Explained
How a car engine works.
The four-stroke cycle
A petrol car engine is a machine for turning chemical energy into rotation. It does this in a repeating four-step pattern called the four-stroke cycle, first made practical by Nicolaus Otto in 1876. Each cylinder runs the cycle on its own, and the cylinders are timed so that their power strokes are spread evenly around the crankshaft's rotation.
On the intake stroke the piston moves down and the intake valve opens, so the falling pressure draws in air mixed with a fine mist of fuel. On the compression stroke both valves are closed and the piston rises, squeezing the mixture into a small space at the top of the cylinder. Just before the piston reaches the top, the spark plug fires. The burning mixture expands rapidly and drives the piston down on the power stroke, which is the only stroke that produces work. Finally, on the exhaust stroke, the exhaust valve opens and the rising piston pushes the burnt gas out towards the exhaust pipe.
From explosions to rotation
Each piston is joined to the crankshaft by a connecting rod. The crankshaft has offset journals called crank pins, so as the pistons go up and down the shaft turns, much like your legs turning the cranks of a bicycle. In an inline-four engine, pistons one and four move together, and so do two and three, which keeps the engine well balanced. Because the cycle takes two revolutions and there are four cylinders, one cylinder fires every half-turn. A heavy flywheel stores energy between those pulses so the output feels smooth.
The valves are opened by camshafts, which are driven from the crankshaft by a toothed belt or a chain. Each valve needs to open only once per cycle, so the camshafts turn at exactly half the crankshaft's speed. Springs close the valves again, and the shape of each cam lobe decides how far and for how long a valve opens.
Why the firing order matters
The order in which the cylinders fire is chosen to share out the forces along the crankshaft. The model above uses 1-3-4-2, the usual order for inline-four engines. Slow it down in Run mode and you'll see the flash move between cylinders while the valves above each one open and close in step. Real engines idle at several hundred revolutions per minute, so this all happens far too quickly to see without slowing it down.
Parts glossary
- Engine block
- The cast iron or aluminium block contains the cylinders and the passages for coolant and oil. In this cutaway the front wall is removed so you can watch the pistons move.
- Cylinder head
- The head bolts onto the top of the block and seals each cylinder. It holds the valves, the spark plugs and the camshafts, and forms the roof of each combustion chamber.
- Pistons and connecting rods
- Burning fuel pushes each piston down on its power stroke. The connecting rod turns that straight-line push into rotation at the crankshaft.
- Crankshaft
- Offset crank pins turn the up-and-down motion of the pistons into rotation. A heavy flywheel on the end smooths out the pulses between firing strokes. Fun fact: In a four-stroke, four-cylinder engine one cylinder fires every half-turn of the crankshaft.
- Camshafts
- Egg-shaped lobes on the camshafts push the valves open at exactly the right moment. A belt or chain drives them from the crankshaft. Fun fact: In a four-stroke engine the camshaft turns at exactly half the speed of the crankshaft, because each valve opens once every two revolutions.
- Valves
- Intake valves let the air-fuel mixture in and exhaust valves let burnt gas out. Springs snap them shut again once the cam lobe has passed.
- Spark plugs
- Near the top of the compression stroke a high-voltage pulse jumps the plug's gap and ignites the squeezed mixture. Watch for the flash as each cylinder fires. Fun fact: This engine fires in the order 1-3-4-2, the most common firing order for inline-four engines.
