Diesel engine - efficiency

Question

The fuel efficiency of diesel engines surpasses that of most other combustion engines due to their high compression ratio, elevated air-fuel equivalence ratio (λ), and the absence of intake air restrictions (throttle valves).

In theory, a diesel engine can achieve a maximum efficiency of 75%. However, practical efficiency falls significantly lower, reaching up to 43% for passenger car engines, up to 45% for large truck and bus engines, and up to 55% for large two-stroke marine engines.

The average efficiency during a motor vehicle driving cycle is lower than the diesel engine's peak efficiency.

For instance, an engine with a peak efficiency of 44% may achieve an average efficiency of 37%. This difference arises because the fuel efficiency of a diesel engine decreases at lower loads, although this decline is not as steep as in the case of Otto (spark ignition) engines.