Radial engine carb position

petertha · Mar 30, 2012

Im having to make some design decisions around carburetor placement & intake tubes & how those elements are going to communicate with one another (. and rapidly realizing Im flying by the seat of my pants at best).

Most of the model radials I looked at have a similar theme: spot the carb near the crank centerline & induction tubes going to the heads in a nice flow path, tarantula style. Makes sense to me. So why did the FS engine lay the carb very low relative to the crankcase? One would think this would create imbalances such as short U-turn path to the lower cylinders vs. the extra long path to the upper cylinders, especially #1. The path lengths from venturi to the intake valves are hugely different.

Was the carb spotted more-so for better intake air position relative to the airstream & suction/timing took care of itself? Or maybe some sort of lean/rich self compensation? (That would infer hotter cylinder on bottom, is that right?) One hears the term 'down draft' (carbs), why would that be important or advantageous in this case?

Dave G · Mar 31, 2012

The carb usually fed into the center of a supercharger impeller. I don't think it matters where the carb is mounted as long as the mixture is fed into the impeller or at the proper location. I built the Hodgdon 9 cyl radial ten yrs ago. This was before Lee designed a diffuser to help atomize the mixture better. Mine has no diffuser or impeller so I have mixture problems. Lee designed his impeller to run at crank shaft speed and mounts to the end of the crank inside the intake manifold. My radial likes to run with the bottom cyls rich when the top cyls are correct. If I lean out the mixture so the bottom cyls are right the top ones are too lean. A diffuser would help this a great deal and I think is needed. The intake tubes are arranged so they are fed from the periphery of the diffuser. I hope this helps, Dave

petertha · Mar 31, 2012

Thanks Dave, that's very helpful info. You confirmed the lower cylinders would typically run richer than the upper. Which is why I was kind of scratching my head on the FS Kinner. There is no supercharger/boost that I'm aware of. Maybe there is another trick hidden in the internals or one of those things that doesnt just scale to miniature size.

I did note the diffuser on the H9 radial. I wasn't aware it was subsequently added somewhere in the design evolution, so likely for reasons you encountered. Interesting.

Here are some sketches where Im at. Its glow ignition, so pre-mixed oil in the methanol fuel. If I try & replicate the induction tube positions connecting the head, the holes would be spotted in the crankcase about as shown. So my thought was to position the carb on a backplate type manifold that basically caps off the cam timing gear back end. The thinking was allow the mixture to mist over the gears & lube them a bit in the process. I was also considering positioning the carb 'high' (closer to upper cyclinder bank) & maybe that would self-compensate the rich/lean issue? The location is not scale, but the thing has to run. Making new backplates would be easier than mods involving the cranckase.

I recognize this is not a diffuser by any means. Its more the opposite, a 'mixture masher chamber'! I wonder if fuel/air charge will actually find its way into tubes & heads properly this way? If a diffuser was the inevitable decision, it would add another chunk to the back end & tube reposition. I can totally see the benefit though, maybe back to the drawing board.

Dave G · Mar 31, 2012

Hi Petertha, The Bentley BR2 of rotary engine fame was lubricated with castor oil mixed with the fuel. I think it was injected somewhere into to the airstream of the mixture which was a hollow crankshaft. The carb was at the end of the crank just in front of the pilot's knees. The fuel-air mixture came out of the end of the hollow crankshaft at the center of the engine. The engine was then lubricated by
centrifical force with the oil carried by the mixture.

I really do feel that something is needed to help atomize the fuel. I don't think it has to be much, maybe a small impeller driven from the end of the crank. Maybe if you mounted the carb at the center of the domed intake manifold and fed the mixture directly into the center of a small impeller at the center of your geartrain. The larger the plenum of your intake, the less effect a small impeller will have on the mixture. The intake tubes are begging for air and anything you can do to help feed them will make it run better. I think I would make the area around the geatrain relative to the intake tube dia. Not to much, not to little.

The H9 has the crankcase vented by a small hole drilled thru the front crankshaft segment. Venting in this way may allow enough oil to flow forward in the engine to keep things lubricated. I don't know.

My experience with glow fuel is that it will lube anything that it touches. I think I would concentrate more on mixture distrubition than lubrication. You could also use an impeller feeding directly into the intake tubes with a small deflector or opening to lube the gears.

I don't know if any of this will help. Keep going cause when it finally runs the rewards can't be matched. I tell my wife, its as close as us men will ever get to giving birth when our engines run. As a mother of 2, she doesn't find any humor in that. Dave

petertha · Mar 31, 2012

I forgot to add this pic of the Edwards radial. It has a more sophisticated dedicated oil pump system so that aspect is taken care of. But in terms of carb position intake tube geometry & lengths, seems like a good reference. The trick is to essentially hide the bottom intake tubes segments coming off the carb.

Hmm.. Im going to have to play with this some more & see if I can get the best of both worlds with a diffuser & still try & maintain some look of the real one.

Thanks again for the insight & ideas.

petertha · Apr 1, 2012

Found one more good reference, Martin Ohrndorf 9-cyl radial (methanol/glow).
http://engineman.de/grafiken/bilder/c_2.jpg

The intake charge gets delivered from carb into the crankcase chamber presumably for the same reason, to lube up the crankshaft & rod assembly. Then it continues on its way to the heads via the ports. The interesting thing is, on any given intake stroke, one could almost visualize the flow being an even tighter U-tube path relative to what I'm trying to accomplish.

gbritnell · Apr 1, 2012

Hi Petertha,
When I was designing my engine I used the Morton as my model. On the Morton the carb is on the bottom of the main crankcase so it has a long path to the upper cylinders but seems to work ok. On mine I moved the carb to the rear crankcase cover (separate chamber) to supply fuel. I would have liked to center it about the cylinders but the oil pump prevented that from happening. As you can see from the PDF picture it's offset from center but here again I don't think it will cause a problem. I'll certainly let you know.
George

View attachment ENGINE RADIAL ASSY REAR.pdf

petertha · Apr 18, 2012

Well, a little more drawing progress, a little more head scratching. By hook or by crook, looks like I also need to position the carb inlet hole quite high in the crankcase backplate if I go down this route. The issue is, the inlet hole needs to avoid 'obstacles' I kind of lost track of until this point, namely the 5 camshaft ends & their bearings plus the crankshaft end bearing TBD. So for sure I cant put the inlet hole on the center of the backplate. And it doesnt fit well between the crank center & #1 camshaft. So I have it spotted kind of like this. It's still centered, but its up pretty high.

Pros
- This layout fits the FS engine pretty good in terms of the crankcase depth & position of the induction tubes.
- The fuel mixture is lubing the gears.
- The higher inlet position would likely richen the upper cylinders & lean the lower cylinders based on flow path which would hopefully serve to balance out the typically lean upper cyl issue Ive heard mentioned

Cons
- its a very crowded crankcase with the gears, therefore maybe a tortuous path for gas to find its way down to the other cylinders? I really dont know, flying by the seat of my pants here.

- I think I could thin the crankcase ID chamber a bit & give more 'radial' flow path if that helps.
- Possibly I have one gear set combination that reduces the whole cluster & gives more flow room
- I could add on another inlet manifold chunk to the back of this backplate, but then the whole inlet tube positions move back quite a distance & the scale-ish aesthetics go for poop.
- I could make 2 smaller holes in the backplate, on either side of vertical center, but these would have to be tied to a mini manifold attached to the carb, ugh!

Comments & opinions?

petertha · Apr 18, 2012

Another question. Is there a rule-O-thumb of carb venturi ID relative to cylinder displacement, or some other comparison metric? I should probably have a handle on this from my RC methanol stuff... but so far Ive just run 'um, havent built 'um!

I noticed on the Edwards 5 radial plans, they called for a Perry #302 or #205 carb. Its single cyl displacement is I believe ~0.677 CI. Kind of looks like those Perry model numbers have been replaced & modernized, so Im not really sure what ID they corresponded to. Anybody know?

The good news is, looks like they offers a huge step-bracketed range now, so thats potentially useful.
http://www.perrypumps.com/prod01.htm

Lakc · Apr 18, 2012

My memory is pretty dusty, but the carb size rule of thumb is something like 27% of the cylinder diameter, irrespective of how many cylinders there are, unless they all draw air simultaneously for some reason.

True forced induction on a model radial engine has always been the holy grail, but its complicated by the strength of the gears at their necessary size. I dont know if there really are any advantages of that, but the diffuser on the hodgson seems to get good reviews. Might be a good question to ask him at NAMES.

Since you have all those nice gears back there anyway... Some type of diffuser would be tempting.

Dave G · Apr 18, 2012

Hi Petertha, My H9 has a carb designed by Jerry Howell and it's bore is .218". The cylinder bore is 1.000". The design for this carb is available in the Strictly IC magazine or from Jerry's son on his website. My carb will get cold when the engine is first started and then as the engine heat warms up the carb the mixture needs to be adjusted. My throttle bore is probably too small but it runs fine and there is no need to change it. Dave

Jerry-howell.com

petertha · Apr 18, 2012

Another cutaway section. Visualizing it from this angle, I think the lower cylinders would need to get fed more by inlet gas coming around the case ID, kind of in a radial/ring fashion? There is just too much gear 'stuff' in the way for a more direct A>B path. I dont think 2 inlet holes on either side of center helps in this regard. Also what Im wondering, if the case holds a certain (biggish) volume of pure induction gas at any point between induction strokes, kind of like a reservoir, would this help the issue in the absence of a smooth direct 'piped' path to each induction tube?

Lakc · Apr 18, 2012

Edwards practice was to extend the bottom tubes above the bottom of his mixture box to avoid the liquid collecting. He had a separate sump and oiling system.
Most all the the model engine 4 strokes just use blowby oiling to lube the internals, do the commercial OS and FS radials oil any differently?
All those gears spinning may help break everything up and keep the fuel and oil vaporized, but there is bound to be some complicated fluid dynamics going on that would probably make that a crapshoot.

Swede · Apr 20, 2012

Did I miss what fuel is going to be used? I think that will influence the final decision. Gasoline vaporizes easier than methanol, and I thing the former will be a bit more forgiving in the sense that distribution will be more even in a given setup vs. methanol.

I also built my H9 before an impeller was part of the official plans set. I wanted to get a little mixer wheel in there, and the design made this pretty easy. Since supercharging is not the goal, the wheel need not be particularly high tech, nor does it need to have any sort of a tight fit in the rear crank case.

In the end, I cannibalized a plastic impeller from a little "dust buster" vacuum, used it as a pattern, and cast it in aluminum:

As far as I can tell, the mixture between cylinders seems good. The lower cylinders smoke a bit more at idle, but that's oil getting by the rings.

As far as the carb, it's counter-intuitive that the throat diameter doesn't change (much) regardless of the number of cylinders. I started first with carbs that were WAAAY too big. I went smaller, smaller, finally settling on a Super Tigre carb designed for a .40 glow engine. What I saw was this - the larger carbs gave a greater RPM, more HP, but mid throttle and idle especially were terrible. The velocity of the air was too low at idle to reliably vaporize fuel. So the smaller carb gives less power at the top end, but the engine runs smoothly and reliably at all throttle settings.

Remember too that carbs for glow are a bit different than gasoline, and mixture adjustment can be tricky when using glow carbs with gasoline.

petertha · Apr 20, 2012

Swede said:
Did I miss what fuel is going to be used? ... mixture adjustment can be tricky when using glow carbs with gasoline....

Hi Swede. Definately planning on typical RC 4-stroke glow fuel. So methanol based, oil pre-blended in, 15% nitro +/- what works best with various glow-plugs. And yes, RC type carb or clone, at lease initially. I'm committed to glow vs ignition just because this will be my first build & my machining skill plate will be full (err...overflowing?) on this beast. Ignition & gasoline I'll save for subsequent engines. I have a lot of respect for you guys that pull off those additional issues.

I got thinking about this flow path thing some more. I bet if I drew up a typical RC 2-stroke & 'froze' the flow path, it would probably be an interesting shape, a twisty, turny tortuous path. Front carb induction, down into the hollow crankshaft, out the end past the counterweight, make a U-turn, past the con rod, divide into 2+ side passages between cylinder sleeve & casting, then out through cylinder liner slotted port into the combustion chamber. Just for kicks I want to try & figure out how to show that in cad one day by eliminating all the 'metal'.

But for this radial, I think I can Im just going to nudge things here & there to make some room so the flow doesnt have to traverse accross the gear cluster quite as much. I probably dont need such a fat lip of the backplate extending into the crankcase. And maybe just extend the back end of the CC a bit more. Its mostly aesthetics at this point.

Swede · Apr 22, 2012

I'm looking forward to what you come up with! Is there anything on the induction side that could benefit from lubrication? In other words, would there be components that might NEED that flow of fuel + oil?

odie · Dec 23, 2012

Hello,
I've built several radial engines. A Morton M-5, from castings by Bruce Satra. This engine has flown on my Fleet Biplane. A seven cylinder, using Satra cylinders. Plane is ready, but too late for this season. Both of these engines are on glo fuel, and they're relatively small engines. My third engine is a P&W R-985 Wasp Jr, Built from Bob Roach's drawings. This engine is 84cc, and was also designed for glo fuel. On an engine of this size, glo fuel is NOT a good option. It drinks alcohol like a drunken sailor. The small engines have updraft carburetors, and I still had bent rods from hydro lock. The nine's carb feeds into the eye of the super charger that turns a crankshaft speed, and is an updraft type. Another problem with all of the radials is, the glo plugs need to be energized during idle operations. Glo plugs draw between 2, and 3 amps. You'll need a glo driver, and a hefty battery. I'm currently running my nine on a gasoline, and oil mix. The ignition system weighs a fraction of the glo plug battery, and it gives much better fuel mileage. You'll find carburetion a problem using gas. Glo carbs don't seem to work well over the full throttle range, and Walbro carbs need a source of pulsing air, to operate the carbs internal fuel pump, which a radial doesn't normally provide. Hope this helps, good luck.
Odie

Bentwings · Jul 12, 2020

petertha said:
Well, a little more drawing progress, a little more head scratching. By hook or by crook, looks like I also need to position the carb inlet hole quite high in the crankcase backplate if I go down this route. The issue is, the inlet hole needs to avoid 'obstacles' I kind of lost track of until this point, namely the 5 camshaft ends & their bearings plus the crankshaft end bearing TBD. So for sure I cant put the inlet hole on the center of the backplate. And it doesnt fit well between the crank center & #1 camshaft. So I have it spotted kind of like this. It's still centered, but its up pretty high.

Pros
- This layout fits the FS engine pretty good in terms of the crankcase depth & position of the induction tubes.
- The fuel mixture is lubing the gears.
- The higher inlet position would likely richen the upper cylinders & lean the lower cylinders based on flow path which would hopefully serve to balance out the typically lean upper cyl issue Ive heard mentioned

Cons
- its a very crowded crankcase with the gears, therefore maybe a tortuous path for gas to find its way down to the other cylinders? I really dont know, flying by the seat of my pants here.

- I think I could thin the crankcase ID chamber a bit & give more 'radial' flow path if that helps.
- Possibly I have one gear set combination that reduces the whole cluster & gives more flow room
- I could add on another inlet manifold chunk to the back of this backplate, but then the whole inlet tube positions move back quite a distance & the scale-ish aesthetics go for poop.
- I could make 2 smaller holes in the backplate, on either side of vertical center, but these would have to be tied to a mini manifold attached to the carb, ugh!

Comments & opinions?

View attachment 54449

View attachment 54451

View attachment 54453

Bentwings · Jul 12, 2020

Here is a thought I’ve been rolling around for a while. I’m glad I read about some kind of impeller for “mixing” the air fuel. You probably can’t make any kind of supercharger but I thought that possibly using an electric rc plane motor to drive an impeller. It would rotate much faster so ai floe through the intake pipes would be better and more positive. I think you would not wan5 to over speed this too much but it would be relatively easy to implement especially if you have a nice cad system to work with. I have tig welding experience with small stuff and can do silicone bronze brazing . My eyesight has been compromised by med issues so typing and fine work is very difficult. I’ve been coaching my grand son on tig welding and brazing this small stuff. I’m about ready to start on the 18 cylinder motor. I can model about anything needed and I thought maybe have a preliminary design 3D printed in plastic for test fitting and actual electric motor testing. Let me know your thoughts on this.

Bentwings · Jul 12, 2020

petertha said:
Another question. Is there a rule-O-thumb of carb venturi ID relative to cylinder displacement, or some other comparison metric? I should probably have a handle on this from my RC methanol stuff... but so far Ive just run 'um, havent built 'um!

I noticed on the Edwards 5 radial plans, they called for a Perry #302 or #205 carb. Its single cyl displacement is I believe ~0.677 CI. Kind of looks like those Perry model numbers have been replaced & modernized, so Im not really sure what ID they corresponded to. Anybody know?

The good news is, looks like they offers a huge step-bracketed range now, so thats potentially useful.
Perry Pumps - Carbs

Radial engine carb position

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