2-stroke Engine Types Help?

[Robin Plummer]

Hi, can someone explain to me what is differnt about a Piston Port, Reed Valve and Rotary Valve 2 stroke engine? How does each type work and what type can develop the most potential power? Can one type rev higher than the other?

[zefarelly]

rotary valve is an opening on the crank web, the size and position of the web dictates the size and duration of inlet port opening (the exhaust port is controlled by the piston) old Vespa engines are like this.

piston porting is just that . . . .as the piston goes up and down it opens and closes the port at the bottom of the barrel . . .Lambretta engines do this, you can modify the piston skirt and port size to alter inlet timing, size and duration.

reed valves, I think, work on a non return basis, and are further up the barrel controlled by the piston top ( I could be wrong here)

from my knowledge of scooters performance is reeds best, piston 2nd, rotary Vespa! (very reliable though)

[KC]

Reed valves open from the down stroke vaccuum and then are closed as the piston pressurizes the crankcase. Many compressors use this type of valving.

Link to 2 stroke engine info and reed valve systems...
http://www.howstuffworks.com/two-stroke2.htm

[zefarelly]

the only downside to reeds Im aware of is they break and the bits damage internals . . . .Ive utilised a cashpoint card as a makeshift reed before . . . .got someone home!

[Warwick]

The old reeds used to be steel and when they broke you would suffer serious engine damage,for quite some time now they have been fibreglass,carbon fibre etc and will normally go through a engine without doing any damage at all.

[boyracer]

I beg to differ Warwick.
Most of the reed valves on 2-stroke bikes are still steel (the ones I have seen anyway) and the chnces of a reed breaking are very, very slim. Even with a reed valve piston port timing is used (you have no choice) as the exhaust port must be closed before the transfer slot is opened (ok, slight overlap, but still) the reed valve simply stops the charge from escaping back up the inlet tract.
Supercharged, 2-stroke diesel motor (General Motors engine, aka Jimmy) Have valves like a 4-stroke motor, as the fuel (being compression ignition) is injected at the top of the stroke, thus no need for the complicated transfer porting.

[Warwick]

I have only once seen steel reeds and that was in a 1982 YZ250J,My 1983 RM125 used fibreglass and I haven't yet seen another engine that uses steel,thats not to say they didn't come out with them,but maybe most people change them.

In my RM we machined away part of the skirt to change the port timing,quite a common mod along with spacers under the barrel.