A heat engine takes heat from something hot, turns part of it into work, and must throw the rest away to something cooler; the larger the temperature difference, the larger the share it can turn into work. A real petrol engine turns only about a quarter of its fuel’s energy into shaft work, and a refrigerator is the same machine run backwards, spending work to move heat from cold to hot.
Part of Internal Combustion
The thermodynamic principles behind every engine: heat in, work out, heat rejected, and the limit on how much of the first can become the second.
Heat Can Only Become Work If Some Of It Is Rejected
An engine works between two temperatures. It takes heat from a hot source, which in an internal combustion engine is the burning mixture. It converts some of that heat to work. The rest has to go to a colder place, which is the exhaust gas and the cooling system. Nothing in nature lets all of the heat become work; the part rejected is not a flaw of poor machines but the price of running at all.
Efficiency is the work out divided by the heat in. The best any engine could do between a hot temperature Th and a cold temperature Tc, both in kelvin, is 1 − Tc ÷ Th. This is the Carnot limit, a standard thermodynamic result recalled here and not derived. It says two practical things: raising the hot temperature helps, and lowering the cold one helps. A real engine, with finite speeds, heat leaking through walls and friction, falls well below it.
A Refrigerator Is An Engine Run Backwards
Reverse the direction round the cycle and the machine is a refrigerator or heat pump: it is fed work and moves heat from cold to hot. The same fluid, cylinder, piston and valves appear in both, which is why a refrigeration compressor and an engine cylinder look alike. The refrigeration tree sets out the other half of this idea in The Refrigeration Cycle and the temperature-lift arithmetic in Compression. A reader who understands the bookkeeping of one can reason about the other: for the engine, heat in equals work out plus heat rejected; for the refrigerator, heat rejected equals heat absorbed plus work in.
The Four-Stroke Cycle Is The Working Pattern
The carrier describes the four strokes: intake draws in the mixture, compression squeezes it to between a tenth and an eighth of its volume, the power stroke follows ignition and is the only stroke that makes work, and exhaust pushes the burned gas out. The crankshaft turns twice (720 degrees) per cycle, and the flywheel carries the engine through the three strokes that make no power. Otto Cycle and Diesel Cycle treat the spark and compression-ignition forms, and 2 vs 4 Stroke compares them with the two-stroke pattern.
An internal combustion engine burns its fuel inside the cylinder, which is why it is compact and starts in seconds, as the carrier says. A steam engine, described in Steam Engine, burns its fuel outside and uses the steam as the working fluid; it is heavier and slower to start but tolerant of cheap fuel.
Worked Example: From The Limit Down To A Real Engine
Assume a hot temperature of 1,000 K and a cold temperature of 300 K, round numbers chosen to show the steps.
- Carnot limit: 1 − 300 ÷ 1,000 = 0.70, which is 70%.
- Ideal Otto cycle at a compression ratio of 8, with the exponent 1.35 the corpus uses in Engine Modifications: 1 − 8^(1 − 1.35) = 1 − 8^(−0.35) = 1 − 0.483 = 0.517, which is 51.7%.
- Real petrol engine: the carrier takes about 25% when it works out the wartime wood-gas figures.
Each step down loses something real. The Otto figure is below the Carnot figure because heat is added and rejected over a range of temperatures and not at the two extremes. The real engine is below the Otto figure because of friction, heat lost through the walls, incomplete burning and the time the burn takes. The 25% is the carrier’s, and the fraction a rebuilt engine reaches must be measured.
Common Mistakes
- Thinking the lost heat can be recovered by a better design. Some can be, by improving the cycle; most is rejected by necessity.
- Ignoring the cooling system as part of the cycle. It is where the engine rejects its heat.
- Quoting efficiency for the fuel without saying which engine. The figure belongs to the machine.
Safety
An engine turns most of its fuel into heat, flame and exhaust gas
The exhaust is hot and contains carbon monoxide, which is colourless and odourless; run engines outdoors with the exhaust clear of people and buildings. Fuel vapour burns, and a hot engine lights it. Guard every belt, flywheel and shaft. The rest of this tree assumes these are in place.
What’s Next
- Learn next: Otto Cycle — the spark-ignition cycle in detail
- Combine with: The Refrigeration Cycle — the same machine, run the other way