Rotax Performance at High Altitude with Different Fuels

[AndrewR]

Originally Posted by mwardle7

Given that moving the jet needle clips only affects the throttle in the mid range, I'm wondering if it will make much difference at high altitude where I fly with the throttle wide open.

I think this is a misconception about the 912 carbs.

There is no connection between the throttle and the carb piston. The carb piston is moved by the difference between the venturi pressure and the pressure at the carb inlet.

This is influenced by the density and volume of air flowing through the carb.

So the carb is on the main jet at low altitude WOT. As the density decreases the airflow is less able to lift the piston even at WOT so the carb will transition to the needle jet. That is how the carbs get their altitude-compensating function.

I don't know at what actual power setting the transition occurs but it's not a throttle position. It must be related to (approximately) mass airflow. I don't know enough about the carb function to be sure but I think the piston probably moves proportionally to air density but the square of the airflow. The needle taper must therefore be a compromise, so the altitude compensation isn't perfect.

It would be interesting to rig a camera looking down the air filter to see the piston move with different altitudes and throttle settings.

In regards to the original question, it sounds like the auto fuel is starting to boil due to the reduced pressure at altitude.

[Mich48041]

There was a 3 part series by Brian & Carol Carpenter about the Rotax carburetor
in the Jan, Feb & March 2017 issues of Sport Aviation (Experimenter section).
PDF copies can be downloaded from EAA HERE.
You will have to sign in with your username and password.
Search for Keyword, "carburetor".
Set the search dates between January - March 2017.

[RFSchaller]

Off topic, but part of the fuel system: the latest EAA Sport Aviation has a good article on how the mechanical fuel pump works.

[mwardle7]

Originally Posted by AndrewR

In regards to the original question, it sounds like the auto fuel is starting to boil due to the reduced pressure at altitude.

This is one of the things I've suspected, but didn't know it could actually happen. It makes sense given the circumstances. I've noticed that the fuel smell is more pronounced when the ambient temperatures are hotter, and particularly when I use fuel with ethanol. Given my understanding that ethanol fuels can boil at a lower temperature than non-ethonol fuels, and given that 100 LL boils at a higher temp than automotive fuels, this just make sense to me.

So, if fuel boiling is the issue (which I'm not sure it is - this is all theoretical at this point) then what is there to do about it? Not using auto fuels, flying lower, and cooling the fuel somehow seem logical. Cooling the fuel seems the most difficult, impractical, and the highest risk due to unforeseen risks/downsides. How could it be tested? I think the simple answer is to just use 100 LL when I'm going to fly high and hot. I may also just avoid ethanol fuel; I'm fortunate to have access to 91 octane Ethonol fuel about 10 miles from Salt Lake International where I am hangared. But it is nice to understand, diagnose, and confirm before choosing a course of action.

Thanks to those who have posted helpful info about carb tuning. Brian and Carol Carpenter's articles are awesome. If you ever visit Rainbow Aviation, Brian 3D printed a giant Bing 64 Carb with all of the moving parts. It's a masterpiece. Here are a few other Bing carb articles that I've found informative:

Using Choke in Carburator Tuning
Tuning the Bing Carburator
Understanding the Mid-Range Circuit

I'm going to experiment with 150 main jets and weigh the floats this weekend. We'll see how that goes.

[h&jeuropa]

Boiling in the carb bowl has been a problem in the past. My 914 has carb drip trays whose main purpose is to keep fuel off the exhaust in case of a carb bowl leak. A secondary purpose is to act as a heat shield, to block heat from the exhaust pipe from the carb bowls. This is particularly a problem when you shut down after a flight and go to restart soon after. The heat will cause the fuel in the carb bowls to evaporate and starting is difficult.

Have a look at the parts manual to see what the drip trays look like and do a search on the Rotax site for examples others have fit.

Brian's talk on Bing carbs at Oshkosh was really good. I'd talk to him about your problems.

Jim Butcher
Rotax 914 in europa

[AndrewR]

Originally Posted by mwardle7

Here are a few other Bing carb articles that I've found informative:
Using Choke in Carburator Tuning
Tuning the Bing Carburator
Understanding the Mid-Range Circuit

Beware: Those articles appear to be about the Rotax 2-stroke carbs, which operate on a different principle to the 912 carbs. I would not rely on tuning information from those articles to apply to the 4 stroke carbs.

Originally Posted by mwardle7

So, if fuel boiling is the issue (which I'm not sure it is - this is all theoretical at this point) then what is there to do about it? Not using auto fuels, flying lower, and cooling the fuel somehow seem logical.

If auto fuel is causing a problem, I think the simple answer is to use a fuel that is formulated for use at altitude i.e. avgas. From what I have read, as the fuel approaches vapor pressure limits bubbles will form in the float bowls, which means the fuel is less dense and the floats sink, giving the same symptoms as bad floats.

Originally Posted by mwardle7

Brian and Carol Carpenter's articles are awesome. If you ever visit Rainbow Aviation, Brian 3D printed a giant Bing 64 Carb with all of the moving parts. It's a masterpiece.

Are these the articles?
https://electricmotorglider.com/2017...uretor-part-1/
https://electricmotorglider.com/2017...g-carb-part-2/
https://electricmotorglider.com/2017...ircuit-part-3/

I agree they're good. They also confirm that you are probably not on the main jet at altitude:

As the aircraft gains altitude, the air density diminishes whereas the fuel density remains the same. This inevitably results in a rich mixture. The CV Carburetor on the other hand, senses the reduced atmospheric pressure on the lower half of the diaphragm, and the piston lowers in the body of the carburetor. This happens automatically even though the throttle valve position is still in the full open position.