When starting an internal combustion engine, it is useful to know the rest position of the engine crankshaft prior to cranking so that fuel and spark settings can be targeted accordingly. In this way, the engine starts in less time and consumes the delivered fuel more efficiently. However, it is difficult to determine the position at which the engine crankshaft stops rotating. This is because crankshaft position sensors are not designed to indicate the direction of rotation, and the crankshaft may reverse directions one or more times due to cylinder air compression before the rest position is finally achieved. If the engine speed at turn-off is too slow for the crankshaft to continue rotating through the next compression cycle, the crankshaft will reverse directions, or “rock-back”. If the engine speed is rotating faster at turn-off, the crankshaft will rotate through the next compression cycle, or “rock-forward”.
Although it is possible to predict or estimate the final position of the crankshaft based on engine speed and crankshaft position measurements, the estimate is only accurate to within 90 crank degrees of the actual crankshaft position. This inaccuracy can cause the engine to begin fueling on the wrong cylinder when the engine is re-started. As a result, the engine must be cranked longer before starting, and the initial exhaust emissions can exceed the regulated limits. Accordingly, what is needed is a method of more accurately determining the final rest position of an internal combustion engine.