Introduction to On Board Diagnostics (II)

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Engine Misfire Detection: Misfiring is the lack of combustion in the cylinder. Misfiring can be caused by worn ignitioncomponents, poor fuel metering, or faulty electrical system. Excessive exhaust emissions will be the result even with fewmisfires. Increased misfire rates can damage the catalytic converter. Engine misfire is detected by monitoring crankshaft speedfluctuations. Engine misfire willcontribute to a deceleration of the crankshaft’s rotational speed d due to the momentary absence of engine torque during thepowerstroke of the cylinder that is misfiring. Using the crankshaft sensor input, theinstantaneous crankshaft speed is calculated, and the speed signal is analyzed to detect the misfire. To eliminate other causes oftorque reduction due to rough roads and other driving events, the e speed reduction is monitored using Exponentially weightedmoving average (EWMA) technique to identify the misfiring cylinder. Other techniques used to identify torque reduction due tomisfire, include signal processing using several algorithms. One signal processing method analyzes the amplitude and phase ofeach of the first twelve frequency components of the crankshaft angular velocity signal taken continuously during the torquereduction time. If a certain percent of misfires within 200 or 10001revolutions is detected , a fault code (DTC) is set. Misfire isdetected if the offending cylinder can be identified. Other advanced anced signal processing algorithms can be used such as PrincipalComponent Analysis and Clustering to compress the data and isolate the misfiring cylinder.If a misfire is detected, all the main engine operating parameters rs such as engine speed , engine load or MAP (Manifold absolutePressure), engine coolant temperature, throttle position, oxygen n sensor, values are stored away in memory. This is called“Freeze Frame”, which is an OBD II requirement. Freeze Frame is used to identify a consecutive misfire in the next driving cycledefined by the EPA as the next driving “Trip” after ignition OFF. . If a second misfire is detected the engine controller will turnon the MIL (Malfunction Indicator Light) to alert the driver. The e specific cylinder experiencing misfire must be identified. Ifmore than one cylinder is misfiring a separate DTI (diagnostic trouble tcode) is required.If misfire is not detected during the next three subsequent consecutive driving “Trips” when similar conditions occur then theoriginal fault will be erased and the MIL will be turned off by the engine controller. In another circumstance , if “similarconditions” are not encountered during next eighty subsequent trips the original fault will be turned off by the enginecontroller.
The Freeze Frame can also be used for Off-Board diagnostics and trouble shooting by service technicians.Misfires can damage the catalyst converters by raising the catalyst temperature beyond safe values.Type A misfire is defined below:For type A misfire, up to three 200 revolutions are evaluated on first driving cycle for misfire detectionbefore illuminating MIL.MIL must be illuminated on misfire detection during first 200 revolutions’ evaluation during the second driving cycle.However MIL need not be steadily illuminated when misfire ceases, , until second driving cycle.Type B misfire (during starting of engine):This misfire is evaluated in first 1000 revolutions after engine is started. Misfire detection will set coolant temperaturefault code since that is the likely cause of misfire detection at this time.MIL and “hard” fault code is set permanently on second driving cycle. cUp to four 1000 revolutions are evaluated for misfire detection excluding the first 1000 revolutionsbefore illuminating temperature fault code.MIL and “hard code” are set on second driving cycle.Thermostat (coolant temperature) monitoring and misfire detection n monitoring are extremely important due toincreasingly tighter controls mandated on emissions.Misfire detection is described in more detail in a later section.
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Functionality of PowertrainControl
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Introduction to On Board Diagnostics (II)

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