General Misfire Processing

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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 and cylinder 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 cold start emission reduction (CSER) monitor uses a threshold multiplier during startup to compensate for the reduction in the signal amplitude during ignition spark retard conditions. The threshold adjustments may also be applied to compensate for torque reduction during gear shift events, and to compensate for changes in driveline coupling with torque convertor lock status.

The calculated deviant acceleration values are also evaluated for noise. Normally, misfire results in a nonsymmetrical loss of cylinder acceleration. Mechanical noise, such as rough roads or crankshaft oscillations at low rpm or high load conditions, will produce symmetrical positive acceleration variations. Noise limits are calculated by applying a negative multiplier to the misfire threshold. If the noise limits are 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 vehicles with 4 cylinder engines, the signals are also evaluated for noise from mechanical resonance in the transmission hardware at low load conditions. This mechanical noise can produce acceleration variations that are indistinguishable from a paired cylinder misfire pattern. The software includes a check of the universal HO2S signal when a paired cylinder misfire pattern is present. Normally, there will be a lean shift on the HO2S if 2 out of the 4 cylinders are misfiring. If the HO2S does not indicate lean, a noisy signal condition is inferred and the misfire monitor is suspended while the conditions are present.

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 is evaluated every 200 revolution period (Type A) and compared to a threshold value achieved from an engine speed and load table. This misfire threshold is designed to prevent damage to the catalyst due to sustained excessive temperature 899deg.C (1, 650deg.F) for Pt/Pd/Rh advanced washcoat and 982deg.C (1, 800deg.F) for Pd-only high tech washcoat. If the misfire threshold is exceeded and the catalyst temperature model calculates a catalyst mid-bed temperature that exceeds the catalyst damage threshold, the 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.

At high engine speed and load operating conditions the monitor continuously evaluates the misfire rate during each 200 revolution period. If a calibrated number of misfire events have been accumulated within a 200 revolution block such that the misfire threshold is already exceeded before the end of the block has been reached, the monitor will declare a fault immediately rather than wait for the end of the block. This improves the capability of the monitor to prevent damage to the catalyst.

If a single cylinder is determined to be consistently misfiring in excess of the catalyst damage criteria, the monitor will initiate failure mode effects management (FMEM) to prevent catalyst damage. The fuel injector to that cylinder is disabled for a calibrated period of time, typically 30 to 60 seconds. Up to 2 cylinders may be disabled at the same time on 6 and 8 cylinder engines and 1 cylinder on 3 and 4 cylinder engines. The fuel control will go open loop and target lambda as slightly lean. The software may also use the throttle to limit the airflow (limit boost) on GTDI engines for additional exhaust component protection. After the calibrated period of time, typically 30 seconds, the injector is enabled and the system returns to normal operation. On some vehicles, the software may continue FMEM beyond 30 seconds if the engine is operating at high speed or load at the end of the 30 second period. The software will wait for a low airflow condition to exit from FMEM. This protects the catalyst should the misfire fault still be present when the fuel injector is turned back on. If a misfire is detected on that cylinder again after 200 revolutions (about 5 to 10 seconds), the fuel injector is disabled again and the process repeats until the misfire is no longer present. Note that ignition coil primary circuit failures trigger the same type of fuel injector disablement.

If fuel level is below 15%, the misfire monitor continues to evaluate misfire over every 200 revolution period to determine if catalyst damaging misfire is present so that the fuel shut off FMEM can be utilized to control catalyst temperatures. When a misfire occurs at low fuel levels, DTC P0313 will set in place of DTCs P0300 to P0308.

The misfire rate is also evaluated every 1, 000 revolution period and compared to a single (type B) threshold value to indicate an emission threshold concern, which can be either a single 1, 000 over revolution event from startup or 4 subsequent 1, 000 over revolution events on a drive cycle after startup. Many vehicles set DTC P0316 if the type B threshold is exceeded during the first 1, 000 revolutions after engine startup. This DTC P0316 is stored in addition to the normal P03xx DTC that indicates the misfiring cylinder. If the misfire is detected but it can not be attributed to a specific cylinder, DTC P0300 is stored.