Reed valves for two-stroke inductionTags : valvetrain
Most of the articles I've written about valves have looked at the design of, or materials in, conventional poppet valves as applied to many four-stroke engines. So I thought it would be good to take a brief diversion from this and look at something completely different.
Two-stroke engines have been used widely for grand prix motorcycle racing (although this era is soon to come to an end), motocross, snowmobiles, jet skis and so on, and are currently enjoying something of a resurgence for applications where low fuel consumption is of importance. Just as the motorcycle race classes and the bike manufacturers are turning away from two-strokes due to emissions, other companies are embracing the technology.
Two-stroke engines have no need for the valvetrain that we find in a four-stroke engine; the movement of the piston past carefully shaped ports controls the admission of charge to the cylinders. Fresh charge is drawn into the crankcases where it is compressed by the descending piston. The piston then uncovers ports, through which the charge is passed to the cylinder.
The principle requires a device to prevent the charge that's compressed in the crankcase from being expelled back to whence it came. This function is usually dealt with by a reed valve but there are other systems, the most common of which is the disc valve system. These systems - the reed or the disc - act as a one-way valve, opening when the crankcase pressure is below atmosphere and closing as it rises again.
The reed valve - sometimes called a leaf valve - is a profiled flat component that sits across an aperture in its closed position. These are commonly used in pairs on a V-shaped block, as shown in the photo here. The opening of the reed valve is restrained by a 'reed stop' that prevents the reed opening so much that it becomes permanently deformed.
The materials used to make reed valves vary widely, although steel was a popular choice for many years. Available in a wide range of thicknesses as shim stock, it is in many ways an ideal material.
In recent times though, steel has been replaced to a large extent by composite reeds. Glass-fibre reinforced plastics were very popular for this application, offering good fuel resistance and low mass. These have, however, as we might expect, been supplanted largely by carbon-fibre composites.
A further development has been the use of two-stage reeds. These comprise an inner reed which itself has an aperture, and a smaller outer reed that opens initially. When the air flow rate reaches a certain level, the pressure differential across the inner reed causes it to open, allowing the full potential airflow into the engine. This is said to improve part-throttle and transient behaviour.
In these two-stage reed valve arrangements there is scope to use different materials for inner and outer reeds, and to change the shape of the aperture in the inner reed to tailor the pressure differential at which each opens.
Fig. 1 - The operation of an induction reed valve as applied to a two-stroke engine
Written by Wayne Ward