Testing carb heat

[Dan Kent]

I apologize in advance for being OT. If carb icing is a concern there is another approach to this. All the US Sting aircraft have this device as standard equipment. It is supposed to not effect engine performance. Being in a very high humidity location (Houston), I was concerned with carb icing and am glad I have this on my plane.

 

Here's the link for the 912 ULS version.

 

http://www.skydrive.co.uk/proddetail.asp?prod=CH-912-3

 

 

[Flying Bozo]

Nobody has mentioned that the Bings are altitude compensating. therefore I would expect almost no drop in RPM since the carbs are supposed to adjust to altitude and with the carb heat on and working they should adjust and keep the RPM almost constant. Carb heat looks like a higher altitude to the engine so I would suggest that if there was a significant drop in RPM then the carbs would not be doing their job.and there would be something wrong. Like Eddie said there is a more roundabout route for the air, hence a slight drop but the actual thinner air with heat on should be compensated by the carbs. So I would be alarmed if there was more than a very slight drop in RPM.

Flying Bozo

[Ed Cesnalis]

FB,

 

That doesn't sound right to me, in this thread there are 3 types of carb heat

1. Warm air from inside cowling on CTSW
   
2. Hot air from heat muff on CTLS
   
3. Hot liquid in carb base plate in Sting aircraft.
   

The bings compensate when the needle jet has control by utilizing the pressure differential to raise the needle and lean the mixture.

 

Is the warm or hot intake air at a significantly lower pressure? Take the CTSW isn't the air inside the cowling now on both sides of the throttle body and therefore no differential at all?

[mocfly]

All,

Carb heat on my Aircraft 2006 CTSW comes thru a small opening located at the bottom of the air filter box......this door is actuated via the carb heat cable which terminates inside the cabin. This warm/hot air comes from the increased ambient temperature inside the engine cowl.

 

The carb heat circuit is not connected to a muff or any other device which would put much warmer and under a higher pressure than ambient/ cowl into the carb openings.

 

I will add that the hot air that exits the cabin heat valve( located in the engine bay and attatched to the frame)when cabin heat is not in use flows directly behind the engine and towards the carb heat opening at a pretty good rate and a very high temp. Neither of which I have measured, I am only guessing at the pressure based on the velocity of the air entering the cabin when the cabin heat is on. I guess you could try the carb heat with and without the cabin heat on to see if my hypothesis is valid.

 

If you look up, download and refer to the parts manual C2K/CTSW you will see how the carb heat system works.

[chanik]

The pressure does not change as a function of the incoming air temperature so the diaphragm position does not move. It is all at ambient pressure. The air density goes down though so that nudges the engine richer. The difference is slight because there really is not much temperature difference from the cowl air and ducted external air. Now in a steep, extended climb, the air in the cowl will have the greatest difference so that is why it is a good tip to check carb-heat operation.

[GlennM]

The Flying Bozo's explanation actually sounds quite reasonable to me since the Bing carbs are constant depression or constant velocity, whichever you prefer.

 

From what I understand, they adjust the venturi size to keep the vacuum constant (constant massflow) and meters the correct amount of fuel for that air mass. Massflow depends on the density of the air, which accounts for pressure AND temperature. Once you set the fuel mixture you want with the fuel jet assembly, no matter what altitude or air temperature, the fuel is dispensed appropriately to keep that mixture. It is really a nifty little mechanism. Very reliable and simple. As was stated elsewhere, the mixture chosen is very rich to keep you out of the very high temperatures and pressures around the stoichiometric ratio, which shorten engine life.

 

As others like to point out, the carb mixture is fixed to keep the mechanism simple. If you add electronics that sample other parameters, like O2 content of the exhaust gases (lambda ratio) with an oxygen sensor, you can adjust the fuel mixture on the fly (pun intended). This results in less fuel used since you can run the engine lean with the electronics. I believe this is one of the sources of the confusion over fuel injection - the electronic control. I like electronic control and fuel injection, but I can't support the notion that carburettors are bad or not good at their job.

[Ed Cesnalis]

Flying Bozo says that carb heat looks like an altitude increase to the engine so the bing will lean. The bing leans by a rubber diaphragm, motivated by pressure diferential raising the needle so unless the throttle setting gives mixture control to the needle jet there is no leaning. If the needle jet is controlling the mixture how would carb heat create the needed pressure differential that you get at altitude?

[GlennM]

When you activate the carb heat, presumably, the temperature of the air is also increased. The increased air temperature results in less air density, which reduces the velocity of the air going through the venturi. This results in less pressure drop throught the venturi. The reduced pressure drop closes the venturi (it acts on the back side of the diaphragm). The decreased venturi area increases the velocity of the air, increasing the pressure drop through the venturi. Since the venturi is connected to the tapered pin going to the fuel jet, the fuel flow is reduced as the venturi closes, keeping the mixture the same as it was before the carb heat was applied. That is why it is called a constant depression carburetor. The venturi adjusts to keep the same depression (pressure drop) at all throttle settings, altitudes, and temperatures. It is magic and a testament to man's mechanical ingenuity.

[Ed Cesnalis]

Glenn,

 

I am not a mechanic, so excuse me if I'm slow, but still I'm having trouble accepting what you say because of the little I know about our carbs. I think what you might be missing is that the needle only controls the mixture at certain throttle settings. Wide open throttle for instance, by design gives mixture control to the main fuel circuit and not the needle jet circuit. In that condition there is no leaning because the needle jet is not controlling the mixture and only the needle jet can lean the mixture when there is a pressure differential.

 

I am based at 7,180' and cruise at 10,000' or higher yet I burn over 5 gal/hour. I seldom see more than 69% power. I contend that I would be burning a lot less fuel if the leaning was working all of the time but for me it only works when my throttle is retarded and the needle jet gains control of the mixture and that happens mostly on descents otherwise I'm at WOT to get the 60+% power available to me at my cruise altitudes and that means no leaning.

 

I see it as less than magic, more a crude adjustment that runs into a design conflict above 7,500DA where you need WOT to get 75% or less power and as a result get no leaning when you would need it most.

[mocfly]

I just uploaded a pdf from Bing describing the Bing64.

 

For inquiring minds

[mocfly]

http://www.bingcarburetor.com/pdf/94x.pdf

 

This extended view shows the latest revisions in the carb.

[mocfly]

Bing Carburetor Technical Information

 

Adjusting the Bing Carburetor

The Bing Carburetor (36mm) is a three stage system: Idle, Midrange, and Top End System. From the moment the engine is started to approximately 1/4 throttle, the carburetor is running according to how the ldle System is set up. As the idler jet number decreases, the mixture gets leaner; (see part# 4), as the number increases, the mixture gets richer. For instance, ldler Jet #45 has a leaner mixture than ldler Jet #50. ldler Jets

The Air Regulating Screw (see part# 10) must be set as stated in the Bing Jet Chart in order to insure smooth operation of the ldler Jet. This screw adjusts the air/fuel mixture at idle speeds and for smooth acceleration. Turning this screw in a clockwise direction creates a richer mixture while turning it counterclockwise creates a leaner mixture. To adjust this screw, gently turn in a clockwise direction until the screw bottoms out, then loosen the screw (in a counter-clockwise direction) the number of turns as recommended in the Bing Jet Chart. For example, the ROTAX 503A would be .5 (1/2 turn) out (effective range 1/2 to 2-1/2 turns out).

Use the carburetor piston Adjusting Screw (see part# 11) to adjust the idle RPM. Turn this screw in a counter-clockwise direction until the Carburetor Piston (see part# 3) is in the lowest position. Then carefully turn the screw clockwise until it just engages the piston and then continue to turn it clockwise for2 to 2-1/2 full turns. This determines the idle RPM of the engine and should be set at 2,000 RPM.

The Midrange System affects the carburetor for approximately 1/4 to 3/4 throttle. Once again, the lower the number of the Needle Jet, the leaner the mixture.

Needle Jets, Jet Needles

(NOTE: Jet needle must always be under plastic spring cup)

The Jet Needle (see part# 2) has three grooves which the Holding Plate (see part# 3) can be snapped onto. The three grooves are numbered 1. 2, and 3 with number 1 in the top position (note: some jet needles have 4 grooves as in diagram #1 at right). When the Holding Plate (see part# 3); is snapped onto the top position (#1) of the jet needle, a leaner mixture is created than if it were mounted in the #2 groove.

1. lnstall sieve sleeve by slightly squeezing it by pushing it all the way up the mixing tube.
   
2. Smaller numbers indicate decreasing jet opening and leaner conditions.
   
3. The #54 marked on the side of the carb is just a casting mark.
   
4. Float guide pins should be polished to insure smooth float action.
   
5. Floats with aluminum guides should be replaced.
   
6. lnspect jet needle grooves for wear every 50 hours.
   
7. Float arm should start out parallel to the float chamber base.
   
8. Fuel pump must be in system to insure correct fuel level.
    
   

How to Check the Float Level

1. Start and warm up engine.
   
2. Run the engine at 3/4 power for 2 minutes.
   
3. Let engine idle back and cool down for 1 minute.
   
4. Shut off engine and fuel system (if the fuel system is an overhead system).
   
5. Remove float bowl.
   
6. Remove the floats from the float bowl while float bowl is sitting on a level surface.
   
7. The fuel level should measure approximately 1/2" from the top edge of the floatbowl.
    
   

Fuel level below 1/2" will cause a lean mixture.

Fuel level above 1/2" will cause a rich mixture.

[Doug G.]

Does anyone actually check their "needle jet grooves for wear every 50 hours?"

[Tom Baker]

Not at 50, but I do at the 200 hour inspection.

[WmInce]

Not at 50, but I do at the 200 hour inspection.

 

Tom,

 

In the course of doing those inspections at 200 hours, have you ever found any negligible wear?

[GlennM]

I do not claim to be an expert in carburetors either, but I was just presenting my understanding of them. This .pdf might be more appropriate.

bing64.pdf

[Tom Baker]

Tom,

 

In the course of doing those inspections at 200 hours, have you ever found any negligible wear?

 

Not yet, but I don't have many that I work on with more than 500 hours total. They did show us some in class that had quite a bit of wear.

[Ed Cesnalis]

I do not claim to be an expert in carburetors either, but I was just presenting my understanding of them. This .pdf might be more appropriate.

 

Glenn,

 

I think it takes 3 pieces of information to see how limited the leaning is.

1. leaning is accomplished by pressure differential acting on the diaphragm (part 16 in your pdf) which adjusts the height of the needle in the needle jet.
    
   
2. 'The Midrange System [needle jet] affects the carburetor for approximately 1/4 to 3/4 throttle.' - from mocfly's post on Adjusting the Bing Carburetor.
    
   
3. Beyond 3/4 throttle the main jet ( by design ) gets control of the mixture.
   

Conclusion; When flying above 7,500'DA and maintaining a cruise power setting using all available power ( less than 75% ) your throttle will necessarily be wide open, beyond the 3/4 throttle where the needle jet leans, and you will have no leaning available to you at altitude until you retard your throttle for descent.

 

912iS NOTE: A very similar limitation exists with the 912iS because it runs full rich at WOT as well. In order to achieve altitude compensating leaning you have to retard the throttle to access "eco-mode"

 

Flying a CT at 14,000' with 58% power available and no climb available it is likely that you will have your throttle fire walled (no leaning for you).

[mocfly]

Ct,

If you were experimental this system would give you the ability to lean.

 

The new mixture control was designed to use as a stand alone unit, or in conjunction with our automatic High Altitude Compensating (HAC) system. It functions on the same principal as our Automatic HAC which mixes and controls veturi and float chamber pressure, but with HACman, mixed pressure is determined by pilot input. In a manual system, it is paramount that control input results are gradual, and anticipated without wild fluctuation. The Green Sky manual control, works incrementally by conventional, precise, vernier-type input that is counterclockwise to lean. The fine vernier movement of our metering needle gives a motion rate of only .213 mm per half turn.

In testing, some notable advantages over automatic systems were discovered but the general benefit is an ability to richen or lean the mixture, at will, during flight. In recent testing, at 10,000' Density Altitude, climb rate was increased more than fifty percent by adjusting mixture for peak power which was about 25 degrees richer than peak EGT. Other testing was conducted to 14,600 ft Density Altitude. Range of adjustment is approximately 80 to 100 degrees F on EGT, which makes it possible to run in the "normal operating range" at most any density altitude extreme without having to rejet. HACman is a bolt-on installation for current production Bing 54 36mm caruburetors. Green Sky Adventures, Inc., can economically modify previous production carburetor bodies to accept mixture control.

Preliminary work is underway to develop a similar system for the Bing 64 Constant Velocity Carburetors used on HKS and Rotax 912 series 4 strokes. Prices start around $150 for the controller, lines, fittings, jets and clamps. For more information Contact: Green Sky Adventures, Inc.

888 887-5625 www.greenskyadventures.com

 

They seem to have figured out how to manually adjust the Bing carb.

 

[Roger Lee]

Hi haven't met a lot that have been happy with the Hacman. They have been around a while. I'm on a few other forums ans some have purchased to Hacman, but weren't as happy with the setup as they thought they would be. If you were interested I would do a good search and try and talk to some of these people. I personally have not tried the Hacman.

[mocfly]

I met a guy who put one on his rv-12 and liked it. He did mention that you need to be diligent in going thru a checklist on your descent or the engine will cough and stop.

Sounded like he had to do some additional piping to really get the most out of it.

Those rv guys can do things like working with things like this, unfortunately unless we change out designation we can't. Another reason I'm gonna go build a -14.