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Engine valve
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- Valves are used to
control gas flow to and from cylinders in automotive internal combustion
engines. The most common type of valve used is the poppet valve. The
valve itself consists of a disc-shaped head having a stem extending from
its centre at one side. The edge of the head on the side nearest the
stem is accurately ground at an angle – usually 45 degrees, but
sometimes 30 degrees, to form the seating face. When the valve is
closed, the face is pressed in contact with a similarly ground seat.
- The valve seat in an
internal combustion gasoline or diesel engine is the surface against
which an intake or an exhaust valve rests during the portion of the
engine operating cycle when that valve is closed. The valve seat is a
critical component of an engine in that if it is improperly positioned,
oriented, or formed during manufacture, valve leakage will occur which
will adversely affect the engine compression ratio and therefore the
engine efficiency, performance (horsepower), exhaust emissions, and
engine life.
- Valve seats are often
formed by first press-fitting an approximately cylindrical piece of a
hardened metal alloy, such as Stellite, into a cast depression in a
cylinder head above each eventual valve stem position, and then
machining a conical-section surface into the valve seat that will mate
with a corresponding conical-section of the corresponding valve.
- Valves are subject to
both thermal and mechanical loads,the latter being applied by the
springs and actuating gear. All these loads are so severe as to justify
an assertion that the valves are the most heavily loaded components in
an engine. Exhaust valves , in particular , operate for long periods at
high temperatures 700-800.
- Valve seats are an
extremely important part of a cylinder head because the seats cool and
seal the valves. They also support the valve when it closes, which
affects both valve train geometry and valve lash. If a seat is damaged,
cracked, loose, receded or too badly worn to be recut or reground, it
can cause a variety of problems: loss of compression, valve burning,
valve failure, valve train wear and breakage, even head and valve damage
if the seat comes loose.
- Seats should not be
replaced until the head has been thoroughly cleaned and inspected. This
includes checking for cracks (especially around and near the valve
seats) and checking the deck surface and cam bore for straightness. Any
welding and/or straightening that may be needed must be done before
remachining the valve seats or installing new inserts.
- Also, the valve
guides should be replaced or reconditioned before the seats are
machined. Concentricity between the seat and guide is absolutely
essential for a proper alignment, good compression and long term valve
durability.
- The cylinder head
must be dimensionally and geometrically within specifications before
seat counterbores are machined. That includes cylinder head thickness,
valve guide clearances, concentricity and perpendicularity. There should
be no warping, twisting or any type of misalignment anywhere in the
head.
- PEP-PRO Engine Valves
were introduced to the Australian market in 1985, in response to a
growing demand from the performance aftermarket industry. The initial
launch with Chevrolet, Datsun, Ford and Holden valves has seen the
Pep-Pro brand become the choice of many engine builders across
Australia.
- The automotive parts
industry has undergone a significant change in the recent past. There
has been a considerable shift in cars and components, production methods
and materials. Driven by expanding demand in the Asian economies and new
product developments, worldwide pistons, piston rings and engine valves
market is forecast to reach 2.5 billion units by 2010.
- Engine valves market
is forecast to witness the fastest growth at a CAGR of over 3% over the
period 2000-2010. Most engine subsystems can be implemented using
different technologies, and better technologies can improve the
performance of the engine. The valve train consists of the valves and a
mechanism that opens and closes them. The opening and closing system is
called a camshaft. The camshaft has lobes on it that move the valves up
and down.
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