What a heat engine does
A heat engine changes the internal energy of fuel (heat) into mechanical energy (movement). The steps are: burn fuel to get heat, heat a gas so it expands, let the expanding gas push a piston or turbine, then cool the gas and start again. The gas is the working substance.
Build an air-engine model
You need: a small glass or metal bottle with a rubber stopper that has one hole, a thin tube or straw, a 20 mL plastic syringe (no needle), small weights (coins or paper clips), a bowl of hot water (a candle is more fun but ask an adult).
- Push the tube through the stopper and fit the stopper tightly in the bottle.
- Join the other end of the tube to the syringe. Put the syringe upright and put a light weight on the plunger.
- Put the bottle into the hot water, or heat it gently. The air inside expands and pushes the plunger up.
- Move the bottle to cold water. The plunger comes down.
A steam engine model works the same way, but with boiling water: the steam pushes a small piston or turns a light pinwheel. It must be done only with an adult and with great care because steam is dangerous.
Safety: use only a little hot water, never seal a heated bottle fully, and keep your face away from steam.
Work and efficiency
The work done in lifting a load is W = m × g × h (g about 10 m/s²). A 50 g load lifted 2 cm gives W = 0.05 × 10 × 0.02 = 0.01 J.
Only a part of the heat in becomes work. The rest warms the bottle, the air and the table. The efficiency is η = work out ÷ heat in. Heat in = work + waste heat. No heat engine can have an efficiency of 100%.
Make it better
- Leaks: a leaky tube lets the gas out. Seal with tape or clay.
- Friction: a dry syringe sticks. Add one drop of oil or water.
- Heat loss: a bigger bottle holds more air and moves the piston further. Wrap it in cloth to lose less heat.
- Cooling: a faster cooling gives a faster return stroke.
Try it: count how many up-and-down cycles you can get in one minute, and how high the load rises. Change one thing at a time.
Key formulas and definitions
- W = m × g × h (work done in lifting a load)
- η = W ÷ Q × 100% (efficiency)
- Q = W + Q_waste (heat in = work + wasted heat)
- Heat → gas expands → piston moves → work
Worked examples
1. A 50 g load is lifted 2 cm. Find the work done (g = 10 m/s²).
m = 0.05 kg, h = 0.02 m. W = 0.05 × 10 × 0.02 = 0.01 J.
2. An engine takes 1000 J of heat and does 150 J of work. Find its efficiency.
η = 150 ÷ 1000 = 0.15 = 15%.
3. For the same engine, how much heat is wasted?
Waste = 1000 − 150 = 850 J.
4. An engine of 30% efficiency takes in 2000 J. Find the work done.
W = 0.30 × 2000 = 600 J.
5. A 200 g load is lifted 5 cm. Find the work done.
W = 0.2 × 10 × 0.05 = 0.1 J.
6. An engine takes 5000 J and wastes 4000 J. Find the efficiency.
W = 5000 − 4000 = 1000 J. η = 1000 ÷ 5000 = 20%.
Common mistakes
- Thinking all the heat becomes work. Some is always lost, so efficiency is always less than 100%.
- Sealing a heated bottle fully. Pressure can build up and pop the stopper, which is dangerous. Use a tube or syringe.
- Forgetting to cool the gas. Without cooling, the piston does not return and there is no cycle.
- Mixing grams and kilograms in W = m × g × h. Change grams to kilograms first.