Engine valves are used to regulate the flow of gases
into and out of a cylinder by opening and closing at correct time in the
combustion process. Each cylinder has
at least one INTAKE VALVE and one EXHAUST VALVE. The intake valve controls the amount of
fuel/air mixture that is drawn into the cylinder through the intake port while the
exhaust valve allows the exhaust gases to exit the cylinder through the exhaust
port.
These valves are faced into the combustion chamber of the cylinder and therefore those are subject to high temperatures, corrosion, and extreme operating
stresses. Therefore, valves must be
constructed of metals that are able to resist these stressful operating factors. For example, intake valves operate at lower temperatures
than exhaust valves and, therefore, are typically made of chrome, nickel, or
tungsten steel.
However, since exhaust
valves must endure much higher temperatures they are usually made of more heat
resistant metals such as inconel, silicon‑chromium or cobalt-chromium alloys.
As mentioned, each cylinder must have at
least one intake and one exhaust valve;
however, on some high powered engines, two intake and two exhaust valves
are provided for each cylinder for better operational performances. There are various types of valves used in piston engines and the most
common type of valve used in aircraft engines is the poppet valve which gets
its name from the popping action of the valve.
Poppet valves are classified according to
their head shape which may come in four basic designs:
- Flat head valve : As its name implies, this valve has a flat head and is typically used ONLY as an intake valve in aircraft engines
- Semi Tulip head valve : This valve has a slightly concave area on its head.
- Tulip head valve : This valve has a deep, wide indented area on its head.
- Mashroom head valve : This valve has a convex head and is NOT commonly found on aircraft engines
The valve face is that portion of the valve
that creates a seal at the ports. A valve face is typically ground to an angle
of between 30° and 60° to form a seal against valve seat when then valve is closed. In
some engines, the intake valve face is ground to 30° while the exhaust valve face is ground to 45°. The engine manufacturer
specifies the exact angle which is based on airflow, efficiency, and sealing
ability. Valve faces are often made more
durable by welding STELLITE, an
alloy of cobalt and chromium, to the valve face. Once applied, the face is ground to the
correct angle. Stellite resists high
temperatures and corrosion and withstands the shock and wear associated with
valve operation.
The Valve stem acts as a guide to keep the valve head properly aligned as it moves
back and forth. Most valve stems are
surface hardened to resist wear and are joined to the valve head at the Valve neck. The TIP
of a valve stem is also hardened to withstand both wear and hammering. A groove is machined around the valve stem near the tip for a SPLIT KEY. This key will keep the valve spring
retaining washers in place and hold the valve in the cylinder head.
To help dissipate heat better, some exhaust
valve stems are hollowed out and then partially filled with Metalic Sodium( Na). When installed in an operating engine, the
sodium melts when the valve stem reaches approximately 98°C. The melted sodium
circulates naturally due to the up and down motion of the valve and helps carry
heat from the valve head into the stem where it is dissipated to the oil circulating at the cylinder head.
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