EGT SPREAD

[Bruce Tuncks]

I've just got a 4 channel EGT digital readout for my Jab 2.2 after many years of no EGT gauge and got a bad surprise.

 

Yesterday I measured on climb, Cylinder 1=724 degrees C, 2=690,3=714 and 4=786.

 

On cruise, 1=705, 2=672, 3=668 and 4=741.

 

Thats 96 degrees difference between the hottest and coldest. And I thought cruise was suppesed to be hotter than climb.

 

Today, I put an aluminium vane ( a rectangle of width equal to the internal diameter of the SCAT hose ) into the inlet to the carby and measured these...

 

Climb : The hot cylinder 4 went to 794 degrees The others were quite a bit cooler, just like the previous day.

 

On cruise: 1=714, 2=667, 3=692, 4=715

 

So my "vane " seemed to help on cruise but made the climb worse!

 

An indication that no 4 is not quite right came from the valve seat falling out, something I still blame on a ground run mistake, but now there seems to be a contributing factor.

 

 

[billwoodmason]

Bruce your temps would appear to be on the hot side. What did you use to record the temps? Did you get advice on the location of the probes and installation etc. Good luck with it.

 

 

[Ultralights]

actual temperature recorded by the probes is irrelevant, as the temperature reading is influenced by probe position, distance from exhaust port, airflow around the exhaust pipes under the cowling eg, my EGT probes are 1 inch from the exhaust port, so of course, they will read very high compared to ones installed 2 or 3 inches from the ports. also, the front cylinder probes on my engine are in the cooling air path so read significantly cooler than the aft ones in the same position in the exhaust pipes.

 

, the purpose of the probes is to be able to indicate when you are at peak EGT and what temperature either rich or lean of that peak, and it can be a great diagnostic tool when you learn how to interpret changes in those figures.

 

considering the Jabiru engine, and the Rotax counterparts, do not have manually adjusted mixtures, then telling weather your X deg rich or lean of peak EGT is not really a concern, i have the 4 EGT probes installed so i can diagnose problems before they become bigger issues. eg, diagnosing a dying or weak spark plug, or intake manifold leak.

 

the only way we can influence EGT with mixture is via carby heat.

 

and just a quick question, whats hotter, 50 deg Rich, or 50 deg Lean of peak EGT?

 

 

[Bruce Tuncks]

The probes are Dynon type as sold by Jabiru and they are all at 11cm from the flange on the exhaust pipe.

 

I think the readings are accurate from checking them with boiling water ( 100 C) before installation. Also before starting the engine they all read the same correct ambient ( 26 degrees today) .

 

It would be easy to swap the no 4 sender to another channel and see if this did the same reading. I'll try that.

 

 

[Ultralights]

if they are all reading the same ambient temperatures before start up, then i dont think there is anything wrong with them. the otehr 3 pipes may just be getting better air flow and a little more cooling of the pipes. what matters is they all rise and fall at pretty much the same rate. the actual numbers mean nothing. as there is way to many variables at play that influence the actual reading at the probe.

 

mine, being just over 2 cm from the exhaust port read around the 850 deg mark. and around 700 at idle. the aft cylinders, about 25 deg warmer.

 

 

[jetjr]

What youve seen is pretty common, 100 deg variation is what I had for years.

 

Tried lots of fixes. I believe the problem is in the intake upstream of carb. Maybe even upstream of filter

 

The variable spread changing with rpm is due to turbulence or swirl changes as air speeds up

 

My target is 700 deg and no higher except maybe when in descent and even then no higher than 740

 

Id say a few things to try, ive done most with varying sucess.

 

Make sure you have adequate fuel flow before mech pump

 

Double check fuel level correct on carb bowl.

 

Work with vanes, install one at air filter box and just before carb

 

Seriously smooth out exit from airbox, fill and sand in needed.

 

Try a little carb heat, can be used to stop those 750 plus levels

 

Back off throttle just a bit, drops needle and can change things

 

Look into jetting, increase main a size

 

An easy test for airflow disturbance from force feeding is to remove hose between airbox and cowl inlet naca duct, fly and see what happens. Probably block off air intake in cowl side too. Air is still filtered just drawn from inside rather than outside cowls.

 

An issue to think about is that dynon probes arent long enough to reach inside cockpit -maybe ok on 2200- and the temp at where wires join needs to be added to readings. If cowls temps are 50 deg that can make readings look pretty serious.

 

No way its safe to run LOP as it currently is. Have to be careful as Jab heads may not be able to handle peak egt the engine can generate

 

 

[Ultralights]

the only thing an intake issue will contribute to EGT readings is a leak, leaning the mixture, causing a change depending on where the mixture is set by the carby. if you already rich of peak, then a lean mixture will cause a rise in egts, if your at peak egt, then it will casue a drop. if your already lean of peak, it will cause a further drop.

 

 

EGT Exhaust gas temp

 

CHT Cylinder head temps

 

ICP Internal cylinder pressures

 

HP, horsepower produced

 

BSFC efficiency, best specific fuel consumption

 

 

[jetjr]

The engine isnt running lean or rich, but both at once. The char is only relevant if cylinders are even and fuel distributed evenly.

 

Swirl or turbulence in intake is well known to be a difficult problem even in auto engines. Its part of the reason port injection is widely used.

 

Another possible improvement is the replacement of the blade splitter downstream of carb with round bar. Helps evenly distribute mixture

 

 

[Ultralights]

according to my rotax 912 ops manual, the optimum carby air temp should be 21 deg C, to help ensure the most fuel vapourisation to get the best chance of equal mixture distribution.

 

 

[jetjr]

Probably

 

912 have dual carbs i thought, so just feeding 2 cyl each, way lower airspeed id think, thats how they battle even mixture distribution.

 

All major engine makers with carbs have fought this problem. Its why they go to lengths to fit carbs centrally to engine. Jab are very compact and having carb at one end is a compromise.

 

I reckon this problem could be one of the root causes of some Jabiru engine problems. Really poor distribution is setting it up for detonation under some circumstances like lots of circuits, bit of old fuel, hot cylinders. Other times its not such a big deal. Some actually have good distribution out of the box

 

 

[Yenn]

Why worry about your EGT. The one to watch is CHT. If that goes too high you are in strife and your EGT could be either high or low when it happens. The EGT is telling you a bit about what is happening with combustion. You can get detonation when EGT is low and the best way to see it is with the CHT.

 

Learn as much as you can about the combustion process, how mag timing and leaning, enriching etc affect both CHT and EGT and you will be ahead of the game, but just aiming for a set EGT is little help.

 

 

[jetjr]

Detonation and burnt valves

 

Both are related to major failures in Jabiru engines

 

Cht is a big issue too, similar 50-80 deg spread on those as well. Youve got to have both low and even befor you canlay with jetting accurately

 

General setups run very rich to compensate, thats an expensive and bad thing too.

 

 

[Bruce Tuncks]

I would like you to be right Yenn. The CHT's are ok, rarely going above 150.

 

But how can one cylinder have an EGT of 790 and another 690 and that be ok?

 

The only way for this to happen is for them to be getting different mixtures, I think. I asked a Cessna pilot if he had a separate EGT for each cylinder and he said no.

 

If the EGT's are different, then wouldn't the combustion pressures be different too?

 

 

[jetjr]

Its the whole concept of Gami injection setup, tailors fuel per cylinder.

 

Balanced power production is important and you cant correctly jet an uneven engine

 

Other engine have better and more even distribution AND likely the ability to handle unwven temps better. They also weigh and cost far more and aren't line competitors with Jabiru engines.

 

Jabiru are more sensitive to temps and irregularities. Traditional engines have manual mixture control, often running quite rich when engine heavily loaded. Self adjusting mixture carbs arent able to do all phases very well

 

Not sure about detonation at rich mixtures, but some poor fuel, hard running at take off and lean runnong you can get it for sure, may not know its happening until holes line up

 

 

[Bruce Tuncks]

Thanks for the good stuff jetjr

 

Here is what I think is happening:

 

The fuel comes out at a point just below the centre of the carb, initially the spray of fuel is as big as a pencil lead in the middle of the air stream which is 40mm diameter.

 

At 3000 rpm the air flow rate is over 30m/sec, so it will travel 30mm in one thousandth of a sec. It is probably not mixed when it gets to the distribution box.

 

Even if this small area of rich mixture is dead center, there can be an imbalance between front and rear offtakes from the distribution box. And if the rich mixture plume is angled, you will get a difference left and right. I see both of these effects in my EGT readings. And if the rich plume angle changes with airflow speed, things will change as well. The upstream fixes seem to be aimed at getting this plume straight.

 

How about putting a relatively small obstruction downstream of the carb jet in the path of the rich plume to spread it out? This would have to go fairly close to the carb outlet and would need to be, I imagine, about 10mm diameter sheet metal, with 4 cross-supports of about 2mm diameter.

 

It would have the downside effect of lessening the total flow into the cylinder, and therefore the power output, ( a reverse of turbocharging ), but this would be a price worth paying if it was not too much and it got the EGT readings more even.

 

I think something like this has been done on engines later than mine.

 

 

[Nobody]

it can also be slight differences in the placement of the EGT probes. This can be distance form the exhaust port but also the position around the pipe. This is because the exhaust gases come out in a very swirled manner. Don't fret too much about temperature difference between cylinders.

 

What you need to determine is that they are all operating relatively as rich or lean as each other. The way to do that in a traditional lycoming is to progressively lean the mixture and watch the rise and then fall of the EGT. As long at they all reach a peak EGT at close to the same mixture setting they are balanced even if the numbers are different.

 

 

[Bruce Tuncks]

Thanks Nobody, yes if each cylinder could be leaned or richened then there would be no problem. I reckon the only way to do this with a Jabiru engine would be to add fuel injection. Or maybe the Rotec throttle-body would give a more even mixture to each cylinder.

 

I do think the no.4 is running too lean also from the spark plug colors and from the valve-seat incident. And richening up the whole engine by increasing the main jet size is an option but with only one cylinder too lean I don't want to do this except as a last resort.

 

 

[Oscar]

Bruce, there's a whole lot of interacting factors here, and there's no 'silver bullet'. It took the guys who adapted the Jab 2200 to a Motor-Falke more than a year of careful experimentation to get everything to work to their satisfaction, and it was a case of tweak, test fly, tweak again.. Swirl coming from the airbox and the intake turns is a serious issue and when it's good for one particular situation, it can change with different revs, quite dramatically.

 

Long ago, when we were building extractors for our racing cars ( before egt's were really known for automobile use), we'd use a pyrometer to determine the pulse points along each pipe to find a low-pressure area at a suitable length for cutting and joining, to make four into two or one exhaust pipe set-ups, at the revs best suited to the specific cam we were using. It was quite easy, because the heat changes along the length of the pipe are really noticeable.

 

 

[facthunter]

It's virtually impossible to get even mixtures with the set up you have. The better the vapourisation prior to the first branch point the better. It's likely there is minimum vapourisation as most modern fuels are designed for (and suit) injected motors. One carb per cylinder would be ok for tuning but is impractical. A mixing device like a supercharger would help. Radial engines use them to aid fuel distribution as well as the benefits of boost. I don't believe the Jabiru is a good engine to supercharge though. A mechanical system for injection would benefit the metering and also reduce the chance of a backfire /burn in the manifold which could force a large charge into a cylinder and cause damage.. It would be interesting to fit Autogas to a test engine and see how even things are, just to prove the mixture variability theory. Nev

 

 

[Bruce Tuncks]

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?

 

 

[Frankie]

Bruce

 

Have a look at this website http://www.saxonpc.com/120mm-airflow-honeycomb-without-hous120.html&usg=AFQjCNEAAAAAVw6jhsiiIX_gbyAlECry46ZrmfulDQUi

 

Tony Spicer is building a Panther and has a good web site documenting the build . He is using a jab 3300, this is a flow straightener he has made a connection in front of the carb, may help or may not, I'm not sure.

 

 

[jetjr]

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?

If you look downstream of carb, theres a central splitter in the plenum thats supposed to do what you say

 

Jabiru have used a sharp edged blade for some time, its secured. by a bolt from underneath.

 

Some tinkerers have tried a piece of round bar in its place and seen disruption improve EGT spread

 

Tilting carb also moves fuel about

 

 

[Bruce Tuncks]

Thanks jetjr, my engine is older than the one you describe and there is nothing downstream of the carb.

 

Today the gauge worked just fine, but everything I tried to even out the EGT's made no difference...

 

Firstly, I tried a splitter/turbulator just on the downstream side of the carb. It was a 2mm vertical wire on the carb downstream exit .

 

On take-off I measured 800 C on no.4 compared with 673C on no.2. Yes that's 127 degrees difference.

 

Then I swapped the senders between no.2 and no.4 just to see if the instrument was faulty. This time I saw 804C on no.2 ( actually no.4) so this didn't help. The instrument is fine I think, but I haven't calibrated it at high temperatures. It reads ambient ok.

 

Next I installed a flow-straightener on the carb input side. Is was a vertical aluminium sheet 50mm x 120 mm in the SCAT hose which goes from the air-filter box to the carb intake. It had horizontal wings front and rear to keep the vertical bit centered.

 

Flying with this I measured 804C on no.4 compared with 648C on no.2. That's 157 degrees difference.

 

These high readings were at full throttle take-off power.

 

On cruise-climb ( 2800rpm and 80 kts) it was more like 748C on no.4 and 665 on no.2. Not so bad but still too big a difference.

 

I need to check if there is an air-leak into no.4 but I doubt it. The air leak would do more harm at lower throttle and I am seeing the opposite. Maybe my 2mm wire splitter is way too small to do anything much.

 

Any ideas welcome ... I really don't like the 800C temps.

 

 

[Ultralights]

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.

 

Any ideas welcome ... I really don't like the 800C temps.

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.

 

 

[jetjr]

UL, egt are critically important in Jabirus. Id doubt any are running lean of peak. Also doubtful std heads can handle temps seen at peak EGT for any length of time. We are talking engines that have known failures of throughbolts and valves. Both can be linked to uneven lean running.

 

I agree numbers are not critical and can be changed by setup and measurement errors BUT having them even is very important and very difficult to achieve. Annoyingly some engines just have good spread. I have seen EGT spread problems remain with an aircraft after entire engine swap, as a result put lots of time into air intake with ok results so far.

 

Bruce the splitter is there, it is part of the plenum just before where pipes go to each cylinder, left and right. Its a very neat fit and could be seen to be part of the central wall of plenum.

 

Try a little carb heat at full throttle, not during take off of course, See if spread evens out. Doesnt need to be full ON, just enough to disrupt airflow in air box.

 

The carb heat can induce a different air flow. IF this works then thats the avenue to follow.

 

800 could see detonation and burnt valves so its a serious issue