Automata
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  1. 1 intake
  2. 2 intake
  3. 3 intake
  4. 4 intake

Four-stroke engine

An inline four of 71 parts. The crankshaft, rods, pistons, belt, camshaft and eight valves move by the laws of mechanics — each part on its own, held to its neighbours by joints, teeth and springs.

Take the crank at the front and turn it — with the mouse or a finger. Drag the background to look from every side.

How it works ↓

What you are looking at

An inline four-cylinder engine, opened up and built from parts. At the bottom is the crankshaft: it is built up from webs and pins, because a straight shaft would cut through the connecting rods that swing round above it. On each of its four pins sits a connecting rod, and on top of each rod a piston in a clear cylinder. The flywheel, a pulley and the crank are at the front.

The crankshaft's pulley drives, by a toothed belt, a pulley twice its size on the camshaft. The camshaft carries eight cams, and under each is a valve with a roller and a spring — two valves per cylinder, intake and exhaust. The cam presses the roller and opens the valve; the spring closes it again.

Four strokes

One cylinder's cycle takes two turns of the crankshaft — four strokes of the piston:

  1. Intake. The piston goes down with the intake valve open, and the cylinder fills with fresh mixture. It is blue on the model.
  2. Compression. Both valves shut, the piston goes up and squeezes the mixture — the blue grows denser.
  3. Power. Near the top the mixture is lit and the gas pushes the piston down. This is the only stroke that turns the shaft — on the model a flame fills the cylinder and lights the parts around it.
  4. Exhaust. The exhaust valve opens, the piston goes up and pushes the burnt gas out — grey.

The strip of four cylinders in the corner shows the stroke each one is on.

Why the camshaft turns at half speed

Every valve opens once per cycle, and a cycle lasts two turns of the crankshaft. So the cam needs one turn for every two of the shaft — which is why the camshaft's pulley is twice as large. The model doesn't depict this, it does it: the belt ties the two pulleys' turning together, and the ratio comes out exactly 1 : 2.

Firing order 1 – 3 – 4 – 2

The crankpins stand at 0°, 180°, 180°, 0°: the outer pistons move together and the middle ones the other way. The cylinders fire in the order 1, 3, 4, 2 — one every half turn — so the power strokes follow one another without a gap and the shaft turns evenly. You can also see that two pistons rising together are busy with different things: one is compressing, the other pushing out exhaust.

Valve timing

A valve stays open not for half a turn but longer — about 240° of the crankshaft. The intake opens 20° before top dead centre and closes 40° after bottom dead centre: the mixture keeps flowing in by its own momentum after the piston has started up again. The exhaust opens 40° before bottom dead centre and closes after the top. Around the top both valves are open for a moment — the valve overlap, which helps the cylinder clear.

How it is computed

This isn't an animation along a path laid down in advance. Every part has a mass and a moment of inertia, worked out from its shape and material. The parts are held together by hinges, sliders and springs; the belt couples the two pulleys; the cams press the valve rollers through a contact with friction. A physics engine of its own solves this whole system many times a second, and the pistons, valves and flywheel move as the joints and forces dictate.

The model is checked against formulas. The pistons follow the slider-crank curve to within a micron, the camshaft turns at exactly half speed, the valves open in the order 1 – 3 – 4 – 2 and stay open for as many degrees as their cam profile sets. At no step does any part pass through another.

The explosions are drawn, not computed: the engine is turned by its crank — by you or by the crank's drive. Let go of the crank while it runs and the flywheel keeps the shaft turning a while, until friction in the bearings brings it to rest.