The energy produced by the combustion is the same but different fractions of it are converted into work. Just like efficiency differences in other types of heat engine.
η = 1 - ( T_c / T_h )
Carnot-Efficiency is the theoretical limit of the cycle’s ability to get work out of the thermal energy where T_c is the exhaust (coldest) temperature after expansion and T_h the hottest temperature (before expansion). Temperatures given in K (Kelvin), so 100% if you manage to get T_c to absolute zero temperature.