Author Topic: Model I.C. Carb with Throttle  (Read 18074 times)

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #15 on: March 13, 2014, 06:35:20 PM »
I guess if you are going to have a throttle, you will want a handle to turn it with.



Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #16 on: March 13, 2014, 09:13:49 PM »
I know, I know---I said to make the handle out of steel. But when I got to my "cupboard of all bits and pieces" the only 1/16" stock I had was brass. I don't think you can bend brass too readily without it breaking, so I heated it up with my oxy acetylene torch till it was dull red and then bent it in my vice. I'm somewhat amazed at how few pieces there are to this carb. All that's left to make is the fuel inlet and the needle and cap. I might solder up the fuel inlet tomorrow and get it ready for when my taps and dies arrive.





Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #17 on: March 13, 2014, 10:29:21 PM »
Okay---At this stage of the game, I am beginning to realize that ---I muffed it!!! On the carburetor main body, the "thru-hole" that the air flows thru has to be the same diameter both entering and leaving the round central chamber where the throttle barrel is.--Otherwise, as I have just figured out (much to my dismay) the amount of opening or closing on one side of the barrel (for instance the inlet side) is totally different than the amount of opening or closing on the opposite side. I am going to have to reconfigure the main carburetor body--as in redesign and remake it. That's okay. I'm learning as I go along here. I think I can save the barrel and the handle.

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #18 on: March 13, 2014, 11:29:10 PM »
There wasn't a lot of redesign involved. I made the hole thru the air intake side the same diameter as the hole in the air outlet side at the point where it breaks into the 3/8" hole that the throttle barrel fits into, and moved the air bleed hole and screw around a bit to optimize their location according to advise from Geo. Britnell.---George had sent me this information earlier, but I couldn't really get my head around what he was saying until I read thru it again today while I was puzzling things out. I can re-use the barrel and handle.--Interestingly enough, this throttle goes from closed to open in 45 degrees, unlike the butterfly valve in an automobile carb that must rotate thru 90 degrees.

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #19 on: March 14, 2014, 02:25:00 PM »
This morning, before diving headlong into another piece of brass, I am doing a bit of taper experimenting. I wanted to know if I can drill a 20 degree included angle taper thru material 0.361" thick and go from a diameter of 0.315 to a diameter of 0.187". My selection of small boring bars is, well, minimal would be a good word. I didn't want to have to make up a special fluted taper cutter. I reproduced the critical geometry in aluminum--a 3/16 centered hole going .361" before breaking thru to a 3/8" diameter cross hole. I found that by drilling a couple of stepped holes first to remove as much meat as I could from the outside end of the aluminum piece without cutting into what would become the tapered hole, I was able to get my much abused boring tool thru and give me the correct taper and correct diameters. That's a big plus!!! Now I am off to my supplier to buy some more 3/4" square brass. The outer body could be made from aluminum, but I like the look of brass better.

Offline cfellows

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Re: Model I.C. Carb with Throttle
« Reply #20 on: March 14, 2014, 03:26:15 PM »
Brian, I think the added feature of the tapered (seamless) inlet and outlet is to reduce or eliminate turbulence which probably has an effect on how well it works.  I've found that a simple, D style reamer works about as well as a fluted reamer for these small applications.

One thing I'm kind of figuring out with these small carburetors is that fit and finish is critically important to their operation.  I think one reason that the sewing needle works so well for a needle valve is the polished finish on the needle's surface. 

Chuck
So many projects, so little time...

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #21 on: March 14, 2014, 04:23:26 PM »
Chuck--I'm really glad you chimed in here. I know you have been fighting these carburetor wars yourself for a while and I value your input. I agree with you about the turbulence in the air stream. I think my ability to put a nice taper on the air inlet side is only going to help. It makes me a little bit crazy that I completely finished the brass piece before having the revelation about the size of holes in the outer body having to match the holes in the barrel. I wasn't that happy with the way that first body turned out anyways, as I had miss-measured something and the bores were not as "in line"as they should have been with the barrel.---Brian

Offline Roger B

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Re: Model I.C. Carb with Throttle
« Reply #22 on: March 14, 2014, 05:01:39 PM »
Brian, as I understand it the hole on the inlet side needs to be larger than the hole on the outlet side. If you have a simple jet and venturi followed by a butterfly throttle the mixture becomes richer as the speed increases (apparently the mass of petrol delivered is increased by the air velocity whereas the mass of air decreases due to the reduction in pressure caused by the increase in air velocity). By making the inlet side hole bigger the amount of incoming air is increased more rapidly and if correctly sized compensates for this effect.

Quite how this works in practise I am not sure, but both my commercial model aircraft type carbs have a bigger hole on the inlet side.
Best regards

Roger

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #23 on: March 14, 2014, 05:57:15 PM »
Roger--I am not exactly sure how that works either. If you take the taper on the inlet side into consideration, the intake side of my carb is much larger than the outlet side. So---And this is only assumption at this point--Air enters the large end of the tapered bore at one speed. As it gets farther down the taper it must flow faster to get the same volume of air thru an ever decreasing hole. When it gets to the center of the barrel where the nose of the needle valve sticks up into the air flow as an obstruction, the air must flow even faster to get the same volume of air that entered the carb thru the center of the carb----And according to my friend Bernouli, as the air increases in speed its pressure drops. So you have the maximum pressure drop and Venturi effect right at the needle valve nose. Now what happens on the other side of the needle valve nozzle, I'm not sure if the passage must become larger again to slow the air down a bit, or if I stay with this reduced diameter all the way into the cylinder head. I will be conducting tests with a vacuum meter t check this out as work progresses.---Brian

Offline Roger B

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Re: Model I.C. Carb with Throttle
« Reply #24 on: March 14, 2014, 07:11:39 PM »
Brian, it's certainly a complex subject. I hope to get outside to try my version of Graham Meek's carburettor this weekend. Chuck had a go at producing a very basic carb which worked, but in the end I don't think that he was satisfied with the low speed throttling.

In the end, I think' a lot depends on what you want. Is it 'display' engine where you want a nice low speed idle? Is it a stunt plane engine where revs and power are the goal? Is it for a tractor (one of my plans) or a locomotive (another plan) where flexibility is important?
Best regards

Roger

Offline PStechPaul

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Re: Model I.C. Carb with Throttle
« Reply #25 on: March 14, 2014, 07:29:46 PM »
The carburetors on my old Honda 350 had vacuum operated diaphragms that operated a fuel metering needle valve, which I think helped control the mixture depending on speed. I found a description of how it operates:
http://www.bikerenews.com/Stories_Archives/Carburetors.html
 

Offline Brian Rupnow

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Re: Model I.C. Carb with Throttle
« Reply #26 on: March 14, 2014, 07:55:04 PM »
Roger--My engines are small bore (3/4" to 1") x similar stroke lengths, and they lead pretty sheltered lives. The twin opposed cylinder was my sixth bar stock internal combustion engine. Three of these engines are hit and miss engines where the speed is controlled by governors and the carburetors have no throttle. I have also built an Atkinson, a Webster, and the Dual Opposed cylinder engine, and they are all running Traxxas model airplane engines because they need to have a throttle. As far as I know, there are no simple/free/small/ suitable for single cylinder engines in my bore range, throttle style carburetor plans out there floating around on the web. What Chuck has done, and what I am attempting to do, is to develop a simple small carburetor which is throttle capable, work up a complete set of plans which I will post free to download as I do with all the engines I design, and post the build as well as an engine being successfully ran by it.

Offline Roger B

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Re: Model I.C. Carb with Throttle
« Reply #27 on: March 14, 2014, 08:15:44 PM »
Brian, my views are much the same. The two engines I have built ( a 25mm bore 50mm stroke horizontal 'slow speed' engine and a 16mm x 16 mm  4 stroke vertical engine) have commercial model aircraft carbs. I felt that the carbs were designed for high performance two stoke engines and did not really suit my engines. I started looking into my own design and came into contact with Graham Meek who had designed a carb based on the same concepts for Seagull (iirc).

My build is illustrated in my 4 stroke vertical thread, but I was not intending to post the drawings until I have tried it.
Best regards

Roger

Offline Roger B

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Re: Model I.C. Carb with Throttle
« Reply #28 on: March 14, 2014, 08:26:24 PM »
This is the build thread. The carb stuff is around page 6.

http://www.modelenginemaker.com/index.php/topic,2093.75.html

I have also attached my usual electronic back of an envelope sketches  ::)
Best regards

Roger

Offline cfellows

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Re: Model I.C. Carb with Throttle
« Reply #29 on: March 14, 2014, 08:54:32 PM »
Brian, did you ever try your version of my carb design on the opposed piston engine after you had worked out some of the other issues?  I'd be interested in getting your views on how well my design works compared to the design you are developing in this thread...

Chuck
So many projects, so little time...