Peak combustion pressure timing detector for internal combustion engine

Apparatus for sensing the timing of peak combustion pressure in the combustion chamber of an internal combustion engine comprising, in combination:

• pressure sensing apparatus including a piezoelectric element adapted for flexure in response to variations in combustion chamber pressure, the piezoelectric element having a capacitance varying with said flexure;
• a first resistance connected electrically in parallel with the capacitance of the piezoelectric element with grounded and ungrounded junctions;
• constant current generating means adapted to supply a constant current to the ungrounded junction, the constant current being effective, when the capacitance remains constant, to establish a predetermined voltage across the first resistance and thereby across the capacitance;
• a second resistance;
• a reference voltage source effective to provide the predetermined voltage;
• an operational amplifier having an output connected through the second resistance to the ungrounded junction and through a third resistance to the grounded junction, an inverting input connected to the output through a feedback resistance and a non-inverting input connected to the reference voltage source, the operational amplifier thus comprising a variable current source and sink effective to maintain the predetermined voltage on the capacitance;
• and
• output circuit means connected to the output of the operational amplifier, the voltage thereon corresponding to the derivative of the pressure sensed by the piezoelectric element and thus indicating the timing of peak combustion pressure as it passes through the predetermined voltage.