at top dead center which value will be denoted by x i the point in time when the piston reaches top dead center is that point when v equals zero and is changing direction from towards the cylinder head to away from the cylinder head the equation for x c according to the invention is therefore as follows x c in equation 7 is the displacement of any point on the piston from the location of the same point when the spring is neither compressed nor extended measured when the piston is at top dead center fig3 is a block diagram of the invention in which signal flow direction is indicated by arrows and the subcircuits required by a preferred embodiment of the invention are indicated by titled blocks inputs proportional to v and i are labelled v signal and i signal respectively the block labelled 34 v computation 34 computes v according to equation 1 the blocks labelled 34 differentiator 34 and 34 integrator 34 compute a and x respectively from equations 6 and 2 the block labelled 34 top dead center sample pulse generator 34 has v as input and generates a pulse using conventional techniques when v is equal to zero and is changing direction from towards the cylinder head to away the block labelled 34 suction phase sample pulse generator 34 has x and or v as input and generates a pulse at some point in time during the suction phase the exact point being determined by a combination of x and v for example v alone could be used as input and.