Valvetrain

valvetrain or valvetrain is a mechanical system that controls operation of the valves in an internal combustion engine,[1] whereby a sequence of components transmits motion throughout the assembly.

A conventional reciprocating internal combustion engine uses valves to control the flow of the air/fuel admix into and out of the combustion chamber.[2]

Cutaway of an ohc engine

A V8’s valvetrain: pressed steel rockers activate poppet valvess via pushrods

The exposed valvetrain of a 5.9 Cummins in a 1991 Dodge Ram

Layout

A typical ohv valvetrain consists of valves, rocker arms, pushrods, lifters, and camshaft(s).[3] Valvetrain opening/closing and duration, as well as the geometry of the valvetrain, 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.

Valvetrains 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

The valvetrain 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 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 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]

References

  1. Brain, Marshall (5 April 2000). “How Car Engines Work”. HowStuffWorks. Retrieved 29 January 2014.
  2. “Sci-Tech Dictionary: “valvetrain””. Answers.com. Retrieved 29 January 2014.
  3. “The Valve Train”. AutoEducation. Retrieved 29 January 2014.
  4. Scraba, Wayne (October 2000). “Camshaft Tips & Definitions”. Hot Rod. Retrieved 29 January 2014.
  5. Cranswick, Marc (2011). The Cars of American Motors: An Illustrated History. McFarland. p. 80. ISBN 9780786446728. Retrieved 29 January 2014.
  6. 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.
  7. Forst, Sarah (2008). How to build performance Nissan sport compacts, 1991-2006. HP Books. p. 29. ISBN 9781557885418. Retrieved 29 January 2014.
  8. Ellinger, Herbert E. (1974). Automotive engines. Prentice-Hall. p. 171. ISBN 9780130554260. Retrieved 29 January 2014.
  9. Tom, David (2013). The Cars of Trans-Am Racing: 1966-1973. CarTech. p. 32. ISBN 9781613250518. Retrieved 29 January 2014.
  10. 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.
  11. Fletcher, Mark; Truesdell, Richard (2012). Hurst Equipped: More Than 50 Years of High Performance. CarTech. p. 63. ISBN 9781934709313. Retrieved 29 January 2014.
  12. Shepard, Larry (1989). How to Hot Rod Small-block Mopar Engines. HPBooks. pp. 74–88. ISBN 9780895864796. Retrieved 29 January 2014.
Piston ring A piston ring is a split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine. The main functions of piston rings in reciprocating engines are: Sealing the combustion chamber so that there is minimal loss of gases to the crank case. Improving heat transfer from the piston to the cylinder wall. Maintaining the proper quantity of the oil between the piston and the cylinder wall Regulating engine oil consumption by scraping oil from the cylinder walls back to the sump. The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore. Piston rings are a major factor in identifying if an engine is two stroke or four stroke. Three piston rings suggest that it is a four stroke engine while two piston rings suggest that it is a two stroke engine. Most piston rings are made of a very hard and somewhat brittle cast iron. Horizontal steam engine. Th...
Stroke (engine) In the context of an Internal combustion engine, the term stroke has the following related meanings: A phase of the engine's cycle (eg compression stroke, exhaust stroke), during which the piston travels from top to bottom or vice-versa. The type of power cycle used by a piston engine (eg two-stroke engine, four-stroke engine). "Stroke length", the distance travelled by the piston in each cycle. The stroke length- along with bore diameter- determines the engine's displacement. Phases in the power cycle] The phases/strokes of a four-stroke engine. 1: intake 2: compression 3: power 4: exhaust Commonly-used engine phases/strokes (ie those used in a four-stroke engine) are described below. Other types of engines can have very different phases. Induction/Intake stroke The induction stroke is the first phase in a four-stroke internal combustion engine cycle. It involves the downward movement of the piston, creating a partial vacuum that draws a fuel/air mixt...
Connecting rod A connecting rod is a shaft which connects a piston to a crank or crankshaft in a reciprocating engine. Together with the crank, it forms a simple mechanism that converts reciprocating motion into rotating motion. A connecting rod may also convert rotating motion into reciprocating motion, its original use. Earlier mechanisms, such as the chain, could only impart pulling motion. Being rigid, a connecting rod may transmit either push or pull, allowing the rod to rotate the crank through both halves of a revolution. In a few two-stroke engines the connecting rod is only required to push. Today, the connecting rod is best known through its use in internal combustion piston engines, such as automobile engines. These are of a distinctly different design from earlier forms of connecting rod used in steam engines and steam locomotives. Piston (top) and connecting rod from typical automotive engine (scale is in centimetres) History Scheme of the Hierapolis sawmill ...
Petrol engine A petrol engine (known as a gasoline engine in American English) is an internal combustion engine with spark-ignition, designed to run on petrol (gasoline) and similar volatile fuels. In most petrol engines, the fuel and air are usually mixed after compression (although some modern petrol engines now use cylinder-direct petrol injection). The pre-mixing was formerly done in a carburetor, but now it is done by electronically controlled fuel injection, except in small engines where the cost/complication of electronics does not justify the added engine efficiency. The process differs from a diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into very hot air at the end of the compression stroke, and self-ignites.   W16 petrol engine of the Bugatti Veyron History The first practical petrol engine was built in 187...
Sleeve valve The sleeve valve is a type of valve mechanism for piston engines, distinct from the usual poppet valve. Sleeve valve engines saw use in a number of pre-World War II luxury cars and in the United States in the Willys-Knight car and light truck. They subsequently fell from use due to advances in poppet-valve technology, including sodium cooling, and the Knight system double sleeve engine's tendency to burn a lot of lubricating oil or to seize due to lack of it. The Scottish Argyll company used its own, much simpler and more efficient, single sleeve system (Burt-McCollum) in its cars, a system which, after extensive development, saw substantial use in British aircraft engines of the 1940s, such as the Napier Sabre, Bristol Hercules, Centaurus, and the promising but never mass-produced Rolls-Royce Crecy, only to be supplanted by the jet engines. Sleeve valve closeup from a Bristol Centaurus Mark 175. Bristol Perseus Description A sleeve valve tak...