A piston is part of a reciprocating engine with the purpose of transferring the force from the expanding gases in the cylinder to the crankshaft. In steam reciprocating engines, valves are necessary to control the entry and exit of gases at the proper time in the piston’s cycle to ensure the engine is functioning properly. Piston engines mounted on aircraft have utilized several different types of valves over the years.
Sleeve valves were first patented by Charles Yale Knight, and were initially used in luxury automobiles such as Mercedes-Benz and Peugeot. Early sleeve valves used alternating twin sliding sleeves, but those used in aircraft, notably by Bristol, used a single sleeve that rotated around a timing axle set at 90 degrees to the cylinder axis. Named the Burt-McCollum valve after the two inventors that developed it separately, the greatest advantage of the sleeve valve was its mechanical simplicity and ruggedness, as well as consuming less oil than other sleeve valve designs. Sleeve valves were frequently used in both aircraft and automobiles up until the 1940s, when their replacement was introduced.
Poppet valves, also known as mushroom valves, control the timing and fuel quantity flowing into an engine like any other valve. Poppet valves operate the marionette as they move in response to remote motion transmitted by the engine, and instead of sliding or rocking over a seat to uncover a port the way that sleeve valves do, they lift in a movement perpendicular to the port. Because there is no movement in the seat, there is no need for lubrication. The issue with piston poppet valves lies in metallurgy and heat management.
In the early days of poppet valves, the metal used to make poppet valves was not resilient enough, and the rapid opening and closing of valves would cause the cylinder heads to rapidly wear out. This meant that the poppet valves would need to be re-ground every two years or so, causing high maintenance costs. Modern poppet valves are made from high-quality stainless steel, which averts this issue. Overheating was also a serious concern, causing excessive piston valve wear and defective sealing. This was solved by adding valve cooling systems, as well as sodium-filled valve stems that acted as a heat pipe to divert heat away from the valve head.
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