In terms of thermal efficiency, what effect does leaning the mixture have?

Leaning the mixture in an engine refers to reducing the amount of fuel in relation to the amount of air in the combustion process. This adjustment plays a crucial role in improving thermal efficiency. When the mixture is leaned, it results in a higher air-to-fuel ratio, which can lead to a more complete combustion of the fuel. As a result, thermal efficiency increases because more of the fuel's energy is harnessed to do work rather than being lost through incomplete combustion or waste heat.

By optimizing the combustion process, the engine can operate at a higher efficiency, converting more of the fuel's energy into useful mechanical energy. Leaning the mixture is often employed in various operational scenarios where maximizing efficiency is desired, such as cruise conditions, where the demand for power is lower, and the engine can run more efficiently with less fuel.

While other factors, such as horsepower and engine cooling, may be affected by leaning the mixture, the primary and noteworthy impact is on thermal efficiency, which is why this option is the most accurate response to the question.