The 4‑Stroke Engine in One Smooth 720° Loop
If you’ve ever wondered “Okay, but how do the four strokes line up with crankshaft degrees?” — you’re about to get it.
A classic 4‑stroke engine completes one full engine cycle in 720° of crankshaft rotation. That’s two full turns of the crankshaft (because one turn is 360°).
We’ll walk it like a clock: 0° → 180° → 360° → 540° → 720°.
First, a quick mental picture (super helpful)
- The crankshaft is the spinning part that eventually turns your wheels.
- The piston moves up and down inside the cylinder.
- The valves are little “doors”:
- Intake valve lets fresh air–fuel mixture in (or just air in many modern engines).
- Exhaust valve lets burned gases out.
One cycle = 4 piston strokes = 720° crank rotation.
The 720° timeline (step‑by‑step)
Think of each 180° chunk as one stroke.
0° → 180°: Intake stroke (slurp!)
- The piston moves down.
- The intake valve opens.
- The cylinder pulls in fresh air–fuel charge.
- The crankshaft spends energy to pull the piston down (but it’s helped a bit by momentum from other cylinders / the flywheel).
180° → 360°: Compression stroke (squeeze!)
- The piston moves up.
- Both valves are closed.
- The trapped mixture gets compressed into a smaller space.
- The crankshaft spends energy compressing (this is like pushing on a spring).
360° → 540°: Power stroke (bang → push!)
- Near 360°, the spark plug fires (timing varies, but this is the “power event”).
- The piston gets shoved down by expanding gases.
- Both valves stay closed so pressure can push hard.
- The crankshaft receives energy here — this is the stroke that actually “pays the bills.”
540° → 720°: Exhaust stroke (blow out!)
- The piston moves up.
- The exhaust valve opens.
- Burned gases are pushed out of the cylinder.
- The crankshaft spends energy to clear the cylinder (again helped by momentum).
At 720°, you’re back where you started: ready for intake again.
Simple table: what’s happening in each 180° stroke
| Crank degrees | Stroke | Piston direction | Intake valve | Exhaust valve | Air‑fuel charge / gases | What the crankshaft gets |
|---|
| 0° → 180° | Intake | Down | Open | Closed | Fresh charge moves in (or air only) | Gives energy (pulling in) |
| 180° → 360° | Compression | Up | Closed | Closed | Charge gets compressed | Gives energy (squeezing) |
| 360° → 540° | Power | Down | Closed | Closed | Charge burns/expands, pushes piston | Gets energy (useful torque) |
| 540° → 720° | Exhaust | Up | Closed | Open | Burned gases move out | Gives energy (pushing out) |
A tiny “map” you can memorize
- Down + intake open = Intake (0–180)
- Up + both closed = Compression (180–360)
- Down + both closed = Power (360–540)
- Up + exhaust open = Exhaust (540–720)
If you remember just that, you’re already ahead of the game.
Common mistakes (quick save!)
Mistake 1: Mixing up compression vs power
Compression is up and costs energy. Power is down and makes energy.
Mistake 2: Assuming a valve is open during compression
In the basic 4‑stroke explanation, both valves are closed during compression (and power). That sealed chamber is the whole point.
Mistake 3: Thinking each stroke is one crank revolution
Nope — each stroke is 180°, and the whole cycle is 720°.
(Real engines also have brief moments when both valves are slightly open near the exhaust/intake transition—called valve overlap—but the table above is the clean, beginner-friendly version.)
Takeaway
A 4‑stroke engine is a simple 720° story: fill → squeeze → push → clear. Only one stroke (power) adds energy to the crankshaft, and the other three mostly spend it — which is why engines rely on momentum, flywheels, and multiple cylinders to feel smooth.
