A camshaft is an engine component that opens intake and exhaust valves in time with the motion of the pistons. In this post, we’ll answer the question, What does a camshaft do?
Physically, camshafts are a hardened alloy or steel rod with egg-shaped lobes called cams, which are directly tied to valve timing. Engine performance requires precise valve timing. Because of that, cam lobes must be designed to open the valve at the right time, keep it open for the optimal duration and allow it to close at the ideal moment. The shape of the cam, or cam profile, controls valve timing.
How do camshafts work?
The valve stays closed when the lifter is resting on the shaft. As the camshaft rotates, the nose of the cam pushes against the valve lifter, which opens the valve. The shape of the nose determines how much the valve is lifted and how long the valve stays open. The closing flank allows the spring-loaded valve to close gently so that it doesn’t pound against its seat. The camshaft is kept in synch with the crankshaft with a timing belt or timing chain.
Profile and performance
The shape of the cam lobes directly impacts engine performance at various speeds. That’s because the speed and momentum of air entering the combustion chamber changes as engine speed varies.
When the intake valve opens and the piston starts its intake stroke, air starts to accelerate into the cylinder. By the time the piston reaches the bottom of its intake stroke, the air is moving at a high speed. By leaving the intake valve open longer, the momentum of the fast-moving air continues to bring air into the cylinder, even as the piston starts its upward compression stroke. And that improves power output.
Therefore, as engine speed increases, it’s ideal to open the intake valve wider and longer. However, fixed camshafts can’t make that adjustment.
For that reason, a fixed camshaft compromises performance at most speeds. Most cams are designed to work at normal driving speeds, while some are designed to provide low-end power or high-speed performance. But no single cam profile can be ideal for all driving conditions.
To work around this shortcoming, automakers have devised solutions to adjust valve timing as driving conditions vary. We will discuss some of those systems a little later.
There are several camshaft designs on engines. The most common are:
- Single-overhead cam
- Double-overhead cam
In a pushrod engine, the camshaft is inside the engine block below the engine head and the valves are located in the head above the cylinders. This system uses long rods to reach from the cam to the valves. Pushrod engines usually have one intake valve and one exhaust valve. These long rods add mass to the system, which increases the load on the valve springs, limiting engine speed and performance.
In an overhead-cam engine, the camshaft is in the cylinder head. A single-overhead cam system has one cam per engine head, so one in an inline engine and two in a V-6 or V-8 engine.
Single- and double-overhead camshafts are driven by the crankshaft via either a timing belt or timing chain. These belts and chains need to be replaced or adjusted at regular intervals. If one breaks, the cam will stop spinning and the piston could hit the open valves, resulting in catastrophic damage.
A double-overhead cam engine has two cams per head, one for intake valves and one for exhaust valves. Double-overhead cams are used on engines with four or more valves per cylinder because one camshaft can’t physically fit the cams needed to operate that many valves. Double-overhead cams allow for more intake and exhaust valves, which allows more air/fuel mix and exhaust flow. That translates to more engine power.
Optimizing valve timing
As mentioned earlier, some cars are equipped with systems that can adjust valve timing as driving conditions change. Variable valve timing is one. It uses hydraulic actuators to advance or delay valve timing. With this system, valves do not stay open longer, but timing is delayed as the engine speeds up, which does lift performance at high speeds.
Some modern vehicles use variable valve timing and lift electronic control, an electronic and mechanical system that employs a second camshaft. One cam is engineered for slower engine speeds and one for higher speeds.
Ferrari has engineered a system that varies the cam profile along the length of the cam lobe. One end is for low engine speeds, the other for fast speeds. The entire camshaft can slide laterally to vary valve timing.
In vehicles with direct fuel injection, like gasoline direct-injection engines and many diesel engines, the high-pressure fuel pump is activated by a lobe on one of the camshafts.
Several engine manufacturers are working to innovate new systems to provide infinite variability in valve timing. When successful, engines will provide maximum engine performance at every speed.
While the camshaft rarely fails in spectacular fashion, occasional problems do arise, especially when vehicles have not been properly maintained.
Worn cam lobes
For example, worn cam lobes won’t push the valve open as much as they should, leading to cylinder misfiring and diminished engine performance. Overhead-cam engines are more prone to camshaft wear as it takes time for the oil pump to bring oil up to the top of the engine. Over time this can result in worn lobes.
Worn lifters won’t lift the valve open enough or at all.
A broken camshaft, often caused by the camshaft seizing, will likely cause catastrophic damage to the engine.
Protection and performance
Protect your camshaft by following original equipment manufacturer (OEM) oil-change interval recommendations, or the recommendations on the oil bottle, and using a quality motor oil. Check your owner’s manual for that information or use the AMSOIL Vehicle Lookup to find the right motor oil for your vehicle. We have products that allow you to safely extend your OEM drain interval.