Why is it that reed valves are so successful in high-performance motorcycle two strokes yet are only used in teeny 'sport' class model airplane engines? Is it just because you can't get suitable reed material in such small sizes?
Two-stroke design has advanced a great deal since I was interested in it, but the general idea of reed valves is that they allow a broader torque range, with fairly even crank-filling over a wide range of rpm.
Disk-type or other crank-timed or piston-timed inlets produce a uniform number of rotation-degrees of inlet opening, but that becomes a shorter
*time* of inlet opening as rpm increase. Thus, they lead to sharper torque peaks, even though they may allow slightly greater peak horsepower at the rpm for which they're tuned. But good reed-valve design hardly steals any peak horsepower.
Tim, In very high performance two strokes both reed valves and rotary valves are used, like in some go-kart engines. In that way, when the reeds go into float, the rotary valve provides the extreme high rpm horse power and at low rpm, the reed valve prevents reversion and low speed torque is retained. Steve
I think model airplain engines are very cost sensitive. Another poster said reed valves provide low end torque but I don't think that model airplane engines need low end torque. So my guess is they are not needed.
It's really fascinating how a 2-stroke works. I've seen some computer modeling of the flow characteristics of a few different engines at varying RPM, which showed all of this in red/blue flow, with the intensity of color showing the density of that location. Of course, I can't google it up right now.
Yes, the Day cycle (did you know it was called that? Invented by a guy named Day, of course) is pretty fascinating. They're doing some amazing things with direct fuel injection and two-part gaseous mixtures to make it clean, too -- basically they keep the same cycle, but the fuel (or fuel-laden air) gets put into the cylinder after the cylinder is scavenged with fresh air.
prevent the engine running backwards; most glow plug engines rely on the induction timing to prevent reversal. The small reed induction engines (e.g Cox 049) use a starter spring to ensure that they only start in the correct direction (you hook it around the prop, wind it backwards a few turns and let go), you couldn't easily do this with a larger engine. I know from experience that if you try and start one of these reed induction engines with your finger, they bounce off compression and start backwards; that's led to a few very short flight on occasions ;-) Martin
There may be another factor. Model engines commonly use the crank as the inlet valve, with a hollow crank for the air-fuel mix passage into the crankcase. The counterweight and crank throw are cantilevered. The big end of the rod slides over the crank throw when the sleeve is removed.
This is very simple and works well for low mass (small) engines. I suspect by the time you're up to motorcycle size engines of high output, the cantilevered design would not be favored compared to reed valves and crank bearings on both sides.
I have flown the Cox engines for years, and yes it's an issue (always check by feeling for airflow). You can learn to flip the thing in such a way that you neither start it backward or chop off your fingers by exerting force on the front of the blade about 2" out from center and following through (and using a finger guard).
Those dang starters rob power and wear out -- I don't use 'em.
whee is right... when I was a young man, I worked at a suzuki dealer unpacking and assembling motorcycles after school... I put together a trails bike and didn't check the timing, trying to start it, it back-fired and started up... I put it in gear and took off... boy, was I suprised when it took off backwards throwing me over bars... timing was to close to top dead center