Indicated mean-effective pressure instrument

An apparatus for measuring indicated mean effective pressure (IMEP) of an internal combustion piston or rotary engine or of an external combustion engine such as a stirling engine is disclosed. An optical shaft encoder measures crankshaft angle of the engine. Changes in volume with respect to changes in crank angle of one or more cylinders (dV/d theta) is determined either empirically or algebraically from engine geometry and stored in a memory. As the crank angle changes, dV/d theta) is read from the memory and multiplied by chamber or cylinder pressure. The product (P dv/d theta) is then added to the total previously accumulated in the cycle. Each time theta changes by an amount equal to delta theta, the process is repeated. At the end of each engine cycle, the total is equal to the IMEP value for that cycle.