What is the efficiency of a diesel engine?
2 Answers
The efficiency of a diesel engine is 34%-45%. Diesel has a higher thermal efficiency than gasoline, and diesel has a higher calorific value, so burning the same mass of fuel releases more energy, resulting in more work done during the power stroke. The compression ratio of a diesel engine is higher than that of a gasoline engine. A higher compression ratio in an internal combustion engine leads to higher working efficiency. The reason diesel engines have a higher compression ratio than gasoline engines is that during compression, only air is being compressed in a diesel engine. Even with a very high compression ratio, premature detonation does not occur. Additionally, diesel engines rely on the temperature of the compressed air itself to instantly ignite the injected fuel, so the air must be compressed to a temperature above the auto-ignition point of diesel. Diesel engines require more compression to increase temperature. The compression ratio directly affects the engine's efficiency and torque, so diesel engines with higher compression ratios can output greater torque. A characteristic of diesel engines is lean combustion. The air-fuel ratio of a diesel engine can range from 18:1 to 70:1. With lean combustion, more air is available for expansion to perform effective work, thus increasing the engine's efficiency. Achieving this effect requires the assistance of a turbocharger, which forces more air into the engine for efficient combustion.
I've been driving diesel trucks for over a decade. Diesel engines typically have an efficiency between 30% to 40%, sometimes reaching around 45%. This is significantly higher than gasoline vehicles, which only achieve about 20% efficiency, because diesel engines have higher compression ratios and more complete combustion, resulting in less wasted heat energy. When driving, I pay attention to maintaining the air filter and fuel injectors, which can slightly improve efficiency. On highways, especially during long-distance heavy-load trips, they're particularly fuel-efficient—a full tank can take you much further. However, if idling for too long or in extremely cold weather, efficiency can drop below 25%, requiring a few minutes of warm-up. Modern diesel vehicles also come with turbocharging technology, boosting power without consuming extra fuel, making them very cost-effective for daily use. Truck drivers love this advantage of diesel engines—fewer trips to the gas station mean more money saved. Overall, efficiency depends on the vehicle model and maintenance, so regular checks on the fuel system are crucial.
