Misfire Detection Monitor
MISFIRE DETECTION MONITORThe misfire detection monitor is an on board strategy designed to monitor engine misfire and identify the specific cylinder in which the misfire has occurred. Misfire is defined as lack of combustion in a cylinder due to absence of spark, incorrect fuel metering, incorrect compression, or any other cause. The misfire detection monitor is enabled only when certain base engine conditions are first satisfied. Input from the cylinder head temperature (CHT), mass air flow (MAF), and crankshaft position (CKP) sensors is required to enable the monitor. The misfire detection monitor is also carried out during an on-demand self-test.
1. The powertrain control module (PCM) synchronized ignition spark is based on information received from the CKP sensor. The CKP signal generated is also the main input used in determining cylinder misfire.
2. The input signal generated by the CKP sensor is derived by sensing the passage of teeth from the crankshaft position wheel mounted on the end of the crankshaft.
3. The input signal to the PCM is then used to calculate the time between CKP edges and the crankshaft rotational velocity and acceleration. By comparing the accelerations of each cylinder event, the power loss of each cylinder is determined. When the power loss of a particular cylinder is sufficiently less than a calibrated value and other criteria are met, then the suspect cylinder is determined to have misfired.
Misfire Monitor Operation
The low data rate (LDR) misfire monitoring system is capable of meeting the federal test procedure monitoring requirements and the full range of misfire monitoring requirements on four cylinder engines. The monitor allows for detection of any misfires that occur six engine revolutions after initially cranking the engine.
Low Data Rate System
The LDR misfire monitor uses a low data rate crankshaft position signal, one position reference signal at 10 degrees before top dead center (BTDC) for each cylinder event. The PCM calculates the crankshaft rotational velocity for each cylinder from this crankshaft position signal. The acceleration for each cylinder can then be calculated using successive velocity values. The changes in overall engine RPM are removed by subtracting the median engine acceleration over a complete engine cycle. The resulting deviant cylinder acceleration values are used in evaluating misfire in generic misfire processing.
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/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/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. Diagnostic trouble code (DTC) P0316 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 P03xx DTC that indicates the misfiring cylinder(s).
Profile Correction
The profile correction software learns the crankshaft tooth spacing under defueled engine conditions. The profile correction requires the engine to be shut down either at ignition off, or during normal vehicle operation, after the keep alive memory (KAM) reset. The learned corrections improve the high RPM capability of the monitor. The misfire monitor is not active until a profile is learned. The profile correction software learns and corrects for mechanical inaccuracies in the crankshaft position wheel tooth spacing. Since the sum of all the angles between the crankshaft teeth must equal 360 degrees, a correction factor can be calculated for each misfire sample interval that makes all the angles between individual teeth equal. To prevent any fueling or combustion differences from affecting the correction factors, learning is done during engine shutdown. In order to minimize learning time for profile correction factors, the correction factors are learned after an engine shutdown is commanded and fuel is disabled while the generator motor spins the engine. In order to protect the traction battery, to provide vehicle starting and to extend the shutdown, traction battery temperature and state of charge must be within operational limits. This condition occurs when either the ignition is turned to the OFF position (typically one ignition off induced engine shutdown), or the normal operating strategy shuts the engine down (typically multiple shutdown events during normal operation). During this shutdown, the generator motor spins the engine at approximately 1,100 RPM, while delta time intervals are captured for computation of the correction factors. Average profile correction factors are calculated for each of the four combustion intervals over approximately 15 engine cycles. This procedure occurs once per KAM reset during the life of the vehicle. Since inaccuracies in the wheel tooth spacing can produce a false indication of misfire, the misfire monitor is not active until the corrections are learned. In the event of the 12-volt battery disconnection or loss of KAM, the correction factors are lost and must be relearned. The software may be unable to learn a profile if the instantaneous profile calculations vary by more than a specified tolerance from the mean values. In this case DTC P0315 is set. Typical profile correction learning entry conditions are, engine in fuel disabled mode for four engine cycles, engine speed between 800 and 1,750 RPM, maximum RPM change during profile correction is 600 RPM, vehicle speed between 0 and 48 km/h (0 and 30 mph), the traction battery voltage above 216 volts, the traction battery temperature above -15°C (5°F), and the traction battery power discharge limit above 12 kW.
Misfire Monitor Specifications
Misfire monitor operation sets DTCs P0300 to P0304 (general and specific cylinder misfire), P0315 (unable to learn profile), P0316 (misfire during first 1,000 revolutions after start-up). The monitor execution is continuous. The misfire rate is calculated every 200 or 1,000 revolutions. The monitor does not have a specific sequence. The CKP and CMP sensors must operate correctly to run the monitor. The monitoring duration is the entire driving cycle (see disablement conditions below).
Typical misfire monitor entry conditions include entry condition minimum/maximum time since engine start-up is 0 seconds, ECT is -7°C to 121°C (20°F to 250°F), RPM range is (full range misfire certified, with two revolutions delay) two revolutions after exceeding 150 RPM below drive idle RPM to red-line on tach or fuel cutoff. Profile correction factors learned in KAM are Yes, and the fuel tank level is greater than 15%.
Typical misfire temporary disablement conditions include closed throttle deceleration, fuel shut-off due to vehicle speed limiting or engine RPM limiting mode, and a high rate of change of torque (heavy throttle tip-in or tip-out).
The profile learning operation includes DTC P0315, unable to learn profile in three 97 to 64 km/h (60 to 40 mph) decelerations. Monitor execution is once per KAM reset, monitor sequence: profile must be learned before misfire monitor is active. Entry conditions include CKP, CMP, no AICE communication errors, CKP/CMP in synch. The monitoring duration; 10 cumulative seconds in conditions, a maximum of three 97 to 64 km/h (60 to 40 mph) defueled decelerations.
Typical profile learning entry conditions are engine in deceleration fuel cutout mode for four engine cycles, the brakes are not applied, the engine RPM is between 800 and 1,750 RPM, the change is less than 600 RPM, the vehicle speed is between 0 and 48 km/h (0 and 30 mph), and the learning tolerance is 1%.