Friday, February 1, 2013

Piston Engine Valves

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.