question Rotax 912 Carburettors

[Kyle Communications]

The early IS engines had a few issues..mainly electrical..most seems to be sorted now but of course with injection and ECU control for fuel flow and timing also the sensors they have on the engine makes it much easier to get the correct burn hence best economy

[Bruce Tuncks]

Thanks Yenn. My plugs look a nice brown color, and I have egt on each cylinder. I reckon that my engine runs rich and not lean but I'd like a gauge to tell me. Jabiru did bring out a "lean running " mod which they later withdrew.

So how does the EGT tell me about the mixture?

[Kyle Communications]

The higher the EGT the leaner the engine is running

[M61A1]

Thanks Yenn. My plugs look a nice brown color, and I have egt on each cylinder. I reckon that my engine runs rich and not lean but I'd like a gauge to tell me. Jabiru did bring out a "lean running " mod which they later withdrew.

So how does the EGT tell me about the mixture?

 

 

 

Here are some charts:

one gives you the relationship between AFR and EGTs, the other AFR and power.

[Thruster88]

Thanks Yenn. My plugs look a nice brown color, and I have egt on each cylinder. I reckon that my engine runs rich and not lean but I'd like a gauge to tell me. Jabiru did bring out a "lean running " mod which they later withdrew.

So how does the EGT tell me about the mixture?

 

Bruce the following video is an excellent explanation of fuel mixture and how it affects the EGT and CHT, forget that he is flying a fuel injected beechcraft?, all engines respond in the same way. Without a mixture control in your jabiru you won't know if you are lean of peak, rich of peak or at best power mixture.

 

The higher the EGT the leaner the engine is running

 

As Martin explains the EGT and CHT will begin to fall if you continue to lean past the peak EGT.

 

[facthunter]

Leaning it right out will cause lower temps. You are putting the fire out and you have power loss and rough running as well. The last thing you want is a manifold backfire as it puts a pile of mixture in another cylinder. You have to keep your leanest pot happy. Jabiru's problems started with trying to run lean.. A good drip fuel injection system and Gami Injectors is a pretty good set up on a Cont or Lycoming that will give comparable specific fuel consumption to most other engines( except diesel and that's not revolutionising anything lately) and it's simple and fail safe.. Six cylinder engines beat fours for smoothness too, anytime. Nev

[kgwilson]

My observations with my Jab 6 are that low level flying up to around 4000 feet ends up with the plugs a dark coffee colour but on a long trip at 8-10,000 feet the plugs are black indicating running rich at high altitude. I use a bit more fuel as well but it doesn't bother me. Fuel is the cheapest part of flying.

[facthunter]

Generally plug colour is only valid when the engine is cut when the range desired to be checked is established and held for a while to stabilise . Idling is usually excessively rich and will quickly show the change. Nev

[Mike Borgelt]

"The higher the EGT the leaner the engine is running"

 

As others just pointed out, that is wrong.

 

The problem with carburetted Lycomings is that the mixture distribution isn't even at partial throttle settings. Above 7500 feet with the throttle wide open it is pretty good to the point where on my O-320 with two Lightspeed ignitions there is no rough running at all as you lean at and above that altitude. It just loses power but remains smooth. Reduced EGT, reduced CHT and reduced fuel flow. What's not to like?

Interesting is that if you pull the throttle back just a little to descend which you have to do as the fixed pitch prop is running not far short of redline, the engine runs rough unless you richen the mixture a little.

[BirdDog]

Hmm... Interesting discussion. I have sat at 9500ft without any issues. Ran like a dream!

[M61A1]

Hmm... Interesting discussion. I have sat at 9500ft without any issues. Ran like a dream!

Behind what engine? Was it fitted with a mixture control?

I fitted an Air Fuel Ratio gauge when fiddling with a different system. I left it on when I put carbs back on and found the metering not nearly as precise as I thought. Taking off at about 1500 amsl with afr ratio of about 12.5:1 at full power you can watch the afr drop as you climb and will get down to just over 11:1 Around 7000 amsl, which significantly reduced power, not only because of lower atmospheric pressure, but an over rich mixture. Maximum power occurs between about 12.1:1 and 12.5:1, any richer or leaner and revs start to drop off, although over rich seems to reduce power quicker than leaning.

With the other system fitted, I could adjust the mixture as I climbed which provided optimum power for the altitude.

[Flightrite]

Like has been said, fuel is the cheap part of running an engine, never been a fan of LOP, I run ROP in my Lyc and leave the Jab donk as it came out of the factory, I sleep well at night?

 

Just out of curiosity does anyone who operates a Rotax 100 carby donk know the FF at 5200 RPM at say 3000'?

[M61A1]

Just out of curiosity does anyone who operates a Rotax 100 carby donk know the FF at 5200 RPM at say 3000'?

That has too many variables. The type of aircraft and prop, the throttle setting giving you 5200 can vary wildly. I can only assume that you mean straight and level in cruise, but pitch settings affect that a lot. the rotax operator manual has a page with fuel flow, but doesn't chart altitude unless you can derive that from manifold pressure.

 

[CrayonBox]

My 912ULS at 10000 feet... purred the whole time... conclusion to video in part 2.

[Yenn]

Having mixture control will not tell you if you are lean or rich, all the control does is allow you to alter it.

An EGT will give an indication of how the mixture is working.

Full rich you are getting s lot of power and the excess fuel is supposed to be cooling the heads, you will have EGT readings less than maximum, or less than peak as they say. Reduce the fuel slightly and the burn will become more efficient, giving more power and also higher EGT. That is the effect of gases being hotter as they exit the cylinder. Lean some more and you will get to peak EGT. The very hottest that the gases will get and just slightly less than maximum power. As you continue leaning there will be an excess of oxygen in the mix and less fuel to burn, so EGT will drop.

Somewhere just lean of peak you will get the most efficient burn.

Running rich of peak is OK if you are lazy, or don't understand what you are doing and so long as you are not burning everything up, which is possible. Lycomings run very well way lean for taxying and they will not have so much tendency to misfiring. Run them so lean that they will cut out if you advance the throttle to take off power and you will be safe

[M61A1]

Having mixture control will not tell you if you are lean or rich, all the control does is allow you to alter it.

An EGT will give an indication of how the mixture is working.

Full rich you are getting s lot of power and the excess fuel is supposed to be cooling the heads, you will have EGT readings less than maximum, or less than peak as they say. Reduce the fuel slightly and the burn will become more efficient, giving more power and also higher EGT. That is the effect of gases being hotter as they exit the cylinder. Lean some more and you will get to peak EGT. The very hottest that the gases will get and just slightly less than maximum power. As you continue leaning there will be an excess of oxygen in the mix and less fuel to burn, so EGT will drop.

Somewhere just lean of peak you will get the most efficient burn.

Running rich of peak is OK if you are lazy, or don't understand what you are doing and so long as you are not burning everything up, which is possible. Lycomings run very well way lean for taxying and they will not have so much tendency to misfiring. Run them so lean that they will cut out if you advance the throttle to take off power and you will be safe

Yes, having a mixture control only allows you to adjust it, that was the point of the question. Not sure what Bird Dog is flying, but a Rotax with standard Bings at 9500 feet will be very rich as I found with the Air Fuel Ratio gauge fitted. I gets as bad as below 11:1 or richer. The mixture control allows you to get the ratio to a point where is makes more power and uses less fuel if that's a concern.

At 9500 feet a 912 is a slug and I would suggest that running LOP, which significantly reduces power will just make it even worse.

If he's not flying behind a 912 then it's a different story, maybe there are horsepower to spare.

With a 912UL I don't care about LOP, I want all the power it can make, when I want it. Sure it runs smooth at 9500 feet, but it's rich as hell and by leaning it back to optimum power it's still smooth but has significantly more power and because of that will cruise faster and use less fuel.

I provided the graph for the O360 at post #29.

[facthunter]

The trouble with a Constant Vaccuum ((CV) carb is the dashpot won't rise very far at altitude and the metering on the needle is not working appropriately. Geoffrey Dehavilland got a Gypsy Moth to about 23,000 ft in a gypsy Moth in the early 20's and I've managed a Tiger to 14,000 on the alt Indicated which is an ICAN calibrated instrument. I don't know what the corrected level would be as I didn't have an OAT gauge. That was as far as I could get without a mixture control and it was as rich is it would still run. Many DH 82's had mixture control and all the Chipmunks did. You get more power being able to get the mixture in the correct range. Many POHs say don't lean till below 75% power. Depending on the day, I take that to be around 4,000 feet with full throttle on a normally aspirated motor.. It's a density Altitude thing. Nev

[Bruce Tuncks]

M61A1, could you tell me more about the air-fuel ratio gauge please. I googled up how they worked and got some stuff about an oxygen sensor which I didn't understand.

The gauge is selling for $17.95 at banggood and this puts it within my budget, depending on the cost of the sender if this is an extra.

Anyway, where is the sender installed? How does it work in plain english? Is it accurate and does it drift over time?

I think such a sender is standard in cars these days, with the output going to the car computer. But this doesn't help me at all.

It seems that such an instrument can solve the rich or lean question easily.

[Bruce Tuncks]

Nev, I have heard of a Jabiru getting to 14,500 ft. A lighter pilot could go a bit higher I guess. Not even halfway up mount Everest.

I have been over 20,000 ft in a glider.

[facthunter]

Works better in winter. For the plane, not always for you, if it's an open cockpit.. Gliders can end up very high if you don't watch your Cb's. Nev

[Downunder]

I've got my 912 uls to a bit over 11500 briefly. -6 OAT!

As per the other posters, it ran perfectly but those egt's were so awfully cold. Maybe around the mid 600's.

I was probably wasting 4 or more litres per hour......at least.

Whereas a nice WOT run at sea level, on a "high oxygen" day can get me easily over 800 degrees C egt....so sweet!

 

As I normally fly at "altitude", I have considered buying a HACMAN but I've not pulled the trigger yet.

http://gsatech.store/hacman-mixture-control-kit/

[Bruce Tuncks]

I didn't know about the HACman. It looks like it allows air into the inlet manifold so the richest would be with it closed. But this could well be wrong. If so, how does the thing work?

[Bruce Tuncks]

One of these and the meter that M6 knows about should give you all the info and control you need.

[Downunder]

I didn't know about the HACman. It looks like it allows air into the inlet manifold so the richest would be with it closed. But this could well be wrong. If so, how does the thing work?

I'm not sure why this bloke made the vid at such low altitude..... make the tuning differences minor I think.

You can download the manual from the link in my post above.

[Flightrite]

That has too many variables. The type of aircraft and prop, the throttle setting giving you 5200 can vary wildly. I can only assume that you mean straight and level in cruise, but pitch settings affect that a lot. the rotax operator manual has a page with fuel flow, but doesn't chart altitude unless you can derive that from manifold pressure.

 

[ATTACH type=full" alt="fuel flow.jpg]53839[/ATTACH]

Airframe won't make any diff, the engine doesn't know if it's strapped to an airboat of billy cart, I'm not after performance. Somebody out there operating a 100 gee gee's carby Rotax must know what they are getting at 5200 rpm?