The idea
Beginner explanation — switch to Technical for the engineering detail.
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.
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
Try it yourself
Watch intake, compression, power and exhaust with crank, piston, cam and valves on one synchronised timeline. Open the full engine lab
0° · Intake · intake 2% open · exhaust 2% open
Intake stroke: the piston moves down while the intake valve is open, drawing in fresh air and fuel.
- Crank
- Cam
- Stroke
~40× slower than a 800 rpm idle
Step by step
Use “Show in 3D” to jump the animation to a step; the step currently playing is highlighted.
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.
Step 2: Intake lobe opens the intake valve
Now playing in the 3D sceneThe intake lobe comes round, presses the follower and spring down, and the intake valve opens wide while the piston goes down.
Step 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.
Step 4: Both valves sealed
Through compression and most of the power stroke both valves stay firmly shut so no pressure escapes.
Step 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.
Step 6: Valve overlap
Around the top, both valves are slightly open at once. The escaping exhaust helps pull the fresh charge in.
Key components
Select a part to highlight it in the 3D scene.
How it flows
Read the diagram as text
- Crankshaft → Timing belt 2 : 1
- Timing belt 2 : 1 → Cam lobe (½ speed)
- Cam lobe → Follower (lobe pushes)
- Follower → Valve opens
- Valve opens → Spring closes it (compresses)
- Spring closes it → Follower (returns)
- Crankshaft: Drives the timing belt
- Timing belt 2 : 1: Cams turn at half speed
- Cam lobe: Profile sets lift and duration
- Follower: Roller turns rotation into lift
- Valve opens: Gas flows through the port
- Spring closes it: Keeps the follower on the lobe
Go deeper
Optional. Open a panel for the engineering detail — or switch to Technical above.
The engineering explanation4
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.
Key concepts5
- 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 misconceptions4
MythThe cam pushes the valve closed.
RealityThe cam only opens the valve. The spring closes it and must be strong enough to keep the follower on the lobe at high rpm.
MythValves open at TDC and close at BDC.
RealityThey open early and close late to use the momentum and pressure of the gas; this engine's intake closes 40° after BDC.
MythOverlap means the engine is leaking.
RealityOverlap 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.
MythTwin-cam means two valves.
RealityDOHC 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 world4
- 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
Short answers to the questions people most often ask about this topic.
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.
Check your understanding
5 questions. Your best score is saved in this browser.
0 of 5 answered
Question 1 of 5
The crankshaft turns 1440°. How far has the camshaft turned?
The story behind it
Who worked this out, and what they were up against. Optional.
1912
Ernest Henry
Swiss mechanical engineer (1885–1950)
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
Retold in our own words from the sources above.
Almost there
- Overview — Read the introduction to the end.
- See it work — 6 steps left to read; try the lab (play, scrub or select a part)
- Go deeper (optional) — Optional: open a deeper panel or switch to Technical.
- Check — Answer the quiz questions.
- Story (optional) — Optional: who worked this out?
Progress is stored only in this browser — there is no account.
Earlier in this route you skipped The Four-Stroke Cycle, Pistons, Connecting Rods and the Crankshaft.
Sources & attribution
Further reading
- How engine timing worksHow a Car Works · consulted 2026-10-21(external site, opens in a new tab)
Further reading. AutoVerse text is original.
- Licence
- AutoVerse original content
This lesson is original AutoVerse writing. The links above are suggestions for further reading, not the source of this text. Numbers shown in the 3D simulation are illustrative, not measured engineering data.