EDITORS NOTE: In each issue of Snow Goer, trained engineer and hard-core sledhead T.J. Krob takes a “scientific” look at something in snowmobiling. This look at reed cage design and development is from the January 2020 issue of the magazine.
For nearly 100 years during the late 1700s and into the late 1800s, European and American inventors built and patented various internal combustion engines. Today, the ingenious two-stroke engine remains a common power source for household items such as the lawnmower and chainsaw.
Throughout the powersports industry, the two-stroke engine was long a chosen weapon for the personal watercraft, moped and most importantly – the snowmobile. While PWC and mopeds have moved exclusively to four-stroke powerplants on this continent, the years of innovation have been kind to the two-stroke snowmobile, creating massive gains in power potential, and clever metallurgical improvements or lubrication qualities to increase lifespan.
One common internal piece of the reciprocating internal combustion engine is the reed valve. The reed valve can be thought of as a check-valve to only permit a medium – in our case either a fuel and air mixture or just air in some injected sleds – to travel in one direction. When flow attempts to push through the intended direction, the reed opens against the block and allows flow. If the flow reverses, the petal of the reed clamps down to the block and ultimately ceases it. In a combustion engine without intake/exhaust valves, a reed petal can open hundreds of times per second and must seal closed again an equal amount without succumbing to excessive fatigue.
Reed valves are not isolated to use in engines, and may be more ear-catching to the general public for their use in musical instruments. For the medically trained, the reed valve concept is essentially what opens/closes during each beat of a heart. But in an engine, the reed petal controls the flow of the air and fuel ratio entering the crankcase. Under vacuum when the piston is ascending from bottom dead center, the suction opens the petal and permits the mixture to enter. Once the piston changes direction after top dead center, the cylinder pressure reverses and closes the reed petal tight against the block, forcing the mixture to be consumed in the combustion chamber (instead of blowing through the carburetor/throttle body intake), and expelled through the exhaust after ignition.
The petals used in a reed block system can be constructed of leather, metal or, in more modern applications, varied composite materials. In decades past, a single flap covered an intake orifice effectively in a low-horsepower application. To pair with increased operating RPM and higher HP levels, the intake systems including the reeds of modern two-stroke snowmobile engines required immense improvements. Sledheads are demanding of their machines, requiring instant throttle response, consistent idle and drive-away – and especially durability. Aftermarket companies have devoted many hours of research to the optimal reed block and petal design, changing materials, configurations, volumes and more to obtain the best performing system specific for the specific engine.
The main tunable factors in reed design include: overall geometric volume, material composition (typically carbon or glass fiber based), reed cross-section opened port length, number of petals and clamping areas.
The total volume of mixed air that can enter an engine links directly to its displacement. As air passes through the bore of the carb/throttle body, the exit into the reed cage can match the diameter of the bore precisely or can be widened greatly to permit a non-turbulent flow. The shape of the reed block itself can lend to hindering the airflow if jagged edges or abrupt transitions are present – whereas a smooth surface with rounded planes encourages freedom of flow. After the bore of the cage is passed, the opening covered by the reed petals is the true obstacle for the suctioned mass of air. When sealed and bombarded by an incoming pulse, the petal itself will exhibit a slight amount of restriction when it resists the actual flexing motion away from the reed block. Once separated, the volume is directed into the awaiting crankcase or cylinder.
The number of petals inside modern reed cages can be two per cylinder, but using four petals per cylinder has become more popular due to their performance characteristics (VForce). To allow a similar volume of air mixture to enter an engine, the petals must move half the required distance per suction stroke with a four-petal design. This favors reduced fatigue and higher lifespan along with offering a combination petal configuration (some stiff and some soft). The softer reed can open at low RPM or, in smaller engines, more often, while the thicker reed only operates when necessary. Reed stops are also added to prevent the petal from flexing backward too far – and also serve double-duty as the clamping surface to hold the reed in place at the base.
Multi-stage reed timing has been utilized to accomplish the same as a four-petal design, but in a two-petal lay-out (Boyesen). In this instance, a thin petal covers a hole in the underlying reed, thereby decreasing the overall permissible flow – until the second stage opens when a greater force/RPM is present.
The stiffness of the reed petal is governed by its mechanical cross-section, material and clamping area. Reeds that are consistent in cross-section and stiff overall work well at high RPM for high horsepower machines (as they must close rapidly to seal for the incoming combustion wave requiring a large pulse to force them open). Low-tension reeds work well at low RPM or for small engines, but aren’t able to seal entirely when forces are great, or will chip easier at the tips.
Classic composite petal thicknesses range from .25mm to .45mm. Reeds with varied cross-sections progressing from the base to the tip and the block to which they meet gain their advantage from aerodynamics. By varying the thickness and the exit angle of the block, the uniﬁed formations become “wings” which can accelerate airﬂow or increase mixed velocity.
Not all petals are flat, either. A pre-curved petal toward the cage body has become normal in two-stroke engine design as the natural deﬂection to force it to seal favors a precise intake with reduced chance of backﬂow, thereby following the engine demand.
By employing modern materials mixed with a blend of airﬂow innovation, the air consumed by an engine has travelled along a journey optimized by advanced reed technology.
Moto Tassinari has been at the forefront of reed valve development in snowmobiling with its VForce design, which allows in more air with less movement.
Editor’s Note: Every issue of Snow Goer magazine includes in-depth sled reports and comparisons, aftermarket gear and accessories reviews, riding destination articles, do-it-yourself repair information, snowmobile technology and more! This story is from the November 2021 issue’s Cold Tested department. Subscribe to Snow Goer now to receive issues delivered to your door or your computer for a low cost.