EGT SPREAD

[jetjr]

Bruce I see what I was saying about downstream deflector is wrong, yours is newer one piece sump/plenum, supposably an improvement?

 

Re 6 cyl fuel trim- sad it cant be done. Yes another $1K would make it hard to justify.

 

I thought we discussed it might be possible for fuel only ECU?

 

 

[bexrbetter]

Thanks Bex, I had something much more drastic in mind. Like a finger coming out of the no4 pipe right into the stream..

I understand your finger idea, I've known about these things since I was a teenager.

 

It's way too long and it would flutter with the sonic pulses and break off in short time.

 

My tube idea only protrudes in a few mm's, prob 5 max and also allows you to rotate it and I suspect that having the pointy end closer to the plenum floor will be required rather than at 90 degrees (fuel falls quite quickly even in fast flowing air and it's already starting at the bottom of the stream so it would be literally coming in along the plenum floor) and of course it's strong.

 

You could put a lip on the end of the tube to be trapped between the manifold stub and intake pipe and tight enough in the rubber connection to not rotate on it's own.

 

 

I am interested why #3 isn't running at the same mixture as 4, it should be in theory so have a good look for some issue between the carb and plenum maybe and of course carefully check #4 intake tract for leaks or other interferences.

 

It is quite possible that the differences in length of #3 and #4 tracts are sending signals back to the mouth's area that's perfectly timed to cause this issue. That would need a bit further thinking out if this simple fix doesn't help. For that I would need all 4 of the inlet tracts lengths from the mouths to the manifolds flange along the centreline as best as to the mm and I can do a bit of math to try to guess what's going on.

 

 

[Bruce Tuncks]

Hi jetjr, that new-looking sump-plenum is actually quite old, serial number 425. I think I am turning the clock back on you. The more modern setups are better I reckon.

 

Thanks Bex for the offer of analysis, I will measure up those pipes today. I can see that no3 is cool compared with no4 and therefore my thinking must be off-track somehow. On one take-off I saw 805 for no4 and 677 for no3 , and yet the take-offs are opposite each other and closest to the carby. There must be a swirl or something in jetjr's list that I am not doing right, as symmetry would indicate no3 and no4 should be equal, except for the slight difference in the lengths of the pipes.

 

On the force on the proposed finger-diverter: ( Caution..there could well be a big error in the following...)

 

If you assume the air has fixed volume and no inertia: The flow into the distribution box will look like a full rectified sine-wave. It will have instants of zero flow and then only flow into one outlet at a time.

 

3000rpm=50rev/sec. at 2 suctions/rev and each suction is 550cc. So averaged flow-rate is 50 times 1,100=55,000cc/sec

 

Average speed of flow through a 35mm pipe = 55,000/pipe area in sq cm=57160 cm/sec=57m/sec

 

Conservative average force on a 2sq cm plate in 57m/sec = 1/2 x rho x 57 squared x Cd x A =.39N since rho= 1.2 kg/sq m and A=2/10000 sq m.

 

I was surprised that there is a real risk of the finger breaking off. I thought the risk was having no4 running rich and the other 3 running lean. Right now, that result would mean progress. I would just trim down the finger .

 

Well I'm missing something big here. I can imagine the steady-flow situation, and with more difficulty I can imagine the stop and go situation, but reality is somewhere in between and complicated by reflections and standing waves and overloading my brain.

 

 

[dlegg]

Well I'm missing something big here

Bruce, as per Jetjr post #6, forced induction via naca duct on cowl through air mixer box has a major influence on tuning. I found with my 3300 that disconnecting the naca duct tube, blocking it off and only drawing air through from internally in the cowling, which also raises the inlet temp, evened out EGT's dramatically. I also removed all scat hose, used a radiator hose between air box and carby, fitted a divider. Being happy with this result, I then fitted a hacman leaning valve to the carby. It works above 5000 or so and at 8500 I lean off to 50 deg LOP, watch EGT's rise to about 680 and CHT's drop below 130 from 150 or so, fuel flow drops to 15 lph. This at 2900 rpm.

 

I wasted many hours trying to address the uneven egt problem, then removed the hose between the cowl naca and airbox.....

 

 

[bexrbetter]

Well I'm missing something big here.

Pressure sound waves being reflected against the valves when they close and shoot straight back up the intake tracts. Not only do they create a pulsating sound wave that will break your finger off eventually, they are important to tuning, you can increase hp, lose hp and upset mixtures depending on the match of the tuned length to the rpm.

 

 

[dlegg]

This is a great primer for tuning the Jab

 

Tuning the Bing Carburetor on Jabiru Aircraft Engines

 

The Bing 94 CV (constant velocity) carburetor found on Jabiru aircraft engines is designed to

 

deliver a fuel / air mixture to the engines that is appropriate for the load demand on the engine

 

and the operational altitude. The carb generally does this job quite well and very efficiently.

 

However, the carburetor must be properly configured to provide the desired results.

 

The mixture that the carburetor provides to the engine is highly dependent on the load placed

 

on the engine. Loading comes from airframe drag and propeller diameter and pitch. The more

 

load the richer the engine will run. Conversely, lighter load delivers a leaner mixture. This

 

phenomenon can be easily demonstrated with an adjustable propeller. Setting the prop for a

 

low pitch (Climb prop) yields higher rpm at any given throttle setting and a lean mixture.

 

Increasing the pitch (on the same airframe) will result in lower rpm per throttle setting and a

 

richer mixture.

 

Properly tuning the carburetor requires installing the correct jetting and/or propeller pitch so

 

that normal operating parameters can be achieved. Normal operating parameters mean rpm

 

on climb out at normal climb attitudes within the 2750 to 2900 rpm band. These climb rpm’s

 

give the best combination of HP and torque to achieve best all around performance. There may

 

be a few airframe combinations that cannot achieve these numbers as normal but most will fall

 

in the range.

 

Tuning is an exercise in trial, adjustment and retrial until the best combination of jetting is

 

achieved. While engine monitors that report all CHT and EGT are not required for a tuning

 

exercise to be successful, full engine monitoring is highly desire able and makes tuning much

 

more precise. Tuning will require more information than can be provided by a single EGT

 

probe. Here is how we tune carburetors at Jabiru USA.

 

1. Make sure the propeller will allow proper rpm. We check this by observing rpm as we

 

are rolling down the runway at take off power. We do not do static run ups as the

 

information we need can more easily be obtained while the aircraft is on the go and we

 

don’t have to worry about overheating and other tie down issues. The minimum target

 

in this phase is 2650 rpm with 2750 desirable.

 

2. Then we observe rpm in a normal climb out at or around Vy. The minimum target here

 

is 2750 rpm with 2900 rpm as the desired result.

 

3. During the climb out phase at full throttle we observe EGT’s. The target for the center

 

of the range is 1225. We might expect to see one cylinder near 1150 and another near

 

or even above 1300 but the center of the range would still be 1225.

 

4. After reaching a safe cruise altitude we set rpm at cruise power at 2850 rpm (the range

 

can be 2750 – 2950 but we find that on Jabiru aircraft with 3300 engines that 2850 is

 

about right). We again observe EGT’s after temps stabilize in cruise. For 2200 engines

 

the rpms should be 100 rpm higher than mentioned here.

 

5. We then reduce throttle as we observe EGT’s to find the peak EGT rpm. Usually EGT’s

 

should increase as throttle is reduced and at a certain throttle setting will begin to

 

decrease. We expect peak EGT’s to occur at 2600 – 2700 rpm.

 

6. One final observation involves comparing EGT’s at cruise power from the left side of the

 

engine to those from the right side - cylinders 2, 4 & 6 VS cylinders 1, 3 & 5 on the

 

right.

 

Once back on the ground we change jets in the carburetor or adjust pitch setting in an

 

adjustable propeller to try to achieve the targets mentioned above.

 

1. Climb out: Since the amount of fuel delivered to the engine at throttle settings of ¾

 

open or more are controlled by the main jet, we make a change in the main jet to come

 

closer to our target of 1225. If climb our EGT’s are higher than target the mixture is too

 

lean and a larger main jet is indicated. If EGT’s are lower than target the mixture is a bit

 

rich and a smaller main jet should be tried. Remove the main jet to see what size is in

 

the engine. Main jet sizes are a three digit number and most likely 255 for a 3300 and

 

245 for a 2200. Adjust up or down by 5: ie 255 to a 250 or 250 to 255.

 

2. Cruise: At settings of ¾ throttle or below the limiting factor for fuel delivery is the

 

needle jet. Cruise flight should be at settings at or below ¾ throttle in most cases. The

 

target for the center of the range for cruise flight throttle settings is 1325 F. Based on

 

your EGT observations in cruise flight install a larger needle jet if temps are above the

 

target or a smaller needle jet if temps are below target. Needle jet sizes are numbered

 

in x.xx format. You may find a 2.85 needle jet in your carburetor and will need to try

 

perhaps a 2.88 if temps are a little high or a 2.90 jet if temps are quite high. If cruise

 

temps are too low then a smaller needle jet is indicated. Since cruise power setting is

 

where the aircraft will spend most of its time make sure you take the time and make the

 

effort to get cruise mixture settings right!

 

3. Balance: If you have observed a consistent difference in temps from side to side (cruise

 

flight only) EGT’s can be adjusted from side to side by tilting the carb a few degrees with

 

the bottom of the carb rotating toward the hotter side. This procedure helps correct for

 

a spiraling airflow through the carb. Fuel is picked up into the airstream at the bottom

 

of the carb throat. As the flow speeds into the intake manifold it is split left and right by

 

a symmetrical airfoil called a diffuser. If, for instance, the airflow spirals in a clockwise

 

movement as it progresses through the carb throat it will pick up fuel at the bottom of

 

the throat and begin moving the fuel to the left. Before the fuel is evenly spread

 

through the incoming air stream the flow hits the diffuser and splits left and right. The

 

result is that the fuel /. Air mixture delivered to the left side (cylinders 2,4,6) has more

 

fuel in it (richer) than that delivered to the right side (1,3,5). The right side will be leaner

 

– therefore hotter EGT’s. By rotating the bottom of the carb toward the right (hot side)

 

you move the fuel pickup toward the right. The spiral is still there but as the flow is

 

carried to the left the fuel distribution is more centered as the flow hits the diffuser and

 

splits. Carb rotation is limited by space between the distributor caps but differences of

 

up to a 100 degree F average difference can be corrected.

 

4. Next step is to go fly and repeat the observations listed in steps 1 – 6 at the beginning of

 

this advisory and making additional changes if warranted.

 

Some airframes are more difficult to tune and some of the rpm parameters do not apply. An

 

Arion Aircraft Lightning, for instance, will not achieve the desired rpm on the take off roll. Since

 

it is such a clean airframe a low enough pitch prop to give 2750 rpm on the take off roll will

 

allow a prop over speed at full throttle in level flight. Conversely a Zenith 701 is so draggy that

 

a 2900 rpm setting on the takeoff roll will result in a full throttle level flight rpm of only 3050.

 

However, most airframes will fall in the usual range.

 

If your aircraft cannot achieve rpm in the normal range a change of prop may be required

 

before you can really tune the engine.

 

Why bother to tune?

 

Engines last longer and produce more power if they run smoothly. Uneven fuel distribution

 

causes uneven power production from the cylinders causing a rougher running engine. Wear

 

effects on valves can increase on cylinders that run lean. Ring seal problems and sticky rings

 

can be caused by a too rich cylinder. For a longer lasting more reliable engine take the steps

 

needed to optimize your installation and tune it to your own individual combination of airframe

 

and propeller.

 

See the Jabiru engine manuals for carb cutaway drawings. Visit www.bingcarburetor.com forcarb technical info or to order carb parts and jets.

 

 

[jetjr]

Jabiru usa have a couple of good resources

 

This is one, the other is on cooling duct tuning

 

Interested in your experience sucking air from inside cowls, do you run like this full time now?

 

How did you block naca duct off

 

I saw one where the naca was covered and changed to just a round hole. Seemed to help a lot

 

 

[Bruce Tuncks]

Yes they do good stuff, I note that 1300F =704C so my 800C is hot by their standard. Alas I have tried the easy stuff like tilting the carby, with no effect so far.

 

One thing I haven't done yet is to remove the scat hose from its external source to try just sucking from inside the cowl. On my older plane the supply comes not from a NACA duct but from a forward-facing scoop. This could well be worse than the NACA inlet, even though the air-filter box has a rubber flap to allow air out of but not into the box.

 

 

[Oscar]

Yes they do good stuff, I note that 1300F =704C so my 800C is hot by their standard. Alas I have tried the easy stuff like tilting the carby, with no effect so far.One thing I haven't done yet is to remove the scat hose from its external source to try just sucking from inside the cowl. On my older plane the supply comes not from a NACA duct but from a forward-facing scoop. This could well be worse than the NACA inlet, even though the air-filter box has a rubber flap to allow air out of but not into the box.

Bruce, that rubber flap is there to prevent backfires blowing the airbox apart / the scat inlet tube off.

 

NACA ducts are not particularly good at scooping air against any back-pressure; they are a low-drag way of allowing air to move from a higher-pressure area into a lower one.

 

The worst problem for Jab. induction, is swirl from the airbox and cobra-head, any flow-straightener in the scat tube will not do anything useful. It took the guys who transplanted a 2200 into a motor-falke a great deal of trouble with flow-straighteners etc. to try to get fairly even chts and egts at all throttle settings; they had the whole inlet tract extensively instrumented, and did many, many test flights over almost a year. The whole issue almost becomes a 'black art', without really serious testing regimes, to get a decent balance across the operating range. The Jab. USA stuff is probably a very good place to start with your experimentation.

 

 

[facthunter]

The even fuel distribution IS a black art. Streamlined ducts seem to be the worst to get right. The divider is critical and doesn't work over the range.

 

Re the Jabiru USA notes. The butterfly position itself doesn't control mixture. The position of the dashpot does, in conjunction with the taper of it and the size of the needle jet. It is positioned by the pressure drop across the area it occupies. If the prop is too big (and engine revs too low) it will not go all the way up to full open and the same goes for the situation at height. Say above 5,000' where atmospheric pressure is dropping. This is where the so called altitude compensation comes in. Nev

 

 

[Bruce Tuncks]

Oscar, thanks for that comment... I've never had a backfire and didn't think of that reason, and just supposed it was there to stop excessive pitot pressure getting to the carb.

 

On flow-straighteners in the scat hose ... I once saw a small wind tunnel where there was a section filled with drinking straws between the fan and the test-chamber.

 

I have actually thought about filling the scat hose with drinking straws, surely this would get rid of any swirl. My guess is that the frictional pressure loss might rob too much power.

 

Changing the scat hose to a radiator hose seems like a good idea too.

 

 

[facthunter]

Any turbulence in the flow will reduce the flow rate. That is a pity because it's probably what is needed for better mixing. Nev

 

 

[jetjr]

Ive found , yet to fit, a silicon hose with 45 deg bend. Upon sample fit its perfect to replace scat and cobra head.

 

I will loose the cross vane in the cobra head but one at airbox exit will still be there

 

Im told silicon hose wont burn and is a good choice. Bit thick and heavy.

 

Can get them in really bright colours which would no doubt help too.

 

Im told the airflow through Bing is up at limits on 3300 engine and this contributes to problem too.

 

Im keen on the info regarding Hacman style leaning and thats its working LOP.

 

 

[Oscar]

JJ - just before you throw on that silicone hose...

 

An engine is a remarkably good vacuum pump, upstream of the cylinders. The reason that SCAT rather then the smoother SCEET tubing is mandated, is because of the possibility of the inner liner detaching and blocking the air inlet, due to vacuum. SCAT does have some resistance to vacuum ( though not much). I like silicone hose - BUT, if it can become pinched, then the vacuum could close it off almost entirely. The possibility of a constriction in the primary air entry route, is the reason why the hot air bypasses the air filter - it's the 'fail-safe' for induction air.

 

 

[jetjr]

good point but being 45mm ID? and just 100mm long. Its also around 7-8mm thick, Id think very unlikely to pinch enough. Id squash SCAT easier than this stuff.

 

Im more concerned its too stiff as its now the joint between engine and firewall.

 

Pretty sure Ian says he has seen one installed with good results.

 

The hot air in most Jab setups doesnt bypass filter?, a good feature but I can see if element sucked in it would stop the noise.

 

I disagree with the bit you deleted,

 

 

[Oscar]

Well, that's why I deleted it!. Didn't fit the circumstances, you were ahead of me already.

 

I can only comment on the early airboxes, but it's normal practice for GA that the hot air will bypass any filter.

 

I agree that silicon hose SHOULD be a better option, but there's a reason that there are requirements for negative-pressure handling. As long as you don't have a serious kink point, I personally believe that silicon hose is a good alternative; the safety factor is probably if the silicon hose is moulded for the turn.

 

 

[jetjr]

Its interesting to note that even if you edit responses the original is emailed out to everyone.

 

Yes its moulded in 45deg, no kink.

 

I think its 100mm x 100mm and for this it will need 70mm cut of one end and maybe 30 off the other

 

 

[Oscar]

There's probably something in the FAR's that doesn't approve it - but personally, I'd use it...

 

 

[jetjr]

Yes an approved one would be 1950 materials technology and much more expensive

 

 

[facthunter]

Anything used downstream of a supercharger would be fine. Nev

 

 

[jetjr]

Not sure the Jab would cope

 

 

[Bruce Tuncks]

Today I found some 10mm diameter drinking straws and I plan to try them in the SCAT hose . I'm getting desperate for something to work.

 

They cost $2 at a cheapo shop.

 

 

[Oscar]

Bruce, I seriously think that's a bad idea. If you do try it, please fly off - if you get that far - in conditions where you can pull carb heat and still climb to a safe height and land again with carb. heat on - assuming that your carb. heat bypasses the air filter completely.

 

May I respectfully suggest at least a primary test, off the aircraft? Take the SCAT hose off, and attach it to a strong vacuum cleaner , set it up with a decent approximation of whatever bend is normally in the SCAT hose ( and secure both ends somehow so that the SCAT hose remains approximately in the same curve as installed; a couple of saddle clamps, a a bit of board and some racer-tape may suffice for that). Run the vacuum cleaner and note the effort introduced by the SCAT tube. Then, set up your straws in the SCAT hose and repeat the test and see how much extra load (if any) that places on the vacuum cleaner, and whether that load tends to distort the SCAT hose.

 

A vacuum cleaner doesn't produce a hell of a lot of vacuum, but it'd be a better than nothing test. An even better one than that, would be to use a real engine of around the same capacity - a Subi 2200 engine would be perfect - and run it at the same revs as your Jab engine runs i.e. up to around 3200, so it's flowing the same air volume.

 

 

[jetjr]

Got to agree with Oscar, i dont like this idea.

 

Too much chance a straw will come loose or melt and block or jam carb

 

Theres some honeycomb panel you could try instead, but anything in intake has significant risk it can come loose and twist, blocking hose or worse the carb

 

The cross vanes i tried are made in fibreglass and flocced to airbox and the hose adapter infront of carb, cant come loose., also very thin and low restriction

 

Try the safe easy options first, remove airfeed scat, maybe even an air pod direct on carb

 

 

[facthunter]

It won't at no load. Air/fuel volume relates to power out, not rpm. RPM can be high at very small throttle openings. (Less than 5%). without load. Nev