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You also increased the mass of air in the cylinder, which increases the compression pressure and the post combustion pressure. Less work to get the mass in the cylinder and more work from the mass when it is ignited.
Also in lean burn you get higher temperatures in the cylinder which increases the pressure on the piston.
The downside is NOX emissions, which killed lean burn, but you may see it come back with things like Transonic Combustion Injectors, which heat the fuel to very high temperatures and inject it under very high pressures for ignition without spark on gasoline.
The key is to have the mixture as close as possible to homogenous, meaning every fuel molecule is evenly distrubuted with the necessary oxygen molecules. This also means there will be more than a single injection point. The Mazda SKY-ACTIV uses mulitple injection points to provide detonation free combustion, even on gasoline with 14 to 1 compression ratios.
regards
mech
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