The combustion engine — how it all works

A combustion engine does one single thing: it turns an explosion into rotation. Everything else is the price you pay to make that last.

The principle, in one picture

A burning gas expands. Trap that expansion in a tube closed at one end and movable at the other, and you get a push. Connect the movable part to a crank, and the push becomes rotation. That's all. A car engine, a chainsaw or a cargo ship does nothing else.

The tube is the cylinder. The movable part is the piston. The crank is the crankshaft, and the rod connecting them is the connecting rod.

The four strokes

The cycle used in almost every car engine takes four strokes of the piston, which is two turns of the crankshaft.

1. Intake. The piston goes down, a valve opens, and the mixture of air and fuel is drawn in.
2. Compression. The valves close, the piston goes up and squeezes the mixture. Compressing heats it and packs the molecules together: the combustion will be faster and more violent.
3. Combustion. The mixture ignites. The gas pushes the piston down hard. It's the only stroke that produces work. The other three consume it.
4. Exhaust. The piston goes up, the other valve opens, and the burned gases are pushed out.

A single-cylinder engine would therefore only produce force a quarter of the time, and would vibrate unbearably.

That's the only reason multi-cylinder engines exist: to stagger the cycles so that one piston is always pushing. A four-cylinder fires every half turn. A six, every third of a turn. The more cylinders, the smoother the rotation — and the more parts to maintain.

What ignites the mixture

This is the only fundamental difference between the two big families.

Petrol engines use a spark. A spark plug produces an electric arc at a precisely chosen moment. The mixture must therefore be accurate, and above all must not ignite on its own under compression — hence a moderate compression ratio, and hence the octane rating, which measures a fuel's resistance to self-ignition.

Diesel engines have no spark plugs. They compress air alone, much harder, until that air reaches several hundred degrees from compression alone. Then they inject the diesel fuel into it, which ignites on contact. The consequences cascade: no ignition system to maintain, better efficiency because compression is higher, much more torque at low revs, heavier construction because the pressures are enormous, and zero tolerance for water and dirt in the fuel, because the injection works at pressures where the tiniest particle scratches everything.

The two-stroke

It does the same thing in two strokes instead of four: intake and compression on the way up, combustion and exhaust on the way down. One explosion every turn instead of every other turn.

It has no valves, no camshaft and no separate oil circuit — the oil is mixed into the fuel and burns with it. The result: very few moving parts, an excellent power-to-weight ratio, and exceptional repairability with few tools. In exchange, it uses more fuel, it smokes, and it releases some unburned oil, which is why it disappeared from cars for emissions reasons. When you have to repair things yourself with whatever you find, that simplicity becomes a decisive argument again.

The ancillary parts, and why each one exists

The timing. The camshaft opens and closes the valves at the right moment, driven by the crankshaft through a belt or a chain. If that link breaks or slips a tooth, the valves open while the piston is coming up: they collide, and the engine is destroyed in a fraction of a second. It's the most destructive failure there is on a four-stroke.

The oil circuit. A pump sends oil under pressure through channels to the crankshaft bearings. Oil doesn't just reduce friction: it forms a film that keeps metal from touching metal, carries away some of the heat, and takes wear particles to the filter. An engine without oil pressure destroys itself in a few tens of seconds.

The cooling. About a third of the fuel's energy turns into pure heat. A circuit of water and antifreeze carries it from the block to the radiator, where the air takes it away. The thermostat keeps the temperature in a narrow range, because a cold engine wears fast and uses too much fuel, while an overheated engine warps its cylinder head.

The air supply. The engine is an air pump: its power is directly tied to how much air it swallows. On the water, this constraint changes completely, see Boat engines — the complete guide. That's the whole point of the turbocharger, which uses the energy of the exhaust gases to compress the intake air and fit more mixture into the same cylinder.

The flywheel. A heavy disc on the crankshaft that stores the energy of the power stroke and gives it back during the other three. Without it, the engine would stop between two combustions.

The starter and the alternator. An engine can't start itself: it has to be turned from outside to begin the first cycle. The alternator then recharges the battery and powers everything else — it's the entry point for any electrical salvage, covered in Electrical energy in everyday objects. When the starter turns but the engine won't catch, the diagnosis is done in order: see An engine that won't start.

Identifying an unknown engine

In front of an unknown engine, the order of identification is always the same.

1. Look for spark plugs. Present: petrol. Absent: diesel. It's the first question because it determines everything else.
2. Look for the oil dipstick. Present: four-stroke. Absent, with fuel mixed with oil: two-stroke.
3. Count the cylinders by following the spark plug leads or the injectors.
4. Find the timing belt or chain and check its condition. A cracked or oily belt is a ticking time bomb.
5. Check the fluid levels before trying anything. Oil and coolant. Milky oil means water in the oil, and therefore a blown head gasket.
6. Note the plate or the numbers stamped on the block. That's what will let you find a compatible part on another engine.

Comparing engine types

You want what will last. An old naturally aspirated diesel, with mechanical injection and no electronics, is the closest thing to an eternal engine: few sensors, little to understand, and high mechanical tolerance.

You have to drive the vehicle. Controls, clutch and driving are in La conduite automobile; flying is in Comment vole un avion.

You want what can be fixed with nothing. A two-stroke. A spark plug, a carburettor, a gasket, and you've covered most of the possible failures.

You've run out of refined fuel. A diesel accepts filtered vegetable oils, within certain limits of viscosity and temperature, with some adaptations. Petrol engines accept almost nothing but petrol. It's a decisive criterion for the long term.

You're cannibalising. The parts that are easiest to reuse from one engine to another are the alternator, the starter, the water pump and the hoses. The block and the cylinder head can only be swapped between identical models.