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 internal combustion engines with pistons, the camshaft is used to operate poppet valves. It consists of a cylindrical rod running the length of the cylinder bank with a number of oblong lobes protruding from it, one for each valve. The cam lobes force the valves open by pressing on the valve, or on some intermediate mechanism, as they rotate.
Camshafts can be made out of several types of material. These include:
Chilled iron castings: Commonly used in high volume production, chilled iron camshafts have good wear resistance since the chilling process hardens them. Other elements are added to the iron before casting to make the material more suitable for its application.
Billet Steel: When a high-quality camshaft or low-volume production is required, engine builders and camshaft manufacturers choose steel billet. This is a much more time-consuming process, and is generally more expensive than other methods. However, the finished product is far superior. CNC lathes, CNC milling machines, and CNC camshaft grinders will be used during production. Different types of steel bar can be used, one example being EN40b. When manufacturing a camshaft from EN40b, the camshaft will also be heat treated via gas nitriding, which changes the micro-structure of the material. It gives a surface hardness of 55-60 HRC. These types of camshafts can be used in high-performance engines.
Camshaft are typically produced by forging steel, casting or CNC machining and more recently using hydroforming technology. Camshaft can be solid or hollow to reduce weight such as camshaft used in BMW N52 motor.
The relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance. Since the valves control the flow of the air/fuel mixture intake and exhaust gases, they must be opened and closed at the appropriate time during the stroke of the piston. For this reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a belt or chain called a timing belt or timing chain. Direct drive using gears is unusual because of the cost. The frequently reversing torque caused by the slope of the cams tends to cause gear rattle which for an all-metal gear train requires further expense of a cam damper. Rolls-Royce V8 (1954) used gear drive as, unlike chain, it could be made silent and to last the life of the engine. Where gears are used in cheaper cars, they tend to be made from resilient fibre rather than metal, except in racing engines that have a high maintenance routine. Fibre gears have a short life span and must be replaced regularly, much like a timing belt. In some designs the camshaft also drives the distributor and the oil and fuel pumps. Some vehicles may have the power steering pump driven by the camshaft. With some early fuel injection systems, cams on the camshaft would operate the fuel injectors. Honda redesigned the VF750 motorcycle from chain drive to the gear drive VFR750 due to insurmountable problems with the VF750 Hi-Vo inverted chain drive.
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 timing of the camshaft can be advanced to produce better low RPM torque, or retarded for better high RPM power. Changing cam timing moves the overall power produced by the engine down or up the RPM scale. The amount of change is very little (usually < 5 deg), and affects valve to piston clearances. Refer to this video.
Duration is the number of crankshaft degrees of engine rotation during which the valve is off the seat. In general, greater duration results in more horsepower. The RPM at which peak horsepower occurs is typically increased as duration increases at the expense of lower rpm efficiency (torque).
Duration specifications can often be misleading because manufacturers may select any lift point from which to advertise a camshaft's duration and sometimes will manipulate these numbers. The power and idle characteristics of a camshaft rated at a .006" lift point will be much different from one with the same rating at a .002" lift point.
Many performance engine builders gauge a race profile's aggressiveness by looking at the duration at .020", .050" and .200". The .020" number determines how responsive the motor will be and how much low end torque the motor will make. The .050" number is used to estimate where peak power will occur, and the .200" number gives an estimate of the power potential.
A secondary effect of increased duration can be increased overlap, which is the number of crankshaft degrees during which both intake and exhaust valves are off their seats. 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 intake and exhaust lobe centers are moved apart to compensate.
The camshaft "lift" is the resultant net rise of the valve from its seat. The farther the valve rises from its seat the more airflow can be provided, which is generally more beneficial. Greater lift has some limitations. Firstly, lift is limited by the increased proximity of the valve head to the piston crown and secondly, greater effort is required to move the valve springs to a higher state of compression. Increased lift can also be limited by lobe clearance in the cylinder head casting. Higher valve lift can have the same effect as increased duration where valve overlap is less desirable.
Higher lift allows greater airflow; although even by allowing a larger volume of air to pass through the larger opening, the brevity of the typical duration with a higher lift cam results in less airflow than with a cam with lower lift but more duration, all else being equal. On forced induction motors this higher lift could yield better results than longer duration, particularly on the intake side. Notably though, higher lift has more potential problems than increased duration, in particular as valve train rpm rises which can result in less efficient running or loss of torque.
Cams that have excessive valve lift, running at high rpm, can cause what is called "valve float", where the valve spring tension is insufficient to keep the valve following the cam at its apex. This could also be a result of a very steep rise of the lobe, where the valve is effectively shot off the end of the cam rather than following the cams’ profile. This is typically what happens when a motor over revs. This is where the engine rpm exceeds the maximum design rpm. The valve train is typically the limiting factor in determining the maximum rpm the engine can maintain either for a prolonged period or temporarily. Sometimes an over rev can cause engine failure when the valves become bent as a result of colliding with the piston crowns.
Depending on the location of the camshaft, the cam operates the valves either directly or through a linkage of pushrods and rockers. Direct operation involves a simpler mechanism and leads to fewer failures, but requires the camshaft to be positioned at the top of the cylinders. In the past when engines were not as reliable as today this was seen as too much trouble, but in modern gasoline engines the overhead cam system, where the camshaft is on top of the cylinder head, is quite common.
Number of camshafts
While today some engines rely on a single camshaft per cylinder bank, which is known as a single overhead camshaft (SOHC), most[quantify] modern engines are driven by a two camshafts per cylinder bank arrangement (one camshaft for the intake valves and another for the exhaust valves); such camshaft arrangement is known as a double or dual overhead cam (DOHC), thus, a V engine, which has two separate cylinder banks, may have four camshafts (colloquially known as a quad-cam engine).
More unusual is the modern W engine (also known as a 'VV' engine to distinguish itself from the pre-war W engines) that has four cylinder banks arranged in a "W" pattern with two pairs narrowly arranged with a 15-degree separation. Even when there are four cylinder banks (that would normally require a total of eight individual camshafts), the narrow-angle design allows the use of just four camshafts in total. For the Bugatti Veyron, which has a 16-cylinder W engine configuration, the four camshafts are driving a total of 64 valves.
The overhead camshaft design adds more valvetrain components that ultimately result in more complexity and higher manufacturing costs, but this is easily offset by many advantages over the older design: multi-valve design, higher RPM limit, and design freedom to better place valves, spark plugs (Spark-ignition engine), and intake/exhaust ports.
The rockers or cam followers sometimes incorporate a mechanism to adjust the valve lash through manual adjustment, but most modern auto engines have hydraulic lifters, eliminating the need to adjust the valve lash at regular intervals as the valvetrain wears, in particular the valves and valve seats in the combustion chamber.
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 lubricant motor oils contain additives specifically 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. You can put new lifters on an old cam but never old lifters on a new cam. In some engines the followers have rollers which eliminate the sliding friction and wear but add mass to the valvetrain.
Camshaft bearings are similar to crankshaft main bearings, being pressure-fed with oil. However, overhead camshaft bearings do not always have replaceable , meaning that a new cylinder head is required if the bearings suffer wear due to insufficient or dirty oil.
In addition to mechanical friction, considerable force is required to compress the valve springs used to close the engine's valves. This can amount to an estimated 25% of an engine's total output at idle, reducing overall efficiency. Some approaches to reclaiming this "wasted" energy include:
- Springless valves, like the desmodromic system employed today by Ducati
- Camless valvetrains using solenoids or magnetic systems have long been investigated by BMW and Fiat, and are currently being prototyped by Valeo and Ricardo
- The Wankel engine, a rotary engine which uses neither pistons nor valves, best known for being used by Mazda in the RX-7 and RX-8 sports cars.
- Koenigsegg has developed an electric valve actuator as a more fuel efficient and space saving alternative to the traditional camshaft.
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.
|Components of a typical, four-stroke cycle, DOHC piston engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug, (V) Valves, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow.|
|Double overhead cams control the opening and closing of a cylinder's valves.
|Valve timing gears on a Ford Taunus four-cylinder engine — the small gear is on the crankshaft, the larger gear is on the camshaft. The gear ratio causes the camshaft to run at half the RPM of the crankshaft.|
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