Introduction to On Board Diagnostics (II)

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.