Inside the cylinder

A car engine has one job: turn the heat of burning fuel into the rotation of a shaft. Almost every petrol and diesel car does it with the same trick, repeated in several cylinders: a piston that slides up and down in a tube, and a mechanism that converts that straight-line motion into spinning.

The key parts of one cylinder, from top to bottom:

• Cylinder and cylinder head. The tube the piston slides in, closed at the top by the head. The space between the piston and the head is the combustion chamber.
• Valves. Two or more small doors in the head: intake valves let fresh air (and, in many engines, fuel) in; exhaust valves let burnt gas out. A rotating camshaft pushes them open at the right moments.
• Spark plug (petrol engines). It makes a spark that lights the fuel and air.
• Piston. The plunger. Rings around it seal the gap to the cylinder wall.
• Connecting rod. A strong rod that links the piston to the crankshaft. It swings as it moves.
• Crankshaft. A shaft with an offset section, the crank throw. As the piston pushes the rod, the rod pushes the throw round in a circle, like a foot turning a bicycle pedal.
• Flywheel. A heavy disc on the end of the crankshaft that stores spinning energy.

The stroke is the distance the piston travels between its highest point, top dead centre (TDC), and its lowest point, bottom dead centre (BDC). The crank throw sits a distance r from the shaft centre, so the piston moves up by r and down by r in one half turn: stroke = 2r.

The piston height follows from the geometry of the crank and rod. With crank angle θ measured from TDC and a rod of length l, the distance from the shaft centre to the piston pin is x(θ) = r cos θ + √(l² − r² sin² θ). You do not need to calculate with it, but notice what it says: every position of the piston is fixed by the crank angle. Turn the crank and the whole engine moves; nothing can be out of step.

The piston does not rotate. It slides straight; the rod swings from side to side; only the crankshaft spins. And the piston actually stops for an instant at the top and at the bottom, where it reverses direction, and moves fastest around the middle of each stroke.

Two sizes describe an engine. The swept volume (displacement) of one cylinder is the area of the piston times the stroke: V = (π/4) × bore² × stroke. For a bore and stroke of 86 mm each, V = (π/4) × 0.086² × 0.086 ≈ 0.0005 m³ = 0.5 litre, so four of those cylinders make a 2.0-litre engine.

The compression ratio compares the biggest and smallest volumes the gas ever has: CR = (V_swept + V_clearance) / V_clearance, where the clearance volume is what is left above the piston at TDC. If one cylinder sweeps 500 cm³ and 50 cm³ remains at the top, CR = 550 / 50 = 11 : 1.

How fast does the piston move? Its mean speed is 2 × stroke × (revolutions per second). At 6 000 rpm (100 rev/s) with an 86 mm stroke that is 2 × 0.086 × 100 ≈ 17 m/s, about 62 km/h, going up and down hundreds of times a second. That is one reason there is a limit to how fast engines can safely spin.