[christofoo]

We can compare a 4 cylinder DCD engine at 3/4 duty-cycle to a hypothetical 4 cylinder variable-displacement engine, similarly with 1 cylinder deactivated and sealed shut. The variable displacement engine is identical except for the elimination of pumping losses in 1 cylinder, so it must be more efficient than DCD.

(A 3 cylinder engine would be even more efficient than the variable displacement engine due to reduction in mechanical friction losses.)

This means by extension that variable-displacement technology is thermodynamically more efficient than lean-burn, all other things being equal.

Seems surprising? Let's try a sanity check.

Try setting up the two engines I hypothesized with equal fuel input and equal manifold vacuum. The lean burn has more displacement than the variable-displacement engine with respect to the same fuel, so it must run leaner than stoich, which isn't so important except to illustrate that this case exists. Bottom line: lean-burn has the same produced work, the same manifold vacuum but with more displacement, so it has higher pumping loss and lower available work and therefore lower efficiency.

EDIT: one key assumption however is that the lean-burn engine has a manifold vacuum at all, which may not be true. If conditions allow manifold to be close to ambient pressure and pumping losses become negligible, then there's nothing left for variable-displacement to improve over. (But a smaller smaller fixed displacement engine would eliminate mechanical friction over a variable-displacement engine.)

(EDIT: FYI, need a break, will get back to assumptions and practicalities later on... probably.)