What is the maximum thermal efficiency of a typical piston engine?

The maximum thermal efficiency of a typical piston engine is around 30%. This efficiency is a reflection of the limitations imposed by the thermodynamic cycle that the engine operates within, typically the Otto cycle for gasoline engines or the Diesel cycle for diesel engines.

In practical terms, a key contributing factor to this efficiency limit is the inherent losses associated with the combustion process, friction, heat dissipation, and other mechanical inefficiencies. Furthermore, the second law of thermodynamics limits the maximum efficiency of any heat engine operating between two temperature reservoirs, with most piston engines falling short of this theoretical maximum in practical applications.

The other options reflect values that are more characteristic of advanced systems or alternative energy conversion technologies, which are not representative of typical reciprocating piston engines. Such higher efficiencies are usually achieved in systems designed specifically for optimized conditions, unlike conventional piston engines that are subject to various operational inefficiencies.