EGT SPREAD

[Bruce Tuncks]

No I didn't know about that installation manual section Oscar, thanks. I'll look at it now.

 

 

[Oscar]

Bruce: JEM 2202-7_inst-1.pdf. Start at Page 31.

 

 

[andrew lott]

Seems like a lot of effort being made to even up some EGTs which in my humble opinion may not have any beneficial effect on the engine with the fuel management systems that you are stuck with.

 

What is critical is the CHT which directly affects engine health and is not directly related to EGT. EGT has many variables so is useful mainly for establishing peak EGT for the purpose of leaning or richening your mixture.

 

After over 3000hrs of running a big bore Continental with the worst induction system (carby) devised, I have found that EGTs are all over the place but have no measurable effect on the running etc.

 

Mine regularly exhibits temps (and higher) to those you are concerned about but no bad effect so far.

 

I religiously keep CHTs below 380f (193c) and the cylinders last. Cooling is critical.

 

 

[Bruce Tuncks]

Hi Andrew, do you have CHT and EGT on each cylinder? What figures are you seeing?

 

 

[Jaba-who]

Seems like a lot of effort being made to even up some EGTs which in my humble opinion may not have any beneficial effect on the engine with the fuel management systems that you are stuck with.What is critical is the CHT which directly affects engine health and is not directly related to EGT. EGT has many variables so is useful mainly for establishing peak EGT for the purpose of leaning or richening your mixture.

After over 3000hrs of running a big bore Continental with the worst induction system (carby) devised, I have found that EGTs are all over the place but have no measurable effect on the running etc.

 

Mine regularly exhibits temps (and higher) to those you are concerned about but no bad effect so far.

 

I religiously keep CHTs below 380f (193c) and the cylinders last. Cooling is critical.

The problem you are overlooking is that this is not a continental its a jabiru. Now that might sound flippant but this is actually very significant. Jabs are a very light engine with tolerances for heat dissipation running right on the edge. Continentals are massive heat sinks and tolerate heat much better by Comparison.

 

In jabs Individual cylinder cooling is often varied and limited by the cowel set up. Fuel delivery to individual cylinders is always varied. Egt readings reflect heat generation while cht reflect the summed effect of both generation and cooling.

 

In jabs since they have limitations in all directions have to have all the parameters considered and addressed.

 

 

[jetjr]

Continental with the worst induction system (carby) devised

Might want to look closer at 6 cyl Jabiru

 

 

[andrew lott]

Yes. Have a JPI 700 with CHT & EGT on all cylinders, fuel flow, carb inlet temp, EGT difference,, oil temp, volts, and other fuel management (linked to GPS) functions. The unit is needed to run LOP which I do all the time in cruise. At max. power in the climb EGTs range from high 1200f/700c to 1470f/800c with very rich mixture.

 

CHT is what I watch for. In cruise and leaned out to LOP, the EGTs could range between the 1400f and 1500f range. Because of the awful inlet system I have, it is not uncommon to have a difference in EGTs of 100 -110*f, however provided the engine is running smoothly, it does not matter. Carb inlet temps of 50*f/10*c is the optimum temp for mixture distribution between the cylinders. I use carb heat to get that temp if needed. If the engine is rough after leaning, applying the correct amount will smooth it up again.

 

Remember that you have four or six single cylinder engines bolted to a common crankshaft. Unless it has tuned Gami style injectors, you are bound to get mixture distribution differences and hence differences in EGT readings.

 

Any roughness or vibration is because one cylinder is not producing the same power as the others. When it is running smoothly, all the cylinders are producing about the same power. You can still have EGT differences with a perfectly smooth running engine. There always differences between the cylinders performance and indeed each cylinder. Personally I consider that as long as my engine is running smoothly, the CHTs are below 380f and the EGTs are close enough, then Ops are normal.

 

The best info on engine operation is at the Advanced Pilot Seminars and there is one graph from them in an earlier post. Look up their website of the same name and there is a section that is open to the public. Also look up John Deakin, Pelicans Perch, as well as The savvy Aviator- Mike Busch. Those three sites have some remarkable reading on engine matters.

 

 

[andrew lott]

The problem you are overlooking is that this is not a continental its a jabiru. Now that might sound flippant but this is actually very significant. Jabs are a very light engine with tolerances for heat dissipation running right on the edge. Continentals are massive heat sinks and tolerate heat much better by Comparison.In jabs Individual cylinder cooling is often varied and limited by the cowel set up. Fuel delivery to individual cylinders is always varied. Egt readings reflect heat generation while cht reflect the summed effect of both generation and cooling.

In jabs since they have limitations in all directions have to have all the parameters considered and addressed.

You are quite correct that all parameters have to be looked at, and in the Jabiru, cooling could be improved. For example the baffling around the cylinders of all air cooled mainstream aero engines and industrial engines is quite extensive and considered critical to cylinder cooling. The Jab doesn't have that level of cooling sophistication.

A Jab engine is really pretty advanced in its power output. If you scaled its volume up to the equivalent of the Continental O-470 it would produce 50 more horsepower . This applies to both the 2200 and the 3300. Quite remarkable really.

 

I don't agree with the Continental being a big heat sink. While it is larger, it has to get rid of much more heat than the smaller engine and once it is up to temp, it runs at the same temps as any other air cooled engine, be it a Jab, a Lycoming 540 or an 1820 Wright. CHTs result from a number of factors including also, the cooling arrangement, the mixture setting, the position of Peak Pressure in the cylinder (refer to APS info re that one.)

 

 

[jetjr]

You are seeing 100-110 deg F difference, we are seeing 100-150 deg C difference, Also theres no way youd run a Jab CHT up to 190 Deg C, they reportably change permanently much over 180 deg.

 

Even the best tuned Jab Ive seen is only at the numbers your seeing in terms of EGT spread. Most are far worse. We are targeting differences you have.

 

You certainly wouldnt want to run an engine LOP with 150 degrees C between richest and leanest.

 

To say rough running due to mixture problems "doesnt matter" is a big call seeing the number of possible detonation related problems out there with Jabiru engines

 

Id go further and say older design engines are simply stronger and more resistance to what you call rough running. Penalty is weight.

 

 

[andrew lott]

You are seeing 100-110 deg F difference, we are seeing 100-150 deg C difference, Also theres no way youd run a Jab CHT up to 190 Deg C, they reportably change permanently much over 180 deg."100 - 150 deg Celcius is a huge spread. I have never seen that spread in mine. Something definitely not right there. Don't know what the limits are on th Jab CHT but on the Continental the max should be considered to be 400F. The tensile strength of the head alloy drops off considerably over 400F"

 

Even the best tuned Jab Ive seen is only at the numbers your seeing in terms of EGT spread. Most are far worse. We are targeting differences you have.

 

You certainly wouldnt want to run an engine LOP with 150 degrees C between richest and leanest.

 

"I don't think you could. I have not seen those differences in any stage of flight."

 

To say rough running due to mixture problems "doesnt matter" is a big call seeing the number of possible detonation related problems out there with Jabiru engines

 

"That's not what I am saying. You don't let the engine run rough. You take steps to smoothen it up which I do by mixture adjustment, throttle position and carby heat. Smooth running shows that all cylinders are producing about the same power each. The differences in EGT when the engine is smooth really don't matter. I have seen EGT spreads of 40degF and still had rough running. I have also had it smooth at 40 degF and the same at the large spreads of 100-110degF. It can vary from flight to flight."

 

Id go further and say older design engines are simply stronger and more resistance to what you call rough running. Penalty is weight.

"Yes they may be but they have also had the benefit of 80 years of development. Yes they are heavy both in weight and in dollars!!!"

 

 

[VFR Pilot]

The position of peak pressure above the piston is governed by the ignition timing, which can't be changed.

 

Jabiru actually retarded the timing 5 deg to 20 deg some time ago.

 

 

[Jaba-who]

The other factor which the jab pilot has to contend with is that you can't lean or richen, run lop or rop or anything. Not sure if you are aware but the jab has no leaning ( in the air) capability apart from a poor mans leaning with the carb heat which drops your power output. The carby is so-called altitude compensating but it's not completely accurate.

 

So talking about maintaining egts and running lean, leaning till just rough or lop etc are fairly irrelevant to the jab story.

 

Sure you can make hardware changes on the ground but not in the air.

 

Even on the ground there are issues because the wide variation means running one cylinder rop may well be at the expense of others which may be lop and vice versa.

 

 

[VFR Pilot]

HACman. greenskyadventures.com

 

 

[andrew lott]

The position of peak pressure above the piston is governed by the ignition timing, which can't be changed.Jabiru actually retarded the timing 5 deg to 20 deg some time ago.

The peak pressure can be changed by fuel air mixture ratios eg Ratios that produce EGTs of 50degF LOP

 

compared to 50degF ROP, result in a lowering of PP and a delaying of the peak several degrees After Top Dead Centre. The peak occurs ATDC anyway. I think it changes from 12 deg ATDC to 18deg ATDC with the mixture change (my figures may not be completely accurate but close)

 

Have a look at the graph put up by Ultralight on pg. 1 which shows engine parameters relative to mixture control

 

and the relationship of Internal Combustion Pressure to Cylinder Head Temperature

 

 

[andrew lott]

Agree. Sadly you do not have that option in the Jab or most small aero engines of that group.

 

 

[jetjr]

HACman. greenskyadventures.com

You can build your own for far less $$ than this, not much to it really.

Its the EMS thats required where the dollars are and without it your really testing faith in fuel distribution.

 

How many are out there being used in Jabiru's?

 

 

[jakej]

From what I've seen on factory built Jabby's (J33) with an EMS built earlier than this year only have 1 or 2 CHT probes fitted, why is that ? money ? oh no can't be that surely ? Do Jabby NOW say you should monitor every cylinder ? If so then iot's all a bit cynical the horse had bolted.

 

Andrew is on the mark above about CHT's etc, why has it taken so long for some to realise that high CHT is the bane of air cooled engines ?

 

One thing I should mention - if your fuel flow is too low at max power (takeoff) you will get high CHT's, that's why many have to do a step climb. So don't mess with the jets to get a lower cruise fuel burn least you ultimately pay the price for that.

 

 

[jetjr]

Yes Jabiru recommend watching all cylinders, No doubt a problem for them is that if some cylinders exceed limits on a new LSA , its going to be their problem to fix it. Fitting just 2 is plain odd. I also see they dont turn on data logging.

 

CHT is the primary parameter for sure, unless you have full EMS - which wasnt required or supplied until recently - how would you know whats going on other than the one you're watching.

 

The fuel flow is an interesting one, Jabiru allow for use of electric pump for fuel supply test, if it isnt on or maybe not working then you can run lean for sure, fuel lines way too small to feed by gravity at required flows. Even mech pump can starve under full load. A clogged filter or even the fuel flow sensor itself creates lots of restriction.

 

Some are up sizing lines.

 

 

[Oscar]

Some aromatics will cause some fuel lines to swell, and restrict flow. Those who think that they are doing their engines a favour by running on 98 PULP, may well be introducing more problems than they are hoping they eliminate ( and frankly, anything less than extremely fresh 98 PULP is a waste of money and relying on it to perform as advertised is false hope).

 

 

[facthunter]

Always bear in mind the two different octane measures used for Avgas and Mogas. Mogas is about 7 lower and subject to being adulterated by who ever wishes to. It's already variable in production in some instances Two stroke enthusiasts shouldn't let the fuel go stale especially after mixing where the life is more likely to be more reduced.

 

A rich mixture will cool the engine by latent heat of evaporation, and increase power because there ia more mass of charge in there. Most engines ( Petrol) that are run at the limit of their power will enrichen the motor(s). Race cars and aero engines. Road cars would not be allowed as much leeway and they hardly ever go there anyway. Nev

 

 

[frank marriott]

The fuel flow is an interesting one, Jabiru allow for use of electric pump for fuel supply test, if it isnt on or maybe not working then you can run lean for sure, fuel lines way too small to feed by gravity at required flows.

That's an interesting observation - completely at odds with my testing. Both pumps rated at 60ltrs/hr. Tested flow rates on both on mine and got 60ltrs/hr on electric and >50 odd/ltrs/hr at idle on the mechanical. Gravity only feed was not a problem under full throttle static but starved on climb (on a 3300 I think is about 38-40ltrs/hr).

 

Cruise climb on mechanical pump only without any problem also. Full throttle climb on electric pump ONLY also no problem.

 

Interesting that your observations are at odd with my findings.

 

 

[jetjr]

Not at odds at all, depending on your setup it could be right

 

You should be able to provide 60 lph (1.5x max usage) with no pumps at all, i doubt many can. This is the benchmark Im talking about. This is an aviation standard i think.

 

I did lots of testing too and found fuel flow sensor is a big restriction, non running elect pump too. Jabirus standard fitment of sensor and fuel filter upstream of electric pump seems a bit odd.

 

Standard setup saw just 20lph without elect pump running at the engine. I got this up to around 50lph eventually.

 

As mech pumps are hard to test, and inflight not many can see their performance, its possible to be running relying totally on electric pump and gravity. If it stops or fogotten there could be problems at sensitive times.

 

Just to prove a point to me, soon after i had the mech pump stop intermittently, gravity almost kept it all going until periods of higher flow.

 

 

[frank marriott]

Jabirus standard fitment of sensor

Haven't been fitted for some time now mate.

 

 

[jetjr]

Not sure i understand, the flosacan or cube fuel flow sensor is reularly fitted, theres plenty around, and theres drawings in tech manual.

 

 

[frank marriott]

Not sure i understand, the flosacan or cube fuel flow sensor is reularly fitted, theres plenty around, and theres drawings in tech manual.

Quite correct, all I was pointing out no longer fitted to new aircraft. (Although with an engine management system there is now a fuel flow readout)