A camshaft is a rotating object-- usually made of metal-- that contains pointed cams, which converts rotational motion to reciprocal motion. Camshafts are used in internal combustion engines (to operate the intake and exhaust valves), mechanically controlled ignition systems and early electric motor speed controllers. Camshafts in automobiles are made from steel or cast iron, and are a key factor in determining the RPM range of an engine's power band.
Among the first cars to utilize engines with single overhead camshafts were the Maudslay designed by Alexander Craig and introduced in 1902 and the Marr Auto Car designed by Michigan native Walter Lorenzo Marr in 1903.
In piston engines, the camshaft is used to operate the intake and exhaust valves. The camshaft consists of a cylindrical rod running the length of the cylinder bank with a number of cams (discs with protruding cam lobes) along its length, one for each valve. A cam lobe forces a valve open by pressing on the valve, or on some intermediate mechanism, as it rotates. Meanwhile, a spring exerts a tension pulling the valve toward its closed position. When the lobe reaches its highest displacement on the push rod, the valve is completely open. The valve is closed when the spring pulls it back and the cam is on its base circle.
Camshafts are made from metal and are usually solid, although hollow camshafts are sometimes used. The materials used for a camshaft are usually either:
Most early internal combustion engines used a cam-in-block layout (such as overhead valves), where the camshaft is located within the engine block near the bottom of the engine. Early flathead engines locate the valves in the block and the cam act directly on those valves. In an overhead valve engine, which came later, the cam follower (lifter) transfers its motion to valves at the top of the engine via a pushrod and a rocker arm lever. As engine speeds increased through the 20th century, single overhead camshaft (SOHC) engines-- where the camshaft is located within the cylinder head near the top of the engine-- became increasingly common, followed by double overhead camshaft (DOHC) engines in more recent years. Note that many modern industrial and automotive engines continue to use the overhead valve design (with the cam mounted low in the engine block) which allows lower overall engine height than a similar overhead cam design.
The valvetrain layout is defined according to the number of camshafts per cylinder bank. Therefore a V6 engine with a total of four camshafts (two per cylinder bank) is usually referred to as a double overhead camshaft engine, although colloquially they are sometimes referred to as "quad-cam" engines.
In an overhead valve engine, the camshaft presses on a pushrod which transfers the motion to the top of the engine, where a rocker opens the intake/exhaust valve. For OHC and DOHC engines, the camshaft operates the valve directly or via a short rocker arm.
Accurate control of the position and speed of the camshaft is critically important in allowing the engine to operate correctly. The camshaft is driven, universally, at exactly half the speed of the crankshaft either directly, usually via a toothed rubber timing belt or a steel roller chain (called a timing chain). Gears have also occasionally been used to drive the camshaft. In some designs the camshaft also drives the distributor, oil pump, fuel pump and occasionally the power steering pump. In severe service applications, such as farm tractors, industrial engines, piston driven aircraft engines, heavy trucks and racing engines, gear driven camshafts are common, given their mechanical simplicity and long service life.
An alternative used in the early days of OHC engines was to drive the camshaft(s) via a vertical shaft with bevel gears at each end. This system was, for example, used on the pre-World War I Peugeot and Mercedes Grand Prix cars. Another option was to use a triple eccentric with connecting rods; these were used on certain W.O. Bentley-designed engines and also on the Leyland Eight.
In a two-stroke engine that uses a camshaft, each valve is opened once for every rotation of the crankshaft; in these engines, the camshaft rotates at the same speed as the crankshaft. In a four-stroke engine, the valves are opened only half as often; thus, two full rotations of the crankshaft occur for each rotation of the camshaft.
The camshaft's duration determines how long the intake/exhaust valve is open for, therefore it is a key factor in the amount of power that an engine produces. A longer duration can increase power at high engine speeds (RPM), however this can come with the trade-off of less torque being produced at low RPM.
The duration measurement for a camshaft is affected by the amount of lift that is chosen as the start and finish point of the measurement. A lift value of 0.050 in (1.3 mm) is often used as a standard measurement procedure, since this is considered most representative of the lift range that defines the RPM range in which the engine produces peak power. The power and idle characteristics of a camshaft with the same duration rating that has been determined using different lift points (for example 0.006 or 0.002 inches) could be much different to a camshaft with a duration rated using lift points of 0.05 inches.
A secondary effect of increased duration can be increased overlap, which determines the length of time that both the intake and exhaust valves are open. It is overlap which most affects idle quality, in as much as the "blow-through" of the intake charge immediately back out through the exhaust valve which occurs during overlap reduces engine efficiency, and is greatest during low RPM operation. In general, increasing a camshaft's duration typically increases the overlap, unless the Lobe Separation Angle is increased to compensate.
A lay person can readily spot a long duration camshaft by observing the broad surface where the cam pushes the valve open for a large number of degrees of crankshaft rotation. This will be visibly greater than the more pointed camshaft bump than is observed on lower duration camshafts.
The camshaft's lift determines the distance between the valve and the valve seat (i.e. how far open the valve is). The farther the valve rises from its seat the more airflow can be provided, thus increasing the power produced. Higher valve lift can have the same effect of increasing peak power as increased duration, without the downsides caused by increased valve overlap. Most overhead valve engines have a rocker ratio of greater than one, therefore the distance that the valve opens (the valve lift) is greater than the distance from the peak of the camshaft's lobe to the base circle (the camshaft lift).
There are several factors which limit the maximum amount of lift possible for a given engine. Firstly, increasing lift brings the valves closer to the piston, so excessive lift could cause the valves to get struck and damaged by the piston. Secondly, increased lift means a steeper camshaft profile is required, which increases the forces needed to open the valve. A related issue is valve float at high RPM, where the spring tension does not provide sufficient force to either keep the valve following the cam at its apex or prevent the valve from bouncing when it returns to the valve seat. This could be a result of a very steep rise of the lobe, where the cam follower separates from the cam lobe (due to the valvetrain inertia being greater than the closing force of the valve spring), leaving the valve open for longer than intended. Valve float causes a loss of power at high RPM and in extreme situations can result in a bent valve if it gets struck by the piston.
The timing (phase angle) of the camshaft relative to the crankshaft can be adjusted to shift an engine's power band to a different RPM range. Advancing the camshaft (shifting it to ahead of the crankshaft timing) increases low RPM torque, while retarding the camshaft (shifting it to after the crankshaft) increases high RPM power. The required changes are relatively small, often in the order of 5 degrees.
Modern engines which have variable valve timing are often able to adjust the timing of the camshaft to suit the RPM of the engine at any given time. This avoids the above compromise required when choosing a fixed cam timing for use at both high and low RPM.
The lobe separation angle (LSA, also called lobe centreline angle) is the angle between the centreline of the intake lobes and the centreline of the exhaust lobes. A higher LSA reduces overlap, which improves idle quality and intake vacuum, however using a wider LSA to compensate for excessive duration can reduce power and torque outputs. In general, the optimal LSA for a given engine is related to the ratio of the cylinder volume to intake valve area.
Many older engines required manual adjustment of the rockers or pushrods in order to maintain the correct valve lash as the valvetrain wears (in particular the valves and valve seats). However, most modern auto engines have hydraulic lifters which automatically compensate for wear, eliminating the need to adjust the valve lash at regular intervals.
Sliding friction between the surface of the cam and the cam follower which rides upon it can be considerable. In order to reduce wear at this point, the cam and follower are both surface hardened, and modern motor oils contain additives to reduce sliding friction. The lobes of the camshaft are usually slightly tapered and the faces of the valve lifters slightly domed, causing the lifters to rotate to distribute wear on the parts. The surfaces of the cam and follower are designed to "wear in" together, and therefore each follower should stay with its original cam lobe and never be moved to a different lobe. Some engines (particularly those with steep camshaft lobes) use roller tappets to reduce the sliding friction on the camshaft. If the lift of a camshaft is increased or the operational revolutions per minute of an engine is increased, valve spring pressure may need to be increased as well, to maintain physical contact of the lifter with the camshaft.
The bearings of camshafts, similar to those for the crankshaft, are plain bearings that are pressure-fed with oil. However, overhead camshaft bearings do not always have replaceable shells, in which case the whole cylinder head must be replaced if the bearings are faulty.
In addition to mechanical friction, considerable force is required to open the valves against the resistance provided by the valve springs. This can amount to an estimated 25% of an engine's total output at idle,.
The following alternate systems have been used on internal combustion engines:
In mechanically timed ignition systems, a separate cam in the distributor is geared to the engine and operates a set of breaker points that trigger a spark at the correct time in the combustion cycle.
Before the advent of solid state electronics, camshaft controllers were used to control the speed of electric motors. A camshaft, driven by an electric motor or a pneumatic motor, was used to operate contactors in sequence. By this means, resistors or tap changers were switched in or out of the circuit to vary the speed of the main motor. This system was mainly used in electric multiple units and electric locomotives.