WHEN evaluating engine efficiencies, mean effective pressures or the work done by the piston in internal combustion engines or compressors, it is generally assumed that the state of the working fluid is uniform throughout its mass. From this it follows that the expression for work where P denotes pressure and V, volume, holds. Now, it is known that, strictly speaking, this expression applies only in the limiting case of zero piston velocity, when the motion of the piston, and the thermodynamic process in the cylinder are said to be quasi‐static. The question, therefore, poses itself as to how far such an assumption is justified, when applied to a modern high‐speed reciprocating engine, say, an aircraft or motorcar engine, when piston velocities of the order of 40 ft./sec. are encountered.