EGT SPREAD

[Jaba-who]

i still dont understand what the exact problem is? the actual numbers on the EGT are meaningless, as they can be effected by many variables giving different indications, as long as they are rising and falling at the same rate with changes in throttle position and mixture, then all is good. as said before, my EGT probes are just 1 inch from the exhaust valve, so read a lot higher than yours, and the front cylinders are in the cooling airstream, so read significantly less than the rear cylinders. this does not mean there is a mixture imbalance. just differences introduced by probe location conditions. a EGT probe 3 inches from the valve, will read far less than ones at 1 inch from the valve.

 

mine read in the 900's! all thats needed to know is, are you rich or lean of peak?

 

quick question,

 

what is hotter? 50 deg RICH of peak EGT, or 50 deg LEAN of peak EGT?

 

The only EGT spread that is of any significance, is the spread between Peak, Rich or lean of that peak in EGT between cylinders. the actual number of the peak is irrelevant, you just need to know when the peak is and which side of it your on.

Sorry but this not true at all. You are throwing in a whole bunches of variables which don't exist. The engine in question appears to set up as per standard jabiru set up. So the variables you keep mentioning are irrelevant. You keep mentioning temp changes with mixture changes - your avatar says you fly jabirus but you seem to forget the pilot can not change the mixture on a jabiru.

 

If - there is standardised position of the probes

 

If - there is standardised airflow around the exhaust

 

If - the jabiru standard test engine is the same set up as the engine in question

 

And it seems it is -

 

All of these seem to be true then yes there is significance in the raw numbers.

 

Jabiru have produced a huge bank of numbers when engines run ideally and when run non-ideally and converted these into ranges which they advise are acceptable and which are not acceptable.

 

The temps this engines cylinder four are achieving are not running in the ranges which jabiru says are acceptable so I can't see how you can suggest things are fine. None of the parameters you say make the temps ok exist in the situation we are talking about.

 

 

[Ultralights]

mixture and internal cylinder pressures are the only thing that can influence the ACTUAL exhaust gas temperature in the cylinder can reach over 2000degC, the probe location, airflow within the exhaust pipe itself can all alter the reading at the probe, which actually reads an average.

 

2 things influence engine reliability, Cylinder head temp, and internal pressures. especially with aluminium head engines, such as the Jabiru. as higher than normal temps do a strange little metallurgical thing called artificial age hardening, resulting in things cracking.

 

Yes, even a pilot of jabiru engined aircraft can influence the fuel mixture.. its called Carby heat. and you will be surprised how much it can change the mixture. with an understanding of the EGT probe data you can determine which side of the peak curve you are on with the addition, or removal of carby heat.

 

Considering the actual temps at combustion can reach 2200DegC, i seriously doubt that its possible to achieve combustion at all and keep temps below 800degC

 

flying a Jabiru, the only temperature i am concerned about is the CHT.. as this directly dictates the health of the engine.. detonation and pre ignition can be detected by monitoring CHT, whereas EGT will not. actually, a falling EGT and rising CHT is the signature of pre-ignition. this does far more damage than even mild detonation. and what i suspect is the contributor to most Jabiru issues.

 

http://www.gami.com/articles/egt_myths.pdf

 

 

[bexrbetter]

Today I took off the carby and had a look at things.The fuel comes in at 5mm from the throat bottom , which is only 88mm from the take-off to cylinder no.4 at the distribution box. You can see how tilting the top of the carby away from the hot cylinder brings the fuel plume closer to the take-off to the hot cylinder.I don't think the fuel has had time to mix uniformly with the airstream and no.4 is getting more air and less of the rich plume than is wanted, especially at full throttle.

What about putting a small scoop out from the no.4 take-off to divert part of the fuel-rich plume into no.4?

 

Think of the take-off hole to no.4 at the distribution box just downstream of the carby. Suppose a "finger" came out of this hole into the richer part of the airstream to divert some of this richer plume into no.4

 

Would this not richen no.4 at the expense of ( mainly) no.2? ( no.2 is the take-off downstream of no.4)

 

I can't believe that I am the first to think of this... does anybody know of experiments along these lines?

Yes Bruce, it's exactly how it works and is actually that logical and easy, i.e. fuel has a lot more weight and inertia than air wanting to travel straight when the air suddenly bends, eg; the air turns without the fuel (lean). These sorts of experiments to help deviate the heavier fuel's path will yield results and do mostly work in logic.

 

Not experiments either, V8 production and aftermarket intake manifolds have been doing it forever, usually a small dam is put at the entrance of typically rich cylinder's runner, on the floor of the plenum and sometimes a trench leading into lean ones. Other more technical tricks are used also.

 

Can you offer some drawings or good pictures of the intake manifold and layout?

 

 

[Jaba-who]

mixture and internal cylinder pressures are the only thing that can influence the ACTUAL exhaust gas temperature in the cylinder can reach over 2000degC, the probe location, airflow within the exhaust pipe itself can all alter the reading at the probe, which actually reads an average.2 things influence engine reliability, Cylinder head temp, and internal pressures. especially with aluminium head engines, such as the Jabiru. as higher than normal temps do a strange little metallurgical thing called artificial age hardening, resulting in things cracking.

Yes, even a pilot of jabiru engined aircraft can influence the fuel mixture.. its called Carby heat. and you will be surprised how much it can change the mixture. with an understanding of the EGT probe data you can determine which side of the peak curve you are on with the addition, or removal of carby heat.

 

Considering the actual temps at combustion can reach 2200DegC, i seriously doubt that its possible to achieve combustion at all and keep temps below 800degC

 

flying a Jabiru, the only temperature i am concerned about is the CHT.. as this directly dictates the health of the engine.. detonation and pre ignition can be detected by monitoring CHT, whereas EGT will not. actually, a falling EGT and rising CHT is the signature of pre-ignition. this does far more damage than even mild detonation. and what i suspect is the contributor to most Jabiru issues.

 

http://www.gami.com/articles/egt_myths.pdf

Sorry but again you are not seeng the actually picture but relying on a theoretical discussion in an ideal engine.

 

In jabirus there is a significant problem with fuel distribution. Typically different cylinders receive different amounts of fuel in the fuel air mix.

 

Jabirus aren't alone here - by the way. If you put EGT on all cylinders of a lyc they do similar - just that big heavy cylinders don't get negatively effected as much.

 

Changing the mixture by some global change ( such as carb heat) may richen the global mix ( in theory) but it doesn't always. But what it does do usually is simply retain the spread of fuel mix to different cylinders but makes each a bit richer. Sometimes it makes the spread worse because the droplets of fuel weigh more than the air and contain more inertia so they tend to travel forward further while the leaner air fuel mix can bend more and make the diversion to the back cylinders. Apart from being furthest back in the ram duct air blast - this is why rear cylinders are usually hottest.

 

Jabiru have changed the shape of the plenum chamber a few years back to try to induce more change in direction of the fuel droplets but I think with limited success.

 

The changing of the aero foil shaped vane in the centre of the chamber to a circular cross section post has been said by some to help - presumably by further disrupting the airflow to a more chaotic impact or maybe higher direction change from increased choanda effect ( where a fluid follows the shape of a surface rather than follow a straight line despite its inertia)

 

Tilting the carby has similar effect as it changes the angle the flow stream impacts the vane.

 

 

[Bruce Tuncks]

Thanks guys, I really appreciate the comments.

 

Ul, the probes are all at 11cm from the flange, measured along the centerline of the pipe. There is not much airflow in the lower cowl past the probes. This cylinder has always been the hottest one on CHT, but the CHT's are ok generally less than 150C.

 

But this cylinder was the one which had the exhaust valve seat fall out, so it does have a history of real problems. ( since this, I got a new head for no4 and naturally want this one to last)

 

On tilting the carb: I forgot to say that I had done this to the extent possible, which is about 5 degrees. The top was tilted away from no4, which brings the below-center outlet closer to no4. It made no difference. Also checked the float level in the carby bowl and nothing was wrong there. I'll take some photos soon.

 

 

[Jaba-who]

Thanks guys, I really appreciate the comments.Ul, the probes are all at 11cm from the flange, measured along the centerline of the pipe. There is not much airflow in the lower cowl past the probes. This cylinder has always been the hottest one on CHT, but the CHT's are ok generally less than 150C.

But this cylinder was the one which had the exhaust valve seat fall out, so it does have a history of real problems. ( since this, I got a new head for no4 and naturally want this one to last)

 

On tilting the carb: I forgot to say that I had done this to the extent possible, which is about 5 degrees. The top was tilted away from no4, which brings the below-center outlet closer to no4. It made no difference. Also checked the float level in the carby bowl and nothing was wrong there. I'll take some photos soon.

Try tilting the carb the other way as well.

I'm sure on my previous 3300 tilting the top TOWARD the hottest side made a drop on that side. On my current 3300 hasn't made any difference either way.

 

 

[jetjr]

You can only tilt a little on one direction, forgotten which way

 

The egt numbers presented AND those on the POH and maint manual, which we must abide by, are in a set of certain conditions. Distance from exhaust port, type of probe etcetc

 

The combustion temps may well be high but not relevant as we are largely comparing the same data for different engines of the same make.

 

800 measured at the correct location is too high

 

Results like Bruce is seeing are quite typical, if youll notice often you have the highest and lowest on the same side, and the rest of engine sees numbers centred between these outliers. The correct volume of fuel is there just going the wrong place.

 

To achieve the same change in mixture and resultant egt would require dramatic jetting changes and big reductiosn in fuel flow. Like dropping flow from 27lph to 19lph. Jabiru tried this with an economy jetting kit. Many engines failed with burn valves and overheating problems.

 

This was reversed and new jetting recommended due to damage caused.

 

The whole argument of LOP cannot be used sucessfully unless mixtures are even.

 

Yes carb heat can influence mixture but not incrementally, it goes very rich and sees reduction of power. Hardly mixture control. Also the self adjusting bing doesnt do much of a job either.

 

 

[Jaba-who]

Jetjr said

 

"Also the self adjusting bing doesnt do much of a job either."

 

Sadly I have to agree. I was always dubious about how good it was and then I confirmed it when I got the actual Bing manual from Bing and read it cover to cover.

 

I forget the actual altitude change it can cope with but its only about 4500 ft as best I recall. So what you are supposed to do is set it up to your regular altitude that you fly at and accept that when you fly outside of the envelope of ( if I have recalled correctly) +\- 4500 ft of that then it becomes progressively out of mixture.

 

Since I fly ( and I suspect everyone else) at lots of altitudes well beyond + 4500 ft from an average I think it's highly likely we are often flying with a badly running carby.

 

I know Rod Stiff's idea is to make it simple for pilots and to remove poor decision making from the loop - but I personally think if your smart enough to master everything else to the level of getting a licence then you're smart enough to lean and richen a mixture.

 

But if running lean has been proven to be problem for lots of engines - as it was with the economy kit of a few years back that they had to recall - then I guess I can understand his reluctance to let pilots go out and run lean etc.

 

But even if people do realise the issues of jabs uneven distribution I accept -seen even by reading this thread - that people can get distorted information and think they are doing the right thing. Especially when they can be led astray by information that is true about other engines ( big heavy cylindered engines) but doesn't necessarily apply to light literally "running on the edge" engines like the jab.

 

 

[aj_richo]

I have an Aerocarb/Aeroinjector on my 2200 (2012 hydraulic lifter), which gives manual mixture control.

 

Instrumentation is a Dynon 180 with 4 x EGT/CHT and fuel flow.

 

I find the fuel distribution on mine at 2850/2900 RPM tends to run rich on 3 and 4, leaner on 1 and 2. Number 1 is the leanest.

 

Maybe because of the longer intake tubes.. I dunno.

 

#3 CHT is always the hottest. I should take some air from #2 and duct it across the engine to #3.

 

I manage fuel flows using the mixture control, I expect to see 25Ltrs per hour on takeoff and EGTs below 700. If they creep over I tend to back the throttle off slightly.

 

Cruise I will lean to around 15-16 litres per hour, any more and it will start to give little burps. EGTs #1 will be around 700-710C, #2 will be around 680C, 3 and 4 around 640C

 

The early aerocarbs are a little rough with their manufacturing tolerances and RPM/mixture tends to wander a bit.

 

This is a shot of my panel cruising at 3500' to Ehuca last Saturday. Fairly typical numbers, I think OAT was around 12C.

 

 

 

[Jaba-who]

I have an Aerocarb/Aeroinjector on my 2200 (2012 hydraulic lifter), which gives manual mixture control.Instrumentation is a Dynon 180 with 4 x EGT/CHT and fuel flow.

I find the fuel distribution on mine at 2850/2900 RPM tends to run rich on 3 and 4, leaner on 1 and 2. Number 1 is the leanest.

 

Maybe because of the longer intake tubes.. I dunno.

 

#3 CHT is always the hottest. I should take some air from #2 and duct it across the engine to #3.

 

I manage fuel flows using the mixture control, I expect to see 25Ltrs per hour on takeoff and EGTs below 700. If they creep over I tend to back the throttle off slightly.

 

Cruise I will lean to around 15-16 litres per hour, any more and it will start to give little burps. EGTs #1 will be around 700-710C, #2 will be around 680C, 3 and 4 around 640C

 

The early aerocarbs are a little rough with their manufacturing tolerances and RPM/mixture tends to wander a bit.

 

This is a shot of my panel cruising at 3500' to Ehuca last Saturday. Fairly typical numbers, I think OAT was around 12C.

 

[ATTACH=full]42558[/ATTACH]

I think I will investigate them.

 

 

[jetjr]

Theres a range of experiences with these and Rotec version. Aerocarb much cheaper but other AU made in Melb.

 

Some have tried and taken off, others very happy

 

Other option is leaning unit which adds little vacuum to carb bowl slightly leaning mixture. Have recently seen some success with this unit for altitudes over 4500. Very simple, just a needle valve and vac from downstream of carb. Can cost around $100.

 

Wouldnt try without full EGT as you'd never know if one cylinder was too lean.

 

 

[Guest rv6ejguy]

You could simply remove the single point fuel delivery and change to EFI, enabling you to safely run LOP with all cylinders at the same AFR. Not cheap but it solves uneven mixture distribution once and for all.

 

 

[Oscar]

I agree that multi-point efi would be a huge step forward - and Jab. engines for drones used by the Israeli drone manufacturers have this already, they come out of the engine factory with multi-point injector housings installed in the inlet tubes - BUT you can't use them on a certificated / certified aircraft - only an e-LSA / 19-reg / VH-experimental. This is not a case of intransigence by the manufacturers, but a failure by the regulators to adopt an intelligent approach to engine reliability risk assessment. Multi-point, egt-responsive fuel mixture control solves WAY more problems than it potentially introduces, but the regulators have an aversion to allowing any 'new' risk into the matrix - instead they prefer to continue with 'known' risks for which they have established 'fixes'.

 

 

[facthunter]

The only way to get even fuelling is to treat each cylinder separately. No manifold will distribute fuel evenly. At each split there will be a mixture bias. Injection close to the port is the answer. I'm not convinced it has to be EFI. Mechanical systems can be just as suitable and don't require electrical power. Nev

 

 

[Oscar]

The only way to get even fuelling is to treat each cylinder separately. No manifold will distribute fuel evenly. At each split there will be a mixture bias. Injection close to the port is the answer. I'm not convinced it has to be EFI. Mechanical systems can be just as suitable and don't require electrical power. Nev

Only, if you can guarantee equal air delivery to each cylinder.

 

 

[jetjr]

If only there was a EFI system that allowed for individual cylinder trim on 6 cylinders........

 

You still there RV6ejguy?

 

Our dollar is looking up right now

 

 

[facthunter]

Where do you stop? The fuel is the big variant due to stratifying of the mixture. The actual manifold Pressure at each port won't have variation of a large order, particularly it the port velocity isn't high. Even the flow in the exhaust system will have a small effect. Your EFI doesn't guarantee anything is better in this respect. Gami type injectors can be tailored to the situation on each cylinder. Nev

 

 

[jetjr]

The efi Im elluding to, allows individual tuning of injectors aimilar to gami.

 

Can also be done in flight i think

 

 

[Bruce Tuncks]

Here's the setup. I guess it looks old-fashioned to guys with new planes.

 

The first pic is the underside of the distribution box.

 

The second pic is the crude and apparently quite ineffective flow straightener in the SCAT hose coming into the carby. The third pic is the ineffective "turbulator" wire in the carby outlet. The last pic is the internal dimensions of the inlet flow system.

 

 

 

 

 

[bexrbetter]

I would start some experiments with a bit of tube, cut on an angle, extended into the plenum to literally 'catch' some fuel.

 

 

 

[KRviator]

The efi Im elluding to, allows individual tuning of injectors aimilar to gami.Can also be done in flight i think

The SDS EFI can trim the fuel to specific injectors +/- 10% in flight. It's something I'm looking at retro-fitting to the RV when I get the flight testing out of the way. I've only flown it twice and decided I need more speed!

[Guest rv6ejguy]

With EFI, you can trim the fuel delivery to each cylinder individually at any MAP/ RPM setting. Can't do that with mechanical systems. On certified aircraft you're out of luck though...

 

 

[Bruce Tuncks]

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

 

After the things I've tried that did nothing, I thought it was time I did something too much. What do you think this would do?

 

On the sketch, the long tail bit with the hole is to fix the thing where the bolt will not interfere with the rubber joiner bit.

 

 

 

[aj_richo]

Assuming you're making it out of metal, Id be worried about potential fatigue and sucking parts it into the cylinder..

 

 

[Guest rv6ejguy]

If only there was a EFI system that allowed for individual cylinder trim on 6 cylinders........You still there RV6ejguy?

Our dollar is looking up right now

We've investigated doing this but given the demand so far, not sure the price point would be palatable for 3300 owners. Estimate if would add almost $1K US to the price of a system. Lycoming 540 folks might pay this but no so sure about Jabiru 3300 folks. Involves a second ECU board and a lot of new software development to make this come about.