When a modern diesel engine is running at optimum efficiency, the percentage of rejected heat would typically be?

In a modern diesel engine operating at optimum efficiency, approximately 60% of the energy generated from the combustion process is typically rejected as heat. This is a result of the inherent inefficiencies present in any internal combustion engine, where not all fuel energy is converted into useful work.

The rejected heat includes heat lost through exhaust gases, cooling systems, and other engine components. Diesel engines are known for their higher thermal efficiency compared to gasoline engines, yet they still lose a significant amount of heat energy, and modernization and advancements in diesel engine design have led to marginal improvements in efficiency. Understanding that around 60% represents a typical figure helps to grasp the challenges engineers face regarding heat management and energy recovery methods, such as turbocharging and improved cooling systems, to optimize performance and fuel efficiency.