Author Topic: Carburretor Thoughts  (Read 20913 times)

Offline Roger B

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Carburretor Thoughts
« on: March 09, 2020, 01:33:17 PM »
There have been many model aircraft and boats powered by home built IC engines but very few land vehicles. ETW designed the road roller and the 1831 locomotive. Roy Amesbury made a V8 powered Hymek. Gail made some hit and miss powered gauge one locomotives. There may be a few others.

A reason for this seems to be carburation. If the engine is driving a propeller (planes and boats) as the revs drop the load drops and visa-versa. For a land vehicle as the revs drop the load can increase such as starting to climb an incline. A simple barrel type carburettor with a suction feed has problems with this. As the revs drop the airflow through the carburettor drops and so the suction is reduced resulting in the mixture leaning until the engine stops firing. Opening the throttle makes the problem worse. As the level in the fuel tank drops the mixture also becomes leaner.

These problems were noted by ETW when designing the 1831 and he developed a carburettor with a  float chamber to maintain a constant fuel level and an auxiliary air valve to maintain a fairly constant vacuum over the jet. This was/is apparently successful.

Roy Amesbury discusses similar problems with his V8 powered Hymek. He tried several designs before settling on an SU like constant vacuum type. This design also had a constant fuel level but used a pump and overflow system which he considered better for small engines than a float chamber.

As part of the experiments with my 12cc twin I am going to design and build a slightly smaller single choke version of the Amesbury carburettor including a small diaphragm pump for the fuel feed. This pump will hopefully also be useable at a higher pressure (~0.8bar) as a feed pump for my petrol injection system. The Amesbury version used a 9mm choke bore, 19mm piston and 0.8mm jet. I will try a 6mm choke bore 12mm piston and 0.5mm jet. Attached are some initial sketches. The carburettor uses a barrel throttle as that is easier to make than a butterfly type in small sizes. I will have to make some experiments to find a suitable mass for the piston as air leakage will play a significant part. The pump cam will pull the diaphragm back  for the suction stroke and a spring will be used to deliver the fuel.
Best regards

Roger

Offline awake

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Re: Carburretor Thoughts
« Reply #1 on: March 09, 2020, 02:31:30 PM »
Roger, I look forward to seeing what you come up with!
Andy

Offline Vixen

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Re: Carburretor Thoughts
« Reply #2 on: March 09, 2020, 03:00:50 PM »
Hi Roger,

I will be following your experiments with great interest. I think they may be very relevant to running my Seal engine in the Bren Gun Carrier. That will be exactly the type of load this carburettor is intended for.

As I understand the SU. The piston goes up and down to maintain a constant vacuum in the venturi between the base of the piston and the jet. When airflow demand is high, the piston is high and the needle releases the right amount of fuel. Conversely, when airflow demand is lower, the piston rides lower and again the needle releases the right amount of fuel for that airflow. The taper on the needle is all important as it sets the mixture for each operating condition.  If the needle's taper is wrong, the mixture will be wrong at one end of the operating range. SU used to offer thousands of different needle tapers for the enthusiast to experiment with. SU also offered a range of springs to control the piston response.

Does the simplicity of Art's chicken hopper offer any advantage over the pump and overflow alternative?

This is going to be very interesting

Mike
« Last Edit: March 09, 2020, 07:19:32 PM by Vixen »
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Offline Roger B

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Re: Carburretor Thoughts
« Reply #3 on: March 10, 2020, 03:33:16 PM »
Hi Mike,

I spent many a happy hour looking through the needle charts to find the best for my modified engines. I expect similar fun making needles for these experiments  ::) At least I will be able to measure the piston and hence needle position whilst moving the jet up and down to find an optimum flow rather than guesstimating the position whilst pulling the choke out at illegal speeds on a deserted M27 at night.

I can't really comment between pump and overflow and bird feeder having not yet tried either. Westbury did not think highly of the bird feeder and prefered float chambers. Amesbury seemed to prefer pump and overflow to float chamber  :headscratch:
Best regards

Roger

Offline Vixen

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Re: Carburretor Thoughts
« Reply #4 on: March 10, 2020, 05:00:35 PM »
Hello Roger,

I expect most of use have cut out teeth trying to tune SU carbs. Going flat out and pulling on the choke may not have been the most scientific approach but we all did it and lived to tell the tale.

I live beside the M27 motorway. So it was probably you who woke me in the middle of the night as you roared past fiddling with the SU carbs.  :thinking: :thinking:

Mike
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Offline Brian Rupnow

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Re: Carburretor Thoughts
« Reply #5 on: March 10, 2020, 05:10:58 PM »
I will follow this post with great interest.---Brian

Offline Roger B

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Re: Carburretor Thoughts
« Reply #6 on: March 10, 2020, 08:26:33 PM »
Thank you for your support  :ThumbsUp: As ever for me this is a long term project, although my travel is currently stopped I will have to catch up with it all when the restrictions are removed.

Mike, if it was a red Triumph Herald Convertible in the 80s I admit responsibility ( the statute of limitations is long passed).

I have started on the diaphragm pump, beginning with a stub of 25mm diameter aluminium bar. I have some 0.5mm thick Viton sheet for the diaphragm on order. The minimum quantity will certainly see me out  :old:
Best regards

Roger

Offline dieselpilot

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Re: Carburretor Thoughts
« Reply #7 on: March 10, 2020, 08:33:05 PM »
There are two separate issues with carburetion. Fuel delivery (to the carb) at consistent pressure is one and relatively easily solved. Air/fuel ratio over the load/speed range is the other, but is difficult with simple carbs seen in models. As noted propellers have reasonable relationship between RPM and engine air consumption which generally works well with simple throttles. Traction or other loads with simple carbs leads to unstable air fuel ratio over the speed/load range. The biggest issue with home built model engines, in my opinion, is oversize carbs. If the vacuum signal is so low at WOT throttle that jets(adjustable needles) have to be large, you have little hope at low throttle settings or low speeds. I've run model aircraft four stroke engines at 3000 RPM(vs the typical 10-11kRPM) with oversize propellers, but it wasn't on the original carb. Area was reduced to account for the anticipated air flow, and there was no trouble at all. As a side note, (glow) methanol fuel allows a wider range of combustible air/fuel ratio, and adding nitromethane widens it even more. Gasoline is much more picky about air/fuel than glow fuels, so the carb must be sized correctly.

Specifically for your engine, 6mm for a 12cc four stroke twin is too large if it only runs 6kRPM. I would try something closer to 1/4 the area. I have made 250W+ electrical with a 7.5 CC single cylinder four stroke at ~8kRPM, maybe 4mm venturi, but no throttle. If your engine only breathes enough to make 150W, even smaller venturi may be required than I suggested. We know that going too small will limit power, but a size should be found where it makes good power and has proper fueling. I think reproducing a functional SU design at model scale would be a miracle. Though, when I considered it I was thinking flying models, not stationary engines.

For the injected engine look at the Kugelfischer mechanical injection pump.

Offline Art K

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Re: Carburretor Thoughts
« Reply #8 on: March 11, 2020, 02:20:06 AM »
The trick with the SU carbs is to always keep an oil can to top off the things. My 140 series Volvo's all had SU's
Art
PS It wasn't me I've never been on the M27. :lolb:
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Offline Roger B

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Re: Carburretor Thoughts
« Reply #9 on: March 11, 2020, 11:53:19 AM »
Greg,

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 will also try the same carb on my 25cc horizontal engine.

My fuel injection system is based on a diesel style pump like the Kugelfischer system and has the same problem of the petrol vaporising when the plunger makes it's return stroke. I am currently using a small electrically driven gear pump and relief valve to supply the petrol at around 0.8 bar. I hope to use the controlled pressure diaphragm pump as a replacement for this.
Best regards

Roger

Offline Admiral_dk

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Re: Carburretor Thoughts
« Reply #10 on: March 11, 2020, 12:23:43 PM »
Interesting that so many had so many problems getting the settings right on SU carbs - where they not standard on those whehicles ?

I only had two motorcycles with the much improved Mikuni CV carbs and they where standard items on those models. The first was a bog standard GSX-1100EZ and I never had any reason to fiddle with the carbs (there where four in-line), other than ensurering that they all where clean, corretly jetted (original sized), needle in correct groove and correct float height.

The second one was a different matter - a GSX-R1100H (Gixxer) and it had an obnosxiously LOUD aftermarked exhaust, open airfilters and a Jet-Kit installed when I bought it - but it was extremely fast for the day. The 1/4 mile in less than 10 seconds and more than 200 Km/h. / 125mph exit speed. The engine itself where bog standard.
I knew when I bought it that I could not live with the racket, so I bought a never version of the exhaust system (can't remember brand) that was a lot less noisy, but almost exactly same power. I also changed the four individual K&N filters out with one that fitted inside the original airbox, as this also reduced noise and DO NOT have any problems running in the rain  ;D

I therefore knew that I had to redo the jetting and thank God I actually wrote the settings I bought it with down above the orignal settings in the original Suzuki Workshop Manual.
I started with adjusting the float level back to the original height, put it all back together and did a test run => put bigger jets in and after a few tries all was apparently good ....
It turned out that it lost over 100Km/h (~60mph) in topspeed and used more fuel  :Mad:

The secret turned out to be the float height that I had written down. The lower fuel height made the carburators mix the air and fuel much better, as it opened more of the air holes in the Needle Jet - premixing more air into the fuel (making the droplets smaller) before it was lifted into the airstream in the venturi ...!... ah the small details  ;)

Offline Vixen

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Re: Carburretor Thoughts
« Reply #11 on: March 11, 2020, 01:17:13 PM »
Hello Roger,

If you make the needle in the dashpot adjustable, you could use the needle to set the correct mixture at tick-over then re-adjust the mixture at WOT and all points in between. This could give you a good indication as to what the final needle taper should look like.         Just a thought.

Cheers

Mike

PS for Per, young lads (would be engine tuners) just love to fiddle with the SU carb, in the hope of higher performance. They could always go back to the factory needles and settings if (when) they cocked it up
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Sometimes, it can be a long and winding road

Offline nj111

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Re: Carburretor Thoughts
« Reply #12 on: March 11, 2020, 01:46:48 PM »
Did anyone here make the 1/4 scale Stromberg Carb from Strictly IC and if so how did it perform?  Pretty sure that incorporated an accelerator pump, fast idle cam and a choke.  It was made using EDM but the author stated that it could be milled.  I think it was by Lee Root somewhere around issue 29 onwards....
Nick

Offline Laurentic

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Re: Carburretor Thoughts
« Reply #13 on: March 11, 2020, 02:11:04 PM »
dieselpilot - you say a 6mm carbie for a 12cc engine is too large, I'll be pleased to take your word for that no worries even if I don't fully understand why.  I have a .33cu.in sleeve valve 4 stroke aero engine (about 5.4cc) that won't go.  Part of the problem, which I'm working on, is lack of compression, the other is the carbie.  The original carbie design that came with the plans I am unconvinced on, I put a Chris Turner designed carbie on, from memory a 7mm, again am not convinced but until I sort the compression out I won't know for certain.  What size carbie would you recommend for that small an engine, and what are the guideline to determine suitable size carbies against engine size? Presumable there is some 'rule of thumb' that applies?

Roger - I hope you don't think I'm trying to deflect or detract from your thread here, rather I am trying to get a feel for why a carbie is deemed too large for an engine by relating it to a known (to me) engine and thus understand what is going on.

! will follow this thread with considerable interest as carbies in general are a bit of a black hole for me; hopefully, here I will learn something!  I look forward to reading your progress........


Chris

Offline gbritnell

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Re: Carburretor Thoughts
« Reply #14 on: March 11, 2020, 02:38:01 PM »
Gentlemen,
There are many different styles of gasoline delivery carburetors, throttle plate, basic slide, CV (constant velocity), air bleed etc. The thing they all have in common is to deliver the fuel at the best possible mixture with the air. 14.7:1 ratio.
We'll stick with the SU (CV type) as this is what the conversation is primarily about. The CV carb has 4 basic circuits, enrichment (for starting) idle or low speed controlled by jetting and needle valve, intermediate throttle controlled by a tapered needle which works in conjunction with it's needle jet, and at wide open throttle a predetermined jet.
The needle valve (intermediate throttle) is mounted to the slide which also controls the venturi area. The slide has a diaphragm attached to the top of it. The diaphragm is balanced by atmospheric pressure on one side and air velocity through the carb on the other side. There is also another element that controls the slide movement and that is the throttle plate (butterfly valve) like the type found in most carburetors. When the engine is started at low speed (idle) the slide is at rest (bottom of it's travel. As the throttle plate is opened it allows air to flow through the port in the carb that is connected to the top of the diaphragm. This air flow or pressure drop is lower than atmospheric pressure on the other side of the diaphragm so therefore the slide raises in it's bore and as it rises it lifts the tapered needle out of the needle jet which maintains the proper air/fuel ratio. At wide open throttle the needle/needle jet is open enough that it will allow as much fuel to flow as the main jet will allow. The main jet is mounted at the bottom of the needle jet.
There is a similarly designed carb for RC aircraft. The venturi opening is controlled by a slide much like the CV carb but rather than being lifted by a diaphragm it has a direct link attached to it. The similarity  is that it has a slide and a tapered needle valve. It doesn't have a butterfly valve so there is no atmospheric pressures involved.
The original question, as I understand it, is having a carb that will react to engine loading to maintain a constant rpm of the engine with the proper air/fuel mixture.
Here's the scenario! You're driving along with the gas pedal at a fixed point. If you accelerate you push on the gas which in turn opens the butterfly valve which then causes the slide to open which increased the speed of the car. The somewhat misunderstanding as I read it is that when you're driving along at a constant speed and you start to go up a hill and therefore load the engine with the gas pedal still at the same setting the car will start to slow down but the slide valve will raise giving more air/fuel to the engine thereby maintaining a constant speed. This is incorrect. The slide will not move unless the butterfly valve is opened allowing more air to flow through the carb and therefore opening the slide valve more.
What you are kind of describing is a governor. The governor senses a drop in rpm (engine load) and opens the throttle plate more and maintains a constant speed. If a governor control were attached to a CV carb it would do the same thing.
When I got into riding and repairing dirt bikes most of them had slide/needle type carbs but they weren't CV types. These engines were of smaller displacement and by quickly opening the throttle the engine would respond (to a point). As the engines got larger if they used the same non CV carb when the throttle was opened quickly the engine would bog because it would take a big gulp of air but the air velocity through the carb hadn't yet affected the fuel flow so there was a lean point until everything balanced out. By going with the CV carb you could crack the throttle wide open (throttle plate) but the slide/needle valve would react only as the air flow would dictate so the engine wouldn't bog. (Here again to a point).
Most dirt bikes and off road machines nowadays are 4 cycle engines and have been fitted with CV type carbs. The carbs are quite sophisticated and have enrichment control for starting, accelerator pumps for quick throttle response and some even have heating elements to warm the carb quicker.
Although I have never run any of my engines under load I have put friction on the flywheel of my Holt and it didn't die but just slowed down. It has a simple air bleed type carb and the fuel metering is controlled by a needle valve adjusted to a particular air flow. In slowing down with a fixed throttle position the air velocity will drop and therefore it should draw less fuel but the same amount of air which in theory would create a lean condition. I can't prove it because I have never done any testing to prove otherwise. Most model engines aren't working engines they are just examples of the builder's talents.
Talent unshared is talent wasted.

 

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