hz rate while the misfire is present. If the threshold is again exceeded on a subsequent driving cycle, the MIL is illuminated.

The misfire rate is evaluated every 1, 000 revolution period and compared to a single (type B) threshold value to indicate an emission threshold, which can be either a single 1, 000 over-revolution event from startup or four subsequent 1, 000 over-revolution events on a drive cycle after start up. A DTC is set if the type B misfire threshold is exceeded during the first 1, 000 revolutions after engine startup. This DTC is stored in addition to the normal misfire DTC that indicates the misfiring cylinder.