What is a heat engine?

Short Answer:

A heat engine is a device that converts heat energy into mechanical work. It takes in heat from a high-temperature source, uses part of it to do useful work, and then releases the remaining heat to a low-temperature sink. The main aim of a heat engine is to produce mechanical power using thermal energy.

Heat engines work on the principle of thermodynamic cycles such as the Carnot cycle, Otto cycle, and Diesel cycle. They are commonly used in cars, power plants, airplanes, and machines that need mechanical movement.

Detailed Explanation:

Heat engine

A heat engine is a system in thermodynamics designed to extract useful mechanical work from the flow of heat energy between two thermal reservoirs — one at a high temperature and another at a low temperature. The working of a heat engine is based on the second law of thermodynamics, which says that heat cannot be fully converted into work, and some part of it must always be released.

Heat engines are very important in mechanical engineering because they form the core of machines such as internal combustion engines, steam engines, and gas turbines.

Working Principle of a Heat Engine

A heat engine operates in a cyclic process, which repeats again and again. The main steps involved are:

  1. Heat Intake (Q_in)
    • The engine receives heat from a high-temperature source like burning fuel or steam.
  2. Work Output (W_out)
    • A part of this heat is used to perform mechanical work, such as moving a piston or turning a turbine.
  3. Heat Rejection (Q_out)
    • The remaining heat is released to a low-temperature sink, such as the atmosphere or cooling system.
  4. Cycle Repeats
    • The working substance (gas, steam, or air-fuel mixture) goes back to its initial state and the cycle continues.

This forms a closed cycle, such as the Otto cycle, Diesel cycle, or Rankine cycle.

Types of Heat Engines

  1. External Combustion Engines
    • Fuel is burned outside the engine.
    • Example: Steam engine, Rankine cycle
    • Heat is used to convert water to steam, which drives a piston or turbine.
  2. Internal Combustion Engines
    • Fuel is burned inside the engine itself.
    • Example: Petrol and diesel engines (Otto and Diesel cycles)
    • Air-fuel mixture is ignited inside the cylinder to produce power.

Efficiency of a Heat Engine

The efficiency of a heat engine is given by:

η = (W_out / Q_in) × 100 = [(Q_in – Q_out) / Q_in] × 100

This shows that not all heat is converted into work. Some heat is always lost. Even the best engine cannot be 100% efficient.

Carnot Efficiency (ideal case):

η = 1 – (T_cold / T_hot)

Where:

  • T_cold = temperature of the sink (in Kelvin)
  • T_hot = temperature of the source (in Kelvin)

This is the maximum possible efficiency any engine can achieve under ideal conditions.

Examples of Heat Engines

  1. Car Engine (Otto cycle) – Converts chemical energy of fuel into motion.
  2. Diesel Engine (Diesel cycle) – More efficient, used in trucks and heavy machinery.
  3. Steam Turbine (Rankine cycle) – Used in thermal power plants.
  4. Gas Turbine (Brayton cycle) – Used in aircraft and power generation.

Applications

  • Power generation plants
  • Transportation systems (cars, trains, ships, aircraft)
  • Pumps, compressors, and industrial machinery
  • Military vehicles and submarines

Heat engines are essential for converting energy into motion and power.

Conclusion

A heat engine is a device that converts thermal energy into mechanical work using a cyclic process. It takes heat from a source, does work, and releases the rest to a sink. Heat engines are used in many machines around us, including cars, turbines, and power plants. Understanding how they work is important for designing efficient energy systems in mechanical engineering.