Final answer:
Diesel engines should ideally operate at 60-75% of their maximum load and around 80-85% of their maximum speed to balance power delivery and efficiency. Automotive engines typically run at 15-25% efficiency, with the most fuel-efficient speeds for cars being 70-80 km/h to reduce air drag.
Step-by-step explanation:
Ideally, diesel engines should run at least around 60-75% of their maximum rated load, and at around 80-85% of their maximum speed. This operating range is important for the engine's efficiency and durability. Diesel engines operate efficiently when the load and speed are balanced to provide sufficient power without undue stress or inefficiency. For reference, real heat engines, such as power plants or automobile engines, often operate at efficiencies far lower than the theoretical maximum due to practical challenges. For example, a typical efficiency for an electrical power plant is 30-40%, while cars are typically in the 15-25% range. However, theoretical combustion temperatures around 700-800°C suggest a maximum efficiency of about 60%. At highway speeds, a significant portion of a car's power is used to overcome air drag. The most fuel-efficient cruising speed for a car is around 70-80 km/h (approximately 45-50 mi/h). This was one of the reasons for the 90 km/h (55 mi/h) speed limit set during the 1970s oil crisis in the United States.