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I always thought that the carburettors fitted to the Rotax 900 range were altitude compensating (auto adjust) to at least 8,000 ft - perhaps not as precisely/accurately as one might like but a fair compromise never the less.

 

I recently read a comment, by Ivan Shaw, where he categorically states that the Bing carburettors, as fitted to Rotax 912 range, are not altitude compensating.

 

Did a little additional reserch, only to find the exact opposite statement from other authoritative figures.

 

So whats it to be::

  • Are they altitude compensating?
  • To some degree?
  • If so to what altitude?
  • If not - where does the myth come from?

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From the heavy maintenance manual.

 

The vacuum in the venturi acts on the top of the diaphragm and the carburetor piston (slide) via 2 holes in the carburetor piston and attempts to lift the piston against its own weight and spring.

 

The reference pressure prevailing between airfilter/airbox and carburetor (e.g. ambient air pressure) is applied to the underside of the diaphragm via the duct. The space in the cover above the piston guide is vented through a bore to prevent hammering of the slide.

 

Mixture is controlled by the combination of spring pressure holding the slide down, venturi vacuum trying to raise the slide by sucking on top of the diaphragm, and atmospheric pressure on the underside of the rubber diaphragm.

 

Simples.

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From the heavy maintenance manual.

 

The vacuum in the venturi acts on the top of the diaphragm and the carburetor piston (slide) via 2 holes in the carburetor piston and attempts to lift the piston against its own weight and spring.

 

The reference pressure prevailing between airfilter/airbox and carburetor (e.g. ambient air pressure) is applied to the underside of the diaphragm via the duct. The space in the cover above the piston guide is vented through a bore to prevent hammering of the slide.

 

Mixture is controlled by the combination of spring pressure holding the slide down, venturi vacuum trying to raise the slide by sucking on top of the diaphragm, and atmospheric pressure on the underside of the rubber diaphragm.

 

Simples.

 

 

Great FlyBoy - but doesnt really address the practical application questions posed - are the 912 carburettors altitude compensating (or not)? ?and if so to what altitude are they effective ??

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Yes, it is in the section explaining that the carburettors do not require any adjustment up to 8000 feet. Then the extract from the heavy maintenance manual explains how it works along with its limitations. Just down load the manual from Rotax-owner.com and do a search on the word then do a search on the word " venturi " or something like that and it will come up straight away

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Great FlyBoy - but doesnt really address the practical application questions posed - are the 912 carburettors altitude compensating (or not)? ?and if so to what altitude are they effective ??

The answer is yes and no ... and even the yes as a little asterisx.

 

Yes they are alitude compensating because they use differential pressure between ambient and the engine BUT they are not perfect as they use a fixed spring as the 'balance'.

This introduces variable error dependant on:

1. absoulte pressure on the diaphrams and

2. differential pressure between the two and

3. the temper state of the actual spring

 

1. and 2. will impact a carb differently as altitude is gained - higher the worse the match between spring tension and pressure differential so you will see failure to lean correctly as you go up.

3. impacts two ways - difference between carb springs will show in unbalanced carbs - the cross feed of air cannot deal with any fixed diferential tension in the springs - and age of springs - please replace BOTH if you relace 1 - heat and manufacture changes will introduce additional tension differences if you only do one of the springs.

 

From my experience on an 80HP 912 fitted with standard carbs plus a sunplimentary mixture control I started to see richer mix on carbs only over 6,000ft AMSL pressure altitude and as I was only ever flying to 8,000 max I decided to remove the suplimentary mix control to remove another point of failure on the system and let the extra few hundred mls of fuel per hour go out the back end.

 

For reference cruise was 11LPH below 6,000 achieved on legs from 2-3 hours in length and it went up to just under 11.4LPH when flying up to 8,000 from a 2,000 starting level.

And the 11LPH rate was the same between 3 aircraft of the same make/model over the exact same legs - we all went touring together and were very careful recorders of detail.

Edited by kasper
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Kasper - That's what I call a comprehensive answer and a half.

 

Your res ponce matches my pretty sketchy understanding and gives me a much more in depth knowledge - my thanks.

 

Ivan Shaw of aircraft design/build fame is wrong - just proves in the experts can err. There is hope for me yet

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This is why I am fitting a O2 sensor to my next 2 aircraft

Then I can jet them properly. So far we have found that at cruise our rotax 912ULS are running quite lean ..the other issue that also affects this is the type of muffler as well that you have and the Sav muffler is crap....its quite complicated. M61A1 has done a lot of work on this and started us off on another mission.

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As the 912iS has a dramatically improved economy (25% I heard) the compensation of the carburetors must be limited. But probably still a lot better in real flying than most lycosaurus are flown.

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My experience in 912 uls is compensation exists up to around 5000 ft.

The exact altitude is variable because atmospheric conditions are variable.

It is easy to see when it is not compensating as egt's lower as the mixture becomes richer.

It is also noticable how poor carbies meter fuel when the fuel injected engine uses noticeably less fuel.

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My experience in 912 uls is compensation exists up to around 5000 ft.

The exact altitude is variable because atmospheric conditions are variable.

It is easy to see when it is not compensating as egt's lower as the mixture becomes richer.

It is also noticable how poor carbies meter fuel when the fuel injected engine uses noticeably less fuel.

I agree with the setup for 5’000 ft. Told that years ago and just remembered it for trivia nights. Can’t back up the tech info, just that our local carby guru told me a number of times while I was learning the carby servicing trade off him.

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The answer is yes and no ... and even the yes as a little asterisx.

 

Yes they are alitude compensating because l..............

 

.

And the 11LPH rate was the same between 3 aircraft of the same make/model over the exact same legs - we all went touring together and were very careful recorders of detail.

 

Thank you very much for this info. It will be digested, compared and shared with future students

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This is why I am fitting a O2 sensor to my next 2 aircraft

Then I can jet them properly. So far we have found that at cruise our rotax 912ULS are running quite lean ..the other issue that also affects this is the type of muffler as well that you have and the Sav muffler is crap....its quite complicated. M61A1 has done a lot of work on this and started us off on another mission.

On exhaust impacts:

3 X Raven EclipsR 912 with 80hp engines all around 3-400hrs ttis flying same legs together returned exactly the same fuel consumption. Two were fitted with the split two exhaust systems - one for each bank - while the third was running the four into 1 rotax system. No performance or fuel differences and we all cruise at 3,400rpm returning 68-70mph for 11lph.

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On exhaust impacts:

3 X Raven EclipsR 912 with 80hp engines all around 3-400hrs ttis flying same legs together returned exactly the same fuel consumption. Two were fitted with the split two exhaust systems - one for each bank - while the third was running the four into 1 rotax system. No performance or fuel differences and we all cruise at 3,400rpm returning 68-70mph for 11lph.

The stock Rotax system is clearly designed to be compact without too much thought.

Did your modified system have tuned length runners for the rpm you were using?

Also to get correct pulse timing the 1 & 2 cylinders should be paired and 3 & 4 cylinders paired together as there is 360° between pulses (between each cylinder of the pair). Where there is an uneven pulse (540° and 180°) between 1 & 3 and 2 & 4 if there are paired together.

I think also what KC is getting at is that the Sav exhaust causes uneven flow between cylinders, making it more difficult to properly setup.

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Posted (edited)

The stock Rotax system is clearly designed to be compact without too much thought.

Did your modified system have tuned length runners for the rpm you were using?

Also to get correct pulse timing the 1 & 2 cylinders should be paired and 3 & 4 cylinders paired together as there is 360° between pulses (between each cylinder of the pair). Where there is an uneven pulse (540° and 180°) between 1 & 3 and 2 & 4 if there are paired together.

I think also what KC is getting at is that the Sav exhaust causes uneven flow between cylinders, making it more difficult to properly setup.

Well the twin system is not one I created - it’s a stock from factory system used by mainair and medway trikes.

 

The 1-3 and 2-4 exhausts run lengthwise on the engine and the inlet pipes are basically the same length. The outlet is 3/4 way up the exhaust towards the front. I’ll go over to the container on the weekend and drag out the two setups and take a couple of pics if you like - I’ve got both exhaust system over there on the a shelf off the airframe at the moment. The twin and the single.

Edited by kasper
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Well the twin system is not one I created - it’s a stock from factory system used by mainair and medway trikes.

 

The 1-3 and 2-4 exhausts run lengthwise on the engine and the inlet pipes are basically the same length. The outlet is 3/4 way up the exhaust towards the front. I’ll go over to the container on the weekend and drag out the two setups and take a couple of pics if you like - I’ve got both exhaust system over there on the a shelf off the airframe at the moment. The twin and the single.

Ok, Thanks. That makes a lot of sense that neither performance or economy changed. I think I have seen some of the exhausts you have described on 912s.

I have been on the lookout for good performance 912 exhaust designs for a while and I'm keen to see anything that is proven to work.

The hard bit for the 912 (or any aero engine) is that some length is required, making it difficult to fit under a cowl.

 

How do you measure if the mixture is rich or lean on a Jabiru?

If it has a muffler and not just straight pipes, you could fit an O2 sensor. The results may be surprising.

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Here are stolen pics of the two setups

My current exhaust looks a bit like the one on the left, with a few more bends in it. Not a factory one though, it looks like someone made a copy.

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My current exhaust looks a bit like the one on the left, with a few more bends in it. Not a factory one though, it looks like someone made a copy.

There is a factory used single can that runs lengthwise under the engine that has many more twists n turns. These two have no real attention to work as headers to help with power at any setting but are clean and simple - and deliver the rated power of the 912 80hp.

 

The power setting of 4300rpm to deliver 11lph actual shows that in that airframe and with that prop and exhaust the power is spot on the expected as the 912 power/fuel graphs show that is the expected fuel at that rpm with full power absorption.

 

Just a good setup that has been sorted by the factory to work ?

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Bruce asks how do you measure the mixture on a Jabiru. Look at the plugs, or better still add an Exhaust Gas Temperature gauge to the panel.

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As the 912iS has a dramatically improved economy (25% I heard) the compensation of the carburetors must be limited. But probably still a lot better in real flying than most lycosaurus are flown.

I think a lot of the improvement comes from the fact that the 912is doesn't have carbi's!

It's fuel injected.

Not to highlight ancient technology, but with good EGT's and mixture control, the old Lycosuarus's probably doesn't do too bad, but remember, your driving an engine more than twice the size of a Rotax, for the same power.

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So far we have found that at cruise our rotax 912ULS are running quite lean .

To be running lean you would have to be using less fuel than a 912is?

This video really proves how bad carbs are at metering fuel.

Edited by Downunder
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The 912is is impressive but is is it cheaper to run over its 2000 hour life. Hard to pin down pricing but I believe it costs about $15k aud more than 912uls to buy. At 5l per hour saving and say long term $1.60 per litre aud break even would be 1875hours. For a recreational flyer doing 50-100 hours a year it makes no economic sense. The carburetors have proven reliability and low maintenance costs. I get that people like to have nice toys.

 

My lycosuarus 160hp burns similar fuel per horsepower as what rotax show on their graphs.

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