Author Topic: Carburettors  (Read 6947 times)

Offline Roger B

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Carburettors
« on: December 09, 2013, 10:21:10 AM »
As I have said in the threads I am not happy with the carburation on both my engines. I have reread George Britnell’s  post on the subject, thought about some things and have come up with these ideas about various possible designs.

Barrel Carburetors
These look quite simple but there are a lot of subtleties.
The fuel jet is generally fitted with a needle valve so this can be adjusted when running. The external shape and insertion depth of the jet may also be important. The two commercial units that I have both have a cylindrical jet set halfway across the venture, essentially blocking half of it. This gives a geometry similar to a SU or Amal type with the piston half open.
The shape of the venture in the barrel will also have an influence. One of my units has a straight bore, the other has a cone at either end with an additional cutout on the air intake side that communicates with an additional adjustable idling air bleed.
The biggest effect on throttle control and mixture, I think, is the sizes of the holes in the body of the carburetor. On both my units the air intake bore is larger than the outlet bore.  This offers a lot of possibilities for experimentation.

Constant Depression Carburetors
These use the vacuum generated in the inlet to raise a piston and keep an approximately constant vacuum over the fuel jet. The jet size is controlled by a tapered needle. The throttle butterfly is downstream of the metering piston. This design offers a lot of manufacturing challenges in smaller sizes. The piston must be light enough in proportion to its area that it can be lifted by the available vacuum. The piston must be well sealed, either by a very precise machining or a very flexible diaphragm.  Getting the correct taper on the needle will also require significant experimentation and some precise machining. I have seen one version described in an old copy of Model Engineer. It was used on a V8 petrol engine fitter to a 3 ½ or 5” gauge loco, possible Hymek style (at least 20 years ago).

Slide Carburetors
These were generally used on motorbike engines and are simpler than the CD carburetors. The piston is controlled directly as the throttle and the fuel is metered by a tapered needle. The design does not give any load compensation, but other than the needle would be simpler to build than the CD type.

Sizes
There seem to be various guidelines and ideas on sizes for carburetors. Malcolm Stride recommends a choke size of between 1/5 and 1/8 of the cylinder bore. I tried a comparison calculation to get a similar air flow velocity between a hypothetical  1200cc four cylinder engine with a typical 35mm choke size and my 25cc engine. For a similar gas velocity of 750 m/min this gave a choke diameter of ~6.5mm  or around ¼ of the bore diameter (ignoring all sorts of subtleties like Reynolds numbers).

Conclusion
Stay with the barrel design as that can be made with my current skills and tools. Keep the basic design very simple as I will be remaking the parts with different hole sizes quite frequently. Don’t consider idling air bleeds etc. until it runs well in the mid-range. Start with similar dimensions to the current (old) Fuji carb and begin by experimenting with the shape/position of the jet in the venture.

Does anyone have any other thoughts or experiences to share?
Best regards

Roger

Offline Stuart

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Re: Carburettors
« Reply #1 on: December 09, 2013, 12:17:32 PM »
In my dim distance past I used to race 3 1/2 cc 1/8 scale cars with highly tuned veco 19 engines , this was prior to the scnurle port jobs ,mine where cross flow

I made my own flat slide carbs for these motors note sizes do not conform to the norm.

These had a 1/4 inch straight bore with a large bell mouth one inch dia. To hold the filter
The slide had a small slit to enable the tick over to be controlled ( the cars had to held on the start line hands off one the brakes ,) so they had to idle

Above the slide was a needle jet with the nose cut off at 45 deg and just in the air flow, below the slide was another needle jet but this time flush with side wall of the bore

The fuel tank was pressurised from the exhaust

The theory was that with the slide shut idle the bottom jet controlled the mixture as the slide opened the air flow  increased over the top jet beak and cause a low pressure to develop at the jet , this then became the main jet and the lower one provided very little at this stage

They worked very well the throttle response was very good as was needed for rc car racing.

Note these sizes equate to a 61 size glow motor but used on a 19 size

But another friend did a lot of research on carbs namely the barrel type he obtained the catalog from irvine engines for the veco barrel carb and found that the air bleed disk varied between engine sizes , after ordering most of the catalog a lot of testing was done and we finished up with a 61 body with a 40 size air bleed this setup out performed the slide in economy and power ( We were limited to 4 fl oz) and had to do pit stops

So in conclusion stick with the barrel type but it will take a lot of testing to get running ok
We did use very hot fuel 95 percent nitro

Stuart
My aim is for a accurate part with a good finish

Offline gbritnell

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Re: Carburettors
« Reply #2 on: December 09, 2013, 01:43:23 PM »
Hi Roger,
I'm not sure which posting of mine you're referring to as I have written several explanations over the years and on different forums. As far as miniature engine carburetion goes I'm no expert but only have the knowledge of many years of experimentation to rely on.

Model aircraft carburetors have made evolutionary changes since the first constant speed, fixed venturi, needle adjusted carbs. With today's exacting manufacturing methods these carbs have developed into quite sophisticated metering devices. There are the old standby rotating barrel, air bleed carbs and then a step up would be the sliding barrel with needle attached similar to some motorcycle carbs. The finest iterations are the ones with the rotating barrels that are screw mounted so when they rotate they also move laterally thereby changing the air/fuel mixture. These carbs have adjustments for idle, mid range and high speed.

I have tried every one of these carbs and have had the most success with the simplest designs. First off these carbs are designed for fuels other than gasoline so therefore the  needle tapers and orifice sizes are larger in relation the venturi sizes.  I'm not saying that they couldn't be made to work but the amount of experimentation is more than I want to do now that I have a design that works. Even with the rotating barrel, air bleed types there is some tinkering required. First of all most fellows start out with too large of a venturi. If you start small you can always enlarge it in small steps, the same with the air bleed port.

Without a background in airflow engineering and miniature vacuum measuring devices there is no way to calculate the requirements for a particular engine.

I have reached the point of where most of my engines run quite well with my carbs.
gbritnell
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Offline Graham Meek

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Re: Carburettors
« Reply #3 on: December 09, 2013, 06:05:17 PM »
At one time I too have had limited success when it came to the carburettor and 4 stroke petrol engines. I made an Air-cooled version of Edgar T Westbury's Seagull engine, made the carburettor to his design and after many hours of adjustment, as described by ETW I eventually reached a stage where the engine would run reasonably well.

This was I felt not good enough as the engine could I thoiught tick-over more slowly and pick up from idle to full throttle a lot better. Plus the plugs were always black indicating a rich mixture. I therefore decided to set about designing a new style carburettor for this engine. It follows the traditional barrel type throttle with an adjustable air bleed, but instead of the plain orifice across the jet as in ETW's design I added an adjustable airflow. I also designed the jet such that it was submerged, there being a small reservoir of petrol available for sudden demands.

The aim of the design was to have a carburettor that would work "straight out of the box" and minimal fiddling. I fitted the carburettor to the Seagull and the engine was tuned and ticking over much slower than before all with-in 15 minutes. The engine would pick-up and accelerate to flat-out without the slightest hesitation, I did write the article up for Engineering in Miniature and it was published sometime back.

If I could fathom out how to load photographs onto this system I would let you see how it was done.

Gray,

Online steamer

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Re: Carburettors
« Reply #4 on: December 09, 2013, 06:12:09 PM »
Graham,

As far as loading photo's is concerned.   We can help you with that.

Dave
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Damned ijjit!

Offline Roger B

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Re: Carburettors
« Reply #5 on: December 09, 2013, 06:31:28 PM »
Thank you all for the feedback so far. It looks that I am generally heading in the right direction.  :thinking:

George, I was referring to a fairly recent post on model engine carburation on this forum.

Graham, when you reply if you look below the box you type in there is a blue plus sign followed by Attachments and other options. Click on this. There will be another box with Attach written above it and Browse at the end. Click on browse and then look through you computer's files until you find the picture, click open. If you want to add more pictures click on (more attachments) under the box and repeat.
Best regards

Roger

Offline Graham Meek

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Re: Carburettors
« Reply #6 on: December 09, 2013, 07:48:38 PM »
I hope this works, The first photograph shows the engine with Edgar's carburettor, the next two photographs are of the final carburettor fitted to the engine and shows the hexagon air bleed across the jet. The angle of the fuel inlet provides the little fuel reservoir that the jet sits in.

Gray,

Offline Graham Meek

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Re: Carburettors
« Reply #7 on: December 09, 2013, 08:06:42 PM »
Sorry for the double hit but I was not sure if the pictures had loaded, here are a couple more that give a view of the internals, I have also included a photograph of the plugs I made for this engine, these are 1/4". I am currently working on some 4.5 mm over the thread for my Fiat Tractor Engine.

Gray,

Offline Roger B

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Re: Carburettors
« Reply #8 on: December 09, 2013, 08:34:44 PM »
Thank you for the pictures. I will probably be back with more questions  :ThumbsUp:
Best regards

Roger

Offline Roger B

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Re: Carburettors
« Reply #9 on: December 09, 2013, 08:36:20 PM »
4.5mm plugs for a Fiat tractor engine? More pictures please  :cheers:
Best regards

Roger

Offline Graham Meek

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Re: Carburettors
« Reply #10 on: December 09, 2013, 10:17:28 PM »
It is early days on the Fiat Tractor engine, it is an unusual engine in that there is no cylinder head the original engine having the head cast integral with the block. The model has a bore of 15.5 mm and a stroke of 26.5 mm this is in keeping with the original in scale, although I have chosen an unorthadox scale which equates to roughly 1/7th full size. Any bigger and the rear wheels would be outside my machining capabilities.

As regards the small spark plugs I am currently in the middle of making the parts for these, because of their small size it has  required that I make another piece of equipment to make the plugs, but I will keep you informed on the progress.

Gray,

Offline Roger B

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Re: Carburettors
« Reply #11 on: December 10, 2013, 11:49:01 AM »
Gray,

Thank you once again for the information. The tractor engine looks excellent. Would you consider putting a build log in the “Your own design” section?

I have a few questions to see if I have understood your carburetor correctly. The barrel is fairly conventional with a parallel bore. The angled cap head screw sets the idling speed, the horizontal one controls the idling air bleed. The bore in the body on the inlet side is larger than the barrel bore and on the outlet side is the same.
The petrol feed goes to the small chamber at the end of the needle?
The silver grubscrew in the middle of the brass jet block is the additional adjustable airflow?
The barrel is held in place by the idling screw and position by the little white plastic washer?

Looking at the drawing your jet is around 2mm OD and 1mm bore.  This seem to be better proportions than my two commercial glow fuel carbs. One has a 4mm barrel with a 2.5 mm diameter jet and the other has a 5mm barrel with a 3.5 mm diameter jet so in both cases almost half the choke diameter is blocked.

Would you be prepared to share the drawings?
Best regards

Roger

Offline Jasonb

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Re: Carburettors
« Reply #12 on: December 10, 2013, 12:18:25 PM »
Roger, you might want to have a look at the Firefly drawings, thats an easy carb to make 2.35 bar in a 5.7mm barrel, remote needle, mine is good for 9500rpm

http://homepage.ntlworld.com/glenn.royds/RCM&E/  card starts on drawing 14

You do need to remember though that the glow engine carbs are designed for a far higher airflow than out 4 strokes so a smaller carb may be needed for the same given engine capacity

Offline Graham Meek

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Re: Carburettors
« Reply #13 on: December 10, 2013, 02:00:51 PM »
Hi Roger,

I have sent you a personal message with details as to how to get the drawings, you have pretty well worked everything out though, but drawings are so much better.

Gray,

Offline Roger B

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Re: Carburettors
« Reply #14 on: December 10, 2013, 05:31:35 PM »
Thank you for that link Jason. As you say that carb is definitely too big for my vertical engine and may even be to big for the horizontal one where I am only looking for 2-3000 rpm max.

Gray, I have sent you an email, thank you.
Best regards

Roger