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What Does Engine Thermal Efficiency Mean? Why Can't It Be 100%?

Author: Evelyn

Mar. 09, 2024

55 0

Engine thermal efficiency is one of the important indicators for evaluating engine performance. The higher the engine thermal efficiency, the more advanced the technology. So what does engine thermal efficiency mean? Let everyone, especially friends who are about to buy a car, take a look at it.


The thermal efficiency of the engine refers to the ratio of the heat of the engine's effective power to the heat of the fuel consumed per unit time. To put it simply, it means how much energy generated after burning fuel is converted into the driving force of the car.

Thermal efficiency is calculated by dividing the actual heat value utilized by the heat value contained in the fuel. For example: one liter of gasoline will release 33580KJ of heat after complete combustion, of which 11000KJ is converted into work output by the engine. Then the thermal efficiency of this engine at this time is:


Generally speaking, the thermal efficiency of a certain engine is basically fixed from the time it is designed to when it leaves the factory. The maximum thermal efficiency of gasoline engines is generally between 30% and 40%, and that of diesel engines is slightly higher, between 35% and 45%. Speaking of this, some friends may ask, why is the thermal efficiency not 100%? The reality is so cruel. It is extremely difficult for engine engineers to increase the thermal efficiency by 1% on the basis of 30% to 40%. So where does the remaining 50~60% of the energy go?

An engine is a machine that converts thermal energy into mechanical energy. According to the laws of thermodynamics, thermal energy cannot be converted 100% into other forms of energy. Friction is inevitable in all machinery, and the heat consumed by frictional resistance cannot be eliminated. In addition, The engine must maintain a certain temperature and volume and cannot increase or decrease indefinitely, so cooling loss is inevitable. Coupled with other factors, such as the impossibility of complete combustion of the fuel, pump gas loss, etc., the thermal efficiency of the engine cannot be 100%.

When the engine was first born, the maximum thermal efficiency was less than 20%. After a hundred years, with the continuous efforts of generations of automotive engineers, the maximum thermal efficiency of automobile engines has only been increased to more than 40%. This is already a remarkable achievement. With current technology, engine thermal efficiency is close to the limit. Every percentage increase is a huge technological progress.

What are the methods to improve engine thermal efficiency?

1. For gasoline engines, the most direct method is to increase the engine compression ratio. This is also one of the main reasons why the thermal efficiency of diesel engines is higher than that of gasoline engines.

2. Reduce mechanical friction resistance by using low-friction pistons, low-elasticity piston rings, and better-performance lubricants.

3. Optimize the fuel supply system and adopt higher pressure in-cylinder direct injection, etc.

4. Optimize the design of the intake and exhaust passages of the valvetrain and adopt variable valve timing and lift technology.

5. Optimize the combustion chamber design to allow more complete fuel combustion.

6. Optimize the cylinder design and have a suitable bore-to-stroke ratio;

7. For diesel engines, technologies such as increasing the injection rate, multiple injections, and high intake pressure can also be used.


Engine thermal efficiency is not constant

The thermal efficiency of the engine changes with changes in operating conditions. Under most working conditions, the thermal efficiency of the engine is very low. For example, in the most common urban congestion conditions, the thermal efficiency of the engine is only about 20%; when driving on the highway, the thermal efficiency can reach about 30%; the working conditions with the highest thermal efficiency are large loads and low speeds, such as when a car is driving When climbing a steep slope, the engine throttle is fully open and the air intake resistance is minimal. At this time, the engine speed is not high and the mechanical friction resistance is small. The thermal efficiency at this time is relatively high.

If an engine has high thermal efficiency, does it necessarily save fuel?

Not necessarily. Engines with higher thermal efficiency are generally more advanced in technology, and the economy of the engine itself is definitely better. However, the economy of the engine is not equal to the economy of the car as a whole. The fuel consumption of the car is not only determined by the engine, but also by many factors such as the gearbox, chassis, and body weight. Therefore, not all models equipped with engines with high thermal efficiency are fuel-efficient.


Since the advent of the internal combustion engine, especially under the trend of energy conservation and emission reduction, the improvement of thermal efficiency has become a top priority in research and development. Although there are already engines with thermal efficiencies as high as 43% in mass-produced models, the remaining nearly 50% of thermal efficiency still needs to be explored. This is also the significance of many car companies' intensive research on fuel engines.







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