Mark E Smith — But that was precisely my point! It's a semantic issue. I'm not confusing "compression" with "compression ratio." What I'm saying is that, for greater thermodynamic efficiency, the power stroke should be longer than the compression stroke. It extracts more of the heat energy from the hot gases. The Atkinson/Miller-cycle engines achieve this difference between the strokes by delaying the closing of the intake valves. My point was really that the "compression ratio" number is not meaningful for such engines. It really represents the "expansion ratio," and not the factor (i.e., the ratio) by which the unburned gases are actually being compressed.

Stan