Fuel manifold - material suggestions

[Marty_d]

Help required please...

 

I want to create the fuel manifold as shown by the Rotax Installation Manual, (image below).

Banjo bolts are easily accessible as are the fittings.  They're available in M10 or M12 in various threads which I can tap myself.

So all I need to manufacture is the block itself which clamps onto the crossover pipe.  Essentially it's just a solid block with a threaded vertical hole right through for two banjo bolts to go into, and an unthreaded hole for the crossover pipe to go through with a small bolt to tighten it onto the pipe.

 

Questions:

1. What would this be best manufactured from?  Alloy?  Any particular type?

2. What about the banjo bolts?  They're available in alloy, zinc coated steel and stainless... what's best for fuel?

3. The barb for the return line to tank needs to be restricted (is it 0.35mm?  The manual says "PILOT JET 35"), and the barb for the fuel pressure gauge needs to be restricted to 0.5mm.  How is this usually achieved - can you buy barbs that are sealed at the end and drill them to the correct size, or is there some kind of restrictor available which seems to be suggested by 6 below?

 

As always any suggestions gratefully received.  Cheers, Marty

 

[facthunter]

 1 In brass if you can lighten it otherwise Duralumin, a high strength aluminium-copper alloy. Put the reducer jet in the shorter banjo bolt and be careful what washers you use. (I wouldn't trust fibre) Lockwire the banjo bolts to a convenient elbow.   Nev

[skippydiesel]

To the best of my understanding.

 

1. Solid billet of aluminium of sufficient gauge/dimension to allow for all the fittings

2. Any, with the exception of SS (doesn't go well in aluminium)

3. For  return line/ pressure restriction - Rotax recommended  is Mikuni  VM 22/120 #35. These slow idle jets come is a wide range of flow rates # 10 - #140 . In my installation the #35 was delivering over 7L/hr back to the tank. I installed a smaller jet #15 jet which brought my return rate down to 5L/hr. Your fuel return rate may vary from mine. The restrictor for the fuel pressure should be as mall as you can fit suggest Mikuni  VM 22/120 #10

 

 

 

 

[Blueadventures]

Hi Marty   This is my set up a 4 way and 3 way 1/4" fittings to supply feed from fuel pump, to each carby (same length hose; important) feed to fuel pressure sensor and return (with restrictor) to starboard wing tank.  Note restrictor is in a 3/16" tail so I never mix the hook up and allows using a smaller diameter hose return (lighter and cheaper:)  Image shows restrictor pressed into end.  Early fit-up image before sealed ends of fire sleeve.  I recon the rotax item is over kill on design and good example of needing a mouse and finished up with an elephant, IMHO.  Cheers.

 

[facthunter]

Some pure aluminium billets will just not machine well and can be quite low in tensile strength and likely to strip threads. .  Nev

[Marty_d]

Thanks gents.  So Bob - that's all brass fittings you're using?

Thanks Skippy re the Mikuni jets.  They seem to have an M5 thread - Bob in your photo have you pressed the jet into the end of the normal barb with the thread outward?

[Blueadventures]

1 minute ago, Marty_d said:

Thanks gents.  So Bob - that's all brass fittings you're using?

Thanks Skippy re the Mikuni jets.  They seem to have an M5 thread - Bob in your photo have you pressed the jet into the end of the normal barb with the thread outward?

It's Mike not Bob; yes all brass and yes pressed into, thread out and its 3/16 not 1/4 for reasons above.

[Area-51]

Check out Speedflow or Morosso catalogues for off the shelf AN manifold blocks; might be something in T6 ready to bolt up

[409tonner]

My Sting runs this 5 way spider type fitting with the tank return tapped for the jet to screw into

[Marty_d]

44 minutes ago, Blueadventures said:

It's Mike not Bob; yes all brass and yes pressed into, thread out and its 3/16 not 1/4 for reasons above.

Sorry Mike!  For some reason the engine photo reminded me of iBob's installation and I obviously went by that instead of looking at who posted it... 🤔

[rodgerc]

6061T6….Leftover from Waiex wing attach blocks….Orifice for fuel return included in one gallery. 

[Marty_d]

That's very nice work @rodgerc.  What's the whitish stuff on the threads?

[rodgerc]

4 minutes ago, Marty_d said:

That's very nice work @rodgerc.  What's the whitish stuff on the threads?

Loctite 567

[skippydiesel]

5 hours ago, Area-51 said:

Check out Speedflow or Morosso catalogues for off the shelf AN manifold blocks; might be something in T6 ready to bolt up

Also Aeroflow  https://aeroflowperformance.com/ - Incredible catalogue (takes a little while to load). Page 56 . This is where I get most of my AN fittings from. Lots of fitting for you to choose from.

Edited July 14, 2023 by skippydiesel

[IBob]

18 hours ago, Blueadventures said:

...............I reckon the rotax item is over kill on design and good example of needing a mouse and finished up with an elephant, IMHO.  Cheers.

 

 

 

Yep, I'd agree there. Here is what ICP supply:
 

[IBob]

And while we are on it: can someone point out to me where in the 912 manual it says the fuel lines to the carbs need to be identical in length?
Certainly in the fuel system section it gives a set of coordinates for the fuel manifold as set out by Rotax....but if there is any note saying these coordinates are important, I haven't found it yet.

I'll stick my neck further on the block: my build was done using, amongst other things, build detail pics from the then agent, which I found invaluable. Following those, my manifold is mounted where the LH upper engine mount meets the firewall, which gives a short run to the LH carb and a a longer run to the RH.

And that seems to be working fine, with smoothe starts and running in all modes.

[Blueadventures]

53 minutes ago, IBob said:

And while we are on it: can someone point out to me where in the 912 manual it says the fuel lines to the carbs need to be identical in length?
Certainly in the fuel system section it gives a set of coordinates for the fuel manifold as set out by Rotax....but if there is any note saying these coordinates are important, I haven't found it yet.

I'll stick my neck further on the block: my build was done using, amongst other things, build detail pics from the then agent, which I found invaluable. Following those, my manifold is mounted where the LH upper engine mount meets the firewall, which gives a short run to the LH carb and a a longer run to the RH.

And that seems to be working fine, with smoothe starts and running in all modes.

Can't recall origins re same lengths; told / learnt many years ago or maybe a rotax owner info thing.  Cheers.

[IBob]

Blueadventures, I think we're all doing it because that's how we all do it.
In broadest principal it makes sense to have shared flow systems physically ballanced. And certainly for high velocity things like air and exhaust, this is so.
But for this instance, I just dialled up the online pipe loss calculator. And for a cruise flow of 17lph and carb feed lines of 150mm and 500mm length respectively (I'm not at the hangar, so guessing here) the pressure loss is approx 0.003PSI different between the two sides. That is 1/700th of the minimum fuel pressure mandated by Rotax to run this engine.
So, yes, if you ever get to the situation where the carbs are competing for fuel delivery, then you will get slightly uneven delivery. But if you get to that point, something is horribly wrong with the system elsewhere anyway.

And here's a vaporlock afterthought: In a land/shutdown/hot restart/takeoff situation..........maybe it's not best to have all that heat absorbing and transmitting ironmongery sat right on top of the engine??????
 

Edited July 15, 2023 by IBob

[skippydiesel]

On unequal carb delivery pipes:

 

One additional point - the delivery system is "toping up" a float bowl/reservoir, (acts a bit like a break tank) any minor differences in supply,  will not  impact on the delivery of atomised fuel/air to the engine

[IBob]

6 minutes ago, skippydiesel said:

On unequal carb delivery pipes:

 

One additional point - the delivery system is "toping up" a float bowl/reservoir, (acts a bit like a break tank) any minor differences in supply,  will not  impact on the delivery of atomised fuel/air to the engine

Yep.
And the more I think about it the less sense it makes to have the manifold etc plus all the split off pipework sitting up there in the hotspot. The alternative mentioned above has just the minimum...feed to and from the mechanical pump....situated up there.

[facthunter]

Below you have all the radiated heat from the exhaust system which is at a much higher temperature,. Carburetters on top of the motor are never ideal..  Let's face it they are outdated on cars and even most motorbikes.Time to inject. Some planes had it 70 years ago . Nev

[IBob]

You're not wrong, Nev.
Though in the above context, the radiated heat can be reduced or eliminated by positioning or shielding, since it is line of sight.

[facthunter]

You also need the extra room..  Nev

[skippydiesel]

True Nev!

All mechanical "systems" are, to some degree a compromise, that the pilot must learn to manage, so as to minimise the disadvantages and if available, make best use of the advantages.

Carburettors are crude, compared with some of the latest computer controlled, ultra high pressure common rail, multi pulse injection systems BUT have the advantage of relatively low cost and complexity (can even be serviced by the amateur mechanic with every day tools) proven reliability and if used with care, not so bad on the fuel consumption.

[facthunter]

The more I observe the more I believe we shouldn't use Carburettors in any serious attempt at powered flight in modern times A manifold full of fuel vapour is a serious risk to the motor's longevity. If it's historic,fair enough but fly it at appropriate localities. Nev