A four stroke engine converts the chemical energy in fuel into mechanical motion that can turn a vehicle's wheels. The main moving part is a piston sliding inside a cylinder, connected by a rod to a crankshaft. Each cylinder repeats four strokes called intake, compression, power, and exhaust.
Understanding these steps helps explain why timing, fuel mixture, spark, and engine speed all matter.
Key Facts
- The four strokes are intake, compression, power, and exhaust.
- One complete four stroke cycle takes 2 crankshaft revolutions, or 720 degrees of crankshaft rotation.
- During the intake stroke, the intake valve opens and the piston moves down to draw in air and fuel.
- During the compression stroke, both valves close and the piston moves up to squeeze the mixture.
- During the power stroke, the spark plug ignites the mixture and expanding gases push the piston down.
- Engine displacement for one cylinder is V = πr^2h, where r is cylinder radius and h is piston stroke length.
Vocabulary
- Piston
- A sliding metal part inside the cylinder that moves up and down as gases expand and compress.
- Crankshaft
- A rotating shaft that changes the piston's back and forth motion into rotational motion.
- Connecting rod
- The rigid link that joins the piston to the crankshaft.
- Valve
- A movable opening that controls the flow of gases into or out of the cylinder.
- Spark plug
- An electrical device that creates a spark to ignite the compressed air fuel mixture in a gasoline engine.
Common Mistakes to Avoid
- Thinking the power stroke happens every crankshaft revolution. In a four stroke engine, each cylinder has one power stroke every two crankshaft revolutions.
- Opening both valves during compression. Both valves must be closed so the air fuel mixture can be squeezed before ignition.
- Confusing piston motion with crankshaft rotation. The piston moves linearly up and down, while the crankshaft rotates.
- Assuming the spark plug fires during the intake stroke. The spark occurs near the end of compression so the burning gases can push the piston down during the power stroke.
Practice Questions
- 1 A single cylinder has a bore diameter of 8.0 cm and a stroke length of 7.5 cm. Using V = πr^2h, find the displacement of the cylinder in cm^3.
- 2 An engine is running at 3000 revolutions per minute. In a four stroke engine, how many complete cycles does one cylinder complete per minute?
- 3 Explain why the intake and exhaust valves must open at different times during the four stroke cycle.