# Valves, Camshafts and Valve Timing

> How spinning cam lobes open poppet valves at precisely the right crank angles: lift, duration, overlap, half-speed drive and why the valves open and close away from the dead centres.

- Level: Intermediate · about 14 minutes · Engine
- Lesson page: https://autoverse.maxrecursion.dev/topics/valves-and-camshaft
- Interactive 3D lab: https://autoverse.maxrecursion.dev/labs/engine (Watch intake, compression, power and exhaust with crank, piston, cam and valves on one synchronised timeline.)

## What you will learn

- Describe how a cam lobe, follower and spring open and close a poppet valve
- Explain why camshafts turn at half crankshaft speed and how a 2 : 1 timing drive achieves it
- Read a valve-timing diagram: opening and closing angles, lift, duration and overlap
- Explain why the intake valve closes after BDC and the exhaust valve opens before BDC
- Relate valve timing to engine behaviour at low and high speed

## The idea, in plain language

Each cylinder has doors that let air in and exhaust out. These doors are valves: metal discs on long stems that sit in round holes in the cylinder head. A spring holds each valve shut.

Above each valve is a camshaft — a rod with an egg-shaped bump, called a lobe. As the camshaft turns, the bump comes round and presses the valve down, opening it. When the bump has passed, the spring snaps the valve shut again. The shape of the bump decides how far the valve opens and for how long.

The camshafts are turned by a toothed belt from the crankshaft. Each valve needs to open only once every four strokes, which is two turns of the crankshaft, so the pulleys on the camshafts are twice the size of the one on the crankshaft and turn at half the speed.

This engine has two camshafts on top ("twin cam" or DOHC): one works the intake valve and one works the exhaust valve. In the laboratory, watch the springs squash as each lobe comes round.

## The engineering explanation

A poppet valve is opened by the cam and closed by its spring. The follower — here a roller riding on the lobe, over a cup on the valve spring — turns the lobe's rotation into linear lift. For a roller follower the lobe surface is the inner offset, by the roller radius, of the curve traced by the roller's centre, so the valve lift is a direct function of cam angle. The model uses smooth sin² lift ramps: zero velocity at the seat, maximum lift at the centre of the event.

The cams are driven at half crank speed through a 2 : 1 toothed-belt drive (cam pulleys with twice the crank sprocket's diameter and teeth). One cam revolution therefore spans the full 720° cycle, and a feature on the lobe moves through 1° of cam for every 2° of crank.

Illustrative timing for this engine: intake opens (IVO) 10° BTDC and closes (IVC) 40° ABDC, giving 230° of crank duration and peak lift at 105° ATDC; exhaust opens (EVO) 40° BBDC and closes (EVC) 10° ATDC, also 230°, with peak lift 105° BTDC. The two lobe centre-lines are therefore 105° of cam apart (the lobe separation angle). Around exhaust/intake TDC the events overlap by 20°.

The reasons are gas dynamics. Late intake closing lets the momentum of the intake air keep filling the cylinder after BDC, improving volumetric efficiency at speed, at the cost of pushing some charge back at low speed. Early exhaust opening sacrifices a little expansion work so blow-down empties the cylinder before the piston has to pump against it. Overlap uses the outgoing exhaust pulse to help draw in fresh charge. Because the best timing differs between idle and high rpm, many engines shift cam phasing (variable valve timing) or switch lobe profiles. If the belt slips or breaks, timing is lost — and in an interference engine an open valve can be struck by the piston.

## Step by step

1. **Half-speed drive.** The belt turns the camshafts at half the crankshaft's speed, so each valve opens once every two crank turns. *In engineering terms:* Cam pulleys have twice the crank sprocket's diameter and tooth count: ω_cam = ω_crank / 2, so the cam angle shown in the lab is always exactly half the crank angle.
2. **Intake lobe opens the intake valve.** The intake lobe comes round, presses the follower and spring down, and the intake valve opens wide while the piston goes down. *In engineering terms:* IVO is 10° BTDC; lift rises along a sin² ramp to 9 mm at 105° ATDC, a little after the piston passes its peak speed (about 74° ATDC) and the incoming air is moving quickest.
3. **Intake closes after BDC.** The intake valve stays open a little after the piston starts rising, because the moving air keeps flowing in for a moment. *In engineering terms:* IVC is 40° ABDC (220°). Between BDC and IVC the charge's inertia continues to fill the cylinder at speed; effective compression only begins once the valve seats.
4. **Both valves sealed.** Through compression and most of the power stroke both valves stay firmly shut so no pressure escapes. *In engineering terms:* From IVC (220°) to EVO (500°) both followers sit on their base circles and the springs' preload holds the valves on their seats against cylinder pressure.
5. **Exhaust opens before BDC.** The exhaust valve opens before the piston reaches the bottom, letting the still-pressurised gas rush out on its own. *In engineering terms:* EVO is 40° BBDC (500°). Blow-down releases most of the remaining pressure before BDC, so the piston has less gas to push during the exhaust stroke; peak exhaust lift of 8.5 mm comes at 615°.
6. **Valve overlap.** Around the top, both valves are slightly open at once. The escaping exhaust helps pull the fresh charge in. *In engineering terms:* EVC 10° ATDC and IVO 10° BTDC give 20° of overlap. Lifts are tiny here (about 2% of maximum at TDC), yet the pressure waves in the ports make overlap a powerful tuning tool.

## Key concepts

- **Valve lift**: How far the valve moves off its seat. Flow area grows with lift until the port itself becomes the restriction.
- **Duration**: How many crank degrees the valve is off its seat. Here 230° for each valve (illustrative). (`duration = closing angle − opening angle (mod 720°)`)
- **Valve overlap**: The crank angle during which both valves are open together, around exhaust/intake TDC. (`overlap = IVO (° BTDC) + EVC (° ATDC)`)
- **Half-speed camshaft**: The camshaft makes one turn per four-stroke cycle. (`θ_cam = θ_crank / 2`)
- **Lobe separation angle**: Cam-degree spacing between the intake and exhaust lobe centre-lines. (`LSA = (intake centre ATDC + exhaust centre BTDC) / 2`)

## Common misconceptions

- **Myth:** The cam pushes the valve closed. **Reality:** The cam only opens the valve. The spring closes it and must be strong enough to keep the follower on the lobe at high rpm.
- **Myth:** Valves open at TDC and close at BDC. **Reality:** They open early and close late to use the momentum and pressure of the gas; this engine's intake closes 40° after BDC.
- **Myth:** Overlap means the engine is leaking. **Reality:** Overlap is deliberate: the departing exhaust helps draw in fresh charge. Too much overlap at idle does cause a rough idle, which variable timing can avoid.
- **Myth:** Twin-cam means two valves. **Reality:** DOHC means two overhead camshafts per cylinder bank. Most DOHC engines have four valves per cylinder; this model shows one intake and one exhaust valve for clarity.

## In the real world

- Timing marks on the crank sprocket and cam pulleys must line up when a timing belt is fitted; one tooth out shifts every valve event by several degrees.
- Replacing a timing belt at the specified interval matters most on interference engines, where a broken belt lets the pistons hit open valves.
- A ticking noise from the top of the engine can mean excessive clearance in the valvetrain or low oil supply to the cams.
- Variable valve timing systems rotate the camshaft relative to its pulley under oil pressure, advancing intake timing at low rpm for torque and retarding it at high rpm for power.

## Common questions

### What does a camshaft do?

It opens and closes the engine's valves. Egg-shaped lobes on the camshaft push the valves open as it turns and springs close them again, so the shape and timing of each lobe decide when a valve opens, how far and for how long.

### Why does the camshaft turn at half crankshaft speed?

Each valve opens once per four-stroke cycle, and a cycle takes two crankshaft revolutions. The cam drive therefore turns the camshaft once for every two turns of the crankshaft.

### What is valve overlap?

Overlap is the short period around top dead centre, at the end of the exhaust stroke, when the exhaust valve has not yet closed and the intake valve has already opened. The outgoing exhaust helps draw the fresh charge in, which improves breathing at higher engine speeds.

### What does DOHC mean?

Double overhead camshaft: two camshafts in the cylinder head, one for the intake valves and one for the exhaust valves. It makes four valves per cylinder practical and lets intake and exhaust timing be set independently.

## The story behind it

### Ernest Henry (1912)

Designed the racing engine whose valve gear still dominates. Henry's 1912 Peugeot race engine used two overhead camshafts, four valves per cylinder and hemispherical combustion chambers. The Peugeot L76 won the 1912 French Grand Prix, and Jules Goux won the 1913 Indianapolis 500 in one. Sunbeam designs drew on his work, Harry Miller later copied the layout, and Fred Offenhauser's engines carried it forward. Peugeot's own managers nicknamed the secret racing team 'Les Charlatans' because they opposed its approach.

Sources: [Wikipedia: Ernest Henry (engineer)](https://en.wikipedia.org/wiki/Ernest_Henry_(engineer))

## Related lessons

- [The Four-Stroke Cycle](https://autoverse.maxrecursion.dev/topics/four-stroke-cycle)
- [Pistons, Connecting Rods and the Crankshaft](https://autoverse.maxrecursion.dev/topics/pistons-and-crankshaft)
- [Spark Ignition and Combustion Timing](https://autoverse.maxrecursion.dev/topics/spark-ignition)
- [Fuel Injection](https://autoverse.maxrecursion.dev/topics/fuel-injection)
- [Lubrication System](https://autoverse.maxrecursion.dev/topics/lubrication-system)
- [Routine Maintenance](https://autoverse.maxrecursion.dev/topics/routine-maintenance)

## Further reading

- [How engine timing works](https://www.howacarworks.com/basics/how-engine-timing-works) — How a Car Works

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Original AutoVerse lesson (AutoVerse original content). Simulation values in AutoVerse labs are illustrative. Source page: https://autoverse.maxrecursion.dev/topics/valves-and-camshaft