What does a camshaft do?

Why the camshaft turns at half the crankshaft's speed

Because the timing belt joins the two shafts, it is easy to picture them spinning together, one turn for one turn. They don't. In every four-stroke engine the camshaft makes one turn while the crankshaft makes two, a ratio of 2:1.

The reason lies in the four strokes. Intake, compression, power and exhaust each move the piston once, up or down, so a full cycle takes the crankshaft round twice, through 720°. In that time each valve has to open exactly once: the intake valve to fill the cylinder, the exhaust valve to empty it. A lobe opens its valve once per turn of the camshaft, so one camshaft turn per cycle is exactly right. If the camshaft ran at crank speed, every valve would open a second time in the middle of compression or power, and the burning gas would escape.

The ratio is fixed by the teeth. In the engine of the tour the crank pulley has 18 teeth and each camshaft pulley 36. For every tooth the belt moves, the small pulley turns twice as far as the large ones. A two-stroke engine, which completes its cycle in one crank turn, works the other way: where it has a camshaft at all, it runs at crank speed, 1:1.

Camshaft vs crankshaft: the difference

Both are long shafts that turn inside the engine, and their names sound alike, but they do opposite jobs. The crankshaft sits at the bottom of the engine block. The connecting rods push on its offset crank pins and turn the up-and-down motion of the pistons into rotation; it delivers the engine's power to the gearbox. The camshaft sits up in the cylinder head (or, in older designs, in the block) and does no work on the car. It only takes a little power from the crankshaft to open the valves at the right moments.

Their motion is linked but not equal. The crankshaft is the driver, the camshaft the driven one, through a belt, a chain or gears, at half the speed. A crankshaft carries crank pins and counterweights; a camshaft carries lobes. Both are watched by sensors: the engine computer compares the camshaft and crankshaft position signals to know which cylinder is on its compression stroke and when to fire the spark plug and the injector.

SOHC, DOHC and pushrod engines

Where the camshaft sits gives engine designs their names. The engine in the tour is DOHC, double overhead camshaft: two camshafts in the cylinder head, one for the intake valves and one for the exhaust valves, each pressing straight down on bucket tappets. That is the most common layout in today's car engines, usually with two intake and two exhaust valves per cylinder; the model keeps it to one of each.

An SOHC engine, single overhead camshaft, has one camshaft in the head for both rows of valves. It often reaches them through rocker arms, little levers that pivot between the lobe and the valve. In an OHV or pushrod engine the camshaft sits low in the block next to the crankshaft. Long pushrods carry each lobe's motion up to rocker arms on the head. Pushrod engines are compact and simple, which is why many large American V8s still use them, but the extra moving parts limit how fast they can rev.

Timing belt, chain or gears, and why the timing matters

Three things can carry the drive from crank to camshaft. A toothed rubber belt is quiet, light and cheap, but it wears and has to be replaced at the interval the carmaker sets, often somewhere between about 60,000 and 160,000 km. A steel timing chain runs in oil and is meant to last the engine's life, though chains and their tensioners can stretch and rattle. Gears are used where the camshaft sits close to the crankshaft, as in pushrod engines and many trucks.

In many engines a broken belt does more than stop the car. In an interference engine, a fully open valve reaches into space that the piston fills at the top of its stroke; only the timing keeps the two apart. If the belt snaps, the camshafts stop with some valves open while the crankshaft keeps spinning, and the pistons smash into them: bent valves, damaged pistons, often a new cylinder head. That is why the belt's replacement interval matters, and why the timing marks have to line up exactly when a new belt goes on.

Lift, duration and variable valve timing

Two numbers describe a lobe. Lift is how far it opens the valve, duration is how long it holds the valve open, in degrees of crank rotation. More lift and longer duration let more air in at high rpm, where each stroke lasts only a few thousandths of a second. That is why sports and racing engines use 'hot' camshafts. At low rpm the same long overlap lets fresh mixture slip straight out into the exhaust, so such engines idle roughly and pull weakly at low speed.

A fixed camshaft is always a compromise between low and high rpm. Variable valve timing gets around it: an oil-pressure actuator on the camshaft pulley turns the camshaft against its pulley, by up to several tens of crank degrees, while the engine runs. That shifts when the valves open and close without changing the 2:1 ratio. Some systems also switch between two lobe profiles or vary the lift.

Signs of a worn camshaft

Camshaft lobes and tappets slide against each other under the full force of the valve springs, so they depend on clean oil. When a lobe wears down, its valve opens less than it should. Typical signs are a ticking or tapping noise from the top of the engine, a rough idle, misfires, less power, higher fuel use, and a check-engine light, often with a camshaft position or timing fault code. A faulty camshaft position sensor can give similar symptoms. A worn camshaft is a job for a workshop: it usually means replacing the camshaft together with its tappets.

FAQ

Frequently asked questions