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Joined: Jan 2009
Posts: 6,926
Likes: 10
Pushrod Honda preferrer
***
I suspect the bike engines you've seen with a reed valve, actually had the intake port positioned so low, and the piston skirt cutaway so high, that the piston was not actually controlling the intake. That would make it a conventional reed valve 2 stroke - tends to have good low speed torque and fairly easy starting (for a 2 stroke).

Joined: Oct 2010
Posts: 1,819
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Junior Technician
***
Nah they have port windows in the piston.


A lot of the crank case inducted motors have a cut away in the bottom of the skirt as with the gearbox and all the other jazz its not possible to get the intake just into the crankcase so with the piston at TDC the skirt would still cover the intake port.


Joined: Jan 2009
Posts: 6,926
Likes: 10
Pushrod Honda preferrer
***
I agree about the cutaway skirt Bob, but the question is whether the intake port is closed by the piston during the crankcase compression phase of the cycle. A port-controlled intake system has to close off the intake port when the descending piston is still fairly high in the cylinder, so there won't be a lot of backflow out through the carburetor due to the early part of crankcase compression. That unfortunately means the intake port doesn't open until the piston is fairly high up in the compression stroke. As a result a port-controlled 2 stroke has a powerful crankcase suction phase, that you can feel quite sharply on the starter cord. A reed valve 2 stroke leaves the intake port open all the time or very nearly all the time, and relies on the reed valve to enable crankcase compression to happen for the full length of the downward stroke of the piston. Because the intake port is open all the time, crankcase induction happens for the full length of the upward stroke of the piston, so there is no crankcase suction that you can feel on the starter cord.

It can be convenient for the design engineer to have what looks like a piston-controlled intake port, but to position it as low as possible on the cylinder while also cutting away the piston skirt so the intake port stays open virtually all the time. Then a reed valve is positioned as close as possible to the intake pipe's flange mount on the cylinder. While it may look like a port-controlled intake system, it is actually a normal reed valve design.

Reed valve designs allow a considerably more effective crankcase induction process at low engine speeds, but they are not very good at high speeds: they restrict the gas flow, and they take a finite amount of time to open and close, so timing is delayed. The result is they are a great way to make low speed 2 strokes, but not a competitive way to make high speed 2 strokes. If you want high power output at more than, say, 6,000 rpm, port control is better than reed valves. (I don't think you will ever find a reed valve in a modern chainsaw.) At lower speed it is the other way around, and reed valves are better. So why do Victa and Villiers 2 stroke engines have port control, despite being low speed engines? It's because reed valves cost more than port control.

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