A camshaft is a shaft that contains a row of pointed cams, in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves),mechanically controlled ignition systems and early electric motor speed controllers.
Camshafts in piston engines are usually made from steel or cast iron, and the shape of the cams greatly affects the engine's characteristics.
Trip hammers are one of the early uses of a form of cam to convert rotating motion, e.g. from a waterwheel, into the reciprocating motion of a hammer used in forging or to pound grain. Evidence for these exists back to the Han Dynasty in China, and they were widespread by the medieval period.
The camshaft was described in 1206 by engineer Al-Jazari. He employed it as part of his automata, water-raising machines, and water clocks such as the castle clock.
Once the rotative version of the steam engine was developed in the late 18th century, the operation of the valve gear was usually by an eccentric, which turned the rotation of the crankshaft into reciprocating motion of the valve gear, normally a slide valve. Camshafts more like those seen later in internal combustion engines were used in some steam engines, most commonly where high pressure steam (such as that generated from a flash steam boiler), required the use of poppet valves, or piston valves. For examples see the Uniflow steam engine, and the Gardner-Serpollet steam cars, which also included axially sliding the camshaft to achieve variable valve timing.
Among the first cars to utilize engines with single overhead camshafts were the Maudslay designed by Alexander Craig and introduced in 1902and 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. As the cam rotates, the lobe presses on the valve (or an intermediate mechanism), thus pushing it open. Typically, a valve spring is used to push the valve in the opposite direction, thus closing the valve once the cam rotates past the highest point of its lobe.
Camshafts are made from metal and are usually solid, although hollow camshafts are sometimes used.The materials used for a camshaft are usually either:
Many early internal combustion engines used a cam-in-block layout (such flathead, IOE or T-head layouts), whereby 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 acts directly on those valves. In an overhead valve engine, which came later, the cam follower presses on a pushrod which transfers the motion to the top of the engine, where a rocker opens the intake/exhaust valve.Although largely replaced by SOHC and DOHC layouts in modern automobile engines, the older overhead valve layout is still used in many industrial engines, due to its smaller size and lower cost.
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. For OHC and DOHC engines, the camshaft operates the valve directly or via a short rocker arm.
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 camshafts per cylinder bank - is usually referred to as a double overhead camshaft engine (although colloquially they are sometimes referred to as "quad-cam" engines).
Accurate control of the position and speed of the camshaft is critically important in allowing the engine to operate correctly. The camshaft is usually driven either directly, via a toothed rubber "timing belt"' or via a steel roller "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.
Alternative drive systems used in the past include a vertical shaft with bevel gears at each end (e.g. pre-World War I Peugeot and Mercedes Grand Prix Cars and the Kawasaki W800 motorcycle) or a triple eccentric with connecting rods (e.g. the Leyland Eight car).
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, therefore the camshaft is geared to rotate at half the speed of the crankshaft.
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. [ citation needed ]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.
The most common methods of valve actuation involve camshafts and valve springs, however alternate systems have occasionally been used on internal combustion engines:
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 train motors (i.e. EMUs and locomotives).
VTEC is a system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM. The VTEC system uses two camshaft profiles and hydraulically selects between profiles. It was invented by Honda engineer Ikuo Kajitani. It is distinctly different from standard VVT systems which change only the valve timings and do not change the camshaft profile or valve lift in any way.
In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.
VVT-i, or Variable Valve Timing with intelligence, is an automobile variable valve timing technology developed by Toyota. The Toyota VVT-i system replaces the Toyota VVT offered starting in 1991 on the 5-valve per cylinder 4A-GE engine. The VVT system is a 2-stage hydraulically controlled cam phasing system.
MIVEC is the brand name of a variable valve timing (VVT) engine technology developed by Mitsubishi Motors. MIVEC, as with other similar systems, varies the timing of the intake and exhaust camshafts which increases the power and torque output over a broad engine speed range while also being able to help spool a turbocharger more quickly and accurately.
The GM Ecotec engine, also known by its codename L850, is a family of all-aluminium inline-four engines, displacing between 1.4 and 2.5 litres. While these engines were based on the GM Family II engine, the architecture was substantially re-engineered for the new Ecotec application produced since 2000. This engine family replaced the GM Family II engine, the GM 122 engine, the Saab H engine, and the Quad 4 engine. It is manufactured in multiple locations, to include Spring Hill Manufacturing, in Spring Hill, Tennessee while the engine block and cylinder heads are cast at Saginaw Metal Casting Operations in Saginaw, Michigan.
The Ford Modular engine is Ford Motor Company's overhead camshaft (OHC) V8 and V10 gasoline-powered small block engine family. Despite popular belief that the Modular engine family received its moniker from the sharing of engine parts across numerous Ford vehicle platforms, in reality, the Modular engine family was named as such by Ford Motor Company for the new "modular approach" to the setup of tooling and casting stations in the Windsor and Romeo engine manufacturing plants.
An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion chamber in the engine block.
The VK engine is a V8 piston engine from Nissan. It is an aluminum DOHC 4-valve design.
The powerplant used in Saturn S-Series automobiles was a straight-4 aluminum piston engine produced by Saturn, a subsidiary of General Motors. The engine was only used in the Saturn S-series line of vehicles from 1991 through 2002. It was available in chain-driven SOHC or DOHC variants.
Honda's first production V6 was the C series; it was produced in displacements from 2.0 to 3.5 liters. The C engine was produced in various forms for over 20 years (1985–2005), having first been used in the KA series Legend model, and its British sister car the Rover 800-series.
The Hyundai Beta engines are 1.6 L to 2.0 L I4 built in Ulsan, South Korea.
The Honda K-series engine is a line of four-cylinder four-stroke car engine introduced in 2001. The K-series engines are equipped with DOHC valvetrains and use roller rockers on the cylinder head to reduce friction. The engines use a coil-on-plug, distributorless ignition system with a coil for each spark plug. This system forgoes the use of a conventional distributor-based ignition timing system in favor of a computer-controlled system that allows the ECU to control ignition timings based on various sensor inputs. The cylinders have cast iron sleeves similar to the B- and F-series engines, as opposed to the FRM cylinders found in the H- and newer F-series engines found only in the Honda S2000.
In a piston engine, the valve timing is the precise timing of the opening and closing of the valves. In an internal combustion engine those are usually poppet valves and in a steam engine they are usually slide valves or piston valves.
A camless or free-valve piston engine is an engine that has poppet valves operated by means of electromagnetic, hydraulic, or pneumatic actuators instead of conventional cams. Actuators can be used to both open and close valves, or to open valves closed by springs or other means.
A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combustion chamber once combustion is completed.
The Lotus-Ford Twin Cam is an inline-four petrol engine developed by Lotus for the 1962 Lotus Elan. A few early examples displaced 1.5 litres, but the majority were 1.55-litre (1557ml) engines. It used a Ford 116E iron cylinder block and a new aluminium cylinder head with dual overhead camshafts. The Twin Cam was used in a variety of vehicles until Lotus stopped production in 1973. It was succeeded by the Lotus 907 engine.
A helical camshaft is a type of mechanical variable valve actuation (VVA) system. More specifically it is a camshaft that allows the valve opening duration to be varied over a wide, continuous, step-less range, with all of the added duration being at full valve lift.
Variable valve timing (VVT) is a system for varying the valve opening of an internal combustion engine. This allows the engine to deliver high power, but also to work tractably and efficiently at low power. There are many systems for VVT, which involve changing either the relative timing, duration or opening of the engine's inlet and exhaust valves.
Variable valve lift (VVL) is an automotive piston engine technology which varies the height a valve opens in order to improve performance, fuel economy or emissions. There are two main types of VVL: discrete, which employs fixed valve lift amounts, and continuous, which is able to vary the amount of lift. Continuous valve lift systems typically allow for the elimination of the throttle valve.