Which factor best accounts for a diesel engine's high thermal efficiency?

The high thermal efficiency of a diesel engine primarily results from the high compression ratio. Diesel engines operate at a higher compression ratio than gasoline engines, typically ranging from 14:1 to as high as 25:1 in some designs. This elevated compression ratio allows the air-fuel mixture to be compressed to a smaller volume, which in turn raises the temperature of the air. When fuel is injected into this highly compressed and heated air, it ignites spontaneously due to the heat, leading to a more complete combustion process.

This complete combustion contributes to higher thermal efficiency because more of the fuel's energy is converted into usable work rather than being lost as heat. The high compression ratio directly correlates with the engine's ability to extract greater energy from the same amount of fuel, making diesel engines notably more efficient in converting fuel energy into mechanical energy compared to other types of engines that operate at lower compression ratios.

While other factors like turbocharging can further enhance performance and efficiency, they do not fundamentally account for the inherent high thermal efficiency characteristic of diesel engines as the high compression ratio does.