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A reed valve is a device that allows fluids to flow in one direction but not the other. These unidirectional check valves have been in use throughout much of human history, first appearing as a component in water pumps, and later used in items such as forge bellows and musical instruments. Another common modern use of the reed valve is in two-stroke petrol engines, such as those used in motorcycles, go-karts, and lawnmowers. Two-stroke engines often use reed valves to let air and fuel into the crankcase. Since the valve will not allow fluids to pass in the other direction, the combustion chamber is then effectively sealed off for the power stroke.
Early reed valves were usually simple in construction, often consisting of a simple leather flap. These valves were used in applications like water pumps, allowing the liquid to pass freely through the flap in one direction, while pressure would force the flap closed and prevent movement the other way. Later reed valves worked in much the same way when they were included in forge bellows. A very simple flap valve would allow air into the bellows when they were spread, then close as they were contracted so that the air could be forced into the forge.
Two-stroke engine applications often use reed valves that operate under these same basic principles, though they are typically constructed of high-tech, durable materials. Some reed valves that are used in performance applications are made of carbon fiber and other composites, resulting in components that are light weight, flexible, and resistant to wear. This can be especially important in racing go-karts, motorcycles, and other vehicles.
The specific function of a reed valve in a two-stroke engine is typically to regulate the entrance of fuel and air into the crankcase. A check valve can allow the mixture in while simultaneously keeping hot, high temperature combustion gases from escaping. This can be accomplished through the action of the moving piston. When the piston moves in one direction, it can create a vacuum in the crankcase that allows air and fuel to enter through the reed valve. Then when the piston moves in the other direction, pressure within the crankcase increases and the valve is forced shut.
Some engines use more precise rotary valves instead of reed valves. An engine with a reed valve will typically operate well under a wide range of engine speeds, while one equipped with a rotary valve may perform better in a particular speed range. The reed valve also has the added benefits of being simpler in design and is typically less expensive. Certain engines also use a configuration of multiple small reed valves that can approach the efficiency of a rotary valve.