An engine has to breathe in and out dozens of times a second. The camshaft keeps the rhythm: egg-shaped lobes push every valve open at exactly the right moment, and springs snap them shut.
The engine is already cut open. Start it, or drag the crank slider to turn it by hand, and watch the chain drive both camshafts at half speed. Drag to turn the view, scroll or pinch to zoom, and click a part to use it.
Try itPress Start engine, or drag Turn the crank. Each lobe on the camshafts pushes its valve open once every two turns of the crank.
Loading the 3D model…
This browser can’t show the 3D model, so here is a still picture.
Meet the parts
Thirteen parts, one rhythm
The engine cut open along its length, with the cam caps lifted off. Tap any part of the picture or pick a name below. The drive is at the front, the camshafts and valves on top, and the pistons and crankshaft underneath.
Hover or tap the picture. Red rings mark every part.
Parts diagram
From the crank to the valves
The engine cut open from the exhaust side. Numbers 1 to 6 follow the drive from the crankshaft up the chain, 7 to 13 the valve train on top, and 14 to 17 the parts of the engine around it.
How it works, step by step
Thirteen steps, one rhythm
The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.
Step 1 of 13
An engine has to breathe
Demo · the lobe is a schedule
Shape a lobe
The height of a lobe sets how far its valve opens. Its width sets how long the valve stays open. Make it taller or narrower and see what the bucket, the spring and the valve have to put up with.
A flat bucket can only follow a lobe whose outline bulges outward everywhere, and the point where the lobe touches it slides across its face, so the bucket must be wide enough. Then there is speed: over the nose the valve is slowing down hard, and only the spring pulls it back. Spin the engine faster than the spring can manage and the valve flies off the lobe.
Stock spring. Holds the valve on the lobe past the red line.
Soft spring. Past 7 500 rpm the valve starts to fly off the lobe; past 8 500 it slams down and bounces on its seat.
–open for, crank degrees (at 1 mm lift)
–contact slides across the bucket, mm
–valve slowing over the nose, g
–the spring holds it up to, rpm
Demo · one cylinder
Four strokes, two turns
Follow cylinder 1 through intake, compression, power and exhaust. The crank turns twice, the camshafts once, and each valve opens just one time. Watch the end of the exhaust stroke: for a moment both valves are open.
–stroke
–intake valve lift, mm
–exhaust valve lift, mm
–crank turns · cam turns
Demo · variable valve timing
Shift the timing
The phaser on the intake sprocket can turn its camshaft a little ahead of the sprocket or a little behind it, while the engine runs. The lobes keep their shape; the whole intake schedule just slides earlier or later.
Inside the phaser, a rotor with four vanes is fixed to the camshaft, and the housing with the sprocket turns round it. A valve sends engine oil to one side of every vane or the other: oil on one side pushes the camshaft ahead of its sprocket, oil on the other side holds it back.
Earlier intake: more overlap, and the valve closes sooner.
Later intake: little overlap, and the valve closes well after the piston turns round.
–intake opens
–intake closes
–overlap, crank degrees
–how well it suits the driving
Demo · four cylinders
Take turns: 1, 3, 4, 2
On each camshaft the lobes for the four cylinders point a quarter of a turn apart. So the cylinders take turns, one every half turn of the crank, and one of them is always on its power stroke. Listen: the firing walks across the engine and back.
–just fired
–fires next
180°crank turn between firings
–cylinders on a power stroke
More to know
Kinds, history and care
Six ways to open a valve
Twin camThe engine on this page: two camshafts in the head, one for the intake valves and one for the exhaust, usually with four valves a cylinder. Also called DOHC.
Single camOne camshaft in the head opens both the intake and the exhaust valves, often through small rocker arms. Simpler and lighter.
PushrodThe camshaft sits low in the block beside the crankshaft. Long pushrods and rockers carry its push up to the valves. Compact; used in many big V8s.
Two lobes per valveSystems like Honda’s VTEC give each valve a mild lobe and a wild one. At high revs oil pressure locks the rockers onto the wild one.
DesmodromicDucati’s way: a second lobe and a rocker pull each valve shut instead of a spring, so the valve can never float.
CamlessAn actuator driven by air, oil or electricity opens each valve on its own, with no camshaft at all. Koenigsegg’s Freevalve works this way.
From water pumps to phasers
Al-Jazari’s book of machines describes camshafts working the pumps of water-raising machines.
Peugeot’s Grand Prix racer has two overhead camshafts and four valves a cylinder: the layout of most car engines today.
The Mercedes-Benz W196 racing car closes its valves with cams instead of springs: desmodromic valves.
Alfa Romeo sells the first production car with variable valve timing.
Honda’s VTEC switches each valve between two lobe shapes while the engine runs.
Oil-pressure cam phasers like the one on this page spread to most makers’ engines.
BMW’s Valvetronic varies the valve lift so widely that the valves, not a throttle plate, set the engine’s power.
Koenigsegg shows a car engine with no camshafts at all.
Keeping the rhythm
Change the oil on time and use the grade the maker asks for. The lobes, the buckets and the phaser all run on it, and sludge blocks the phaser’s small oil passages.
Replace a timing belt at its interval; a chain usually lasts much longer. In most modern engines the valves hit the pistons if the drive breaks or jumps a tooth.
A rattle for a second or two at start-up can be a worn chain tensioner, or a phaser that drains its oil overnight.
A light tick that speeds up with the engine is often valve clearance. The small gap above each bucket, 0.2 mm here, is checked and reset now and then.
A warning light with a camshaft timing fault means the phaser or its oil valve is not doing what the engine computer asks. Get it checked early.
Don’t rev a cold engine hard. Give the oil a few seconds to reach the camshafts first.
Things worth knowing
Half speed, alwaysAt idle or at the red line, the camshafts turn exactly once for every two turns of the crank. The chain’s teeth make sure of it.
Fifty times a secondAt 6 000 rpm each valve opens and closes 50 times every second, and it still seals tight in between.
Hundreds of millionsOver a car’s life each valve opens and shuts hundreds of millions of times.
Floating valvesSpin an engine past its limit and the springs can’t pull the valves back in time. The rev limiter cuts in before that happens.
Too close for comfortIn this engine the valves and the pistons share the same space at different moments. Only the timing keeps them apart.
The tick of the gapThe tiny clearance above each bucket is there so the valve can still seal when the engine is hot and everything has grown a little.
Every Mechanism
