1. What Is a Heat Engine?
A heat engine is a device that converts heat energy into useful mechanical work. It takes heat from a high-temperature source, converts part of it into work, and releases the rest to a low-temperature sink.
All engines—from car engines to power plants—follow this basic idea.
2. Basic Components of a Heat Engine
Every heat engine has three essential parts:
2.1. 1. Hot Reservoir (Source)
A source at high temperature that supplies heat to the engine.
Example: Burning fuel in a car engine or steam from a boiler.
2.2. 2. Working Substance
The material inside the engine that absorbs heat and does work. This is usually a gas or steam.
Example: Air–fuel mixture in engines, steam in turbines.
2.3. 3. Cold Reservoir (Sink)
A sink at low temperature where leftover heat is released.
Example: Atmosphere, cooling water, or a condenser.
3. How a Heat Engine Works
A heat engine absorbs heat \( Q_h \) from the high-temperature source. Part of this heat is converted into work \( W \). The rest, \( Q_c \), is rejected to the low-temperature sink.
3.1. Energy Flow
\( Q_h = W + Q_c \)
This is simply the First Law applied to the engine cycle.
3.2. Why Heat Must Be Rejected
The Second Law of Thermodynamics says no engine can convert all heat into work. Some heat must always be released to the surroundings.
4. Efficiency of a Heat Engine
Efficiency tells how much of the input heat is converted into useful work.
\( \eta = \dfrac{W}{Q_h} = 1 - \dfrac{Q_c}{Q_h} \)
Higher efficiency means less waste heat and more useful output.
4.1. Factors Affecting Efficiency
- Temperature difference between source and sink
- Energy losses due to friction
- Heat lost to surroundings
- Design of the working substance cycle
5. Thermodynamic Cycles in Heat Engines
Heat engines operate through cycles—sequences of processes that repeat again and again. Each cycle represents expansion, compression, heating, and cooling steps.
5.1. Common Engine Cycles
- Otto cycle – used in petrol engines
- Diesel cycle – used in diesel engines
- Rankine cycle – used in steam power plants
5.2. Role of P–V Diagrams
P–V loops help calculate the work output—equal to the area enclosed by the cycle.
6. Examples of Heat Engines in Real Life
- Car engines: Fuel burns, creating high-pressure gases that push pistons.
- Steam turbines: Steam expands and spins turbine blades to generate electricity.
- Jet engines: Hot gases expand rapidly, providing thrust.
- Power plant engines: Heat from coal, gas, or nuclear fuel turns water into steam.
7. Why No Heat Engine Is 100% Efficient
The Second Law of Thermodynamics requires some heat to be rejected to a colder sink. This makes perfect efficiency impossible.
Energy losses, friction, turbulence, and heat leaks also reduce real efficiency.
7.1. Practical Insight
Even the most advanced engines convert only a fraction of heat into work. The rest becomes waste heat.