Reed Valve

The reed valve is a device which opens under vacuum, allowing the fuel/ air mixture to enter the crank case of a two stroke engine. The vacuum is generated as the piston makes its up stroke. Then as the piston makes its down stroke vacuum in the crank case changes to pressure, forcing the reed valve closed. This keeps the fuel in the engine and aides in pressure forcing the fuel charge up the transfer ports into the combustion chamber. As the piston starts to make its up stroke the transfer ports are closed allowing the piston to compress and fire the fuel. The case reed valve works in the same way, but is located in a different part of the intake tract.

Before reed valves the popular set up was piston port which did work, but had some issues. With no reed valve, there were occasions where fuel would get pushed back through the carburetor and end up on the air filter, which could be a fire hazard, or significantly reduce the amout of air that could pass through the filter, resulting in poor performance. Also, when running at slow speed the engine could load up with excess fuel, also causing poor performance, and spark plugs to foul. With the reed valve, the above issues were solved, and with the valve there was also a noticeable change in the power curve of the two stroke, as it now could run cleaner from idle to peak horsepower. Although all Japanese manufacturers eventually adopted the reed valve, several after market firms took the basic design and drastically improved it, not only in terms of performance, but with innovative new materials.

Yamaha was the first of the Japanese manufacturers to use the reed as standard equipment beginning in 1974. Their design was to place the reed in the intake, just ahead of the carburetor, known as piston port reed valve. Suzuki introduced their reed valve, but placed it on the bottom of the cylinder at the crank case, known as case reed valve. Both systems worked well, but eventually, the system pioneered by Yamaha, was used by the big four.

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 (fiberglass or carbon fiber).

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. This vacuum opens the valve and admits the fuel-air mixture 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.

10 Things to know about reed valves

(1) Two-stroke engines suck their fuel/air mixture through the intake tract into the engine when the piston rises. This action creates a vacuum in the crankcase. On its descent, the piston forces the mixture up through the transfer ports into the combustion chamber. With earlier, piston-port engines, a part of that mixture would be backwashed into the intake instead of entirely through the transfer port. Reed valves act as one-way check valves that prevent this backwash.

(2) A reed valve consists of flexible reed petals that sit over an opening in a wedge-shaped block. The block fits between the carburetor and the engine. With a lower pressure zone on the engine side, the reed petals flex open to allow fuel/air to pass, when the lower pressure switches to the intake tract side, the reed petals are forced tight against the reed block to seal off the intake tract.

(3) The reed petals pulse as the engine cycles, at roughly a one-to-one ratio. When the engine turns 8000 engine revolutions per minute, a reed opens 7980 times per minute. Needless to say, when the engine is running close to peak rpm, the reeds are really buzzing. With each cycle, the reed petals slap against the reed block, shortening their life span.

(4) The first reeds, like GEM reeds, were made of stainless steel. These reeds were very durable and could be run a long time, but when they did break, the metal was sucked into the engine? destroying it. Eventually, epoxy-based glass fiber laminate (fiberglass) reeds were developed. Fiber reeds didn’t last as long as stainless steel, but when they started to fray, they didn’t cause catastrophic damage. Carbon fiber reeds are similar to glass fiber reeds, but they are a little lighter and stiffer at the same thickness. To achieve a compromise between lighter carbon and less expensive yet more durable glass fiber, a hybrid was developed. Manufacturers have experimented with other materials as well, including Kevlar and titanium.

(5) Durability isn’t the only advantage to different reed materials. A stronger, stiffer material allows the reed to be thinner and lighter. The lighter reciprocating mass of a reed increases performance. If the reed is too thin, however, it can start to flutter at high rpm. This means that the reed isn’t keeping up with the frequency of pulses of the engine and is disrupting airflow. Throughout its life span, a reed petal flexes millions of times, and its individual fibers start to lose their springiness. The reed’s modulus of elasticity decreases so that in equal engine conditions the reed will deform and open farther. This is called hang-open and affects performance. It can even make the bike run a little richer off the bottom.

(6) In addition to thickness, a reed’s performance can be further tuned by its construction. Layup is the orientation of the plies of material of the reed. Many petals have three plies, and by laying the plies perpendicular to the area of flex, stiffness is increased. The shape of the reed’s tip can also be modified for performance. A wing-shaped tip can help air flow past it. A reed’s length affects its stiffness and the distance it must travel.

(7) The specific dimensions and design of every part of the reed valve change performance. The size and shape of the overall duct of the cage, as well as the angles it may have to avoid interference with the shock, are important for airflow. Reed designers try to keep air velocity constant. The volume of the reed cage should be approximately equal to the volume in the carb and crankcase to prevent any bottlenecking. A nylon “stuffer” may be inserted into the cage to help maintain proper flow.

(8) The reed block’s design can affect durability and performance. Since the reed is slapping against it over a hundred times a second, rubber or plastic can be used to cushion where the reed petal contacts the block.

(9) Moto Tassinari uses a multiple reed block design that has a larger number of shorter petals that have less distance to travel. Boyesen pioneered the dual-stage reed petal, where a smaller, lighter petal is overlaid on a longer, stiffer petal to provide a longer duration of fuel/air flow.

(10) It’s a good idea to visually inspect the reeds on a bike for fraying or chipping whenever you examine the piston rings. Boyesen and Moto Tassinari dominate the reed business. As a rule of thumb, when the MXA crew wants more midrange, we call Moto Tassinari, and when we want to increase high rpm power, we hit up Boyesen (although this is not a hard-and-fast rule).

Sources: Wikipedia, Motocross Action Magazine, Classicjapcycles