2-stroke radial engine

[kcnegemen]

http://www.rcmplans.com/issues/requested/content/reviews/pdf/r-rv-berger-121997-1-1.pdf


this site made ​​with atmospheric pressure compressor.

interesting.

 

[aonemarine]

Better up that belt drive by 20......then its a maybe....

 

[MuellerNick]

There are some valid objections. But some numbers should help getting the blower right.
2-strokes have a CR of the crankcase of 1.3 … 1.5. But you do have that pressure only at the BDC. Obviously, it drops to 1 at the end of scavenging. So there is an average pressure of 1.15 … 1.25. If you consider the efficiency of sucking in the air that might be at 90%, the pressure drops even more.

Assuming a pressure of 1.2, you would need a blower that:
* makes a pressure increase of 1.2
* delivers a volume of 3 * displacement (number of cylinders * displacement) at 1.2 pressure

If you can't find how to design blowers, you should at least build a model and verify that it makes enough pressure and volume. The equipment to measure that is really easy to make by your own. Some tube and water. Makes a pressure indicator and a Prandtl tube (to measure speed).
Build that blower, at the outlet add some tube with a throttle at its end. Adjust the throttle to get 1.2 pressure, measure the speed of air going out. With the speed and the cross-section (and an adjusting factor) you get the volume.

Nick

 

[dman]

http://www.rcmplans.com/issues/requested/content/reviews/pdf/r-rv-berger-121997-1-1.pdf


this site made ​​with atmospheric pressure compressor.

interesting.

it's not that you need a compressor. it's that you need an air displacer. the air density has little to do with it so the pressure ratio calcs are not an exact science in this application. the timing on most 2 strokes leaves the exhaust open after the intake so the the nominal pressure in th cylinder is never more than exhaust back-pressure until after the exhaust (and therefore the intake) is closed. but since the exhaust is full of pressure waves bouncing around the exhaust evacuation and cylinder pressure is all over the place in a running engine. with a positive displacement type pump you will mechanically overcome any intermittent back-pressure but with a centrifugal pump you will not.


also you list no dimensions. centrifugal compressors don't scale like a pump. they give pressure per rpm*diameter while a positive displacement type gives a volumetric flow rate per rpm. coming up with adequately pressure to reliably scavenge the exhaust is one thing but achieving it is another. if the engine dimensions dictate a 1" compressor wheel you will need rpms in the hundreds of thousands just to create 3psi and you will not be able to do that through a belt drive. the mechanical forces wont allow it. it would be too difficult to change the compressor speed putting huge strain on the belts and the compressor would act like a flywheel, due to the great rpm it would appear too heavy to the engine to drive it and the belt system would rip its self to pieces. the only way to drive something to these speeds is with a turbine which is a whole other can of worms. like the logistics of getting the compressor up to speed for start up. there is another design that could in theory drive something high speeds which is a type of planetary arrangement that uses wheels instead of gears and is driven by an outer ring that is slightly flexible so it's a sort of rigid belt in a way. this design is limited to about a 1:14 ratio geometrically so 10,000 rpm engine could drive a 140,000 rpm compressor in theory but i don't know how reliably it could do it. and you are still faced with the dilemma of what happens when the engine idles.


the engine in that article has a vane type pump which is positive displacement and has already been recommended to you several times. the vanes slide in the central hub which is concentric to the crank but the housing is offset. with this arrangement the vanes create chambers in the housing that change is size during rotation drawing air in through a channel in the section where the chambers expand that expands and squeezing it out through another on the opposite section.

 

[kcnegemen]

it's not that you need a compressor. it's that you need an air displacer. the air density has little to do with it so the pressure ratio calcs are not an exact science in this application. the timing on most 2 strokes leaves the exhaust open after the intake so the the nominal pressure in th cylinder is never more than exhaust back-pressure until after the exhaust (and therefore the intake) is closed. but since the exhaust is full of pressure waves bouncing around the exhaust evacuation and cylinder pressure is all over the place in a running engine. with a positive displacement type pump you will mechanically overcome any intermittent back-pressure but with a centrifugal pump you will not.


also you list no dimensions. centrifugal compressors don't scale like a pump. they give pressure per rpm*diameter while a positive displacement type gives a volumetric flow rate per rpm. coming up with adequately pressure to reliably scavenge the exhaust is one thing but achieving it is another. if the engine dimensions dictate a 1" compressor wheel you will need rpms in the hundreds of thousands just to create 3psi and you will not be able to do that through a belt drive. the mechanical forces wont allow it. it would be too difficult to change the compressor speed putting huge strain on the belts and the compressor would act like a flywheel, due to the great rpm it would appear too heavy to the engine to drive it and the belt system would rip its self to pieces. the only way to drive something to these speeds is with a turbine which is a whole other can of worms. like the logistics of getting the compressor up to speed for start up. there is another design that could in theory drive something high speeds which is a type of planetary arrangement that uses wheels instead of gears and is driven by an outer ring that is slightly flexible so it's a sort of rigid belt in a way. this design is limited to about a 1:14 ratio geometrically so 10,000 rpm engine could drive a 140,000 rpm compressor in theory but i don't know how reliably it could do it. and you are still faced with the dilemma of what happens when the engine idles.


the engine in that article has a vane type pump which is positive displacement and has already been recommended to you several times. the vanes slide in the central hub which is concentric to the crank but the housing is offset. with this arrangement the vanes create chambers in the housing that change is size during rotation drawing air in through a channel in the section where the chambers expand that expands and squeezing it out through another on the opposite section.

Dman
Using 55 mm compressor wheel.

I also have a problem in my head to fill.

140 000 cycles can not.

40 000 rpm max.

max engine speed is about 10 000.

7.5cc x 5 = 37.5 cc Cylinders

positive displacement vane pump and the corresponding image of the project, for example, the drawing you show an example.

Which do you think would be successful if I choose.

 

[dman]

Dman
Using 55 mm compressor wheel.

I also have a problem in my head to fill.

140 000 cycles can not.

40 000 rpm max.

max engine speed is about 10 000.

7.5cc x 5 = 37.5 cc Cylinders

that's my point. it's just not gonna work at this scale

positive displacement vane pump and the corresponding image of the project, for example, the drawing you show an example.

Which do you think would be successful if I choose.

there seems to be somewhat of a language barrier. but some air tools use a vane style motor. same thing but in reverse...

[ame]http://www.youtube.com/watch?v=qJ1QccN7LXg[/ame]

this should help. maybe another member has a blueprint.

 

[kcnegemen]

perry carbüratör....

 

[dman]

perry carbüratör....

sounds good. once you get the air pump sorted out. the renderings look great by the way. just want to make sure you have some workable engineering before you start.

 

[canadianhorsepower]

maybe this would help

http://en.wikipedia.org/wiki/Rotary_vane_pump

 

[kcnegemen]

have a bit of a language problem

Thank you dieselpilot dman power aonemarina canadionhorse

continue to design

 

[kcnegemen]

 

[kcnegemen]

Has there been carburator? dman.

 

[canadianhorsepower]

very nice drawing

 

[dave-in-england]

.
Wot ? No piston rings ??
and those connecting rods look a little .... Butch !!

This project will involve a hell of a lot of machining, and time.

I can see that the fan will just swirl the inlet mixture around endlessly inside the fan housing, without any pumping action to force any mixture into the cylinders.

What about having a re-design and making the engine a four cylinder model,
where two smaller diameter opposing pistons are actually used as suck-squeeze compressor cylinders to positively force the mixture into the other two working cylinders.

:hDe:

.

 

[Art K]

I normally wouldn't be very interested in a two stroke but I find the concept of a two stroke radial and the discussion on pressurizing the intake charge fascinating. I will be following this build.

Art

 

[dman]

Has there been carburator? dman.

everything looks good to me now. this will be a great project.

 

[dman]

.
Wot ? No piston rings ??
and those connecting rods look a little .... Butch !!

This project will involve a hell of a lot of machining, and time.

I can see that the fan will just swirl the inlet mixture around endlessly inside the fan housing, without any pumping action to force any mixture into the cylinders.

What about having a re-design and making the engine a four cylinder model,
where two smaller opposing pistons are actually used as suck-squeeze compressor cylinders to positively force the mixture into the other two working cylinders.

:hDe:

.

there is no fan. it's a pump that has been used successfully in the type of engine before. read the whole thread. the air blower vs air pump has been addressed the latest renderings have a proper pump.

who needs piston rings? model aviation engines have none and they make pretty high power levels for the displacement..

 

[dave-in-england]

My comments about the fan were directed at the cad model on page 1, which shows a fan being used to blow the mixture into the inlet ports.
However, the comments I made seems to have been placed under the model of the vane pump on page 4.

 

[Till]

It will prove to be very useful to create a small handwritten essay specially on the cylinder ports... Please build a running single cylinder engine first before you start with the radial to avoid the big disappointment.

I strongly recommend using a third party (chainsaw?) carburetor instead of your own design, because from what I've seen so far in this thread, you still need to learn quite a bit and carburetors are a mayor source of avoidable errors. As soon as the engine actually runs, you can still design your own caburetor.

The blower vanes should be slightly spring loaded and of material with good surface slip, such as teflon. Lots of oil in the fuel mixture will establish proper sealing.
I recommend building a prototype with the blowers case supported by an adjustable excenter, so you can determine the proper pressure for your engine.

If you can't exclude flexing of the crankshaft under load conditions, you better use rather thin, flatsided connecting rods with the flexibility to compensate for the angular missalignment of crankpin and gudgeon piston pin. This will partly compensate for bad manufacturing clearences and unexpected vibration issues, too.

Ringless pistons without special coatings and conical cylinder bores require material combinations of equal thermal expansion, so you'll likely go for grey cast iron.

The cylinder head is exposed to maximum thermal stress and therefor needs larger fins, specially if you have no experience with lean mixture conditions.

 

[kcnegemen]