1. The Heart of the Automobile
The Four Stroke Engine
The Heart of the Automobile

2. Engine Operation

3. Four-Stroke Cycle

4. The First Stroke (also called intake stroke)
intake valve open
camshafts
gas/air mixture
The downward moving piston draws a mixture of air and gasoline vapor into the cylinder.
Piston
Piston is moving down
Crankshaft

5. The Second Stroke (also called compression stroke)
Intake and exhaust valves are both closed
The piston moves up, compressing the gas/air mixture. During this stroke, notice that both the intake and exhaust valves are closed.
Piston is moving up

6. The Third Stroke (also called combustion stroke)
Spark Plug
Just before the piston reaches the top of the cylinder a spark from the spark plug explodes the fuel mixture. This pushes the piston down and drives the crankshaft around.
Piston is moving down

7. The Fourth Stroke (also called exhaust stroke)
The piston moves up and pushes the burned gases out through the exhaust valve. Next, the piston moves down again, and pulls more gas/air mixture in to begin the cycle again.
burned gas out
Piston is moving up
exhaust valve open

8. Review
Stroke 1: The downward moving piston draws a mixture of air and gas vapor into the cylinder.
Stroke 2: The piston moves up, compressing the gas mixture.
Stroke 3: Just before the piston reaches the top of the cylinder a spark from the spark plug explodes the gas mixture. This pushes the piston down and drives the crankshaft around.
Stroke 4: The piston moves up and pushes the gases out through the exhaust valve. As the piston moves down, it pulls more fuel/air mixture in to begin the cycle again.
Notice the up-down/linear motion of the pistons
Stroke Number
Linear motion
Rotational motion of the crankshaft