Final answer:
The critical engine is the engine whose failure negatively affects aircraft performance the most, determined by factors like thrust line and propeller rotation. In thermodynamics, engine efficiency is maximized by eliminating irreversible processes as indicated by the Carnot cycle. For real engines, like car gasoline engines, actual efficiencies are compared to theoretical maximum efficiencies calculated from reservoir temperatures.
Step-by-step explanation:
The term critical engine refers to the engine in a multi-engine aircraft that, when failed, would have the most adverse effect on aircraft performance or handling qualities. The factors used to determine it typically include, but are not limited to, the engine's thrust line, the direction of propeller rotation (which can cause yaw due to P-factor), and the location of the engine relative to the aircraft's centerline.
When studying the efficiency of engines, particularly in the context of thermodynamics, reference is often made to the Carnot cycle. In the Carnot cycle, the crucial factor is the use of only reversible processes, which eliminates dissipative factors such as friction and turbulence that can reduce the engine's efficiency.
To compare the efficiency of a real car's gasoline engine with the maximum theoretical efficiency, one would calculate the efficiency using the temperatures of the effective hot and cold reservoirs, bearing in mind that actual efficiencies are usually lower due to real-world irreversible processes.