Given the paucity of 1/2 engines these days (especially r/c). I'm considering turning my hand to designing one. I've been studying 2-stroke design for a little while now, and have given the matter some thought. About the only real difficulty I face is the fact that I've never machined a thing in my life. Anyhooo.....here's a short list of design features I'm considering;

Desaxed cylinder, purported to improve port timing.

Reed valve induction, although with an inclined reed instead of perpendicular like Cox. Should improve flow.

AAO piston/cylinder like the Norvels. Nice and light.

I've come up with a novel cylinder porting configuration, but I've no idea if it will work or not...I've attached a graphic to show what I mean. I'm thinking it will improve scavenging - any experts care to weigh in?

R/C carb o'course.

digital_trucker

I'm no expert, but I'll play one.

It looks like it'd work, BUT:

1. When you get very small passages boundery layer is going to be a big factor in airflow.
2. Your port timing is set by the hieght of your ports, round ones won't flow much for the first part of the opening, but it might make it a quite engine.


I like the idea of driling in the intake passages, then boring the cylinder out, AAO is cool.

Desaxed cylinder would be heavier and not as powerful, a bad idea. My brother had a Sears Twingle that had desaxed cylinders, a 250 cc, the thing had a vibations that was killer, it almost felt like an electrical shock.

It's kinda hard to tell, but the exhaust ports have a steeper angle than the transfer ports. My idea there is that it'll held shunt the exhaust out with the transfer gasses rising in the center. Why would a desaxed cylinder be heavier? It's just a slightly offset cylinder...? I didn't know about the vibration issue...maybe that sears didn't have a sufficient counterbalance on the crank?

I'm even less of an expert, but I would think that a reed valve setup would limit your use of throttle to exhaust only (throttle ring) which would eliminate being able to use a muffler...Rog

Rog, even worse is the huge carb setup would have to stick out the rear of the engine. If you just have to have that, go with a drum rotor or disc rotor. Performance would be drastically improved and it would throttle better.

I would get good at making conventional engines first, get the process for making perfect P/L sets down pat. I think you would need to get the recipe for low expansion, high silicone piston material and crank out dozens of pistons, same goes for liners, you'll need dozens to mix and match your best P/L sets from. Once you are armed with lots of well matched parts, then let the fancy porting begin. I would make the R&D crankcases real meaty to allow different porting ideas. A flow bench is all you need to test volumetric efficiencies of different ideas, you don't even have to fire the engines up, just use a high speed electric motor to turn the engines over. I would baseline some of the world class engines to get a target to aim for as far as air movement is concerned. I have no idea what it would cost to rig this up, but all successful engine developers use similar apparatus.

Every desaxed cylinder I've seen was in a twingle. If you are just going to offset it, what for? Mopar used to put the piston pin offset to cure piston slap when cold, the racers used to put them in backwards and gained maybe one horsepower in a three fifty horse engine, that's 1/3 of 1%.

It was made by Sachs, it's probably a tough engine to balance.

Desaxe offset offers several advantages in a 2-stroke. You get lower piston side loads, but that's probably not significant in 1/2A size. What is more to the advantage is longer piston dwell time at TDC. It also changes port timing in a 2-stroke. The ports are open longer on the power stroke, so there should be better cylinder scavenging. However, the ports close faster on the intake stroke, so there should be better crankcase charging.

As far as reed valves and throttling, the Queen Bee I had did okay on throttling, but I suspect the main problem was a sloppy carb. Cox reeds are set vertically, this makes 'em easy to make, but it forces the incoming charge to change direction and possibly lowers the response time of the reed. A lot of motorcycle 2-strokes use reeds, but they're angled. That should do two things; it should increase the response time and it will give the incoming charge a smoother, more direct route to the crankcase.

Considering how much performance the CL Mouse Racer guys get out of Cox reed engines and the success of reed motorcycle engines I have often thought there a high performance reed 1/2A project just waiting for someone with the smarts and time to work it out.

Take a look at this Japanman thread on more efficient reed configurations.

http://www.rcuniverse.com/forum/m_14...tm.htm#1464571

Yea, the reeds are downstream from the carb, so I see no problem having both. I've got a 52cc twin that has reed valves in a dog house under the crankcase.

Guess I better get busy with the TurboCAD...again. Shame is I have no equipment (or experience) to actually MAKE one.

Have a question...I heard somewhere about using floppy discs for reed material. Is this in fact true?

Yes -- and some use them for hinge material, too. Cut a strip about 1/4" wide and see if you can pull it in two with your hands.

After witnessing a KillerBee turn a 4.2x4 at a steady 28,500 with a vertical reed, I wonder what more could be gained from another reed design? A Fora turns the same prop about 4000 rpm better with shaft induction, so I have to think that a reed can only buzz so fast before it goes into valve float mode.

I think I'm going to put the desaxed cylinder aside for now. It'll be much more difficult to make as a bar stock engine. Now, as a casting it'd be much simpler.

Quick question....what's the mounting hole pattern for a TD .049?