A valvetrain is a mechanical system that controls the operation of the intake and exhaust valves in an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) into the combustion chamber, while the exhaust valves control the flow of spent exhaust gases out of the combustion chamber once combustion is completed.

Layout

The valvetrain layout is largely dependent on the location of the camshaft. The common valvetrain configurations for piston engines, in order from oldest to newest, are: *

Flathead engine A flathead engine, also known as a sidevalve engine''American Rodder'', 6/94, pp.45 & 93. or valve-in-block engine, is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, a ...

: A single

camshaft 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 syst ...

and the valves are located in the

engine block In an internal combustion engine, the engine block is the structure that contains the cylinders and other components. The engine block in an early automotive engine consisted of just the cylinder block, to which a separate crankcase was attach ...

below the

cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

or cylinder bank. *

Overhead valve engine An overhead valve engine, abbreviated (OHV) and sometimes called a pushrod engine, is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with flathead (or "sidevalve") engines, where the v ...

: A single camshaft remains in the block below the cylinder(s), however the valves are located in the

cylinder head 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 ...

above the combustion chamber. *

Overhead camshaft engine 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 combustio ...

: Both the valves and one or more camshafts are located in the cylinder head above the cylinders or cylinder banks.

Components

The valvetrain consists of all the components responsible for transferring the rotational movement of the camshaft into the opening and closing of the intake and exhaust valves. Typical components are listed below in order from the crankshaft to the valves.

Camshaft

The timing and lift profile of the valve opening events are controlled by the camshafts, through use of a carefully shaped lobe on a rotating shaft. The camshaft is driven by the

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

and, in the case of a four-stroke engine, rotates at half the speed of the crankshaft. Motion is transferred from the crankshaft to the camshaft most commonly by a rubber timing belt, a metallic ''timing chain'' or a set of gears.

Pushrod

Pushrods are long, slender metal rods that are used in overhead valve engines to transfer motion from the camshaft (located in the engine block) to the valves (located in the cylinder head). The bottom end of a pushrod is mated to a lifter, upon which the camshaft makes contact. The camshaft lobe moves the lifter upwards, which moves the pushrod. The top end of the pushrod pushes on the rocker arm, which opens the valve.

Rocker arm / Finger / Bucket tappet

Depending on the design used, the valves are actuated by a

rocker arm A rocker arm is a valvetrain component that typically transfers the motion of a pushrod in an overhead valve engine, overhead valve internal combustion engine to the corresponding intake/exhaust poppet valve, valve. Rocker arms in automobiles are ...

finger A finger is a prominent digit (anatomy), digit on the forelimbs of most tetrapod vertebrate animals, especially those with prehensile extremities (i.e. hands) such as humans and other primates. Most tetrapods have five digits (dactyly, pentadact ...

, or bucket tappet. Overhead valve engines use rocker arms, which are actuated from below indirectly (through the pushrods) by the cam lobes. Overhead camshaft engines use fingers or bucket tappets, which are actuated from above directly by the cam lobes.

Valves

Most modern engines use poppet valves, although sleeve valves, slide valves and

rotary valve A rotary valve (also called rotary-motion valve) is a type of valve in which the rotation of a passage or passages in a transverse plug regulates the flow of liquid or gas through the attached pipes. The common stopcock is the simplest form of ro ...

s have also been used at times. Poppet valves are typically opened by the camshaft lobe or rocker arm, and closed by a coiled spring called a ''valve spring''. Valve float occurs when the valve spring is unable to control the inertia of the valvetrain at high engine speeds (RPM).

Valve guide

Valve guides ensure that engine valves are properly aligned, support the intake/exhaust valve stem, stabilize valve angle, and allow heat to escape to the cylinder head.

References

{{Internal combustion engine , state=expanded Engine components