Vehicles with hybrid powertrains are able to shutoff the engine, and rely on torque supplied solely from one or more motor/generators in certain operating modes, thus improving fuel economy. When operating conditions require positive engine torque, the engine is restarted by a hybrid electronic controller in what is referred to as an autostart. A damper is often used in the driveline connection between the engine and the transmission to reduce vibration., filtering out high-frequency engine torque oscillations while transferring engine torque. Some hybrid powertrains also employ torque pulse cancellation, energizing a motor-generator to oppose excessive driveline vibrations caused by engine cranking and cylinder firing events. Other hybrid powertrains utilize a damper bypass clutch (DBC) which is engaged during an engine autostart, in order to bypass the damper to avoid vibrations caused by the damper encountering its natural frequency during that range of speeds, to prevent engine torque pulses from being conveyed to the transmission. The DBC is released during normal operation, restoring the damper function.
Torque pulse cancellation requires knowledge of the position of the engine crankshaft. Crankshaft sensors are typically unidirectional in that they cannot discern the direction of rotation of the crankshaft. As the engine slows to a stop, a reversal in the direction of rotation is common. With a unidirectional sensor, travel past position indicators, such as missing teeth or missing magnetic poles is necessary once in forward and once in reverse before an accurate estimation of crankshaft position can be determined from the sensor information. However, direction reversals may not always occur through the missing teeth/pole area. Thus, position errors occur when travel in the reverse direction is accumulated by the sensor as travel in the forward direction. Thus, even if the previous stop position is stored in the controller, it may not accurately reflect the position of the crankshaft.