The camshaft is the piece of equipment that covers the mix of fuel and air entering the cylinder and also the exhaust gases leaving it. Since this is a process that’s repeated thousands of times each second, having a good camshaft is vital. There are various configurations of camshafts and they each work slightly differently. As well as the standard camshaft, which is under the engine, there’s also the overhead camshaft and the dual overhead camshaft.
The camshaft spins with the car’s crankshaft. Protruding from the camshaft are small pieces called lobes. These are what govern the valves opening and closing on the cylinder. The shape of the lobes is important, as it can affect the engine performance at different speeds.
The lobe starts to open the intake valve as the piston begins its downward intake stroke and then closes when the piston is at the bottom of that stroke. When the piston is on its combustion stroke, the exhaust valve opens and the piston hits the bottom of the stroke. This piston pushing up forces the gases out, and the valve closes as the piston reaches the top.
In practice, the intake valve will actually remain open a little longer, especially as the vehicle’s speed increases. It’s the camshaft lobe’s profile that’s in charge of all this.
Single Overhead Cam
If your car has an overhead camshaft, it will have a single overhead cam if the cylinders are all in line. You only need one camshaft for each line of cylinders. In the case of an engine with a V in front of the number of cylinders, such as V8, there will be a total of two camshafts, one for each of the lines of cylinders.
An overhead camshaft operates differently to a regular camshaft. The lobes in an overhead camshaft move the rockers at the top of the cylinder, which in turn open and close the valves. Very strong springs operate the valves. With both styles over overhead camshafts, the camshaft itself is driven by the crankshaft, via the timing belt or the timing chain.
Dual Overhead Cams
With more powerful engines, where each cylinder has four (or possibly more) valves, dual overhead cams are employed. This means two camshafts for each line of cylinders, to be able to accommodate all the valves, which would be too many for a single camshaft. More fuel and air enter the cylinder, and the gases are expelled harder, making for more power.
Although most car engines really allow for just one timing, which is the rotation of the camshaft and the speed with which the valves open and close, there are variations. In some vehicles it is possible to alter the valve timing, or at least move it ahead a little. It shifts when the valve closes, for instance, or when it opens.
Similarly, the VTEC engines have a second camshaft just for the intake valves. This means those intake valves are open longer and so take in more fuel and air to be more powerful.