ROTAX Engines - Quirky or some bad design choices?

[FastEddieB]

 

Lower velocity means lower vacuum generated, and thus more difficult to raise the piston to full rich.

 

 

 

Also means you don't have full throttle opening either, right?

 

Kinda has to since the needle is attached to the slide.

[Anticept]

Also means you don't have full throttle opening either, right?

 

Kinda has to since the needle is attached to the slide.

Correct. The difference is likely minimal however. It is still quite a large opening, and we're dealing with two of those openings, not just one. I guess for a high altitude guy who wants literally every little fraction of a hp out of it he or she can get, this can be a sticking point, but then I say if it's worth that much, buy a CTLSi

[Ed Cesnalis]

Not sure what "limited" vs "determined" is supposed to mean, but we're saying the same thing. Changing the main jet changes how much fuel is metered in the full rich position.

 

As for mass air flow: I made the mistake of calling it mass air flow, it should be volumetric air flow. It's so easy to get in the habit of calling air flow "mass air flow" since it's so common in cars now.

 

However, even volumetric air flow is a little skewed in this case. These carbs are a bit more advanced than that. There is an upper and lower chamber, with the diaphragm in the middle. The lower chamber uses static air pressure, and the upper chamber uses vacuum generated by the flow of air through the venturi. I say it's a little skewed because we're using a calibrated spring to push the piston and needle down, but also counter-balancing that spring pressure with static air pressure in the lower chamber, which is quite a step up from simple volumetric air flow sensors used in cars; it's self adjusting for altitude!

 

At higher altitude, there is a lower static pressure in the lower chamber. It takes more vacuum to lift the piston against the spring now. In addition, there is a lower pressure flowing into the system, so when the air goes through the venturi, it's going to have a lower velocity than you would have at sea level. Lower velocity means lower vacuum generated, and thus more difficult to raise the piston to full rich.

 

Anyways, you're asking me for more concrete evidence. Technically, the burden of proof is on you to justify that 92% claim. I've never seen this "92%" figure for the 912 carbeurated engines in any official references. I think you are mixing that up with the full rich setting for the 912iS, which IS fixed at 92%.

 

To answer Roger, it was Corey that brought MAF into the carb discussion and led to carb vs injected.

 

Corey,

 

I think we all get that the carbs self adjust for altitude via pressure differential adjusting the diaphragm / needle. For almost all of your flights this works well and you find no fault.

 

I included this issue in this thread for 2 reasons.  1: for almost all of my flights I cruise at 10,000' where leaning is called for but not available at WOT.  Only limited cruise power, less than 75% is available at 10,000' so WOT is the normal cruise setting and at this setting the Bing's altitude adjustment is bypassed because the Bing defaults to full rich when at some point (92%?) prior to wide open the main jet gets full control.  Given that Rotax has Austrian / Alpine roots I find it amazing that high altitude leaning is not available. Past Google searches have provided me with the 92% number which is only a design number and not precisely where the needle circuit actually gives up control.

 

#2 for this issue in this thread is the way the injected version mimics the carbed limitation.  The carbed system is dumb and defaulting to full rich protects the engine in the case of WOT resulting in a high power setting but the injected system is smart and doesn't need to default to full rich at high altitude.  Why does it do this at 92%?

 

Lack of high altitude leaning in both designs qualifies for this thread. All other aircraft engines that I have been around provide for high altitude leaning at WOT.

[Anticept]

I agree that the carbs don't compensate well at extremely high altitude. Might be worthwhile for you to put a turbo on .

 

Anyways, again, you can adjust the needle position to compensate for that high altitude operation. If your plugs (all of them) are coming out coated in carbon, you will definitely benefit from that adjustment. Don't do it if only the front bank of plugs are carbon coated, that's pretty common.

[Ed Cesnalis]

I agree that the carbs don't compensate well at extremely high altitude. Might be worthwhile for you to put a turbo on .

 

Anyways, again, you can adjust the needle position to compensate for that high altitude operation. If your plugs (all of them) are coming out coated in carbon, you will definitely benefit from that adjustment. Don't do it if only the front bank of plugs are carbon coated, that's pretty common.

 

My best friend has a 914 and it does solve the problem.  Changing the needle position does not provide high altitude leaning at WOT because the needle does not have control at WOT.

[Anticept]

Changing the needle position does not provide high altitude leaning at WOT because the needle does not have control at WOT.

 

I don't think you can claim that, but nor can I claim otherwise. The needle position was suggested by Eric Tucker 2 years ago when you asked the first time. We'll see once I find time to set up a bench test.

 

I still think you should adjust your needle position and see if it helps. If your WOT fuel flow changes with the needle position, then you're likely not hitting full rich.

[Roger Lee]

Hi Ed,

 

If I lived and flew where you did I would probably have my needle clip in the #1 position.

[Ed Cesnalis]

I don't think you can claim that, but nor can I claim otherwise. The needle position was suggested by Eric Tucker 2 years ago when you asked the first time. We'll see once I find time to set up a bench test.

 

I still think you should adjust your needle position and see if it helps. If your WOT fuel flow changes with the needle position, then you're likely not hitting full rich.

 

http://www.leadingedge-airfoils.com/CARBTECH.HTM

 

 

The Midrange System affects the carburetor for approximately 1/4 to 3/4 throttle.

 

3/4 throttle is a range you can find many reference for both Rotax and BMW https://www.google.com/search?newwindow=1&rlz=1C1LEND_enUS544US544&espv=2&q=bing+64+3%2F4+throttle+main+jet&oq=bing+64+3%2F4+throttle+main+jet&gs_l=serp.3...142901.147732.0.149434.16.15.1.0.0.0.228.1977.0j13j1.14.0....0...1c.1.64.serp..1.5.742...30i10.FpLPquSpucc

 

If the needle jet circuit's range is approximately 1/4 throttle thru 3/4 throttle then the transition to full rich would happen even sooner than 92%.

 

It doesn't matter if the number is 75%, 92% or higher, as long as it is less than 100% then I am going to be crossing mountain ranges in the west without the benefit of the leaning that occurs when the needle circuit is determining mixture and providing leaning.

 

When the throttle is wide open and the main jet is determining the mixture there is no mechanism to provide leaning.

 

When crossing the Sierra at 12,500' or higher I will have my throttle wide open to access all fifty some available horse power.

[IrishAl]

You guys are going to fry some brain cells.

How did you guys get from quirky bad design Rotax parts to fuel injection versus carb fuel flows?

Here's how.

 

 

CharlieTango: I don't know why you're stuck on this, but it's not fixed at 92% throttle. 

[IrishAl]

Interesting post, Eddie, but where do you stop as a manufacturer?

If you turn the prop backwards you can create a problem, but you could just as easily create a problem by putting oil in the coolant or coolant in the oil reservoir.

 

At what point do manufacturers stop playing nanny and trying to boil everything down to the lowest common denominator of operator stupidity? 

 

Our Roads Service did a very interesting experiment here.   One notoriously accident-prone road in a city ended up with a forest of road markings, warning signs and bollards to try to reduce the accident rate, but without any real success.     The experiment was to remove all the warning signs, all the road markings and all the bollards, which made every driver do a double take and pay real attention to what was happening.  Result: the accident rate plummeted.  

 

I think a lot of pilots enjoy the fiddling, fixing and gaining the understanding they need to keep it all going.  It sort of builds a relationship with your aircraft, if you know what I mean.  

 

I might be old fashioned, but I think removing the requirement to think and learn and understand things (engines or anything else for that matter) doesn't do people any favours in the long run. Everything is spoon fed to us these days.  I mean, self driving cars....what the.......!

[GravityKnight]

A notch leaner on the needles might help out.. my experience was that it ended up a little too lean in the lower mid throttle/rpm area (4k-4500 ish iirc) even though it was much more right on in the upper part throttle cruise type areas... the zipper may exaggerate the lean condition though as I ended up with 158 main jets in prep. for low altitude ops at some point (it was just ok on stock main jets way up here, but I'd rather have some headroom and use the hacman to bring it all back when up high).

 

They are all a little different though, and after building headers for it, air fuel ratios and egts changed again.. the rans headers I had are not what comes on a "normal" 912uls with the muffler under the motor like the FD's have (I believe they use the rotax headers/muffler correct?) and when I did the needle testing it was still on stock headers. FWIW

[FastEddieB]

 

Interesting post, Eddie, but where do you stop as a manufacturer?

 

...At what point do manufacturers stop playing nanny and trying to boil everything down to the lowest common denominator of operator stupidity? 

 

I think a lot of pilots enjoy the fiddling, fixing and gaining the understanding they need to keep it all going.  It sort of builds a relationship with your aircraft, if you know what I mean.  

 

I might be old fashioned, but I think removing the requirement to think and learn and understand things (engines or anything else for that matter) doesn't do people any favours in the long run. Everything is spoon fed to us these days.  I mean, self driving cars....what the.......!

 

 

 

I think you kinda missed the point of my post.

 

I just spent about 5 hours getting the annual on my Sky Arrow started, so I must "enjoy the fiddling, fixing and gaining the understanding (I) need to keep it all going." I like knowing all the ins and outs and idiosyncrasies and, well, quirks of anything mechanical and I like to get it right.

 

But none of that stops me from noticing where the engineers might have done a better job - and that was meant to be the point of my post.

[GravityKnight]

My biggest complaint (and it's not that big) would be the intake manifold design. It doesn't flow evenly and creates EGT spreads between the front and rear cylinders. I can cut them some slack because there is only so much you can do with carbs and even distribution with the space given, but I did some port work on a spare set of manifolds and cut the spread almost in half.. and it was tricky because the hot cylinders change from the front to rear cylinders depending on WOT or cruise throttle positions.

 

Point is, if some guy in his hanger with a 'dremel' and no flow bench can improve the EGT spread, I feel rotax could have done a little better job. Still a really, really great engine for the job at hand!

[FredG]

Gravity - you raise an interesting point, why doesn't someone with a flow bench do some "stage 1" porting and advertise that service to the experimental community.  Seems like a natural market niche for Hal S.  

[Roger Lee]

mechanic or "operator stupidity? " Will always be there for any product ever MFG. Classes are offered to help reduce that problem and if these people don't go or they listen to John Doe next door then they have no one to blame, but themselves.

 

Education can reduce ignorance and stupidity. Although ignorance is fairly easy to correct stupidity seems to be a personality trait which is hard to fix. Even Forest Gump knew that.

 

 

 

Remember a lot of things were done for a good reason like saving weight and adding HP. That engine with what many think are bad ideas are designed in because of weight. Keeping it simple in some areas was in the design. 

What you don't know can make comments misleading and gives you a misunderstanding of why.

 

If you want to know the whys take the Heavy Maint. class and ask.

[IrishAl]

Please let me clarify - I'm not attacking the shortcomings of the engine. 

I'm saying those shortcomings might actually be a good thing because they require owners to have some knowledge and understanding of the engine, as well as providing the satisfaction many of us gain from having to adjust and repair things from time to time.

 

No offence intended. 

[Roger Lee]

No offence taken by me.

 

"because they require owners to have some knowledge and understanding of the engine"  

 

Now if you can get some  owners and mechanics to do this we'll be in great shape.