An internal combustion engine changes the chemical energy in fuel into the spinning motion that turns a vehicle's wheels. The piston and crankshaft are the key parts that make this energy conversion possible. Hot expanding gases push the piston in a straight line, and the crankshaft converts that up and down motion into rotation.
Understanding this mechanism helps explain engine power, torque, vibration, and fuel use.
Inside the cylinder, a fuel and air mixture burns and creates high pressure above the piston. The piston moves downward, pushing on a connecting rod that turns an offset part of the crankshaft called the crank throw. As the crankshaft spins, it also pulls the piston back up for the next part of the cycle.
In a four stroke engine, intake, compression, power, and exhaust strokes repeat rapidly to keep the crankshaft rotating smoothly.
Key Facts
- Combustion pressure creates a force on the piston: F = P A, where P is gas pressure and A is piston area.
- The piston moves in a straight line, but the crankshaft rotates because the connecting rod pushes on an off-center crank throw.
- Torque from the crankshaft depends on force and lever arm: τ = F r sin θ.
- One complete four stroke cycle takes two crankshaft rotations, or 720 degrees.
- Engine displacement is the swept volume of all pistons: V = cylinder area × stroke × number of cylinders.
- Power is the rate of doing work and is related to torque and angular speed: P = τω.
Vocabulary
- Piston
- A sliding metal part inside the cylinder that is pushed by combustion gases and transfers force to the connecting rod.
- Crankshaft
- A rotating shaft with offset crank throws that converts piston motion into rotational motion.
- Connecting rod
- The strong link between the piston and crankshaft that transmits force while changing angle during rotation.
- Stroke
- The distance the piston travels between the top and bottom of the cylinder.
- Torque
- A turning effect produced by a force acting at a distance from an axis of rotation.
Common Mistakes to Avoid
- Thinking the piston spins inside the cylinder is wrong because the piston moves mostly up and down while the crankshaft is the part that rotates.
- Using τ = F r without considering angle is incomplete because only the perpendicular part of the force creates torque, so τ = F r sin θ.
- Assuming combustion happens on every piston stroke is wrong for a four stroke engine because the power stroke occurs once every two crankshaft rotations.
- Confusing displacement with total engine size is misleading because displacement is only the swept volume of the cylinders, not the volume of the entire engine block.
Practice Questions
- 1 A piston has an area of 0.0050 m² and the gas pressure above it is 600,000 Pa during combustion. What downward force acts on the piston?
- 2 A crank throw has a radius of 0.040 m. If the connecting rod applies a 2500 N force perpendicular to the crank throw, what torque is produced?
- 3 In a four stroke engine, explain why the crankshaft must keep spinning during the intake, compression, and exhaust strokes even though only the power stroke is driven directly by combustion.