the ideal modified jab engine

[kgwilson]

Some pilot/maintainers I know are what I call fiddlers. They constantly fiddle with things and tweak this and that to try & get everything perfect & get worried if their fancy all singing and dancing electronic engine monitoring is showing slightly different values for each cylinder. I try to work it out first get it right & then never touch it again. If it aint broke, don't fix it.

[spenaroo]

yeah and that's the problem with more data.
sure we can put a sensor on everything... but how many variables can we actually change especially in flight.
how many of us had to learn to look out the window and not to chase the instruments when learning to fly

[RFguy]

My feeling is that with the Jabiru engine, because there isnt margin for misoperating them, you NEED data. Can't make 'tough' and 'low cost' in same sentence with aircraft engines it seems. 

KG I like what you did with the intake.

[turboplanner]

22 hours ago, RFguy said:

I dunno.  Some users I know have done alot of work  with different main jets.  Jabiru have fiddled with the jet sizes from the beginnning of time....

too lean and you will cook something, for sure  IE beyond exhaust valve sustained capability at WOT .  Too rich and  the chambers etc will tend to cake up more, and also power will be left on the table if too rich... bore and ring wear can increase also since the fuel dissolves the lubricant. 

I am no expert or even have much experience with knowing how much main jet size different will affect WOT EGTs by how much. But others have that knowledge. 

I dont like to see any more than 680C under any weather condition at WOT.

 

Range: We would set the main jet size above your too lean and below your too rich; below the loss of power point.

 

These pistons, designed by GM for cars, are designed for intermittant power mode. In an aircraft application they are in constant power mode. So we have a compromise, but it's the base we have to work on.

 

The mounting point of aluminium is 660.3 degrees C.

The Combustion chamber temperature is 2000 degrees C

Without some intervention the aluminium is going to melt

EGT doesn't measure Combustion temperature

CGT doesn't measure Combustion temperature

You could graph the three temperatures but EGT and CGT would show up as a very gentle bell curve from start up to shut down whereas Combustion temperature would show up as a graph full of almost vertical lines up and down.

Not terribly usable.

Steel has a much higher melting point, yet you can ge both pistons and valves burnt away.

It comes down to how long does it take for aluminium/steel to melt, and can the piston/valve be cooled faster? 

It can, by the incoming fuel that hasn't been combusted yet.

On a constant power application like a tractor, outboard boat motor or aircraft, you wouldn't be looking to set the main jets at the smaller end of the scale.

 

Can you meaure the combustion temperature?   Yes you can, (reasonably) I use spark plug ceramic to measure the working range of combustion temperatures.

There is a range of colours which shows us when the main jet is too small all the way up to too big. (I don't have one but you'll find them on racing engine sites.)

 

We are looking for a honey brown ceramic for maximum power/cooling and might set to a slightly darker brown.

 

However, there's a catch; the colour charts are based on operation at WOT  long enough to colour the plugs then instant switch off/clutch in so the plug isn't masked during the normal intermediate jet, idle jet phase.

 

So you aren't going to be able to get the reading in a WOT climb in an aircraft without some masking.

 

 

 

 

 

 

 

[facthunter]

High valve  temps , plug ceramic and piston crown temps will cause detonation. Lean mixtures tend to  detonate easier.  and burn metals and the oil off the bores. The rich mixture cools by evaporation and that's why take off mixtures are rich excessively at times.  Nev

[RFguy]

detonation or preiginition ? I consider pre ignition maybe more likely  (and more destructive) in the jab with the low compression, little hot spots of build up , but that's just a guess.

[Thruster88]

I wonder if the jabiru engine would have had less issues if fitted with a Marvel Schebler manual mixture control carburetor. Full rich for takeoff and climb,  lean mixture for 75% power or less. Seems to work for Lycoming and Continental.

 

  

[RFguy]

I think the  Bing works very well.  I dont think there is any advantage to providing a mixture control . It would be another item that could be mis-handled.

Certainly there is mixture distribution variation over cylinders. But- correctly jetted Jabiru's without plenum and  intake etc leaks with good leakdowns , and not running high CHT and WOT,   rarely have problems with valves and pistons failing.

 

What kills Jabiru's is people flying with bad leakdowns which is the #1 indicator that something  needs attention.

Jabiru engines because they do not have margin, operators, IMO, MUST heed  a warning sign and take action before further flight. 

 

Sustained WOT and excessive CHTs ( usually due to a lack of airflow like  sucessive power on stalls etc) seems to lead to piston failures by many accounts . LIkely very  hot heads dont pull enough heat of the the tops of the cylinders, leading to not enough heat getting out of the pistons, and elevated CC temps, likely precursor to detonation and preignition ,  and so on !

Edited November 5, 2022 by RFguy

[jackc]

Whilst I cant do a thing to my 24 rego J-230D,  got to stay factory etc.  I wondered IF water injection would be a bonus? Can’t see what not?  Might be something the 19 rego owners could try?

[RFguy]

A well performing Jabiru airframe and engine- cooling is rarely a problem once established airborne I have found. taxi then takeoff (from hot)  in hot weather is a bit of a problem. 

[facthunter]

The water-Meth has been covered fairly extensively, Using it goes back to  WW2.  Nev

[turboplanner]

1 hour ago, RFguy said:

What kills Jabiru's is people flying with bad leakdowns

From RAA official publications - TTIS failures

155

280

382

320

358

310

260

197

40

285

300

196

8

and at least two flying home from a factory rebuild so <6

 

[RFguy]

wow that is interesting !

I have seen with my bare hands, 300 hours is the beginning of the danger zone for :

1) AVGAS buildup inside CC, on  piston top, guides, valves, stuck rings

2) sticking valves (oil burning, gunk buildup),

3) cylinder head interface aluminium recession (aluminium nicely over tempered by now) . By this time,also, some maintainers have been retightening the heads several cycles....

 

There are of course maintenance errors- which account for a few I reckon (flight back from engine work) .

I consider 400 hours to be the TBO on AVGAS  for the top end (piston rings and lands , head CC, guides, rocker bushes .....

The engines cannot run hot enough to burn all the gunk like a Lyco. 

[kgwilson]

I have run mine on Mogas since new. Leakdowns 80/80. Borescope shows clean pistons, heads & valves. Plugs are always black but that may be due to the long taxi back to the hangar. Jabiru cooling is air/oil/fuel. Don't try to run the engine lean. 400 hours and no oil needs to be added between changes at 25 hours and the oil stays fairly clean right up to the change. It is hard to see the level on the dipstick. I add 3 quarts of Aeroshell 100+ only & a new filter each change. This brings the level to about 3-4mm below the top of the knurled section which is about right. Any more and it gets thrown out into the oil catch bottle. The level drops about 3-4mm over 25 hours.

[RFguy]

KG do you run any fuel return loop  for the MOGAS in your setup ? The fuel kettle in the Jab engine does get awfully warm...

[Old Koreelah]

5 hours ago, jackc said:

Whilst I cant do a thing to my 24 rego J-230D,  got to stay factory etc.  I wondered IF water injection would be a bonus? Can’t see what not?  Might be something the 19 rego owners could try?

Jack this was duscussed recently:

 

[kgwilson]

4 hours ago, RFguy said:

KG do you run any fuel return loop  for the MOGAS in your setup ? The fuel kettle in the Jab engine does get awfully warm...

No not necessary. My fuselage tank is 100 litres and will drain by gravity to the engine. Both Wing tanks feed in to the fuselage tank via auxiliary 4-6psi electric pump and a L/R selector. The main electric boost pump sits under the main tank in the fuselage & pushes fuel through the in line filter to the engine driven mechanical pump & on to the carburettor. When it is on, it keeps the line pressurised to only 1.5 psi. There is no point trying to force more fuel into the carburettor than necessary. It would just put pressure on the fuel line and float intake valve. This all ensures there is no possibility of vapour lock.

 

I always run  the electric pump for 10 seconds before startup. Jabiru recommend this. This is especially important when the engine is hot after shutdown for a while. If I do not do this the engine may start and then stop after a few seconds. This has  happened a few times so I always run the electric boost pump every time prior to startup but always start with the pump off. This proves the mechanical pump is working. The engine heat is the likely cause of vaporisation in the fuel lines in the engine bay so once the fuel in the carb bowl is used, it sucks vapor and stops. 10 seconds of the electric boost pump solves this entirely.

[RFguy]

Indeed. And well written up KG. aux fuel pump should have enough pressure at WOT flow to keep things pressurized. There are, on evidence, few problems with Jabiru airframes and engines and MOGAS. Only thing left to hurt you there is a clogged fuel filter. 

Edited November 5, 2022 by RFguy

[turboplanner]

24 minutes ago, RFguy said:

Indeed. And well written up KG. aux fuel pump should have enough pressure at WOT flow to keep things pressurized. There are, on evidence, few problems with Jabiru airframes and engines and MOGAS. Only thing left to hurt you there is a clogged fuel filter. 

I've previously mentioned that MOGAS is a US term for US gasoline.  It comes from different sources, the raw material will be different, the refining may also be different to Australia which has its own grades of petrol.

 

For a group of people who are anal when a reporter calls a crashed aircraft a "Cessna", which can't kill you, then calling Australian petrol "Mogas"  when many people on this site have pointed out issues on certain Australian blends which will clog carburettor galleries, eat hoses, and delaminate fuel tank linings which CAN kill you if you misjudge a forced landing, or are flying over topography you can't land on, we should be past this.

 

[RFguy]

alright , no more MOGAS talk. how about autoULP ?

[turboplanner]

1 minute ago, RFguy said:

alright , no more MOGAS talk. how about autoULP ?

No broad terms when the fuel system clogging is just in two or three of the grades. We need the exact grade, and the best discussion was probably the last one which I thing included the grades to avoid at all costs even if it means staying on the ground, and the calayzing of aromatics which, when understood, get an engine started without a lot of wasted time and drained fuel tanks.

[kgwilson]

Avgas is paraffin based (the flammable component of candle wax) whereas Petrol is aromatic hydrocarbon based ( all the ene's like toluene, benzene xylene etc). They have completely different compositions