What is engine thermal efficiency?
Engine thermal efficiency is one of the key indicators for evaluating engine performance. It is defined as the ratio of the heat converted into useful engine power to the heat content of the fuel consumed per unit time. Simply put, it measures how much of the energy released by fuel combustion is converted into the vehicle's driving force.
Calculating thermal efficiency
Thermal efficiency is calculated by dividing the actually utilized heat value by the total heat value contained in the fuel. For example, one liter of gasoline releases about 33,580 kJ when fully burned. If 11,000 kJ is converted to output work by the engine, the thermal efficiency is:
(11,000 ÷ 33,580) × 100% = 32.8%
Generally, an engine's thermal efficiency is largely fixed by its design and production. Gasoline engines typically have a maximum thermal efficiency in the 30% to 40% range, while diesel engines are usually somewhat higher, around 35% to 45%. Improving thermal efficiency by 1 percentage point from this baseline is a significant engineering challenge. What happens to the remaining 50% to 60% of energy?
Why thermal efficiency cannot reach 100%
Engines convert thermal energy into mechanical energy. According to the laws of thermodynamics, thermal energy cannot be converted entirely into other forms of energy. Mechanical friction is unavoidable, and the heat lost to friction cannot be eliminated. Engines must also maintain operating temperature and volume limits, so cooling losses are inevitable. Additional losses include incomplete fuel combustion and pumping losses. For these reasons, thermal efficiency cannot reach 100%.
When engines were first developed, their maximum thermal efficiency was below 20%. After many decades of engineering improvements, the maximum achievable thermal efficiency for automobile engines has increased to above 40%, which represents substantial progress. With current technology, thermal efficiency is approaching practical limits, and each additional percentage point requires substantial technical advancement.
Methods to improve engine thermal efficiency
- Increase the engine compression ratio. This is one of the primary reasons diesel engines tend to have higher thermal efficiency than gasoline engines.
- Reduce mechanical friction. Use low-friction pistons, low-tension piston rings, and higher-performance lubricants.
- Optimize the fuel delivery system. Adopt higher-pressure direct injection into the cylinder.
- Optimize the intake and exhaust paths in the valve train. Employ variable valve timing and lift technologies.
- Improve combustion chamber design to promote more complete combustion.
- Optimize cylinder geometry, including appropriate bore-to-stroke ratios.
- For diesel engines, use higher injection rates, multiple injections, and higher intake boost pressure.
Thermal efficiency varies with operating conditions
Engine thermal efficiency changes with operating conditions. In most real-world conditions, thermal efficiency is relatively low. For example, in congested urban traffic, thermal efficiency may be only in the low 20s percent. On the highway, thermal efficiency can reach about 30%. The highest efficiency occurs under high load and low engine speed, such as when climbing a steep hill: the throttle is wide open, intake restriction is minimal, engine speed is not high, and mechanical friction is relatively low, resulting in comparatively higher thermal efficiency.
Does higher thermal efficiency always mean lower fuel consumption?
Not necessarily. Engines with higher thermal efficiency are typically more advanced and often more economical, but engine efficiency alone does not determine overall vehicle fuel economy. Fuel consumption also depends on the transmission, chassis, vehicle weight, and other system-level factors. Therefore, a vehicle equipped with a high-efficiency engine does not automatically guarantee lower fuel consumption.
Since the introduction of the internal combustion engine and especially with the focus on energy conservation and emissions reduction, improving thermal efficiency has been a central objective of engine development. Some production engines already achieve thermal efficiency levels around 43%, but a large portion of the lost energy remains to be addressed, which continues to motivate further engine research and development.
