New radial engine project concept

[sheeny]

Hi nlancaster.

Apparently you have read somewhere that the carb size is related to the diameter/bore of a single cylinder of your engine.

Without wishing to upset anyone, I am afraid your info is rather inaccurate.

The carb as we know it performs 2 functions.
1. Allows us to throttle the engine, ie restrict air/fuel flow into the engine and thus the speed of the engine.
2. To control the air/fuel ratio, Normall through the use of needle valves, jets and the use of a venturi.

Basics.
If you put a large diameter/volume carb onto and engine the total volume of air able to pass aided by atmospheric pressure will be great, with the correct air/fuel mixture your engine will obtain its maximum speed and torque dependant on all other factors, to include cam profile, C/R, piston and combustion chamber design and the entire intake/exhaust system.
This sounds great but this same carb will have big problems keeping that air/fuel ratio constant whilst you are opening the throttle and at slower speeds where the air flow through the carb is less, which causes a loss of control of the fuel mixing proccess.
People often have symtoms such as poor throttle response, or if bad, very fast idle and cuts out when you open the throttle.

A smaller carb will have a really great airflow speed through itself on the same engine at much lower engine speeds, this in general will allow smoother/lower idle speed, much better mid range and part throttle response, generally better fuel econemy but you may loose some full throttle max rpm power and speed.

With your project, say a 9 cylinder radial, or any other multi cylinder engine you will NOT need a carb that flows 9 times the air of a single cylinder.
This is due to the fact that a single cylinder engine intake air flow increases to a peak and then decreases, maybe stops, during the engine cycle. This is due to the intake system opening and then shutting again ie the intake valve opens and shuts.
If a multi cylinder engine is made, normaly the intake system has many valves opening and shutting throughout the engine cycle and this will create a much smoother intake/air flow through the carb, in this instance the carb can operate near max flow at all times without starting and stopping creating an overall flow great enough to provide for the entire engine.

I would suggest you find your overall engine capacity. Look and find specs on the carb other people fit to this size of engine, if it is a single cylinder then drop the diameter down a size or 2. Whoever is suggesting the .2-.3 rule has found yes it works, glad to hear it. However it is air flow that is important not size. A carb with a huge butterfly valve restricting airflow will be less capable than a smaller cylinder valve carb with a side fuel metering jet.

Sorry to ramble, and the info above is really missing loads of detail but I hope you get the picture.

[nlancaster]

You have a Point what I said was very simplistic, but I have been studying small engine design as a hobby for the last 4 years. Not to say I am an expert.

One of the people listed as stating the .2-.3(actually supposed to be .25-.35) diameter of one piston is Ron Colonna a very successful model engine designer. On Swedes website where he is talking about the first run of his radial engine, here, he states that he had to go to ever smaller carburetor just to get the engine to run at all. And you have stated the same thing. If the carburetor is too big with a large butterfly, the engine will not run well at all throttle positions. And I have looked at many engine plans and universally they are all designed with a Carburetor that is .25-.35 cylinder diameter. Now that being said large engines run very large carburetors like V-8 car engine may run 2, 4, 6, or even 8 barrel carburetors. But the individual size of each barrel on the carburetor gets smaller as you go to larger numbers of barrels. Now more barrels do make the engine run better and that is because they flow more efficiently then fewer smaller barrels. As an example the Holley 4 barrel carburetor on my friends Mustang opens only 2 barrels for the first portion of the throttle range, then as the throttle is pushed the the full open postition, and the engine picks up RPM, it opens the last 2 barrels. If all 4 barrels are open too soon in the RPM range the engine bogs down and runs very badly. Also I believe (don't know for sure) that large engines have much larger useful range of total carburetor area ( >.25-.35 diameter) to give good operating characteristics across the engines operating range.

I have looked very closely at pictures of model IC engines with more then one carburetor on them and as far as I can tell they stick to the .25-.35 size constraint per carburetor no matter how many that 1 carburetor is feeding. If you were to build a Multi-barrel carburetor for a small IC engine I expect you could bring the affective area of the carburetor higher the .35 cylinder diameter and get a very powerful little engine. But I have yet to see one in action.

So what I meant above is that the carburetor that comes with a Saito 4-cylce engine should work fine for any number of cylinders because it is, most likely, within the .25-.35 diameter of a piston. I have not measured one closely to be able to say that definitively but I have seen many RC 4-cylce engines up close and that does appear to be the case.

[sheeny]

Hi again.

I hear what your saying, and I have looked at the radial that was finished and read the article relating to the carb issues ( The radial looks way cool by the way, well done to the builder).
I can see why builders of these small engines are using this small size carb, all I was trying to point out, is that, venturi size is not determined by engine bore size but should allways be determined by airflow capacity.

Yes the builder has issues with the os carb being too big however, this carb was designed for a .90 2 stroke (The OS carbs of this type even have retracting jets to minimise flow restriction). which would equate to a 4 cycle engine of twice the capacity i.e. a 1.80 engine turning the same rpm as the 2 stroke! or, a .90 4 stroke turning TWICE the max rpm of the 2 stroke! yes this carb is probably too big,

However to say that your saito .40 carb can run any number of cylinders is misguided. What happens if you turn your project into a double row 9 making an 18 cylinder radial. At this point using your std carb - yes it will run, but the engine will suffer from low rpm and power due to the carb being unable to flow enough air (it should start and idle great though).

You mention using multiple venturi carbs, this is one way of having massive airflow capability and also low speed controlability and idle. I.E. open more than one venturi one after another, with overlap, so that at low speeds a single venturi is opened thus maintaining a good "signal" at the carb, and when the engine speed is sufficient you can open a second venturi and keep a good "signal" but allow the engine to breath much better at higher speeds.

Example:-
If you have 2 engines both of a similar bore dimension.

The first engine has a single cylinder, shortish stroke and only revs to 5000 rpm. the carb for this engine will need to flow relatively low amounts of air. and a small venturi will have to be used just to make it run at all.

The second engine is an 18 cylinder and has a longer stroke making it a much larger capacity engine and is also designed to run at 15,000 rpm. This engine will require a much larger carb/venturi otherwise it will never meet its full potential ( this too would probably start and idle though).

The point being the bore may be identical but the carb requirements are VERY different.

Anyway, a carb thats too small will still work especially with engines that are not loaded, whereas a carb thats too big will have a hard job starting and may only work properly at high revs.

I just think its misleading to say the venturi size is linked to bore size, it must match the airflow requirement of the engine as a whole.

[ESjaavik]

It seems to me we could make it simple and say the rule is to use a small carb to get the engine running (works better in the low RPM range). Then increase size until starting/low RPM problems becomes a problem.

Or use more carbs. Then we can think of the engine as several engines with a common crankshaft. The difference is that with fewer cylinders/carb the flow will not be continous but pulsating. This is one reason why cars with multiple carbs often use long venturis in the entry to the carb. The flow is reversed and they breathe out a "cloud" of gas that is kept in the venturi and re-digested in the next stroke.

Point is that the flow through one carb feeding several cylinders works quite differnet from one feeding one or two cylinders because the flow goes from pulsating to continous. I guess I'm just repeating what has been written in the previous 3 postings now just using other words.

[Swede]

Guys, don't overcomplicate miniature engine construction! Unless you are one of the rare builders whose goal is to create a race engine, or you want to pursue theoretical limits, remember what your father may have taught you... if it has fuel/air, compression, and ignition, you can probably make it run! I think most hobbyist's goal is to have an engine that is above all reliable and easy to start, and has good idle characteristics, because to me at least that is where an engine is most interesting.

The cam is a good example. Cam theory can be incredibly exotic and difficult. You can set up a CNC grinder to cut a race cam. On the other hand, if you cut a crude cam that has a base circle and generates some valve lift, at a moderate pace, again, the engine will run, and probably run well. Unless you enjoy it, avoid the math and the exotic cam theories and cut a cam that lifts the valves at the correct points, and run the engine.

I leaned about carbs the hard way. First, a carb designed for glow fuel will NOT perform the same with gasoline. Gasoline's volatility is different and I've found that a glow needle valve, when used with gas, is very sensitive. 1/2 a turn will go from way too rich to way too lean. Likewise, the tank's head pressure will cause big variations in mixture; hence the addition of the float chamber to my own setup, which solved all tank head pressure issues.

I started with a carb from a small Briggs and Stratton! What a joke. That setup didn't have a prayer. Then I went with the O.S. .90 CI carb. Again, way too big. The air velocity through the venturi was waaaay too slow, and the carb did not work.

The very first successful runs happened with a .60 CI-sized glow carb. Great high end speed; pathetic idle characteristics. Next, a .40 CI sized O.S. carb. That one worked well. Finally, I used a Super-Tigre helicopter engine carb, I believe it was from a .32 CI engine. That by far delivered the best overall performance. I lost maybe 600 RPM from the top end of 5500 RPM, but the idle was superb, throttle response excellent, and the engine starts every time with no more than 2 flips. And again isn't reliability what most of us were after?

I don't mean to speak for others, but that at least was my goal. If anything, like Einar says, I'd start really small with the carb, then try bigger ones. Remember to make an adapter plate so you can try different carbs, too!

[mcfgomes]

Originally Posted by Swede Hi Einar, Wow it's the Wright Whirlwind from SIC! I didn't know that the castings were still available. I've always admired that engine. Did you produce one? I'd love to see some pictures if you have, or whatever progress pics you can find. Hemispherical tappets - true, not optimum, but my Hodgson uses them. The cam is 4340 steel Rc 46 or so, while the tappets are A2. After many, many hours, I can detect no wear on either the cam or the tappets. Perhaps roller tappets/followers could be fitted. Thanks very much for the link. There is one other radial that I am interested in, the P&W Wasp Jr in 1/6th scale as designed by Bob Roach and head castings by Bruce Satra. I have the prints for the Pratt, but no castings yet, and Satra has zero presence on the www. I have no problems machining valves, etc. My overall desire is to use the Saito jug as a foundation for a number of engine projects which might ultimately be marketable as a "kit", whereby the user need machine only a crankcase, crankshaft, and of course any other "lower" engine parts. Again, photos would be very appreciated!! And a word of advice to anyone interested in IC engines that require castings... when they are available, I recommend that one buy them and store them. A lot of great engine designs out there use castings and were designed by older fellows who may not want to support or continue to create castings for years and years. I think I am going to go for this Whirlwind while the castings are available. I really missed out on some other designs from the 80's and 90's in which castings are no longer available... Zimmermans deHavilland designs, for example.

Where can i find the P&W Wasp Jr in 1/6th scale as designed by Bob Roach and head castings by Bruce Satra

[Andy Smith]

For those interested if you go to the photo section of my sight have a look at Paul's model 7 cylinder radial , then after have a look where he ended up...........
www.hobbyrobbers.com.au

Regards
Andy Smith

[Andy Smith]

Have a look at the photo section of my website www.hobbyrobbers.com.au to view Rotec Radials. Paul is a keen aeromodeler, things just kept on getting bigger.......

Cheers

Andy Smith

[bmarch]

I am following this thread since I too thought that Saito components would make construction of a very nice 7/9 cyl radial engine much less complicated. There might be enough interest from others to 'chip-in' in order to get a "kit" to assemble. I think there would be quite a few "takers" - what the heck you just have to run a few more pieces right , and a little extra materials.

[sketchyrabbit]

check out this site www.massdispensationengine.com
this will donkey kick your brain...
josh