Method for error correction in measurement of engine speed

A fuel system for a compression ignition internal combustion engine (10) comprising a fuel pump (13), a controller (14) for controlling the operation of the fuel pump so that fuel is delivered to the engine at the required time and in the desired quantity, means (15) for supplying a demand signal to the controller, a wheel (12) driven by the engine, a plurality of indicia on the wheel and positioned about the axis of rotation thereof, transducer means (16) responsive to the passage of said indicia as the wheel rotates, means responsive to the signals generated by said transducer means for measuring the time intervals between the passage of the indicia past said transducer means, characterized by means for determining indicia spacing errors from the signals provided by the transducer means, said means using matrix techniques to solve the equations

• T[2].ER[1]-T[1].ER[2]=6(T[1]-T[2])
• T[3].ER[2]-T[2].ER[3]=6([2]-T[3])
• T[n].ER[n-1]-T[n -1].ER[n]=6(T [n-1]-T[n])
• where
• T[i] is the time taken for an ith indicium cycle and
• ER[i] is the spacing error between indicium i and indicium i+1
• for T[1] to T[n] and ER[1] to ER[n] and further means responsive to the generally sinusoidal fluctuation of engine speed for correcting the determined indicia spacing errors, the corrected determined indicia spacing errors being utilized with the measured time intervals to provide an engine speed signal to the controller.