Hydrocarbons have very similar energy/mole and therefore energy/volume, energy/mass ratios. This relationship breaks down for very short HC (methane, basic alc

Hydrocarbons have very similar energy/mole and therefore energy/volume, energy/mass ratios. This relationship breaks down for very short HC (methane, basic alcohols). Differences in effective energy density result from the combustion cycle used: Otto is better than Diesel for the same compression ratio, but you can’t easily get high compression ratio w/ Otto. Therefore Diesels are, typically, more efficient. However, if you use methane (high compression) in an Otto cycle with turbo compounding, you’ll probably beat diesel. But then you have NOx emissions