question Rotax 912 Carburettors

[Flightrite]

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

 

Saying Running ROP is lazy is a stupid comment!

[M61A1]

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?

A good point, perhaps what I should have said is that just 5200 rpm isn’t enough information. Whereas 5200 at a specific manifold pressure will get some data. Did you look at the Rotax graph?

[M61A1]

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.

https://www.justraceparts.com.au/spartan-2-wideband-gauge-kit-52mm-lcd-air-fuel-bosch

Needs to be a wideband sensor, narrow band are much cheaper, but useless for this application.

[Downunder]

I contacted a company some years ago about a similar A/F gauge.

They said it wouldn't work in an aircraft because it was calibrated to sea level, so lower oxy levels at altitude give incorrect readings.

I couldn't work it out.... perhaps they baulked at the word "aircraft"...

[Geoff_H]

A better gauge would be CO. When the value just starts to rise above zero best performance. Another gauge is residual oxygen in exhaust. Both are monitoring the combustion b

[M61A1]

A better gauge would be CO. When the value just starts to rise above zero best performance. Another gauge is residual oxygen in exhaust. Both are monitoring the combustion b

According to the graph in post #29 CO is sky high at best power. I’m fairly sure the AFR gauge is measuring residual oxygen as it uses a Bosch O2 sensor.

I found that regardless of altitude with a fixed power setting and attitude rpm would drop off ( an indication of power loss) at ratios below 12:1 and above about 12.8:1

there was less power available at higher altitudes obviously, but you could get the most out of it with the right mixture settings.

[Geoff_H]

Yes you are right. I should have said the best operation conditions for cruise.

[Bruce Tuncks]

Downunder, you could be right. Once a spray painter refused to allow an aircraft into his booth. I reckon he was worried about a crash or something.

I can't see why the air-fuel ratio meter would not be useful in a plane. Even if altitude changes things, surely that is good to know?

[facthunter]

When you have manual leaning capability it's pretty obvious when it runs better at higher levels. You will see an instant response when it's needed. If you have some gadgets it may be better. With charts and fuel flow and MP you just apply the proven figures. That works fine even with supercharging. BUT you don't need it on most of our stuff. The BING is a pretty ordinary Carburetter you are pretty much stuck with and it's up high on the motor not a good place for THEM. By the time you are above say 6,000 ft you need to lean. Float carburetters are not good things on an aeroplane but inevitable on some because of cost. Adding air downstream of the carb has to be done carefully as it may stratify and lean some cyls more than others unless it's well mixed. Nev

[IBob]

Adding air downstream of the carb has to be done carefully as it may stratify and lean some cyls more than others unless it's well mixed. Nev

I'll second that. I've seen air in ducting (in a commercial bakery proving room) where heating elements did not extend all the way across the ducting, so some air bypassed the heaters. And nothing we did downstream would effectively mix the hot and cold streams of air: all we succeeded in doing was moving the two streams around. (In the end, we narrowed the duct at the heaters, so that all the air was passing through them.)

And I have also seen an industrial hot water system (where you commonly check what's going on by feeling the pipes) where one side of a quite small pipe was too hot to touch, while the other side was quite cold. Same problem: cold water added in but not mixing as expected........(

[Geoff_H]

I'll second that. I've seen air in ducting (in a commercial bakery proving room) where heating elements did not extend all the way across the ducting, so some air bypassed the heaters. And nothing we did downstream would effectively mix the hot and cold streams of air: all we succeeded in doing was moving the two streams around. (In the end, we narrowed the duct at the heaters, so that all the air was passing through them.)

And I have also seen an industrial hot water system (where you commonly check what's going on by feeling the pipes) where one side of a quite small pipe was too hot to touch, while the other side was quite cold. Same problem: cold water added in but not mixing as expected........(

In the steam power industry this happens all the time. It is wall attachment. A low flow of air, less than turbulent if my memory serves me right, will flow against one wall and not mix. I have done velocity traverses of ducts at low volume and found that the air was only using around 25% of the available duct. As the volume increased the flow became turbulent and filled the whole duct evenly ,(except dropping to zero velocity at the wall). If this is happening in an engine inlet manifold you have some problem with a ventury carb. Too big a carb will often give worse performance.

[onetrack]

So ... Hiclones really do actually work, then??

[Geoff_H]

I expect that they would correct a badly designed system. I expect that most if not all engine design engineers know of the problems that hiclones solve and prevent them from occurring. I owned a BMW some years ago, that used their M44 engine. A truly great engine. It had a variable length inlet manifold, short at high power for less losses in inlet pressure and long at low powers for better air fuel homogeneous mixing.

[Bruce Tuncks]

What about a vacuum gauge in the inlet manifold? Will this tell you useful information? It does have the benefit of being cheap and easy to put in, but unless it gives useful information then its not worth the effort.

[Thruster88]

What about a vacuum gauge in the inlet manifold? Will this tell you useful information? It does have the benefit of being cheap and easy to put in, but unless it gives useful information then its not worth the effort.

A vacuum or manifold pressure gauge will give useful info, it tells the pilot what percentage of power is being produced with a fixed or variable propeller when combined with a manufacturer's chart. Essential equipment with a constant speed prop.

[Downunder]

What about a vacuum gauge in the inlet manifold? Will this tell you useful information? It does have the benefit of being cheap and easy to put in, but unless it gives useful information then its not worth the effort.

 

I have one. It's not really useful with a fixed prop and/or non turbo engine I think.

It does vary with throttle position. I don't take much notice of it.

Most "loading" on the uls is based around RPM.

 

Speaking of turbo's, a turbo normalized set up would not require any leaning at all would it?

You're effectively at sea level all the time.

Any Jabs with turbo's around Bruce?

[Thruster88]

An example of a power chart, note there is no mention of propeller or aircraft type. If we are flying at 5000', 2300rpm and 19 inches manifold pressure the engine is producing 100hp and burning 7.8us gal or 29.6 litres per hour. A Rotax 912 burns 28.5 lph at 100hp.

[Downunder]

How would that relate to prop pitch settings?

A Rotax ground adjustable prop set super fine would pull 5800 easily.......

And going the opposite, lets say WOT at cruise pulls 5200 max rpm. (Rotax's minimum WOT spec).

If the "fine" engine pulls throttle back to 5200 will manifold pressure be the same?

Surely manifold pressures aren't the same?

[Thruster88]

How would that relate to prop pitch settings?

A Rotax ground adjustable prop set super fine would pull 5800 easily.......

And going the opposite, lets say WOT at cruise pulls 5200 max rpm. (Rotax's minimum WOT spec).

If the "fine" engine pulls throttle back to 5200 will manifold pressure be the same?

Surely manifold pressures aren't the same?

 

The manifold pressure has to be higher for a coarser pitch at a given rpm. You can see that in the chart, as the rpm falls with increasing propeller pitch the mp (throttle) has to be set higher to keep the same power. 5200 rpm and 22 mp is more power, fuel and airspeed than 5200 and 20 mp. Rotax have power charts available.

[skippydiesel]

..............................................A Rotax 912 burns 28.5 lph at 100hp.

 

Please explain ??

 

A Rotax 912 ULS only puts out 100 hp at take off power (5800 rpm) max 5 minutes and in my installation this is about 22- 24 l/h. I say about because my fuel flow instruments are not calibrated and I maintain this setting for such a short time, in a high work load situation, the actual fuel flow may be +/-

Also I am inclined toward Downunders statements above - fuel flow must be related to the "work" being done (prop pitch) not just rpm

[Bruce Tuncks]

Thanks downunder. My guess is that a vacuum gauge would help find if there was an inlet manifold leak. Have you any experience with this? Apart from that, I guess it would only show, as you said, that higher throttle gives less manifold suction, and we already knew that.

[Downunder]

Thanks downunder. My guess is that a vacuum gauge would help find if there was an inlet manifold leak. Have you any experience with this? Apart from that, I guess it would only show, as you said, that higher throttle gives less manifold suction, and we already knew that.

It varies so much. Atmospheric pressure, altitude, throttle, load, rpm......too many variables.

Technically, you do have a "leak" via the throttle butterfly........

[facthunter]

The engine's a suction pump .The throttle is a strangler. The ambient airpressure limits your MP as well if you are normally aspirated and climb higher. .You RPM is limited by prop load and usually varied by a CS prop in flight. In that situation the MP /RPM figures will indicate HP. They relate to MASS of combustible GASFLOW which is proportional to power. Your prop converts that to thrust with varying degrees of efficiency. Your mixture (fuel/air ratio) will affect fuel consumption.. Engine friction obeys the squared law. Double the rpm equals 4 times the friction so the lower RPM used the better the specific fuel consumption where there are options and they are useable. This usually occurs with supercharging. Pretty limited when you are not. Nev

[Thruster88]

Please explain ??

 

A Rotax 912 ULS only puts out 100 hp at take off power (5800 rpm) max 5 minutes and in my installation this is about 22- 24 l/h. I say about because my fuel flow instruments are not calibrated and I maintain this setting for such a short time, in a high work load situation, the actual fuel flow may be +/-

Also I am inclined toward Downunders statements above - fuel flow must be related to the "work" being done (prop pitch) not just rpm

 

Ok so only 27.5lph at 100hp, I don't think rotax would put out a chart showing higher than actual consumption.

At cruise power consumption can be lower than shown on the chart if the propeller is not loading the engine as much as the one in the chart, ie lower manifold pressure. Rpm and manifold pressure equals work.

[Downunder]

I've never seen fuel usage that high either, nor quoted figures from anyone I know.

Rotax need to quote maximum outright "possible" use ...... perhaps for liability reasons...

 

Currently sitting on 5150 at 16 lph....."real time" figures...lol.