Reply To: Carburetor principles

[frankr]

So your question is about the purpose of the reed plate? Actually, it is so simple that most of us don’t even give it a second thought, nevertheless it is a worthy question. It all begins with the basic theory of operation of a two-stroke engine. On the piston’s up-stroke, a partial vacuum is formed within the crankcase. That low pressure draws in air and fuel through the carburetor. Then on the piston’s down-stroke, that air/fuel charge is compressed. When the piston nears the bottom of it’s stroke, intake ports open and the compressed air/fuel escapes the crankcase and enters the cylinder. Very soon, the intake ports close again as the piston starts it’s up-stroke and the process begins all over again. Since the question concerns reeds, I am deliberately leaving out the rest of the story,

OK, so we now see how the air/fuel gets into the crankcase. There is one other thing to understand. The air is being drawn in on the up-stroke and being compressed on the down-stroke. So why doesn’t it simply blow back out from where it came, back through the carburetor? Aha, enter the reed valves. They are simply a one-way flapper valve. They pull open in one direction (vacuum) and slap shut in the other direction (pressure). No, obviously they wouldn’t work if installed backwards if that were even possible.

The other part of the question concerns why some two-strokes don’t have reeds. Well they may not have reeds, but they do have some sort of valve to allow air to enter the crankcase on the up-stroke, but not blow back out on the down-stroke. Most often, that is accomplished by some sort of rotating valve (rotary valve engines), or a special port or ports opened and closed by the piston skirt (third port engines). Or even a combination of the two, such as the Johnson TD-TN series.

Reeds were not used on the real old designs, because metal technology didn’t exist yet. Those things are constantly flexing back and forth and would soon break if not made of a metal or other substance able to withstand all the bending. But once invented, reeds pretty much became the standard because of the simplicity, suitability over a wide RPM range, and low cost. An exception might be stuff like chainsaws where high RPM takes a back seat to slow idle. They most often use a third port design for durability, light weight, and low cost.

Hope this helps.