How Does a Car Engine Work?


Internal Combustion Engine

  • A car engine is an internal combustion gasoline engine, which works on a four-stroke system: intake, compression, power, and exhaust. These four strokes are driven by the pistons, which slide up and down inside each of the four, six, eight, or twelve cylinders, depending on engine size. Each cylinder has its own intake valve, exhaust valve, spark plug, and piston. Fuel comes in from the gas tank through the fuel system, where it mixes with air and is atomized. This air/fuel mixture is what feeds into the cylinders and drives them.


  • The intake stroke begins when the intake valve of a cylinder opens, letting the air/fuel mixture inside. The piston slides downward inside the cylinder. This downward motion creates a large vacuum, which helps suck in the air/fuel mixture. (See diagram.)


  • Both valves are closed for the compression stroke. The piston moves upwards, compressing the air/fuel mixture up towards the spark plug. This compression makes the fuel more volatile for easier ignition.


  • Both valves remain closed for the power stroke. The spark plug fires, igniting the air/fuel mixture. The resulting explosion dramatically forces the piston downward.


  • The exhaust stroke begins when the exhaust valve opens. The piston is moving upwards again, forcing the remaining byproducts of the burned air/fuel mixture out to the exhaust system.

Pistons and Crankshaft

  • The cylinder pistons are attached by the piston pin and connecting rod to the crankshaft. The crankshaft is not straight, but has high and low areas that help drive alternating pistons up and down. When the power stroke forces two of the pistons in a four cylinder engine down, it pushes on the crankshaft, which turns and simultaneously pushes up on the other two cylinders' pistons. The system is in constant motion, half the pistons sliding up while the other half are being driven down.

Camshaft and Distributor

  • The intake and exhaust valves are driven similarly by the camshaft. This camshaft is also connected by interlocking gears to the distributor. The spark plugs are attached to the distributor with plug wires. When the cam shaft turns, opening and closing the cylinder valves, it also activates the distributor, which powers the spark plugs at the right point in the cycle.

Timing Chain

  • The crankshaft and camshaft are connected at the front of the engine by the timing chain. This timing chain must be properly aligned in order for the system to work in continuous harmony, with perfect timing.


  • The flywheel is bolted to the back of the crankshaft. When you turn the key in the ignition, the starter engages the gear on the flywheel. This starts the whole motion of the engine. Once the cylinders ignite, however, the engine continuously drives itself with the four-stroke system.

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  • Photo Credit Four-stroke system diagram c2008 by Valerie David.
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