An engine works by burning fuel and air in cylinders, which pushes pistons that turn the crankshaft and power the wheels.
Four strokes
- Intake: the piston moves down and draws air and fuel in.
- Compression: the piston moves up and squeezes the mixture.
- Power: the spark ignites it and pushes the piston down.
- Exhaust: gases leave through the exhaust valve.
Parts
Cylinders, pistons, crankshaft, valves and spark plugs.
Diesel
Uses compression to ignite fuel, with no spark plugs.
Efficiency
Gasoline engines convert about 20% to 30% of fuel energy into motion.
Cylinder and piston
In a typical gasoline engine, each cylinder holds a piston that moves up and down. The piston connects to a crankshaft via a connecting rod, turning up and down motion into rotation.
The four strokes in detail
- Intake: the intake valve opens and the piston moves down, drawing in air and fuel.
- Compression: valves close, and the piston compresses the mixture to about 1/10 of its volume (a compression ratio of about 10:1).
- Power: the spark plug ignites the mixture, and expanding gases push the piston down.
- Exhaust: the exhaust valve opens and the piston pushes out burned gases.
It takes two crankshaft revolutions to complete the cycle.
Valves and timing
A camshaft opens and closes valves in sequence, driven by a timing belt or chain from the crankshaft.
Fuel delivery and ignition
Modern engines use fuel injection, controlled by the engine computer (ECU), which adjusts fuel based on sensors. Spark plugs fire thousands of times per minute.
Cooling and lubrication
Coolant circulates through the engine and radiator, and oil lubricates moving parts. Without them, the engine can overheat or seize.
Engine layouts
Inline, V and flat (boxer) arrangements with 3 to 12 cylinders, or more.
Efficiency
Gasoline engines convert about 20% to 35% of fuel energy into motion, with the rest lost as heat. Diesel engines reach 35% to 45% because of higher compression, which is why they are common in trucks. Electric motors are about 85% to 90% efficient.
Two stroke engines
They complete a cycle in one crankshaft revolution and are used in chainsaws, small outboards and some motorcycles.
The four stroke cycle
Most car engines are internal combustion engines that convert fuel’s chemical energy into motion through the four stroke cycle: intake, compression, power and exhaust. On intake, the piston moves down and draws in air and fuel. On compression, the piston rises and squeezes the mixture, with typical compression ratios of 8:1 to 12:1 for gasoline engines and 14:1 to 25:1 for diesels. A spark plug ignites the mixture in a gasoline engine, and the burning gas pushes the piston down in the power stroke. The exhaust stroke pushes burnt gases out. The crankshaft converts the piston’s up and down movement into rotation.
Supporting systems and efficiency
Valves, controlled by camshafts and timing belts or chains, open and close at precise moments. The cooling system keeps coolant temperature around 195 to 220 F, and oil lubricates parts and reduces wear. Typical gasoline engines convert about 25 to 30 percent of fuel energy into useful work, while diesel engines reach 35 to 45 percent, according to the Department of Energy. Engines are arranged in layouts such as inline four, V6 and V8. Turbochargers add boost, and direct injection improves efficiency.
Based on data from the US Department of Energy and automotive engineering references
