Camshaft

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A camshaft operating two valves Nockenwelle ani.gif
A camshaft operating two valves

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), [1] [2] mechanically controlled ignition systems and early electric motor speed controllers.

Contents

Camshafts in piston engines are usually made from steel or cast iron, and the shape of the cams greatly affects the engine's characteristics.

History

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.

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 1902 [3] [4] [5] and the Marr Auto Car designed by Michigan native Walter Lorenzo Marr in 1903. [6] [7]

Piston engines

DOHC cylinder head with intake camshaft lobe highlighted in blue Cabeca de motor vista em corte cames.PNG
DOHC cylinder head with intake camshaft lobe highlighted in blue

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. [8]

Construction

Camshaft produced from billet steel Nockenwelle 2005.jpg
Camshaft produced from billet steel

Camshafts are made from metal and are usually solid, although hollow camshafts are sometimes used. [9] The materials used for a camshaft are usually either:

Location in engine

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. [13] 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. [13]

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). [14]

Drive systems

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. [15] 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.

Performance characteristics

Duration

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. [16] [17] [18]

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. [16] [18] 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. [16] [18] 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 of the lobe 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 lobe bump that is observed on lower duration camshafts.

Lift

The camshaft's lift determines the distance between the valve and the valve seat (i.e. how far open the valve is). [19] 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). [20]

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. [18] Secondly, increased lift means a steeper camshaft profile is required, which increases the forces needed to open the valve. [19] 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. [21] This could be a result of a very steep rise of the lobe, [18] 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. [20] [21]

Timing

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. [22] The required changes are relatively small, often in the order of 5 degrees.[ citation needed ]

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.

Lobe separation angle

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. [23] A higher LSA reduces overlap, which improves idle quality and intake vacuum, [22] however using a wider LSA to compensate for excessive duration can reduce power and torque outputs. [20] In general, the optimal LSA for a given engine is related to the ratio of the cylinder volume to intake valve area. [20]

Functionality

Camshafts are integral components of internal combustion engines, responsible for controlling the opening and closing of the engine's intake and exhaust valves. As the camshaft rotates, its lobes push against the valves, allowing the intake of air and fuel and the expulsion of exhaust gases. This synchronized process is crucial for optimizing engine performance, fuel efficiency, and emissions control. Without precisely engineered camshafts, the smooth and efficient operation of an engine would be compromised. [24]

Alternatives

The most common methods of valve actuation involve camshafts and valve springs, however alternate systems have occasionally been used on internal combustion engines:

Electric motor speed controllers

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). [27]

See also

Related Research Articles

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A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum in the cylinder through its downward motion.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
  3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
  4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust port.
<span class="mw-page-title-main">Cylinder head</span> Component of an internal combustion engine

In a piston engine, the cylinder head sits above the cylinders, forming the roof of the combustion chamber. In sidevalve engines the head is a simple plate of metal containing the spark plugs and possibly heat dissipation fins. In more modern overhead valve and overhead camshaft engines, the head is a more complicated metal block that also contains the inlet and exhaust passages, and often coolant passages, Valvetrain components, and fuel injectors.

<span class="mw-page-title-main">VTEC</span> Automobile variable valve timing technology

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.

<span class="mw-page-title-main">Variable valve timing</span> Process of altering the timing of a valve lift event

Variable valve timing (VVT) is the process of altering the timing of a valve lift event in an internal combustion engine, 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.

<span class="mw-page-title-main">VVT-i</span> Automobile variable valve timing technology

VVT-i, or Variable Valve Timing with intelligence, is an automobile variable valve timing petrol engine technology manufactured by Toyota Group and used by brands Groupe PSA, Toyota, Lexus, Scion, Daihatsu, Subaru, Aston Martin, Pontiac and Lotus Cars. It was introduced in 1995 with the 2JZ-GE engine found in the JZS155 Toyota Crown and Crown Majesta.

<span class="mw-page-title-main">MIVEC</span> Automobile variable valve timing technology

MIVEC (Mitsubishi Innovative Valve timing Electronic Control system) 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.

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<span class="mw-page-title-main">Ford Modular engine</span> Engine family by Ford Motor Company

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<span class="mw-page-title-main">Overhead camshaft engine</span> Valvetrain configuration

An overhead camshaft (OHC) engine is a piston engine in which 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.

<span class="mw-page-title-main">Nissan VK engine</span> Reciprocating internal combustion engine

The VK engine is a V8 piston engine from Nissan. It is an aluminum DOHC 4-valve design.

<span class="mw-page-title-main">Nissan RB engine</span> Reciprocating internal combustion engine

The RB engine is an oversquare 2.0–3.0 L straight-6 four-stroke gasoline engine from Nissan, originally produced from 1985 to 2004. The RB followed the 1983 VG-series V6 engines to offer a full, modern range in both straight or V layouts. It was part of a new engine family name PLASMA.

The Hyundai Beta engines are 1.6 L to 2.0 L I4 built in Ulsan, South Korea.

<span class="mw-page-title-main">Tappet</span> Internal combustion engine part

A tappet or valve lifter is a valve train component which converts rotational motion into linear motion in activating a valve. It is most commonly found in internal combustion engines, where it converts the rotational motion of the camshaft into linear motion of intake and exhaust valves, either directly or indirectly.

<span class="mw-page-title-main">Honda K engine</span> Reciprocating internal combustion engine

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.

<span class="mw-page-title-main">Toyota S engine</span> Reciprocating internal combustion engine

The Toyota S Series engines are a family of straight-four petrol engines with displacements between 1.8 and 2.2 litres, produced by Toyota Motor Corporation from January 1980 to August 2007. The S series has cast iron engine blocks and aluminium cylinder heads. This engine was designed around the new LASRE technology for lighter weight – such as sintered hollow camshafts.

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.

<span class="mw-page-title-main">Valvetrain</span> Mechanical system in an internal combustion engine

A valvetrain 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 gases out of the combustion chamber once combustion is completed.

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.

References

  1. ↑ "The 4 Strokes of an Engine". help.summitracing.com. Retrieved 2020-06-10.
  2. ↑ "How Camshafts Work". HowStuffWorks. 2000-12-13. Retrieved 2020-06-10.
  3. ↑ Georgano, G. N. (1982). The New Encyclopedia of Motorcars 1885 to the Present (Third ed.). New York: E. P. Dutton. p. 407. ISBN   0525932542. LCCN   81-71857.
  4. ↑ Culshaw, David; Horrobin, Peter (2013). The Complete Catalogue of British Cars 1895 – 1975. Poundbury, Dorchester, UK: Veloce Publishing. p. 210. ISBN   978-1-845845-83-4.
  5. ↑ Boddy, William (January 1964). "Random Thoughts About O.H.C." Motor Sport (1). London, UK: Teesdale Publishing: 906. Retrieved 7 June 2020.
  6. ↑ "Marr Auto Car Company". www.marrautocar.com. Archived from the original on 8 February 2014.
  7. ↑ Kimes, Beverly Rae (2007). Walter L Marr: Buick's Amazing Engineer. Racemaker Press. p. 40. ISBN   978-0976668343.
  8. ↑ "Lunati Cam Profile Terms". www.lunatipower.com. Retrieved 2020-06-10.
  9. ↑ "What kind of camshaft – made of steel or cast iron?". www.camshaftkits.com. Archived from the original on 2020-09-20.
  10. ↑ "Custom Ground Cam - Affordable Custom Cam Grind - Circle Track". Hot Rod. 2004-04-19. Retrieved 2020-06-10.
  11. ↑ "Custom-made billet camshafts: – Moore Good Ink" . Retrieved 2020-06-10.
  12. ↑ "Linamar Buying Mubea Camshaft Operations". www.forgingmagazine.com. Retrieved 7 June 2020.
  13. 1 2 Sellén, Magnus (2019-07-24). "DOHC Vs. SOHC - What's The Difference Between Them?". Mechanic Base. Retrieved 2020-06-10.
  14. ↑ "What is Quad-cam engine?". www.carspector.com. Retrieved 7 June 2020.
  15. ↑ "The V8: Birth and Beginnings". www.rrec.org.uk. Archived from the original on 15 March 2016. Retrieved 12 July 2020.
  16. 1 2 3 "Secrets Of Camshaft Power". www.hotrod.com. 1 December 1998. Retrieved 18 July 2020.
  17. ↑ "Camshaft RPM Range". www.summitracing.com. Retrieved 18 July 2020.
  18. 1 2 3 4 5 "Understanding Camshaft Fundamentals". www.jegs.com. Retrieved 18 July 2020.
  19. 1 2 "Camshaft Lift". www.summitracing.com.
  20. 1 2 3 4 "Be The Camshaft Expert". www.hotrod.com. 14 June 2006. Retrieved 18 July 2020.
  21. 1 2 "What is valve float?". www.summitracing.com. Retrieved 18 July 2020.
  22. 1 2 "COMP Cams Effect of Changes In Cam Timing and Lobe Separation Angle". www.compcams.com. Retrieved 19 July 2020.
  23. ↑ "Camshaft Lobe Separation". www.summitracing.com. Retrieved 19 July 2020.
  24. ↑ "Vehicle Camshafts". www.kelfordcams.com. Retrieved 4 January 2024.
  25. ↑ "Koenigsegg Gemera - Technical specifications". www.koenigsegg.com. Archived from the original on 3 March 2020. Retrieved 19 July 2020.
  26. ↑ "The Future of the Internal Combustion Engine – Inside Koenigsegg". www.youtube.com. The Drive. Archived from the original on 2021-11-18. Retrieved 7 June 2020.
  27. ↑ "Electric Locomotives – The Railway Technical Website". www.railway-technical.com. Retrieved 7 June 2020.
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