engine 1 more practically during a u201c no in cylinder pressure required u201d period in every combustion cycle during which the in cylinder pressure is not required for engine control an output signal of the first voltage generation circuit 18 and an output signal of the second voltage generation circuit 19 are input to the signal process circuit 17 in place of the output signal of the in cylinder pressure sensor 3 for the correction of the constants a and b in every combustion cycle of the engine 1 in the present embodiment such a period i e the u201c no in cylinder pressure required u201d period may be defined between a crank angle of 115 degrees u00b0 ca and a crank angle of u2212 115 degrees u00b0 ca where 0 u00b0 ca is top dead center the flowchart of a process in the microcomputer 9 for the present embodiment is illustrated in fig4 the flowcharted process of fig4 starts at a time of turning on of an ignition switch i e at a time of a switching of the ignition switch from a turn off state to a turn on state first the microcomputer 9 waits for a start time of a period during which the in cylinder pressure is not required for an engine control that is waits until a crank angle becomes 155 u00b0 ca step s 201 then by switching the switch 20 the output signal of the first voltage generation circuit 18 is input to the signal process circuit 17 step s 202 then it is determined whether an output voltage of the first voltage generation circuit 18 and an output voltage of the second.