A traditional reciprocating
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable...
uses valves to control air and fuel flow into and out of the cylinders, facilitating combustion.
Valvetrain is an all-encompassing term used to describe the mechanisms and parts which control the operation of the valves.
Valvetrain: The valvetrain consists of valves, rocker arms, pushrods, lifters, and the cam shaft.
A traditional reciprocating
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable...
uses valves to control air and fuel flow into and out of the cylinders, facilitating combustion.
Valvetrain is an all-encompassing term used to describe the mechanisms and parts which control the operation of the valves.
Layout
Valvetrain: The valvetrain consists of valves, rocker arms, pushrods, lifters, and the cam shaft. Valvetrain opening/closing & duration controls the amount of air and fuel entering the combustion chamber at any given point in time. Timing for open/close/duration is controlled by the camshaft which is synchronized to the crankshaft by a chain or belt.
Valvetrains are built in several configurations, each of which varies slightly in layout but still performs the task of opening and closes 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:
Overhead camshaftOverhead camshaft, commonly abbreviated to OHC, valvetrain configurations place the engine camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters in a more direct manner compared to overhead valves and pushrods.Compared to OHV pushrod systems with the...
- The camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam featuring a camshaft was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206...
(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.
Cam-in-blockThe cam-in-block valvetrain layout of piston engines is one where the camshaft is placed within the cylinder block, usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine...
- 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.
CamlessMost piston engines today employ one or more camshafts to operate poppet valves. The lobes on the camshafts operate cam followers which in turn open the poppet valves. A camless uses electromagnetic, hydraulic, or pneumatic actuators to open the poppet valves instead...
- This layout uses no camshafts at all. Technologies such as Solenoids are used to individually actuate the valves.
Parts
As stated above, the valvetrain is the mechanical system responsible for operation of the valves. Valves are usually of the
poppetA poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
type, although many others have been developed such as
sleeveThe sleeve valve is a type of valve mechanism for piston engines, distinct from the more common poppet valve. They saw use in some pre-World War II luxury cars, sports cars, the Willys-Knight car and light truck, the British Daimler and French Avions Voisin luxury cars, also used the same...
, 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 cylinder heads in which pneumatic pressure closes the valves, while motorcycle manufacturer
DucatiDucati Motor Holding S.p.A. is a privately owned Italian motorcycle manufacturer located in Bologna, Italy. Ducati has achieved prominence in the motorcycle industry and in motorcycle racing.-Ownership:...
uses desmodromic mechanisms to 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 cam-in-block 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
liftersA tappet in mechanical engineering is a projection which imparts a linear motion to some other component within an assembly. Properly speaking, a tappet is only that part of a rocker arm which makes contact with an intake or exhaust valve stem above the cylinder head of an internal combustion...
, either solid or hydraulic, upon which the camshaft, located within the
cylinder blockThe cylinder block or engine block is a machined casting containing cylindrically bored holes for the pistons of a multi-cylinder reciprocating internal combustion engine, or for a similarly constructed device such as a pump...
, 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 cycleToday, internal combustion engines in cars, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle...
. In order to accomplish this the camshaft is linked to and kept in synchronisation with the
crankshaftThe crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
(the main shaft upon which the
pistonsPistons may refer to:* Piston, the engine and engineering part* Detroit Pistons, an NBA basketball team...
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.