page_title: Reed valve
section_title: Two-stroke engines
hierarchical_section_title: Reed valve / Applications / Two-stroke engines
caption_attribution_description: A pair of en:Malossi en:reed valve blades made from en:carbon fibre. Picture taken and released into the en:public domain by en:User:Kallemax.
context_page_description: Reed valves are a type of check valve which 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 composite materials.
context_section_description: Reed valves are commonly used in high-performance versions of the two-stroke engine, where they control the fuel-air mixture admitted to the cylinder.  As the piston rises in the cylinder a vacuum is created in the crankcase beneath the piston.  The resulting pressure differential opens the valve and the fuel-air mixture flows 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. The Swedish motorcycle company Husqvarna produced a two-stroke, 500 cc displacement single cylinder engine with a reed-valve controlled intake, one of the biggest in using this arrangement. Reed valves in two-stroke engines have been placed in the intake ports and also in controlling the intake to the crankshaft space.
  Composite materials are preferred in racing engines, especially in kart racing, because the stiffness of the petals can be easily tuned and they are relatively safe in failure. High-speed impact takes its toll on all reed valves, with metal valves suffering in fatigue. The physical inertia of reed valves means that they are not as entirely precise in action as rotary valves, a rotary valve engine may run better than a reed valve engine at a small rpm range but the reed valve engine often runs better over a wider rpm range. 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 and less rotational mass.
caption: A pair of Malossi reed valve blades made from carbon fibre
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