What is a heat engine?
A heat engine is a machine that turns heat into mechanical work (movement).
Every heat engine has three parts:
- a hot source – burning fuel gives heat;
- a working substance – a gas or steam that expands when hot;
- a cold sink – the air or water that takes away the leftover heat.
The energy chain is: chemical energy of fuel → heat (internal energy of gas) → kinetic energy of piston or turbine.
Two families
- External combustion: fuel burns outside the cylinder, e.g. a steam engine or a steam turbine in a power station.
- Internal combustion: fuel burns inside the cylinder, e.g. petrol and diesel engines, jet engines and gas turbines.
The four-stroke engine
A car or motorbike engine has a cylinder, a sliding piston, two valves (inlet and exhaust), a spark plug and a crankshaft that turns up-and-down motion into rotation.
- Intake: the inlet valve opens and the piston moves down, sucking in air and fuel.
- Compression: both valves close and the piston moves up, squeezing the mixture.
- Power: the spark plug fires, the fuel burns, hot gas pushes the piston down. This is the only stroke that does work.
- Exhaust: the exhaust valve opens and the piston moves up, pushing burnt gases out.
Four strokes take two turns of the crankshaft. A heavy flywheel keeps the shaft turning through the three strokes that do no work. A diesel engine has no spark plug: it squeezes air so much that it gets hot enough to light the fuel.
Heat value of fuels
The heat value (also called specific heat of combustion or calorific value) q is the heat given out when 1 kg of fuel burns completely. Unit: J/kg (often MJ/kg).
Q = q × m
| Fuel | q (about, MJ/kg) |
|---|---|
| Wood | 15 |
| Coal | 30 |
| Petrol / diesel | 46 |
| Methane (natural gas, biogas) | 50 |
| Hydrogen | 140 |
A good fuel has a high heat value, burns easily but safely, is cheap, easy to store and leaves little smoke or ash.
Efficiency of a heat engine
No heat engine turns all its heat into work. Some heat always goes out with the exhaust gas, into the cooling water and through friction.
η = W ÷ Q × 100%, where W is useful work and Q is heat from the fuel.
- Petrol car engine: about 25–30%
- Diesel engine: about 35–45%
- Steam turbine in a power station: about 35–40%
- Large gas turbine with steam stage (combined cycle): about 55–60%
Efficiency is always less than 100% because the engine must dump some heat into the cold sink to keep working. Hotter burning and better cooling raise efficiency.
Heat engines and the environment
Burning fossil fuels releases:
- carbon dioxide (CO₂) – a greenhouse gas that warms the planet;
- carbon monoxide (CO) – a poisonous gas from incomplete burning;
- nitrogen oxides and sulfur dioxide – cause smog and acid rain;
- soot – tiny particles that harm the lungs.
Waste heat also warms rivers and cities. When hot water is piped to homes (district heating), heat also leaks from the pipes on the way, so pipes are wrapped in insulation.
How we reduce the harm
- Better efficiency: more work from less fuel.
- Catalytic converters and filters on exhaust pipes.
- Cleaner fuels: CNG, biogas, hydrogen; electric vehicles charged from solar and wind.
- Using waste heat (combined heat and power plants).
Try it: feel the waste heat
Ask an adult to start a scooter or a generator for one minute. Hold your hand near (not on) the engine and the exhaust. You can feel the heat leaving – that is energy which did not become motion. Now in the 3D, set efficiency to 30% and count the red cubes: 7 out of 10.
Key formulas and definitions
- Heat from fuel: Q = q × m
- Efficiency: η = W ÷ Q × 100%
- Useful work: W = η × Q
- Waste heat: Q_waste = Q − W
- 1 MJ = 1 000 000 J
Worked examples
1. How much heat is released when 2 kg of coal (q = 30 MJ/kg) burns completely?
Q = q × m = 30 MJ/kg × 2 kg = 60 MJ.
2. An engine gets 200 MJ of heat and does 50 MJ of useful work. Find its efficiency.
η = W ÷ Q × 100% = 50 ÷ 200 × 100% = 25%.
3. A diesel engine (η = 40%) burns 5 kg of diesel (q = 46 MJ/kg). How much useful work does it do?
Q = 46 × 5 = 230 MJ. W = 0.40 × 230 = 92 MJ.
4. In Example 3, how much heat is wasted?
Waste heat = Q − W = 230 − 92 = 138 MJ.
5. A car engine must do 69 MJ of work and is 30% efficient. What mass of petrol (q = 46 MJ/kg) is needed?
Q = W ÷ η = 69 ÷ 0.30 = 230 MJ. m = Q ÷ q = 230 ÷ 46 = 5 kg.
6. Which is better for 92 MJ of heat: 2 kg of petrol or 6 kg of wood (q = 15 MJ/kg)?
Petrol: 2 × 46 = 92 MJ. Wood: 6 × 15 = 90 MJ. Almost the same heat, but petrol needs 3 times less mass, so it is easier to carry. That is why vehicles use liquid fuels.
Common mistakes
- Thinking an engine can be 100% efficient. Some heat must always be thrown away to the surroundings.
- Forgetting to convert MJ to J or g to kg before using Q = q × m.
- Saying all four strokes push the car. Only the power stroke does work; the flywheel carries the other three.
- Mixing up heat value (energy per kg of fuel) with specific heat capacity (energy to warm 1 kg by 1 °C).