1. What is reed valve? What are reed petals? How do reed valve works?
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.
For two stroke engine reed petals pulse as the engine cycles. The pulse is basically at the one-to-one ratio. When the engine turns 3000 engine revolutions per minute (rpm), a reed opens almost the same amout of times in a minute, i.e. almost 3000 times per minute. Some top go-cart engines work at 15000rpm then the petals do the same (open and close 250times a second).
So there is a lot of stress on them and high expectations from the cylinder and the piston to work properly at low and high RPMs.
2. What material is used for reed petals?
- stainless steel
- carbon fibre
- glass fibre
First option, stainless steel, is outdated. Although, it’s still marketed, it would not be wise to use those on performance engines. Stainless steel sheets are not reliable at high RPM and cause damage to the engine if they break.
Glass fibre and carbon fibre reed petals
Glass and glass fibre are technologically similar but the glass fibre to achieve a similar stiffness level it has to be thicker compared to carbon fibre. This in return makes it heavier at the same stiffness level. That is the reason why carbon fibre is preferred choice.
Material choice is important to get durable, strong and stiff but at the same time lighter and thinner option. The lighter the mass the better the performance. If too thin and therefore too little stiffness, the petal will start to flutter or so to say resonate at high rpm. This means that the reed is not keeping up with the engine pulses and disrupts the air and fuel flow into the cylinder. If too stiff then low to mid end performance may suffer as the engine does not generate sufficient pressure at lower rotational speeds to open valve(s) for effective engine performance
3. What valve design to use? How does the valve design affect performance?
The design of reed valve and its parts change how the intake behaves. The aim is to improve the fuel-mix flow into the engine and constant velocity. Reed cage inlet surface area should match the inlet area and crankcase to prevent bottlenecking. Often stuffers are used to improve the flow. Tuners also use anomemeter to measure the flow not just through the entire engine but focus on the inlet side to make relevant changes.
Reed valve design will affect the duration of the fuel mix flow into the crankcase. For example Tassinari valves present often multiple reed blocks with several petals on each block. There is even a dual stage reed petal design from Boyesen, where a small petal is overlaid a longer petal to provide improved and longer duration of fuel and air mix.
There are simple flat design valves and sharp corner Tassinari produced valves.
Also the larger the valve body the less room in the crankcase. What it means that it does affect the primary compression if for example you use flat type valve vs V-type or W-type valves. It reduces the crankcase volume.
So it definitely affects performance because:
- valve body size affects primary compression volume
- different designs use different flow directions because of number of petals and the amount of fuel mixture it allows to enter the engine as effectively at different RPM levels as possible.
4. How do I know if I have a reed valve issue? How do I know if my reed valves are worn?
Typical symptom of valve petal damage is that the engine is difficult to start or it will start but idle is very erratic and engine cuts soon after it starts.
Why is that? In the event of the piston moving from the top dead centre (TDC) to bottom dead centre (BDC) it pressurises crankcase primary compression chamber for the fresh air/fuel mixture to enter the combustion chamber. If reed petals for example are worn out then the moment piston is moving down the air/fuel mixture from crankcase is pushed back through intake valve to the carburettor. This does not allow new mixture to be pulled into the cylinder and nothing really to burn in the combustion zone.
So, check the reeds occasionally for proper sealing for lack of chipped material or cracks. Especially if your bike is difficult to start or idle is erratic. One of the ways to check for proper sealing is to direct reed valve cone towards a light source. If no light can be seen from the inside of reed vale assembly this means that sealing is good. A suggestion is also to try to breathe through the valve assembly side and if you cannot do a fast breathe-in through the valve then the sealing is good enough.
However, keep in mind that, when straight, non-curved reeds are used, small gaps up to approx a one or two tenth of a mm (up to 0.007-0.008in) are usually not a problem. The reason for this is that crankcase pressure fluctuations from underpressure to overpressure make reeds to open as well as to close.
Change the reed petals if cracks are visible or material is chipped, usually at the corner of the petals or the edges.