Reed valves allow fluids to flow in one direction and have been used throughout history in water pumps, musical instruments, and forging bellows. They are commonly used in two-stroke gasoline engines to regulate the entry of fuel and air into the crankcase. They are generally made of durable, high-tech materials and are simpler in design and less expensive than rotary valves.
A reed valve is a device that allows fluids to flow in one direction but not the other. These one-way check valves have been used for much of human history, first appearing as a component in water pumps, and later being used in items such as forging bellows and musical instruments. Another common modern use of the reed valve is in two-stroke gasoline engines, such as those used in motorcycles, go-karts, and lawn mowers. Two-stroke engines often use reed valves to allow air and fuel to enter the crankcase. Since the valve will not allow fluids to pass in the other direction, the combustion chamber is effectively sealed off for the power stroke.
Early reed valves were generally of simple construction, often consisting of a simple leather flap. These valves were used in applications such as water pumps, allowing liquid to pass freely through the flapper in one direction, while pressure would force the lid closed and prevent movement in the other direction. Later reed valves worked the same way when included in forge bellows. A very simple flapper valve would allow air into the bellows when they extended, then close as they contracted so air could be forced into the forge.
Two-stroke engine applications often use reed valves that operate on these same basic principles, although they are generally constructed of durable, high-tech materials. Some reed valves used in performance applications are made of carbon fiber and other composites, resulting in components that are lightweight, 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 to regulate the entry of fuel and air into the crankcase. A check valve can allow the mixture to enter while preventing hot, high-temperature combustion gases from escaping. This can be achieved by 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, the pressure inside the crankcase increases and the valve is forced closed.
Some engines use more precise rotary valves instead of reed valves. An engine with a reed valve will generally perform well over a wide range of engine speeds, while one equipped with a rotary valve may perform better over a particular speed range. The reed valve also has the added benefits of being simpler in design and generally less expensive. Certain engines also use a multiple small reed valve configuration that can approach the efficiency of a rotary valve.
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