Author Topic: Carburretor Thoughts  (Read 20912 times)

Offline Roger B

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Re: Carburretor Thoughts
« Reply #15 on: March 11, 2020, 03:10:22 PM »
Thank you for thoughts George  :) I am indeed looking for mixture compensation rather than governing.

If you load an engine the speed will drop and so will the air velocity through the carburettor. This will reduce the depression and hence the fuel feed resulting in a lean mixture. As you say with a CV/CD type design under load condtions the piston or slide will drop maintaining the air velocity and depression and hence the fuel flow. The (correctly profiled) needle will then adjust the fuel flow to match the reduced air flow.

I started down this route after carrying out some load tests on my vertical twin. If  I bought the engine up to speed I could add the load step by step and by adjusting the throttle and needle keep things running. If I put too much load on in one go it would die. If I put the full load on the generator at low revs and then tried to accelerate at some point it would also die.

Chris, all my threads are open for discussion. I have seen various rules of thumb for carburettor size, they are typically between 1/4 and 1/3 of the engine bore. My 12cc twin is 20mm bore so 6mm is at the top end of the range. My 3cc single is 16mm bore and is quite happy with 3-4 mm bore carburettors.

My Triumph Herald was ~74mm bore and used 38mm carburettors.
Best regards

Roger

Offline Laurentic

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Re: Carburretor Thoughts
« Reply #16 on: March 11, 2020, 05:23:21 PM »
Ah Roger, the lesson to me is to read what people write more carefully!  I had missed the vital word 'twin' in your OP!  The bore for my single cyl engine is 0.7875", or approx 20mm in new money, so the 6mm bore carbie I have should not be too far out!  Many thanks for the info.

Chris

Offline Roger B

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Re: Carburretor Thoughts
« Reply #17 on: March 11, 2020, 08:07:10 PM »
Chris, I checked a few dimensions. The homemade carb on my 3cc engine has a 3.2mm venture. It develops more power with the modified commercial carb with a 4.2 mm bore. I am using the 4.2mm bore carb on the 12cc twin at the moment. The 25cc engine runs well with the 4.2 bore carb but develops more power with a commercial slide carb with 5.6mm bore. I get similar power and better throttle response with fuel injection. maybe that's the way to go  :headscratch:

I made a centring plug and a 2.2mm punch for the diaphragm pump and punched out a trial diaphragm.
Best regards

Roger

Offline Art K

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Re: Carburretor Thoughts
« Reply #18 on: March 11, 2020, 10:06:09 PM »
Just to follow up on the conversation that wandered over to Stromberg carb in Strictly IC. I looked it up in the index and here is where it is.
Quote
Carburetor, A Stromberg in Miniature, by Lee Root/Washburn: 31-3; 32-3; 33-3
I don't have the time right now to look up the articles butit is spread over 3 issues.
Art
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Online Vixen

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Re: Carburretor Thoughts
« Reply #19 on: March 11, 2020, 10:14:11 PM »
Art, I looked.  :ThumbsDown:

Unfortunately, it is a straight downdraft fixed jet carb, NOT a constant depression/velocity carb with the flexible diaphragm.

Mike
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Sometimes, it can be a long and winding road

Offline Roger B

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Re: Carburretor Thoughts
« Reply #20 on: March 14, 2020, 06:39:31 AM »
A start on the piston and cover. Getting the various diameters concentric is going to be interesting  :thinking: I think that I will be able to use a 2mm endmill to bore the 3mm holes  ::)
Best regards

Roger

Online Vixen

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Re: Carburretor Thoughts
« Reply #21 on: March 14, 2020, 11:55:49 AM »
Hi Roger,

You are off to a good start with the CD carburettor.  :ThumbsUp: :ThumbsUp:

Are you going to make several of each part to allow for future experiments?

What do you think about the idea of making the central needle screw adjustable? You can then adjust the mixture to be spot on at tickover and again at WOT and all points in between. That should give you a clearer indication of what the 'true' needle taper should be. I think the adjustable needle will be give more finesse than an adjustable main jet.

Mike
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline Roger B

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Re: Carburretor Thoughts
« Reply #22 on: March 14, 2020, 12:19:20 PM »
Thank you Mike  :ThumbsUp:

I intend to just start with one offs as there may need to be significant changes to the design.

The emission controlled Stromberg CD carburettors had the needle adjustable using a special tool but using the tool disturbed the piston due to it's weight. I always found the older versions where the jet housing was screwed up and down to be easier to adjust. I intend to have some form of adjusting screw (not designed yet) to adjust the height of the jet.
Best regards

Roger

Offline Roger B

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Re: Carburretor Thoughts
« Reply #23 on: March 14, 2020, 02:39:14 PM »
The body is from a piece of DIY store (Afler) 15.6mm square aluminium. This was faced, drilled 5.3mm and reamed 5.5mm. I have decided to reduce the bore to 5.5mm but the piston will remain at 6mm. The manifold end was turned down to 8mm to fit the existing adaptors. The body was then milled to 15mm square and the seating for the cover was cut.

I have attached a PDF of the current state of the design.
Best regards

Roger

Offline dieselpilot

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Re: Carburretor Thoughts
« Reply #24 on: March 14, 2020, 05:14:28 PM »
I agree that 6mm is on the large size for this engine however that is the smallest I think I will be able to make. As it has a variable venturi the problem is not so great, the piston will just not rise all the way.

I meant for a standard design carb, you could simply install a reduction sleeve in the carb you're using now. I agree that in theory SU would use only as much area as required, but this assumes it's well calibrated. Even so you are giving up a lot of "resolution", by using only a small portion of the needle if it only opens slightly. Looking at needle profiles, variable tapers in microns over ~50mm, seemingly brutal to duplicate at model scale. Fit of the piston to the suction chamber also looks important. This will be an interesting project.

Offline Roger B

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Re: Carburretor Thoughts
« Reply #25 on: March 15, 2020, 07:54:44 AM »
Thank you Greg  :ThumbsUp:

If the carb works at all I will make a 'guess' constant taper needle based around the dimensions of the one used in the barrel carb which also has a 0.5mm jet and then try and determine a better profile by moving the jet up and down. I think that getting a dimension every mm and smoothing in between should be sufficient.

Next one the trickier bits. The holes to connect the vacuum by the jet to the space above the piston end up at 0.8mm diameter to give a reasonable thickness of metal around them. These are 9mm deep so plenty of stops for swarf clearance, the cutting fluid tends to make the swarf stick to the drill. This was done at 4800 rpm in my Proxxon drilling machine. Still in the RT I moved to the Proxxon mill to cut the two lead in slots and the anti rotation keyway with a 1mm end mill at 20 000rpm.
Best regards

Roger

Offline Roger B

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Re: Carburretor Thoughts
« Reply #26 on: March 15, 2020, 11:03:52 AM »
As the Hobbymat was in milling mode I made the rocker arm support for the diaphragm pump as well as a holder for my 3mm shank Proxxon end mills that I am planning to use as micro boring bars. I should really have moved to the bigger mill to cut the 6mm spring seat but it worked ok.
Best regards

Roger

Offline dieselpilot

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Re: Carburretor Thoughts
« Reply #27 on: March 15, 2020, 12:40:12 PM »
Have you done any math on pressures and spring rates, which make this carb function?

My thoughts on carbs.

The piston in the SU carb is a flow metering device. It measures pressure drop across the venturi(created by the slide itself) and sets venturi area for that air flow rate. In addition the needle affixed to the slide is used to meter fuel accordingly. Really an ingenious device, though somewhat complex and not trivial in manufacture. The main benefit of this design is that needle profiles directly influence air/fuel ratio in very specific regions of operation directly related to volume flow(torque). The throttle plate is there to limit engine torque, just like any other carb. In simplest form, the throttle plate has no direct influence on slide position. The slide in a SU/CV carb is positioned only by air flow demand. Transition shouldn't be confused with basic operation, various methods are used in all carbs to assist when the throttle is opened quickly.

The slide carb used in RC models doesn't really compare to the SU/CV other than the similarity in layout. There is nothing to control the flow to slide position. I understand characteristics are slightly different than a rotary barrel, but not enough to matter. You can open the venturi all the way just the same as a rotary barrel, long before the the engine is actually drawing enough air to need such a large opening. RC helicopter are setup to run a fixed RPM, but obviously need a wide range or throttle setting depending on load. The one thing RC cars and heli have in common is a thirst for nitromethane, which allows more deviation from optimal air fuel ratio.

The trouble with simple venturi is that you are stuck with the pressure drop created by the venturi and volume flow. While the characteristic pressure drop(fuel draw) of a venturi is in our favor (almost linear going slightly richer at higher volume), there is a critical point below which the pressure drop is very non proportional and goes lean very quickly as flow drops. If a carb is so large that it's operating in this nonlinear-region the needles are very difficult to tune, and you may not get enough fuel draw to run at all. Having played with a variety of venturi sizes and throttles on production engines, I can tell you this effect is clear. Small carbs may limit power, but the needle settings are very obvious and fuel draw excellent. Oversize carbs have fuzzy needle setting and are sensitive to fuel level. Muffler pressure is also used to allow larger carbs with production engines. Once you start enlarging the carb to make the most power possible, they get much more difficult to tune and require pumps and regulators. Rotary barrel carbs are different than throttle plates downstream of a simple venturi in that they need air bleeds or extra fuel metering to prevent very rich mixtures at very low throttle. This is apparently due to the very high velocity of the flow in the bore of the barrel even with low volume.

So with some minimum required some pressure drop to generate fuel suction (and sufficient atomization) in the carb, what is the determining factor? Engine air volume flow. It boils down to obtaining a minimum velocity through the venturi. For gasoline in model size engines 120-140m/s (taking into account spraybar area in the venturi) seems to be very reliable with no pump and small suction height(basically at spray bar level with a small fuel tank or float reservoir) assuming flow happens in 180° of the 720° cycle for a single cylinder four stroke cycle engine. I took some time to check these values for a full size application. I picked a Holley carb (rated by CFM) found it’s venturi bore size and did the math. It turns out a 600CFM (.283 m^3/s) carb has a 85m/s venturi velocity. This is low, however, these carbs have double venturi design which essentially double velocity through the venturi in which fuel is introduced. We can also see that if you split flow among two cylinders which don’t draw at the same time, a carb doesn't have to be as large as a single of the same displacement. Specific to model engineering and probable less developed engine designs, meaning volumetric efficiency is low, further venturi size reduction may be necessary. The shaft run/no load condition for display models adds another complication, but further reduction in venturi size(to prevent overspeed) should allow for even better fuel draw. 6mm on a 12cc four stroke twin is only 30m/s so it's no surprise to me that it has trouble.

Offline Roger B

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Re: Carburretor Thoughts
« Reply #28 on: March 15, 2020, 02:35:25 PM »
Thank you again Greg,

I haven't done any detailed work as there are too many unknowns, especially leakage rates and the pressure on the bottom of the piston which can be atmosphere one side and manifold pressure the other. I intend to assemble the carb without the jet and needle and then see how the piston responds while turning the engine with the motor/generator.

The cover was held in a 15mm collet and faced. I removed the bulk of the material with drills and end mills and then started boring using a 6mm HSS boring bar. Using the DTI as a guide I slow opened up until the 12mm plug gauge would go in. The 3mm hole was then spotted and drilled 2.4 mm. I then mounted my endmill holder and cautiously increased the depth 0.1mm at a time until it cut. I made a further pass with 0.1mm depth of cut and finally reamed the hole 3mm.

The fits seem good  :) The plug gauge takes about 5 seconds to fall out under it's own weight if I block the 3mm hole. I have no means of measuring the concentricity so that I will have to wait until I finish the piston to check that.
Best regards

Roger

Offline Roger B

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Re: Carburretor Thoughts
« Reply #29 on: March 16, 2020, 07:12:09 PM »
Next step check that my 3mm silver steel is straight then part off 29 mm for the piston guide. I then moved to the ER11 collet check as the piece was a bit short for the ER25 system. The ends were faced and then one was drilled 1.3mm and reamed 1.5mm for the needle. Next step Loctite the piston onto the guide and let it cure before finishing both parts.
Best regards

Roger

 

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