reed valves
11-12-2015, 08:38 AM
#3
Senior Member
Reed valves are nothing more than one-way valves used to limit fluid flow to one direction only. Typically, they will open when the crankcase pressure drops below ambient pressure (piston goes up) and close when above (piston comes down). The stiffer the reed is the more pressure drop required to open it and the less air delivered to the engine - and - the more limited the upper rpm. I am not sure that I totally understand your question, but that's my answer, and I'm sticking to it.
11-15-2015, 10:34 AM
#6
Most of the newer model airplane gasoline engines are reed valve engines including "DA", "DLE", & the engines marketed by Valley View.
The reed valve system is convenient and compact to make a easy to cowl model airplane engine.
A rear drum valve engine has greater breathing potential but I'm not familiar with any recent gasoline model airplane engines which have rear drum valves? A variation of this idea is the rear rotary valve but it's limited compared with the drum valve.
The popular gasoline model single cylinder engines of 20 yrs or more ago were piston ported with a side carburetor. The engines were chainsaw adaptations and worked very well but the carburetor had to stick out the side of the cowl.
50 years ago reed valves were popular in chainsaws but not in the last 30 years.
The reed valve system is convenient and compact to make a easy to cowl model airplane engine.
A rear drum valve engine has greater breathing potential but I'm not familiar with any recent gasoline model airplane engines which have rear drum valves? A variation of this idea is the rear rotary valve but it's limited compared with the drum valve.
The popular gasoline model single cylinder engines of 20 yrs or more ago were piston ported with a side carburetor. The engines were chainsaw adaptations and worked very well but the carburetor had to stick out the side of the cowl.
50 years ago reed valves were popular in chainsaws but not in the last 30 years.
Last edited by w8ye; 11-15-2015 at 10:36 AM.
11-15-2015, 11:12 AM
#7
My Feedback: (19)
I always though of a reed valve engine as one that exhibits a better low and mid-range torque curve than a piston ported engine. A piston ported engine seems to breath much better in the high RPM ranges and if ported correctly and provided with a proper fuel, air and ignition source can easily rev to the point of mechanical destruction. A reed valve engine will just sort of level off in the high RPM ranges as the reeds become inefficient .... kind of like valve float in a 4 stroke engine where an engine only rev's so high and then it just stops turning any faster. Of course there are exceptions to all of these generalizations.
11-16-2015, 05:33 AM
#9
Our gasoline model airplane engines rarely run over 8500 rpm and the majority are 6500 - 7000.
The reeds seldom float.
Most of my chainsaws will run 14,000 when not cutting wood. In the wood they are 9500. But my chainsaws don't have reed valves either. They are all piston ported.
The reeds seldom float.
Most of my chainsaws will run 14,000 when not cutting wood. In the wood they are 9500. But my chainsaws don't have reed valves either. They are all piston ported.
11-16-2015, 09:59 AM
#11
My Feedback: (19)
Our gasoline model airplane engines rarely run over 8500 rpm and the majority are 6500 - 7000.
The reeds seldom float.
Most of my chainsaws will run 14,000 when not cutting wood. In the wood they are 9500. But my chainsaws don't have reed valves either. They are all piston ported.
The reeds seldom float.
Most of my chainsaws will run 14,000 when not cutting wood. In the wood they are 9500. But my chainsaws don't have reed valves either. They are all piston ported.
W8YE, are you a saw collector? I'm not and sadly, I let my family's old saws get away at an auction when I wasn't thinking quite right. Oh well.
11-16-2015, 02:52 PM
#13
Anyway, reed valves really are best suited to engines that need good low end tractability, a broad torque band, and not high rpm max power engines like chainsaw engines, etc., as already mentioned.
AV8TOR
Last edited by av8tor1977; 11-16-2015 at 02:57 PM.
11-17-2015, 05:47 AM
#18
Senior Member
I have a ZDZ 40cc with the rotary rear valve intake. I think you're right about the ZDZ's. It's one reason they tend to be slightly more expensive.
However, it's a dawg compared to say an OS 33, DLE35 or even a PTE36, all reed valve engines. The rotary valve requires proper running in to fully seat, over many runs. Point is that the humble and simple reed valve enables an engine to run at it's best straight from the box and that's been proven with at least half dozen engine makes.
But it has it's limitations as Aviator, Truckracer and Wy8e have already stated. All engines I've played with were operated at less than 10K, in the wheel house for reeds
Wy8e, curious about what porting is in a racing boat engine....These guys operate at 15K +
However, it's a dawg compared to say an OS 33, DLE35 or even a PTE36, all reed valve engines. The rotary valve requires proper running in to fully seat, over many runs. Point is that the humble and simple reed valve enables an engine to run at it's best straight from the box and that's been proven with at least half dozen engine makes.
But it has it's limitations as Aviator, Truckracer and Wy8e have already stated. All engines I've played with were operated at less than 10K, in the wheel house for reeds
Wy8e, curious about what porting is in a racing boat engine....These guys operate at 15K +
11-17-2015, 07:56 AM
#19
The porting scenario is pretty much the same for a racing boat engine and a competition chainsaw engine.
Typical mods are .020" squish with most probably a widened squish band in the combustion chamber to get the combustion chamber smaller which also involves cutting the cylinder base down.
The relative positions of the upper intake transfers, exhaust port, & piston skirt width are all plotted to widen the exhaust port and direct the intake transfers towards the back of the piston. The transfers are smoothed and timing of the intake, exhaust, and piston port timing (where applicable) are adjusted for optimum flow compromise. Minimum case volume is always a consideration.
Finger boost ports are usually added to the piston ported engines.
Have seen closed port engines have the cylinder way cut away to convert them to open port.
Single thin ring.
Typical mods are .020" squish with most probably a widened squish band in the combustion chamber to get the combustion chamber smaller which also involves cutting the cylinder base down.
The relative positions of the upper intake transfers, exhaust port, & piston skirt width are all plotted to widen the exhaust port and direct the intake transfers towards the back of the piston. The transfers are smoothed and timing of the intake, exhaust, and piston port timing (where applicable) are adjusted for optimum flow compromise. Minimum case volume is always a consideration.
Finger boost ports are usually added to the piston ported engines.
Have seen closed port engines have the cylinder way cut away to convert them to open port.
Single thin ring.
