Short Answer

Thermal efficiency is a measure of how effectively a heat engine converts heat energy into useful work. It tells us what fraction of the heat supplied to the engine is turned into work and how much is wasted. A higher thermal efficiency means the engine is better at using heat to do work.

In real machines like cars, turbines, or power plants, some heat is always lost to the surroundings. Therefore, thermal efficiency is always less than 100%. It helps compare different engines and understand how well they use energy.

Detailed Explanation :

Thermal Efficiency

Thermal efficiency is an important concept in thermodynamics that describes the performance of heat engines. It measures how much of the heat energy taken in by an engine is converted into mechanical work. Since no engine can convert all heat into work due to the Second Law of Thermodynamics, thermal efficiency helps us understand how close an engine is to being efficient.

Thermal efficiency plays a key role in engineering, energy production, automobile design, and power generation. By studying thermal efficiency, we can identify where energy losses occur and improve engine performance.

Definition of Thermal Efficiency

Thermal efficiency can be defined as:

“The ratio of the useful work output of a heat engine to the heat energy supplied to it.”

It is usually expressed as a percentage.

Mathematically:

η = (Work Output / Heat Input) × 100

Or,

η = 1 − (Q₂ / Q₁)

Where:

• Q₁ = heat absorbed from the hot reservoir
• Q₂ = heat rejected to the cold reservoir
• η = thermal efficiency

A smaller Q₂ means less heat is wasted and efficiency is higher.

Why Thermal Efficiency Is Always Less Than 100%

According to the Second Law of Thermodynamics:

• It is impossible to convert all heat into work
• Some heat must always be rejected to the surroundings
• Entropy increases in all natural processes

Therefore:

• Efficiency can never reach 100%
• Even ideal engines have limitations
• Real engines suffer additional losses like friction, air resistance, and leakage

This makes full conversion of heat to work impossible.

Thermal Efficiency in Heat Engines

Heat engines work on the principle of converting heat into work. They operate between two reservoirs:

• A hot reservoir (source of heat)
• A cold reservoir (sink for waste heat)

Heat flows from the hot reservoir to the engine. Part of this heat becomes work, and the remaining part is released to the cold reservoir.

The efficiency of this conversion determines how good the engine is.

Examples of Heat Engines:

• Car engines
• Steam turbines
• Diesel engines
• Jet engines
• Power plant turbines

Each of these engines has a thermal efficiency depending on design, fuel used, and operating conditions.

Carnot Efficiency

The Carnot engine is an ideal heat engine that gives the maximum possible efficiency. Its thermal efficiency depends only on the temperatures of the hot and cold reservoirs.

Carnot efficiency:

η = 1 − (T₂ / T₁)

Where:

• T₁ = temperature of hot reservoir (in Kelvin)
• T₂ = temperature of cold reservoir

Even Carnot efficiency is less than 100% unless T₂ = 0 K, which is impossible to achieve.
Thus, no engine—not even a perfect one—can be 100% efficient.

Factors Affecting Thermal Efficiency

Several factors influence thermal efficiency:

1. Temperature Difference

• Higher temperature of the hot reservoir increases efficiency
• Lower temperature of the cold reservoir also increases efficiency

2. Type of Fuel

Better fuels produce more heat and improve efficiency.

3. Engine Design

Advanced engine designs reduce heat loss, friction, and leakage.

4. Friction and Losses

Real engines lose energy through friction, sound, vibration, and radiation.

5. Working Substance

Using gases like steam or air can affect efficiency.

Examples of Thermal Efficiency in Daily Life

1. Car Engines

A typical petrol engine has efficiency of around 25–30%.
This means only 30% of heat becomes work; the rest is lost.

2. Diesel Engines

Diesel engines are more efficient (35–40%) due to higher compression ratios.

3. Steam Turbines

Thermal power plant turbines have efficiencies between 35–45%.

4. Gas Turbines

Modern jet engines have efficiencies around 40%.

5. Electric Power Plants

Combined-cycle plants can reach 60% efficiency by using waste heat effectively.

These examples show how different machines convert heat with different efficiencies.

Importance of Thermal Efficiency

Thermal efficiency is important because it:

• Helps evaluate engine performance
• Reduces fuel consumption
• Saves energy
• Decreases pollution
• Lowers operational costs
• Guides engineers in designing better engines
• Helps select engines for different applications

Improving thermal efficiency is a major goal in modern energy systems.

Conclusion

Thermal efficiency is a measure of how effectively heat energy is converted into useful work in a heat engine. Because of the Second Law of Thermodynamics, no engine can achieve 100% efficiency; some heat is always wasted. Thermal efficiency helps compare engine performances, design better machines, save energy, and reduce environmental impact. Understanding thermal efficiency is essential in thermodynamics, engineering, and energy technology.