This engine burns a little fuel to push a piston, then turns that push into spin. Use the buttons to look inside it and follow one piston through the four strokes of a full cycle.
A four-stroke engine is a machine that turns burning fuel into rotation. A piston slides up and down inside a cylinder, and the crankshaft below it turns that up-and-down motion into a spin. It takes four separate piston movements — called strokes — to complete one cycle: intake, compression, power and exhaust.
The name counts those movements. During intake the piston moves down and pulls in fresh air and fuel. During compression it moves up and squeezes that mixture into a small space. During the power stroke a spark lights the mixture, the hot gas expands, and the piston is driven down hard. During exhaust the piston moves up again and pushes the burnt gas out. Then the whole cycle repeats, many times every second.
From the outside a running engine is just a humming lump of metal. Everything interesting happens inside, so hiding the outer block shows you which part moves in which stroke and what each one actually does.
Two ideas hold the whole cycle together. The first is the valvetrain: the intake and exhaust valves must open at exactly the right moment, which the rotating camshaft handles for you. The second is the fact that only one stroke in four produces power, so a heavy flywheel stores momentum during that stroke and gives it back during the other three. Together they turn four jerky movements into a smooth, steady spin.
The crankshaft pulls the piston down the cylinder. As it drops, the intake valve opens and the falling piston acts like a syringe: it draws fresh air and fuel in through the intake port. At the bottom of the stroke the intake valve closes and the cylinder is sealed with a fresh charge trapped inside.
Now the crankshaft carries the piston back up. Both valves are shut, so nothing can escape, and the rising piston squeezes the air and fuel into a much smaller space in the combustion chamber. Squeezing does two things: it makes the mixture burn faster and more completely, and it makes the explosion more forceful.
Right at the top of the compression stroke, the spark plug fires and ignites the mixture. The burning gas becomes very hot and expands rapidly, pushing the piston down with real force. This is the only stroke that produces power, and it is the one that actually drives the car. The connecting rod passes that push to the crankshaft, which turns it into rotation.
As the piston reaches the bottom, the exhaust valve opens and the crankshaft pushes the piston back up one last time. The rising piston sweeps the burnt gas out through the exhaust port and away down the exhaust pipe. With the cylinder now empty, the intake valve is ready to open again and the whole four-stroke cycle starts over.
The valves cannot open and close at random — they must follow the piston exactly. A camshaft, turning at half the speed of the crankshaft, uses egg-shaped lobes to press each valve open at the correct moment. Meanwhile the heavy flywheel bolted to the crankshaft keeps spinning through the three strokes that make no power, so the engine runs smoothly instead of lurching once per cycle.
It counts the piston movements needed to complete one cycle: intake, compression, power and exhaust. The crankshaft turns twice for every four strokes, and the whole cycle then repeats.
A gasoline engine mixes air with fuel, and that mixture needs a spark to start burning. The spark plug fires once per cycle, at the end of the compression stroke. A diesel engine has no spark plug because squeezing the air alone makes it hot enough to ignite the fuel.
Any number. The piston movements are the same in every cylinder; extra cylinders simply overlap their power strokes so the shaft is driven more often and the engine runs more smoothly.
Only the power stroke pushes the piston. The connecting rod passes that push to the crankshaft, which turns the up-and-down motion into rotation. A heavy flywheel stores momentum on the power stroke and gives it back on the other three, so the spin keeps going.
The camshaft turns with the engine and its lobes push the intake and exhaust valves open at set moments, so fresh air and fuel come in and burned gases go out at the right time. The timing between the valves and the piston is fixed by the camshaft, and getting it right is what keeps the cycle running smoothly.
A gasoline engine draws in a mix of air and fuel and ignites it with a spark plug. A diesel engine draws in air only, squeezes it until it is hot, then injects fuel that ignites on its own. Diesels run at higher compression because they rely on that heat instead of a spark.
A turbocharger uses the exhaust gas to spin a turbine that compresses the incoming air, pushing more air into the cylinder. With more air, more fuel can be burned, so the engine makes more power.