A valve train or valvetrain is a mechanical system that controls
operation of the valves in an internal combustion engine, 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
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The valve train consists of valves, rocker arms, pushrods, lifters,
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.
Timing for open/close/duration is controlled by the camshaft that is
synchronized to the crankshaft by a chain, belt, or gear.
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:
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.
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.
This layout uses no camshafts at all. Technologies such as solenoids
are used to individually actuate the valves.
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
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. 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. 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. 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.
High-output and engines used in competition feature camshafts and
valvetrain components that are designed to withstand higher RPM
ranges. These changes also include additional modifications such as
larger-sized valves combined with freer breathing intake and exhaust
ports to improve air flow. Automakers offer factory-approved
performance parts to increase engine output, and numerous aftermarket
parts vendors specialize in valvetrain modifications for various
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Animation (OHV, OHC and DOHC)
Part of the Automobile series
Cylinder head (crossflow, reverse-flow)
Starter ring gear
Pneumatic valve springs
Variable valve timing
Cold air intake
Electronic throttle control
Naturally aspirated engine
Short ram air intake
Variable-length intake manifold
Warm air intake
Gasoline direct injection
Stratified charge engine
Turbo fuel stratified injection
High tension leads
Electrics and engine
Air–fuel ratio meter
Automatic Performance Control
Car battery (lead–acid battery)
Crankshaft position sensor
Drive by wire
Electronic control unit
Engine control unit
Engine coolant temperature sensor
Idle air control actuator
Mass flow sensor
Throttle position sensor
Automobile emissions control
Diesel particulate filter
Antifreeze (ethylene glycol)
Viscous fan (fan clutch)
Cylinder head porting