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The Info List - Valvetrain


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A valve train or valvetrain is a mechanical system that controls operation of the valves in an internal combustion engine,[1] in which a sequence of components transmits motion throughout the assembly. A traditional reciprocating internal combustion engine uses valves to control air and fuel flow into and out of the cylinders, facilitating combustion.[2]

Contents

1 Layout 2 Parts 3 See also 4 References 5 External links

Layout[edit] The valve train consists of valves, rocker arms, pushrods, lifters, and camshaft(s).[3] Valve
Valve
train opening/closing and duration, as well as the geometry of the valve train, controls the amount of air and fuel entering the combustion chamber at any given point in time.[4] Timing for open/close/duration is controlled by the camshaft that is synchronized to the crankshaft by a chain, belt, or gear. Valve
Valve
trains are built in several configurations, each of which varies slightly in layout but still performs the task of opening and closing the valves at the time necessary for proper operation of the engine. These layouts are differentiated by the location of the camshaft within the engine:

Cam-in-block The camshaft is located within the engine block, and operates directly on the valves, or indirectly via pushrods and rocker arms. Because they often require pushrods they are often called pushrod engines.

Overhead camshaft The camshaft (or camshafts, depending on the design employed) is located above the valves within the cylinder head, and operates either indirectly or directly on the valves.

Camless This layout uses no camshafts at all. Technologies such as solenoids are used to individually actuate the valves.

Parts[edit] The valve train is the mechanical system responsible for operation of the valves. Valves are usually of the poppet type, although many others have been developed such as sleeve, slide, and rotary valves. Poppet valves typically require small coil springs, appropriately named valve springs, to keep them closed when not actuated by the camshaft. They are attached to the valve stem ends, seating within spring retainers. Other mechanisms can be used in place of valve springs to keep the valves closed: Formula 1 engines employ pneumatic valve springs in which pneumatic pressure closes the valves, while motorcycle manufacturer Ducati
Ducati
uses desmodromic valve drive which mechanically close the valves. Depending on the design used, the valves are actuated directly by a rocker arm, finger, or bucket tappet. Overhead camshaft
Overhead camshaft
engines use fingers or bucket tappets, upon which the cam lobes contact, while pushrod engines use rocker arms. Rocker arms are actuated by a pushrod, and pivot on a shaft or individual ball studs in order to actuate the valves. Pushrods are long, slender metal rods seated within the engine block. At the bottom ends the pushrods are fitted with lifters, either solid or hydraulic, upon which the camshaft, located within the cylinder block, makes contact. The camshaft pushes on the lifter, which pushes on the pushrod, which pushes on the rocker arm, which rotates and pushes down on the valve. Camshafts must actuate the valves at the appropriate time in the combustion cycle. In order to accomplish this the camshaft is linked to and kept in synchronisation with the crankshaft (the main shaft upon which the pistons act) through the use of a metal chain, rubber belt, or geartrain. Because these mechanisms are essential to the proper timing of valve actuation they are named timing chains, timing belts, and timing gears, respectively. Typical normal-service engine valve-train components may be too lightweight for operating at high revolutions per minute (RPM), leading to valve float.[5] This occurs when the action of the valve no longer completely opens or closes, such as when the valve spring force is insufficient to close the valve (it does not fully rest on its seat even though the cam would allow the valve to close) causing a loss of control of the valvetrain, as well as a drop in power output.[6] Valve float will damage the valvetrain over time, and could cause the valve to be damaged as it is still partially open while the piston comes to the top of its stroke.[7] Upgrading to high pressure valve springs could allow higher valvetrain speeds, but this would also overload the valvetrain components and cause excessive and costly wear.[8] High-output and engines used in competition feature camshafts and valvetrain components that are designed to withstand higher RPM ranges.[9] These changes also include additional modifications such as larger-sized valves combined with freer breathing intake and exhaust ports to improve air flow.[10] Automakers offer factory-approved performance parts to increase engine output, and numerous aftermarket parts vendors specialize in valvetrain modifications for various engine applications.[11][12] See also[edit]

Cam-in-block Overhead camshaft Camless

References[edit]

^ Brain, Marshall (5 April 2000). "How Car
Car
Engines Work". HowStuffWorks. Retrieved 29 January 2014.  ^ "Sci-Tech Dictionary: "valve train"". Answers.com. Retrieved 29 January 2014.  ^ "The Valve
Valve
Train". AutoEducation. Retrieved 29 January 2014.  ^ Scraba, Wayne (October 2000). " Camshaft
Camshaft
Tips & Definitions". Hot Rod. Retrieved 29 January 2014.  ^ Cranswick, Marc (2011). The Cars of American Motors: An Illustrated History. McFarland. p. 80. ISBN 9780786446728. Retrieved 29 January 2014.  ^ Vizard, David (1992). How to Build and Modify Chevrolet Small-Block V-8 Camshafts and Valves. Motorbooks International. p. 114. ISBN 9780879385958. Retrieved 29 January 2014.  ^ Forst, Sarah (2008). How to build performance Nissan sport compacts, 1991-2006. HP Books. p. 29. ISBN 9781557885418. Retrieved 29 January 2014.  ^ Ellinger, Herbert E. (1974). Automotive engines. Prentice-Hall. p. 171. ISBN 9780130554260. Retrieved 29 January 2014.  ^ Tom, David (2013). The Cars of Trans-Am Racing: 1966-1973. CarTech. p. 32. ISBN 9781613250518. Retrieved 29 January 2014.  ^ Sessler, Peter C. (2010). "24: American Motors Corporation V-8s, 1958-1991". Ultimate American V-8 Engine Data Book (Second ed.). MBI Publishing. p. 229. ISBN 9780760336816. Retrieved 29 January 2014.  ^ Fletcher, Mark; Truesdell, Richard (2012). Hurst Equipped: More Than 50 Years of High Performance. CarTech. p. 63. ISBN 9781934709313. Retrieved 29 January 2014.  ^ Shepard, Larry (1989). How to Hot Rod Small-block Mopar Engines. HPBooks. pp. 74–88. ISBN 9780895864796. Retrieved 29 January 2014. 

External links[edit]

Animation (OHV, OHC and DOHC)

v t e

Automotive engine

Part of the Automobile series

Basic terminology

Bore Compression ratio Crank Cylinder Dead centre Diesel engine Dry sump Engine balance Engine configuration Engine displacement Engine knocking Firing order Hydrolock Petrol engine Power band Redline Spark-ignition engine Stroke Stroke ratio Wet sump

Main components

Connecting rod Crankcase Crankpin Crankshaft Crossplane Cylinder bank Cylinder block Cylinder head
Cylinder head
(crossflow, reverse-flow) Flywheel Head gasket Hypereutectic piston Main bearing Piston Piston
Piston
ring Starter ring gear Sump

Valvetrain

Cam Cam
Cam
follower Camshaft Desmodromic
Desmodromic
valve Hydraulic tappet Multi-valve Overhead camshaft Overhead valve Pneumatic valve springs Poppet valve Pushrod Rocker arm Sleeve valve Tappet Timing belt Timing mark Valve
Valve
float Variable valve timing

Aspiration

Air filter Blowoff valve Boost controller Butterfly valve Centrifugal-type supercharger Cold air intake Dump valve Electronic throttle control Forced induction Inlet manifold Intake Intercooler Manifold vacuum Naturally aspirated engine Ram-air intake Scroll-type supercharger Short ram air intake Supercharger Throttle Throttle
Throttle
body Turbocharger Twin-turbo Variable-geometry turbocharger Variable-length intake manifold Warm air intake

Fuel system

Carburetor Common rail Direct injection Fuel filter Fuel injection Fuel pump Fuel tank Gasoline direct injection Indirect injection Injection pump Lean-burn Stratified charge engine Turbo fuel stratified injection Unit injector

Ignition

Contact breaker Magneto Distributor Electrical ballast High tension leads Ignition coil Spark plug Wasted spark

Electrics and engine management

Air–fuel ratio meter Alternator Automatic Performance Control Car
Car
battery (lead–acid battery) Crankshaft
Crankshaft
position sensor Dynamo Drive by wire Electronic control unit Engine control unit Engine coolant temperature sensor Glow plug Idle air control actuator MAP sensor Mass flow sensor Oxygen sensor Starter motor Throttle
Throttle
position sensor

Exhaust system

Automobile emissions control Catalytic converter Diesel particulate filter Exhaust manifold Glasspack Muffler

Engine cooling

Air cooling Antifreeze
Antifreeze
(ethylene glycol) Core plug Electric fan Fan belt Radiator Thermostat Water cooling Viscous fan (fan clutch)

Other components

Balance shaft Block heater Combustion chamber Cylinder head
Cylinder head
porting Gasket Motor oil Oil filter Oil pump Oil sludge PCV valve Seal Synthetic oil Underdrive pulleys

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