Short Answer:

Thermal efficiency is a measure of how effectively a system converts heat energy into useful work. It shows the percentage of input heat that is actually used for producing output work, and how much is wasted. The higher the thermal efficiency, the better the performance of an engine or machine.

It is calculated by dividing the useful work output by the total heat energy input. Thermal efficiency helps engineers understand and improve the performance of engines, turbines, boilers, and other thermal systems.

Detailed Explanation:

Thermal efficiency

In thermodynamics, thermal efficiency is a very important concept that tells us how well a machine or process is using the heat energy it receives. Whenever we burn fuel or transfer heat into a system, not all of that heat is turned into useful work. Some part of it is always lost — mostly as waste heat to the environment. Thermal efficiency shows us how much of the input heat is actually turned into output work.

This efficiency is especially useful for machines like:

• Internal combustion engines
• Steam turbines
• Gas turbines
• Power plants
• Boilers

Formula for Thermal Efficiency

The general formula for thermal efficiency (η) is:

η = (Useful Work Output / Heat Input) × 100

Or in energy terms:

η = (W_out / Q_in) × 100

Where:

• W_out = Work done by the system (in joules or kJ)
• Q_in = Total heat energy supplied (in joules or kJ)

The result is usually expressed in percentage (%).

Example:
If an engine receives 1000 kJ of heat and produces 300 kJ of work:
η = (300 / 1000) × 100 = 30%

This means 70% of the heat is lost, and only 30% is used to do useful work.

Importance of Thermal Efficiency

1. Performance Measurement
   • It tells us how good a system is at converting heat into work.
   • Higher efficiency means less fuel is required for the same work output.
2. Cost Saving
   • Efficient systems reduce fuel usage and operating costs.
   • Helps in reducing maintenance and operational losses.
3. Environmental Impact
   • Lower fuel consumption means less pollution and fewer greenhouse gases.
   • Efficient power plants are better for the environment.
4. Comparison of Systems
   • Thermal efficiency allows engineers to compare different machines and cycles.
   • For example, comparing Otto cycle (petrol engine) vs Diesel cycle.
5. Design Improvement
   • Helps identify where heat is wasted and how it can be recovered (like using regenerative cycles or combined heat systems).

Typical Thermal Efficiencies

• Petrol engines (Otto cycle): 25% – 30%
• Diesel engines (Diesel cycle): 30% – 40%
• Gas turbines (Brayton cycle): 35% – 40%
• Modern steam power plants (Rankine cycle): 40% – 45%
• Combined cycle power plants (Gas + Steam): up to 60%

These values show that no system is 100% efficient, and improving thermal efficiency is a key goal in engineering design.

Limitations of Thermal Efficiency

• Cannot be 100% due to the second law of thermodynamics.
• Some energy is always lost as heat to the environment.
• Real engines have friction, heat loss, and other inefficiencies.
• Ideal efficiencies (like in Carnot cycle) are not achievable in real life.

Still, thermal efficiency helps us to get as close as possible to ideal performance.

Conclusion

Thermal efficiency is the ratio of useful work output to the total heat energy input in a system. It indicates how effectively a machine converts heat into work and helps in evaluating and improving energy performance. Although 100% efficiency is not possible, understanding thermal efficiency helps in reducing losses, saving fuel, and designing better energy systems. It plays a key role in engineering, power generation, and environmental protection.