Short Answer:

Thermal efficiency is the ratio of the useful output energy of an engine or system to the total heat energy supplied to it. It shows how effectively the heat energy from fuel is converted into useful mechanical work. The higher the thermal efficiency, the better the performance of the engine because less energy is wasted as heat.

In simple terms, thermal efficiency measures how well an engine or machine uses the heat energy produced by burning fuel. Since some energy is always lost due to friction, exhaust, and cooling, thermal efficiency is always less than 100%.

Detailed Explanation :

Thermal Efficiency

Thermal efficiency is one of the most important performance parameters used to measure the effectiveness of engines, turbines, and heat engines in general. It indicates how efficiently a device converts the heat supplied to it into useful work output. In every practical system, a portion of the supplied heat energy is lost to the surroundings in the form of exhaust gases, friction, cooling, and incomplete combustion. Therefore, the thermal efficiency value is always less than 100%.

It is defined as:

In the case of an internal combustion engine:

Here, the heat supplied by fuel is calculated by multiplying the mass flow rate of fuel and its calorific value (amount of heat energy released by burning 1 kg of fuel).

Thermal efficiency depends mainly on how much of this supplied heat is successfully converted into mechanical work.

Types of Thermal Efficiency

1. Indicated Thermal Efficiency:
   It is the ratio of indicated power (power developed inside the cylinder) to the heat supplied by the fuel. It indicates how efficiently the engine converts fuel energy into power inside the cylinder before frictional losses.

1. Brake Thermal Efficiency:
   It is the ratio of brake power (actual useful output) to the total heat supplied by the fuel. It considers all the real losses in the system, such as friction and exhaust. Hence, it gives a more realistic measure of performance.

Usually, brake thermal efficiency is lower than indicated thermal efficiency because it includes frictional losses.

Factors Affecting Thermal Efficiency

1. Compression Ratio:
   Higher compression ratio increases the pressure and temperature of the air-fuel mixture before ignition, leading to better combustion and higher efficiency.
2. Type of Fuel:
   Fuels with higher calorific values and better combustion properties provide more energy and thus improve efficiency.
3. Engine Design:
   Efficient combustion chambers, lightweight moving parts, and proper cooling systems reduce energy losses.
4. Operating Conditions:
   Proper engine speed, load, and temperature help maintain maximum efficiency.
5. Friction and Heat Losses:
   High friction between parts and excess cooling reduce the amount of useful energy output.

Typical Values of Thermal Efficiency

• For petrol engines, the thermal efficiency ranges from 25% to 30%.
• For diesel engines, it is higher, about 30% to 45%, due to a higher compression ratio.
• For steam turbines, it is around 35%.
• For gas turbines, efficiency can go up to 40% or more.

Even in modern engines, complete conversion of heat into work is impossible because of the second law of thermodynamics, which states that some energy will always be lost to the surroundings.

Methods to Improve Thermal Efficiency

1. Increasing Compression Ratio:
   Higher compression ratio allows better combustion and increases the work done per cycle.
2. Using High-Quality Fuel:
   Better fuel leads to more complete combustion, releasing more heat energy efficiently.
3. Improving Combustion Process:
   Proper air-fuel ratio and correct ignition timing improve energy conversion.
4. Heat Recovery:
   Using waste heat from exhaust gases to preheat air or water improves overall efficiency.
5. Reducing Friction:
   Lubrication and improved material design help minimize friction losses in mechanical components.
6. Efficient Cooling System:
   Proper cooling maintains optimum temperature and prevents excess energy loss through heat dissipation.

Importance of Thermal Efficiency

1. Performance Evaluation:
   Thermal efficiency helps to compare the performance of different engines or power plants.
2. Fuel Economy:
   Higher thermal efficiency means less fuel consumption for the same amount of work output.
3. Environmental Benefits:
   Better efficiency leads to lower emissions and reduced fuel wastage.
4. Economic Operation:
   Improved efficiency reduces operational costs and enhances the life of the engine.

Practical Example

If an engine uses 0.2 kg of fuel per minute with a calorific value of 42,000 kJ/kg and develops a brake power of 25 kW, then:

 

This means that only about 17.85% of the total heat energy is converted into useful mechanical work, while the remaining energy is lost through exhaust, cooling, and friction.

Conclusion

Thermal efficiency is a measure of how effectively an engine converts the heat energy from fuel into useful work output. It is always less than 100% due to heat losses and friction. By improving combustion, increasing compression ratio, and reducing mechanical losses, the thermal efficiency of an engine can be significantly improved. It plays a vital role in achieving better performance, fuel economy, and sustainability in mechanical systems.