In aircraft engines, the fuel and air mixture used for power production enters the cylinders through intake valves while burned gasses are expelled through the exhaust valve ports; these cylinder ports are opened and closed by valve heads. The valves utilized in aircraft engines are of the conventional poppet type, and they tend to be classified according to their shape. They are often called mushroom or tulip valves because of their similarity in shape to these plants. In order to understand aircraft reciprocating engine valves, it is important to delve into their construction. The following blog will review the different types and construction of aircraft reciprocating engine valves.
The valves in aircraft engine cylinders are frequently subjected to extreme temperatures, corrosion, and other operating stresses; for these reasons, it is important that the metal alloy in the valves is resistant to these factors. For example, intake valves can be made of nickel-chromium steel since they operate at lower temperatures than the exhaust valves, while the exhaust valves tend to be made of more heat-resistant materials, such as nichrome, Silchrome, or cobalt-chromium steel.
The valve head features a ground face which forms a seal against the ground valve seat located in the cylinder head when the valve is shut. The face of the valve is typically ground to 30 or 45 degree angles; in some situations the intake valve face is ground to a 30 degree angle, while the exhaust-valve face is ground to a 45 degree angle. Stellite is a material applied to valve faces to make them more durable in harsh environments because it is resistant to high-temperature corrosion and valve operation wear and tear. Other manufacturers use nichrome as the facing on valves for the same purpose.
The valve stem serves as a pilot for the head, and it sits in the valve guide installed in the cylinder head. The surface of the stem is hardened to counter wear while its neck creates the junction between the head and stem. The valve rocker arm is hammered which opens the valve, and to do this, the valve must feature a hardened tip. Near the tip, a machined groove on the stem receives the split-ring stem keys which form a lock ring to hold the valve spring retaining washer in place.
Metallic sodium fills some intake and exhaust valve systems because this material is an excellent heat conductor. As such, the sodium melts at around 208 degrees Fahrenheit, and the valve then circulates the liquidized material so that it can carry heat away from the valve head to its stem; it then dissipates through the valve guide to the cylinder head and cooling fins which lowers the operating temperature by as much as 400 degrees Fahrenheit. Valves filled with sodium should be protected because if they rupture, the exposed sodium will cause a dangerous explosion or fire.
The typical intake valve features solid stems with a flat- or tulip-shaped head; flat-headed valves tend to be utilized in lower power engines, while tulip-shaped intake valves feature either a smaller stem than the exhaust valve or a valve that is similar to the exhaust valve, but with a solid stem and head. The exhaust and intake valves are similar, but not interchangeable, because the faces are made of different materials.
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