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What a reed valve does

A reed valve is a one-way door into the crankcase. It lets fresh mixture in and will not let it back out, and it comes in two halves, the cage and the petals, that some rulebooks treat differently.

What this rests on

Basis
Documents held in this archive, each listed under Where this comes from
Difficulty
Intermediate

A one-way door into the crankcase

In a two-stroke, fresh mixture goes into the crankcase, the closed space around the crankshaft under the piston, before it goes up into the cylinder. Mixture means fuel, air and oil together; a direct-injection engine sends air through the crankcase and puts its fuel straight into the cylinder. The two-stroke cycle article in this topic explains the whole cycle. As the piston rises it pulls mixture into the crankcase, and as it falls it squeezes that mixture. Left alone, the squeeze would push some of the mixture straight back out the way it came. The reed valve sits where the mixture enters the crankcase and stops that.

The cage and the petals

A reed valve has two halves. The cage is a rigid frame with openings in its faces, often shaped like a wedge reaching in towards the crankcase. The petals are thin, flexible flaps, each fixed along its upstream edge, lying flat over those openings. Nothing driven by the engine opens them. The petals are moved by the difference in pressure across them, and they spring back to shut.

The petals flex open, then close

When the rising piston pulls on the crankcase, the mixture outside pushes harder than the mixture inside. That lifts the free ends of the petals away from the cage, the openings uncover, and mixture flows through into the crankcase. When the falling piston pushes back, the petals return flat against the cage and the openings close. This happens once for every turn of the crankshaft. A petal that no longer seals cannot hold the mixture in, so some of it is pushed back the way it came.

Piston rising: the petals lift off the ports A passage runs left to right between two lines. A plate across it carries a wedge pointing right, into the passage, and each of the wedge's two faces is broken by an opening. A thick strip lies along each face, held at the wide end; its far end curves away from the face, and a dashed line along the face shows where it lies when shut. An arrow enters the wedge from the left and two arrows leave through the openings, under the lifted strips, to the right. Piston rising: the petals lift fixed end petal: free end lifts mixture in cage dashed: shut into the crankcase Piston falling: the petals lie back on the cage The same passage and wedge, with both strips lying flat along the faces and covering the openings. Two arrows come from the right towards the tip of the wedge and stop short of it. Piston falling: the petals spring back petal flat on the cage ports shut squeezed mixture held in
Show it moving The petals opening and closing, over and over The same passage and wedge, the strips lying flat. A few times over, the flat strips fade out as bent ones fade in, their far ends curved away from the faces, with arrows coming in from the left and out through the openings; then the bent strips and arrows fade away and the flat strips return. When it stops, both strips lie flat. Rising, then falling, a few times cage how far they bend is not to scale

Pressure, not any part the engine drives, moves the petals: they lift off the ports while the crankcase pulls and lie back flat when it pushes, so the mixture goes in and is kept from coming back. The fixed cage and the flexing petals are separate parts, and two rulebooks rule on each in its own right.

How we know: the wedge cage, its ports, and petals fixed at their upstream end that suction lifts and that spring back are US patent 6,609,535 (lines 62 and 65 of the held copy); the converging pair opening and shutting with the crankcase pressure, US patent 4,633,825 (lines 57, 58 and 62); the cage reaching into the intake passage, US patent 5,370,088 (lines 37, 57 and 62); the two halves ruled on apart, IJSBA's Ski Lites SKL.4.3 and the IHRA rulebook's O.3.3.

Not to scale.

Why the two halves are named separately

The two halves do different jobs. The petals flex and strike the cage on every turn, and in time that fatigue can break them. They are sold and fitted on their own. The cage is the frame whose ports set the size of the openings. Some rulebooks name the petals and the cage separately: IJSBA (the International Jet Sports Boating Association) in its 2024 Ski Lites document at SKL.4.3, and IHRA (the International Hot Rod Association) in its rulebook V.20260603 at O.3.3. So a part sold as one reed valve can be half allowed. The Ski Lites article under Class rules in Tuning sets out what that means there, and this page does not restate it.

What this page leaves out

Not every engine uses reeds. Some let mixture into the crankcase by other means, such as the piston itself or a rotary valve, a disc driven by the crankshaft that opens the intake through a recess for part of each turn. This page does not say which engines use which. It gives no size, stiffness, material or shape for any engine's reed valve, names no maker, and says nothing about what any rulebook allows.

Where this comes from

  1. Reed valves at the crankcase inlet let the charge in on the upstroke and stop reverse flow while the piston compresses it; the caging members reach down into the intake passage. US patent 5,370,088, Two cycle engine, lines 37 and 55 to 62 of the copy this archive holds.
  2. A converging pair of reed valves opens and closes with low and high crankcase pressure, and striking the plate fatigues them until they fail. US patent 4,633,825, Reed valve assembly, lines 35 and 57 to 62 of the copy this archive holds.
  3. A generally wedge shaped cage has ports covered by flexible reeds fixed at their upstream end, lifted by suction and springing back; port size is set by the cage; disc and piston port valves are named as other ways. US patent 6,609,535, Reed valve and method of making same, lines 61 to 65 of the copy this archive holds.
  4. Fuel, air and oil pass a check valve into the crankcase and are held there while compressed; a direct-injection engine puts its fuel into the cylinder instead. US patent 6,379,411, Two stroke engine exhaust emissions separator, lines 110 and 138 to 143 of the copy this archive holds.
  5. In the direct-injection design it describes, the crankcase pumps air to the cylinder. US patent 5,730,099, Reduced emission two-stroke engine, line 100 of the copy this archive holds.
  6. One revolution of the crankshaft makes one complete two-stroke cycle. US patent 6,026,641, on exhaust temperature control in a two-stroke engine, line 56 of the copy this archive holds.
  7. A rotary disc valve driven by the crankshaft opens the induction passages intermittently through recesses in the disc. US patent 4,079,706, Two-stroke internal combustion engine, lines 21 and 54 of the copy this archive holds.
  8. A set of reed petals was fitted on its own as a bolt-on part. Personal Watercraft Illustrated, 1996 WaveRaider test, line 28 of the copy this archive holds.
  9. SKL.4.3 rules on the reed petals and the reed cage assemblies separately. IJSBA 2024 Ski Lites document, lines 179 to 180 of the copy this archive holds.
  10. O.3.3 rules on the reed petals and the reed cage assembly separately. IHRA rulebook V.20260603, lines 2052 to 2054 of the copy this archive holds.
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“What a reed valve does”, BaddJet archive, https://baddjetarchive.com/tech/engine/what-a-reed-valve-does/

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