Four stroke. True, higher compression helps the diesel but probably less than reduced pumping loss.
Intake manifold vacuum is relative to atmosphere or "gage" pressure. Intake manifold pressure is relative to zero or "absolute" pressure. Really the same thing, just less confusing to do calculations if you use absolute pressure instead of manifold vacuum.
Maybe it's helpful to think of the Fig2 P-V diagram this way: A diesel engine is unthrottled and always has a small pumping loop area. Load control is achieved by underfueling which makes the power loop area smaller to match the load.
A gasoline engine at WOT is unthrottled and also has a small pumping loop. But when throttled for load control the pumping loop gets bigger and the power loop gets smaller to match the load. This bigger pumping loop for a throttled gasoline engine is a major power loss at cruise. Really nothing you can do about it except to get a smaller engine or overdrive.
