Reed valves restrict the flow of fluids to a single direction, opening and closing under changing pressure on each face. Modern versions often consist of flexible metal or fiberglass.
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Applications
Traditional
Reed-valves, normally a leather flap covering a hole, are amongst the earliest form of automatic flow control for liquids and gases. They have been used for thousands of years in water pumps and for hundreds of years in bellows for high-temperature forges and musical instruments such as church organs and accordions. Earlier still, heart valves operate in a somewhat similar fashion.
Pumps
Reed valves are used in some reciprocating compressor designs, and in the pumping element of some musical instruments, large and small.
Two-stroke engines
Reed valves are commonly used in high-performance versions of the two-stroke engine, where they control the fuel-air mixture admitted to the crankcase. As the piston rises in the cylinder a vacuum is created in the crankcase beneath the piston. This vacuum opens the valve and admits the fuel-air mixture into the crankcase. As the piston descends, it raises the crankcase pressure causing the valve to close to retain the mixture and pressurize it for its eventual transfer through to the combustion chamber.[1]
High-speed impact takes its toll on light-weight metal valves which suffer fatigue and fail to seat properly while fiberglass ones are liable to snap off and be digested by the engine. Reed valves are not as precise as rotary valves since physical inertia causes them to open later than the optimum time. More sophisticated designs partly address this by creating multi-stage reeds with smaller, more responsive reeds within larger ones that provide more volume later in the cycle.
Nevertheless, current technology favors reed valves almost to the exclusion of rotary valves due to their simplicity and low implementation costs.
Pulse jets
Reed valves are used in the cheap but inefficient pulse jet engine, such as the one used by the Argus engine in the German V-1 flying bomb or doodle-bug. The valves at the front of the cylindrical engine are opened by the low pressure in the combustion chamber caused by the resonance of the air column in the engine, fuel is squirted into the combustion chamber and ignited by the hot combustion gases of the previous cycle. Once the charge has expanded and mostly left the engine, pressure inside drops again to below-atmospheric values and the reed valve allows fresh air to enter and the cycle be repeated. Some ram-air pressure due to forward motion helps scavenging and filling the combustion chamber with the new, fresh air charge, thus improving the power of the engine at higher speeds.
See also
References
- ^ Motorcycle Mechanics Institute,The Complete Guide to Motorcycle Mechanics, 1984, p. 79-80, Prentice-Hall, Inc., ISBN 0-13-160549-6
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