Heat Engines
Machines that turn heat into work, and how well they do it.
Lesson at a glanceShow the key points
- A heat engine takes Q₁ from hot bodies, does work W, rejects Q₂ to cold bodies, working in cycles: W = Q₁ − Q₂.
- Efficiency η = W/Q₁ = 1 − Q₂/Q₁ (26.4).
- Steam engine: steam chest and D-valve (driven by an eccentric) alternately admit steam above and below the piston; crank and flywheel; 3–10% efficient.
- Petrol (Otto) engine: charging, compression (~500°C), working (spark; ~2000°C, 15 atm), exhaust.
- Diesel: compresses air to ~1000°C and 36 atm; injected fuel self-ignites; no spark plug.
Work turns into internal energy easily: just rub. Getting work back out of internal energy needs a machine: a heat engine.
What a heat engine does
A heat engine takes heat Q₁ from hot bodies, turns part of it into work W, and rejects the rest, Q₂, to colder bodies. Its working substance returns to its starting state each time: it runs in a cyclic process. Since ΔU = 0 over a cycle, the first law gives W = Q₁ − Q₂.
An engine takes in 800 J per cycle and rejects 600 J. Find its work output and efficiency.
The steam engine
A sturdy main cylinder holds a tight-fitting piston, whose rod leaves the cylinder. Beside it is the steam chest, sharing a wall pierced by three holes: the upper and lower holes open into the two ends of the cylinder, the middle one leads to a condenser. A box-shaped sliding valve (the D-valve, from its shape) always covers the middle hole and one of the other two.
The piston rod drives a crank and flywheel through a crank-rod. An eccentric disc on the same shaft drives the valve rod, so the valve moves up as the piston moves down. Steam from the boiler enters above the piston and drives it down while used steam below escapes through the lower and middle holes to the condenser; then the valve shifts and steam drives the piston up. Burning fuel to make steam, the whole machine typically delivers only 3–10% of the fuel’s energy as work.
Internal combustion engines
These burn their fuel inside the cylinder itself, with no separate furnace.
Petrol (Otto) engine, 1876
Petrol and air are mixed in the carburettor. Two valves (inlet and outlet) and a spark plug sit at the top of the cylinder. Four strokes:
- Charging: inlet open, piston moves down, drawing in the petrol–air mixture.
- Compression: both valves shut, piston moves up, squeezing the mixture (it warms to about 500°C).
- Working: a spark ignites it; the gas reaches about 2000°C and 15 atm and drives the piston down, turning the crankshaft.
- Exhaust: outlet open, piston moves up, pushing out the burnt gases.
Diesel engine
No spark plug. Air alone is drawn in and compressed so strongly that it reaches about 1000°C and 36 atm. Diesel is then injected and ignites by itself, driving the working stroke.
- 1A heat engine runs in cycles: takes Q₁ from hot bodies, does work W = Q₁ − Q₂, rejects Q₂ to cold bodies.
- 2Efficiency η = W/Q₁ = 1 − Q₂/Q₁.
- 3Steam engine: steam chest, D-valve driven by an eccentric, crank and flywheel; 3–10% efficient.
- 4Petrol engine: charging, compression, working (spark), exhaust. Diesel: compression ignition, no spark plug.
Check yourself
Keep exploring
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