An improved volume signal generator for use with a thermodynamic cycle analyzer for determining the mean effective pressure of an internal combustion engine, includes an axially rotatable cylindrical cam having a hollow truncated end providing an axially gyrating cam surface about the periphery thereof. A light source located within the hollow truncated end directs a radially disposed beam of light towards a photo diode so as to be cyclically interrupted by the cam surface during operation. The same light source cooperates with openings in the cylindrical wall of the cam and with a second photo diode to produce timing signals representative of the extreme positions of the engine's piston.
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