A method of determining the relative phasing between a crankshaft and a crankshaft driven high pressure piston pump via a cam mechanism 4, said piston pump adapted to provide high pressure fuel to a common rail 5, comprising: a) measuring the pressure P1 in common rail 5 at a time before a pumping stroke/event; b)measuring the pressure P2 in common rail 5 at a time after the pumping stroke/event; c) determining the average pressure of said measured pressure from steps a) and b); d) determining the time point where the measured pressure reaches the average pressure determined form step c) e) correlating said time point from d) with crankshaft position to determine said relative phasing. This method may allow for accurate rail pressure control by a digital inlet valve without additional manufacturing constraints such as machining tolerance.
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