# The Four-Stroke Cycle

> Intake, compression, power, exhaust: how a petrol engine breathes in, squeezes, burns and breathes out over two turns of the crankshaft — with the piston, valves, camshafts and spark all kept in step.

- Level: Beginner · about 15 minutes · Engine
- Lesson page: https://maxrecursion.github.io/AutoVerse/topics/four-stroke-cycle
- Interactive 3D lab: https://maxrecursion.github.io/AutoVerse/labs/engine (Watch intake, compression, power and exhaust with crank, piston, cam and valves on one synchronised timeline.)

## What you will learn

- Name the four strokes in order and say what the piston and both valves are doing in each
- Explain why one complete cycle takes two crankshaft revolutions and why the camshafts turn at half crank speed
- Describe why the spark fires before top dead centre and why valves open and close away from TDC and BDC
- Calculate swept volume and compression ratio from bore, stroke and clearance volume
- Relate engine speed to how often each cylinder fires

## The idea, in plain language

Most car engines work in a repeating pattern of four steps, called strokes. A stroke is one trip of the piston, either down the cylinder or back up.

On the intake stroke the piston moves down with the intake valve open, drawing a mixture of air and fuel into the cylinder, a bit like pulling back the plunger of a syringe. On the compression stroke both valves are shut and the piston rises, squeezing that mixture into a small space at the top. Just before the piston reaches the top, the spark plug makes a spark. The mixture burns very quickly, the hot gas expands, and it shoves the piston down: this is the power stroke, the only one that produces work. Finally, on the exhaust stroke the exhaust valve opens and the rising piston pushes the burnt gas out, ready to start again.

The piston is joined to the crankshaft by a connecting rod, so its up-and-down motion turns the crankshaft round. Each stroke is half a turn of the crankshaft, so the whole cycle takes two full turns. The camshafts, which open the valves, are driven by a belt at half the crankshaft's speed so each valve opens just once per cycle.

In a real engine at idle all of this happens about five or six times a second in every cylinder, which is why the laboratory slows it down dramatically.

## The engineering explanation

The four-stroke spark-ignition engine completes one thermodynamic cycle in 720° of crank rotation. Taking 0° as top dead centre (TDC) at the start of intake: intake runs 0–180°, compression 180–360°, power (expansion) 360–540° and exhaust 540–720°. Because each valve event happens once per cycle, the camshafts are driven at ω_cam = ω_crank / 2 by a belt, chain or gears.

Real valve events do not coincide with TDC and BDC. The intake valve opens before TDC and closes well after BDC (40° ABDC in this illustrative engine) so that the momentum of the incoming column of air keeps filling the cylinder while the piston has already begun to rise. The exhaust valve opens before BDC (40° BBDC) so that the remaining cylinder pressure blows most of the gas out (blow-down) before the piston has to push against it, and it closes shortly after TDC. Around exhaust/intake TDC both valves are briefly open together; this overlap lets the outgoing exhaust help draw in the new charge.

During compression the trapped charge is squeezed from the swept-plus-clearance volume towards the clearance volume. The ratio of those volumes is the compression ratio, about 10.5 : 1 here, which raises temperature and pressure and makes the subsequent expansion more efficient. Combustion is not instantaneous: the flame needs a few milliseconds to cross the chamber, so the spark is advanced to fire before TDC (15° here) and peak pressure arrives roughly 15–20° after TDC, where it can push on a crank that has started to turn.

Only the power stroke delivers positive work; the other three strokes absorb a little. A single cylinder therefore produces torque in sharp pulses, carried between them by the flywheel's stored energy, while multi-cylinder engines stagger their power strokes (every 720°/n) to overlap them. The lab's pressure curve and combustion glow are illustrative models for teaching the shape of the cycle, not measured data.

## Step by step

1. **Intake stroke.** The piston moves down and the intake valve is open, so fresh air and fuel are drawn into the cylinder — like pulling back a syringe plunger. *In engineering terms:* 0–180°. The descending piston lowers cylinder pressure slightly below the intake manifold's, and charge flows past the open intake valve. The valve opened 10° before TDC and stays open until 40° after BDC, using the inertia of the moving air to improve filling (volumetric efficiency).
2. **Compression stroke.** Both valves close and the piston rises, squeezing the mixture into the small space at the top of the cylinder. Squeezing it makes it hotter and ready to burn well. *In engineering terms:* 180–360°. After intake-valve closing the trapped charge is compressed roughly polytropically (pV^n ≈ constant, n ≈ 1.3) towards the clearance volume. With a 10.5 : 1 compression ratio the illustrative model reaches roughly 15 bar by the time the spark fires, and the charge temperature rises with it, improving the expansion work that follows.
3. **The spark.** Just before the piston reaches the top, the spark plug sparks. The flame starts small and spreads through the squeezed mixture. *In engineering terms:* 345° (15° BTDC). Ignition is timed before TDC because the flame kernel needs time to grow and the flame front a few milliseconds to cross the chamber; the goal is peak pressure shortly after TDC. The engine management system varies the advance with speed and load.
4. **Power stroke.** The burning gas gets very hot and pushes hard on the piston, driving it down. This push turns the crankshaft — it is the only stroke that makes power. *In engineering terms:* 360–540°. Pressure peaks about 15–20° after TDC (around 40 bar in the illustrative curve) and falls as the gas expands. Gas force on the crown is transmitted through the rod to the crank pin; crank torque = F·r·sin(θ + φ) / cos φ, so useful torque appears once the crank has moved away from TDC.
5. **Exhaust stroke.** The exhaust valve opens and the rising piston pushes the burnt gas out of the cylinder, clearing it for the next intake. *In engineering terms:* 540–720°. The exhaust valve actually opened at 500° (40° BBDC), so blow-down has already released most of the pressure; the piston then displaces the remaining gas. The exhaust valve stays open until 10° after TDC.
6. **Overlap — and round again.** At the very top, for a moment both valves are slightly open. The last exhaust leaving helps pull in the new charge, and the cycle starts again. *In engineering terms:* 710–10°. With both valves off their seats, exhaust momentum scavenges the clearance volume and helps start intake flow. Overlap suits high speed; too much overlap at idle lets exhaust flow back and makes the idle rough, which is one reason variable valve timing exists.

## Key concepts

- **Top and bottom dead centre (TDC / BDC)**: The highest and lowest points of the piston's travel, where it momentarily stops and reverses. Crank angles are usually measured from TDC.
- **Stroke**: One piston trip between TDC and BDC — half a crankshaft revolution. The word also means the distance travelled, which is twice the crank throw. (`S = 2r`)
- **Swept volume (displacement)**: The volume the piston sweeps in one stroke. Multiply by the number of cylinders for engine capacity. (`V_s = (π/4) · B² · S`)
- **Compression ratio**: Total cylinder volume at BDC divided by the clearance volume left at TDC. (`CR = (V_s + V_c) / V_c`)
- **Camshaft speed**: Each valve opens once per 720° cycle, so the camshafts turn once for every two crankshaft revolutions. (`ω_cam = ω_crank / 2`)
- **Firing frequency**: Each cylinder fires once every two revolutions, so at N rpm one cylinder fires N/120 times per second. (`f = N / 120`)

## Common misconceptions

- **Myth:** Each stroke is one full turn of the crankshaft. **Reality:** Each stroke is half a turn. The four strokes together take two turns (720°), which is why the camshafts run at half speed.
- **Myth:** The spark fires exactly when the piston is at the top. **Reality:** It fires before TDC so the flame has time to build pressure; peak pressure arrives a little after TDC, when the crank can turn it into torque.
- **Myth:** The fuel explodes and blasts the piston down. **Reality:** Normal combustion is a fast but controlled flame front. A sudden, uncontrolled explosion of the end gas is knock, which engines are designed to avoid.
- **Myth:** Valves open and close exactly at TDC and BDC. **Reality:** Intake closes well after BDC and exhaust opens well before it, and both valves overlap around TDC, to make the most of gas momentum and pressure.
- **Myth:** An engine produces power continuously. **Reality:** Each cylinder pushes for only about a quarter of its cycle. The flywheel and the staggered firing of other cylinders smooth the gaps.

## In the real world

- At a typical idle of 800 rpm a whole four-stroke cycle takes 0.15 s, so the lab's 6-second cycle at 1× is about 40 times slower than idle.
- A four-cylinder engine at 3000 rpm makes 50 crank revolutions per second; each cylinder fires 25 times a second, giving 100 power strokes per second in total.
- A lawnmower or small generator often uses exactly the single-cylinder four-stroke shown here, with a heavy flywheel to keep it turning between power strokes.
- A compression test measures the pressure each cylinder builds on the compression stroke; a low reading points to leaking rings, valves or head gasket.

## Common questions

### What are the four strokes of an engine?

Intake: the piston moves down and draws in air and fuel. Compression: the piston rises and squeezes the mixture. Power: the spark ignites it and the hot, expanding gas pushes the piston down. Exhaust: the piston rises again and pushes the burnt gas out.

### Why does a four-stroke engine need two crankshaft turns per cycle?

Each stroke is one trip of the piston, up or down, and each trip is half a crankshaft revolution. Four strokes therefore take two full turns, so each cylinder fires once every two revolutions and the camshaft turns at half crankshaft speed.

### What is the difference between a two-stroke and a four-stroke engine?

A two-stroke engine completes its cycle in one crankshaft turn by combining intake and exhaust with the compression and power strokes, usually through ports in the cylinder wall. It is simpler and lighter; a four-stroke uses valves, is more fuel-efficient and runs cleaner.

## The story behind it

### Nikolaus Otto and Eugen Langen (1876)

Turned the four-stroke cycle into a working engine. Otto started out as a travelling salesman of coffee, tea, rice and sugar. In 1864 he and Eugen Langen founded N.A. Otto & Cie in Cologne, and their atmospheric engine won a gold medal at the 1867 Paris World Exhibition. In 1876 Otto's engine, which compresses the charge before igniting it, appeared; the sequence is still called the Otto cycle. Gottlieb Daimler, a former manager at the firm, later left and took Wilhelm Maybach with him.

*Worth knowing:* A French engineer, Beau de Rochas, held a four-cycle patent from 1862. According to the source it cost Otto one of his patents, which let Daimler sell engines without paying royalties.

Sources: [Wikipedia: Nicolaus Otto](https://en.wikipedia.org/wiki/Nicolaus_Otto)

### Étienne Lenoir (1860)

Sold the first widely used gas engine, without compressing the charge. Lenoir's engine burned a mixture of coal gas and air, lit by sparks from a Ruhmkorff coil, and he patented it in 1860. Because the mixture was not compressed first it ran quietly but wastefully: real fuel use was far above the advertised figure, and it could overheat and seize without cooling water. Still, 143 had been sold in Paris alone by 1865, and in 1863 his gas-fuelled three-wheeled 'Hippomobile' covered the 18 km from Paris to Joinville-le-Pont in about three hours.

Sources: [Wikipedia: Étienne Lenoir](https://en.wikipedia.org/wiki/%C3%89tienne_Lenoir)

## Related lessons

- [How a Car Works: The Big Picture](https://maxrecursion.github.io/AutoVerse/topics/how-a-car-works)
- [Pistons, Connecting Rods and the Crankshaft](https://maxrecursion.github.io/AutoVerse/topics/pistons-and-crankshaft)
- [Valves, Camshafts and Valve Timing](https://maxrecursion.github.io/AutoVerse/topics/valves-and-camshaft)
- [Spark Ignition and Combustion Timing](https://maxrecursion.github.io/AutoVerse/topics/spark-ignition)
- [Fuel Injection](https://maxrecursion.github.io/AutoVerse/topics/fuel-injection)
- [Diesel Engines](https://maxrecursion.github.io/AutoVerse/topics/diesel-engines)
- [Torque and Power](https://maxrecursion.github.io/AutoVerse/topics/torque-and-power)
- [Engine Layouts: Inline, V and Flat](https://maxrecursion.github.io/AutoVerse/topics/engine-layouts)

## Further reading

- [The engine - how power is created](https://www.howacarworks.com/basics/the-engine-how-power-is-created) — How a Car Works
- [The engine](https://www.howacarworks.com/basics/the-engine) — How a Car Works

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Original AutoVerse lesson (AutoVerse original content). Simulation values in AutoVerse labs are illustrative. Source page: https://maxrecursion.github.io/AutoVerse/topics/four-stroke-cycle