From our insanely detailed guide:
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- Needs illustrations
The camshaft controls the opens and closes the valves at the correct times for the engine to actually run. The valves and camshaft together make up the valve gear, or valvetrain.
[Illustration of camshaft]
A camshaft is a rotating shaft that consists of a number of pear-shaped lobes, known as cams. The cams push against the valves and the bulge in the cam will cause the valve to open. The camshaft is the most precisely engineered part of the entire car - it can be thought of as a mechanical computer program which tells each valve when to open, and how far to move when it does open.
[Illustration showing timing belt]
The valves need to open open and close at the right moment in relation to the pistons. That is to say, on the intake stroke we want the intake valve to be open, and on the exhaust stroke the exhaust valve is open. At all other times, the valves are closed. The valves must therefore be synchronised with the pistons, and this happens by connecting the camshaft to the crankshaft using a timing belt, timing chain or system of gears. The pistons turn the crankshaft, which turns the camshaft, which controls the valves.
[OHC vs OHV layouts]
There are two different arrangements in use that determine where the camshaft is placed in the engine. In the predominant engine design, known as overhead-cam or OHC, the camshaft is above the valves. The other arrangement is overhead-valve (OHV) and in this layout the camshaft will be installed in the engine block, below the valves. The valves are then driven by pushrods which act on rocker arms. There are advantages and disadvantages to both systems, but overhead cam engines are almost universal nowadays, with the exception of big V8 engines produced by Chevrolet. In this guide we’ll talk mainly about OHC arrangements because they are more direct and easily understood.
[Twin cam layout and single cam]
For OHC engines, there may be a single camshaft in an engine, or there may be one for the intake valves and one for the exhaust valves - this is the most common layout for modern engines, known as a twin-cam engine or a double overhead-cam (DOHC). Where a single camshaft is used, known as a single-overhead cam (SOHC) arrangement, the camshaft will act through rocker arms on each bank of valves.
[V and W shaped engines with heads]
In V-shaped and W-shaped engines with an OHC layout, there will be a one or two camshafts for each bank of cylinders. In the Bugatti W16 engine, which has a DOHC layout, there are two banks of cylinders and so two camshafts for each bank, giving four camshafts in total.
The camshaft is installed into the cylinder head with caps bolted down to secure the camshaft in its bore. The camshaft, like the crankshaft, rotates on a film of oil which is forced into the clearance between the camshaft journal and the bore through a hole. As with the crankshaft, the clearance between the camshaft and its bore is finely specified to ensure the appropriate flow of oil to maintain the crankshaft clearance.
The layout and shape of a camshaft determines the performance of the engine to a large degree. The profile of a cam dictates not only when each valve should open and close, but also the speed that it moves, and how far it should open.
The shape of the cam lobes set the performance parameters of the engine. We’ll have a full article on camshaft geometry including explanations of cam profile, lift, duration and timing.
As discussed, the rotating cam lobes push down on the valves to control their opening. The camshaft is rotating, and so the cam lobes exert a sideways force as they rotate. If this sideways force acted on the thin valve stem it would cause bending and distortion. To avoid this a cylindrical spacer called a cam lifter (also cam follower, tappet, or lifter) is installed between the tip of the valve stem and the cam. Followers are used for several reasons, but the main being to ensure that the force acting on the valve stem is purely pushing it down and not acting sideways. The wider, flat surface of the follower allows the cam lobe to slide more easily.
Valve clearance (lash)
When closed, the valve face should be tightly held against its seat. To ensure this, a small clearance is maintained between the lifter and the cam lobe. It’s important that this clearance is accurate, too little clearance and there is a risk that the valve doesn’t seat properly. Too much and there will be excess noise from the valve gear as the parts rattle against each other.
[Graph of valve clearance]
Valve clearance effectively cuts off the bottom profile of the cam lobe, the very first section from the valve being closed, to beginning to open.
The camshaft is driven around by a toothed gear, which receives drive through the timing belt or chain, from the crankshaft. The four-stroke cycle requires two rotations of the crankshaft for a complete cycle, and so the camshaft is driven at half the speed of the crankshaft. That is to say, the crankshaft rotates twice for every one rotation of the camshaft.
Because the camshaft encodes the entire four stroke cycle for all cylinders, it is possible to determine the engine’s position in the cycle from the position of the camshaft. A camshaft position sensor (also called a camshaft angle sensor) is used to read this information, and the data is consumed by the engine’s management computer, then used to determine the timing of other operations such as fuel injection and ignition. The camshaft position sensor is the most important sensor in the engine management system - it’s failure almost guarantees that the engine won’t run at all - because the computer won’t know which cylinders need sparks or fuel at a given moment.
Installing a camshaft with a different profile can change the performance of an engine. Camshafts for engines that are popular for modifying will be available in a range of profiles. Depending on their purpose, they may increase high speed performance by holding valves open longer, or decrease the valve lift to increase low speed torque.
Symetrical - different ramp on the opening and closing sides of the camshaft.
Aggressive - opens valves fast.
Pointy - High lift.
Asymetrical will have a steeper ramp to get the valve open fast, and more gentle closing side to cushion the valve on closing.
Camshafts can alter the sound of an engine - longer exhaust duration pushing more growl into the exhaust.
Valve lift vs lobe lift depends on the rocker arm ratio.
Higher duration is better at high RPM, where more time is needed to get gases into and out of the cylinder. At low RPM there's time to do this, and so longer duration would lead to reduced pumping pressure.