Mogas

[derekliston]

To the best of my knowledge Jabiru say you should only run their engines on avgas. Does anyone on the forum have experience of running Jabs on 98 octane unleaded?

 

 

[Kyle Communications]

I thought you had a rotax in yours

 

 

[derekliston]

No mate, it worked out about $10,000 cheaper to use a rebuilt Jab. Bit of a concern with the sometimes questionable reliability, could work out more expensive in the long run although I do know people who have had no reliability problems. I guess it's going to be a case of "suck it and see" I liked the lighter weight and less complication of a Jab installation. By the way, just heard back from Jab that it is acceptable to use 95+ mogas if avgas unavailable.

 

 

[facthunter]

THAT surprises me. Mogas meets few quality control standards, and the octane ratings do not directly transpose. They are not equivalent. IF I ran it in a Jab it would want to be a cool running engine. Nev

 

 

[frank marriott]

To the best of my knowledge Jabiru say you should only run their engines on avgas. Does anyone on the forum have experience of running Jabs on 98 octane unleaded?

Derek

 

The is some interesting advice in "From the Engineer’s Desk notes " June 2013, on the Jabiru web site. Worth a read if using mogas re additives.

 

Personally I run avgas full time (assured quality control)

 

 

[Yenn]

I ran a 1600 Jab engine in the Corby on avgas. No problems except oil leaks and a bit low on power in very hot conditions, so I changed to a jab 2200.

 

I ran on avgas with no problems, but when the airport refused to let me fill my 200 litre drum, I started using mogas. Supposedly the high octane caltex at my local servo which has high turnover of fuel.

 

I had a big mag drop one day at moderate revs, Checked out plugs, coils etc and replaced one coil. Still had the mag drop. Swapped coils side to side and the mag drop was on the same side. Wierd!

 

Eventually I did a mag check at high rpm and it ran beautifully. I had 2km of runay in front of me so I took off, knowing I had somewhere to go. no mag drop, running fine. Climb up high and head for home with plenty of safe landing options.

 

Get home, still have the mag drop at low revs, so engine out and dismantle and luckily the first cylinder I took off was the problem.

 

Clearly detonation damage to the piston, broken 2nd ring and ring lands. That was the opinion of our Tech manager at the time. No other problems, valve good, other cylinders good so I replaced 1 piston, all rings, honed the bores and it now goes really well. I will not be using mogas again, it must have been a bad batch. Possibly full CHT instrumentation could have warned me.

 

By all means use mogas if you want, but there is no guarantee of quality and having said that I remember when BP had a massive problem with their avgas, since solved.

 

 

[Russ]

Personally know a chap that runs Mogas, but adds Lucas top end lub, has been doing this for a long time, machine is his trainer, so full power,cruise, etc is it's operation.

 

He swears by his method, and has absolutely no troubles at all. ( jab BTW )

 

 

[derekliston]

I would prefer to use avgas but since I have just assembled the aircraft at Coominya, I need to put fuel in for the first time but have the same problem as you, namely Archerfield aviation fuels will not fill my jerry cans even though I can produce a certificate of registration, so I am in a bit of a Quandary! Actually I haven't checked with John Walmsley whether he has avgas on the field although I suspect the answer will be no so if anyone has a solution I'd like to hear it.

 

 

[Ultralights]

the Lead in Avgas does nothing when it comes to lubrication.

 

 

[facthunter]

It aids in the sealing of the exhaust valves when the right temperatures exist as it works like a flux when brazing. many of the other virtues of it are incorrect, as you state. It can cause problems used in a Rotax 912 with uneven build up on the valve seats and compression loss( and deposits in the engine.). This mostly recovers when run later on mogas, but any leakage at the seats can do damage. Nev

 

 

[Ultralights]

i dont know about that, lead in the fuel, at the combustion event turns into Lead Bromide, which is abrasive, from everything i have been taught, and seen, is the lead only serves one function, and thats to decrease the chance of detonation by increasing the latency, or when combustion begins after the ignition event.

 

Lead Bromide also contaminates oil with an abrasive sludge.. and it can be seen in the exhaust residues as a fine grey powder. Burnt valves are almost always caused by poor sealing at a small point on the exhaust valve seat and valve interface. can be caused by a carbon deposit creating a small gap in the valve/valve seat area, but most often it is poor manufaturing, as getting valve seats and valves perfectly matched is a difficult process. its almost like the argument i have heard that running 50 deg Lean of peak will burn out valves and running 50 deg rich of peak will not.. How can it? when both temperatures are exactly the same!

 

 

[Old Koreelah]

It aids in the sealing of the exhaust valves when the right temperatures exist as it works like a flux when brazing. many of the other virtues of it are incorrect, as you state. It can cause problems used in a Rotax 912 with uneven build up on the valve seats and compression loss( and deposits in the engine.). This mostly recovers when run later on mogas, but any leakage at the seats can do damage. Nev

Can anyone suggest the result of long-term use of avgas in a cool-running Jab? Is more lead likely to build up due to lower temp?

 

 

[coljones]

From http://www.iaopa.eu/mediaServlet/storage/gamag/dec06/Petrol2.pdf

 

Euro AOPA

 

"Your friend, ethylene dibromide

 

When it is burned in an engine, tetraethyl lead – now called avtel by makers Innospec – can

 

leave a deposit which has a corrosive effect, particularly on valve heads and stems. This is called

 

lead fouling. The lead oxides must be scavenged as far as possible, and for this purpose ethylene

 

dibromide is added to avgas.

 

Ethylene dibromide reacts with the lead oxides to form lead bromide, a gas (at high

 

temperatures) which is exhausted from the cylinders. It doesn’t work very well at low

 

temperatures, which is why lead fouling is at its worst during operations at low power settings

 

such as taxying and hanging around waiting for take-off clearance.

 

Because of its effects on the environment, ethylene dibromide was banned by international

 

treaty 20 years ago.

 

It’s worth running the engine up before shut-down, especially if you’ve had a long taxy at

 

relatively low power, to get rid of lead oxides. Increase power from the ground idle range to

 

around 1800 rpm for 15 or 20 seconds. This should raise the temperature enough to let the

 

ethylene dibromide do its work.

 

When you’ve gone back to ground idle, check the magnetos. Lead fouling also affects plugs

 

– temperatures are lower around the plug, before the flame really gets going. Some pilots also

 

lean the mixture off as much as possible when faced with a long taxy, to reduce the amount of

 

fuel and increase the temperature in the pot."

 

 

[ozbear]

Mogas 98oct is the only fuel I use in my 2200 with no problems

 

 

[Old Koreelah]

If only we could be sure of the quality of mogas. Perhaps there is a simple test kit we could carry.

 

What a bind for a greenie like me: use AvGas and save the engine, or use mogas and save the planet?

 

I'd be much happier burning pure ethanol, like the Brazilians do.

 

How much extra weight would it cost to convert to LPG?

 

 

[facthunter]

Once Avgas has been in a non supplier provided container it is not certified either, as far as the law goes. With LPG The tank is most of the weight. I have a thin stainless steel 90 litre that is probably as light as that item goes. It's a bit harder on your motor. Need good valves and seats. You have to heat the regulator or it freezes. How are you going to FILL it . Nev

 

 

[Old Koreelah]

I sure wouldn't relish flying behind a tank of compressed gas, but there is so much of the stuff maybe that will happen.

 

 

[Yenn]

Ultralights.

 

The same EGT temps do not mean the same burn conditions in the combustion chamber. The EGT is measured usually about 50mm down the exhaust pipe from the cylinder face, so it is measuring what is left after combustion. 50 deg rich of peak is a much hotter burn in the cylinder than 50 lean of peak. At 50 lean you have excess oxygen and a slower burn than at 50 rich, where you have less oxygen. You will harm the engine running 50 lean of peak at full throttle, due to poorer cooling and it will show in a rapid rise of CHT. You can run lean of peak all day if you use less than 75% power. Plus you will save fuel and your plugs will be cleaner.

 

 

[Ultralights]

that doesn't quite make sense, if peak EGT is say 1450 deg F, and 50 deg either side means the temps are 1400 deg F lean or rich, how can one 1400 deg temperature be a different temperature than 1400 deg? sure, you are correct in that the conditions are different at the point of combustion, with different amounts of oxygen/fuel ratios, but the temperatures are identical and always will be. as for harming your engine running LOP (lean of peak) at full throttle, i have to strongly disagree.

 

i have seen live data on an engine run, and CHT's drop just after the EGT's drop on the Lean side of peak, and internal cylinder pressures drop off rapidly as well (less fuel and more air = slower combustion and better cooling as more air is left over doing the same job as excess fuel at Rich of peak) , and from the data, it is the Internal combustion pressures that have the strongest influence on CHT's its the reason that Pre ignition, and detonation causes sudden increases in CHT's as its increasing the internal pressures quite substantially, a normal running engine will have internal pressures up to about 800 PSI at the peak of pressure, whereas during a detonation event, it might hit 1200 to 1500 PSI depending on its severity, but the real killer is a pre ignition event, where as the peak combustion pressures hit at TDC, causing pressures to spike well over 3000Psi, destroying rings,burning holes in pistons, valves etc.

 

Now think of every cylinder in the engine as a completely separate, single cylinder engine that shares a common crankshaft with other engines.

 

the bad rep for Lean ops came from poor ability to control the mixtures accurately, to operate very lean of peak, you need to ensure that every cylinder has the same or very similar mixtures reaching it, so if you lean to say 50 deg Lean, one or 2 cylinders might be just at peak EGT, another might be 40 deg Rich, right in the middle of highest internal pressure zones and severely detonating, and as you could imagine, every cylinder creating a different amount of power, running at different mixtures creates a rougher running engine.

 

but with modern instrumentation, on every cylinder, you can now see where every cylinder sits on the EGT trace and get them all in the safe Lean of Peak temperature zone. Sadly a lot of aircraft engines do not have any way of altering the mixtures between cylinders unless fuel injected, or through redesigning the intake manifolds. (have a look at the difference in lengths of each intake tube to every individual cylinder and see how vastly different some are. or how similar some are and think how it might effect the mixture reaching each separate cylinder and its output.)

 

have a look at the video on this page, http://www.gami.com/articles/frugalflyer.php and have a look over this site http://www.advancedpilot.com/, and yes, i have done the seminars and seen the results in real life,i was amazed by the stuff i thought was true, and had to re-learn, but the real time data doesnt lie.

 

and here is a vid taken from a Baron, showing the results of leaning past peak EGT, and note the CHT as he does it.

 

 

this is also a good read and explains a lot. (more starting at the 1/3 down the page)

 

http://www.advancedpilot.com/articles.php?action=article&articleid=1838

 

 

[facthunter]

Temperature is not a measure of HEAT unless everything else is the same.

 

Leaning off will reduce power and power is dependent on heat energy. Simply less fuel less heat less power. You can almost use a fuel flow meter as a power indicator especially on a jet engine.

 

More fuel adds mass and can burn a lot slower. In fact it may not have time or the conditions to burn all the fuel and there will be actual soot in the exhaust. One other of over rich mixture effect is to resist detonation and this is used on the highest power settings on most aero piston engines.. It is used on V8 supercars all the time. watch the amount of visible soot coming out of the engines.

 

At lower power settings only the engine can be leaned. There will be unused oxygen around in that situation and it will be able to oxidise anything that will do it. It is certainly going to keep the combustion chambers cleaner and the oil. I suggest it will be harder to maintain an oil film on the cylinder surfaces. 2 strokes don't survive lean mixture episodes at all. Nev

 

 

[derekliston]

Mogas 98oct is the only fuel I use in my 2200 with no problems

Good to hear that! What airframe and for how many hours? I ask because the CH701 is only a 70-80Kt airframe. By the way, it is interesting to see where this thread has gone from my original question.

 

 

[ozbear]

Good to hear that! What airframe and for how many hours? I ask because the CH701 is only a 70-80Kt airframe. By the way, it is interesting to see where this thread has gone from my original question.

It's in a xair Hanaman approx 75 hrs so airframe is 75 to 80 kt cruise as well

 

 

[Old Koreelah]

It's in a xair Hanaman approx 75 hrs so airframe is 75 to 80 kt cruise as well

What revs do you cruise at?

 

 

[Bob Llewellyn]

i dont know about that, lead in the fuel, at the combustion event turns into Lead Bromide, which is abrasive, from everything i have been taught, and seen, is the lead only serves one function, and thats to decrease the chance of detonation by increasing the latency, or when combustion begins after the ignition event.Lead Bromide also contaminates oil with an abrasive sludge.. and it can be seen in the exhaust residues as a fine grey powder. Burnt valves are almost always caused by poor sealing at a small point on the exhaust valve seat and valve interface. can be caused by a carbon deposit creating a small gap in the valve/valve seat area, but most often it is poor manufaturing, as getting valve seats and valves perfectly matched is a difficult process. its almost like the argument i have heard that running 50 deg Lean of peak will burn out valves and running 50 deg rich of peak will not.. How can it? when both temperatures are exactly the same!

Er - 50 deg lean means excess oxygen means hot metals oxidise rapidly; 50% rich means excess carbon, so they don't.

 

 

[Bob Llewellyn]

If only we could be sure of the quality of mogas. Perhaps there is a simple test kit we could carry.What a bind for a greenie like me: use AvGas and save the engine, or use mogas and save the planet?

I'd be much happier burning pure ethanol, like the Brazilians do.

 

How much extra weight would it cost to convert to LPG?

Nope. There's no such thing as a simple Reid Vapour Pressure tester; or a simple E-35 test engine. One of the problems is the oil companies use a wide range of additives to meet the commercial spec for the fuel, so the various properties vary wildly, including the rate of aging (both loss of volatility and loss of effective octane number can be significant within days, if dissolved butane is used; or months, if (carcinogenic) aromatics are added).