At the SAME compression ratio the Diesel cycle is worse than the Otto one â 51% against 61% at 10.5:1. It wins in the world by running 18:1, which a petrol engine cannot.
Ideal efficiency is governed by how far the gas is expanded, and for the Otto cycle that is 1 â r^(1âγ) â compression ratio and nothing else. Burning at constant pressure, as a diesel does while the piston is already moving away, spends the heat less effectively than burning it all at the top, so the diesel cycle carries a penalty. What removes the penalty is that a diesel compresses AIR ALONE. There is no fuel in the charge to light itself early, so there is no knock limit, and the ratio can go to 18 or 20:1 where a petrol engine stops around 10 to 12. The advantage is real; the usual explanation for it is the wrong way round.
This is the air-standard ideal and says nothing about the two engines as built. Real diesels also run lean and unthrottled, which is worth more at part load than anything in this comparison â a petrol engine spends much of its life pumping against a closed throttle. And the numbers are ceilings no engine reaches.
Estimate with the rule, then check it against the calculator that models it properly.
Open Engine Cycle Simulator →At the SAME compression ratio the Diesel cycle is worse than the Otto one â 51% against 61% at 10.5:1. It wins in the world by running 18:1, which a petrol engine cannot. Ideal efficiency is governed by how far the gas is expanded, and for the Otto cycle that is 1 â r^(1âγ) â compression ratio and nothing else.