Unleaded auto fuel

Well, I reached the point in my phase I flight testing where I wanted to see how the engine would run on unleaded auto fuel. I have a Superior IO-360 with one mag and a single Lightspeed Plasma II+ EI. I had flown down to about 5 or 6 gal in each wing tank, so I filled the right tank up with 91 octane no ethanol auto fuel ($1.70/gal) and the left tank with 100LL ($4.00/gal). I did my takeoff and climb with the 100LL, and when I reached my cruise altitude (7.5K) I leaned the mixture to 30 degrees LOP. After letting things stabilize, I switched tanks to the auto gas and waited several minutes to see if I could see any change. The winds were very strong, and the ride was bumpy, but as near as I could tell there was no change in performance. I would have flown more tests including climb performance, etc., but given the winds and ride I didn't think I would get much valuable info. I returned to the pattern and landed uneventfully burning the auto gas, but when I taxied in I noticed that the engine didn't feel as smooth at idle as I was used to. I played with the mixture some, but no help. Back at the hanger, I tried one ignition at a time. What I found was that when I was idling with only the mag, the engine was idling very poorly. When I used the EI, it was better, but not as smooth as I'm used to. I then switched tanks back to the 100LL, and within a few minutes the engine was idling as smoothly as it aways does. I guess from here I'll wait to a better day to fly more testing, but I'd classify today as mixed results. I would sure have liked to have seen no difference between the two, but that wasn't the case, on the other hand at cruise power the auto fuel seemed great. If anyone else has any experience along these lines I'd love to hear about it. Perhaps I should just adjust my idle up to 900 from the 650 I have now and say "good enough." I really like saving $2.30 a gal.

Mark

Mark,

The vapor pressure of the mogas is not as good as 100LL, you could have had some bubbles in the fuel from the fuel controller up to the divider. Winter blends of the stuff test out worse than summer blends.

The STC's for mogas for certificated airplanes require increasing the idle RPM, not sure how much, but I don't think it was a high as 900rpm. You might also need to enrichen the idle mixture a bit.

Check your engine manual

Superior recommends a change to the timing of your engine (can't remember if it is advance or retard) if you intend to use auto fuel for extended periods. I regularly burn auto gas in my Superior engine, but I have two magnetos. The idle does seem a little lower for me too, but that hasn't stopped me from running mogas.

Don

Mark, keep the data coming as you try that out. I plan on doing the same thing this spring/summer. I have the O-360 with one Lightspeed and Airflow Performance fuel injection. As soon as I can acquire a tank to fill with MoGas, I'll probably give it a try.

Whose injector setup were you running?

Mark,

Yes, please keep posting your results. I am currently at 20 hours and considering going to 91 on my Superior IO-360, so am very interested in whatever you find.

greg

More info

I'm running the Precision Airmotive Silver Hawk fuel injection system. The Superior manual does address the timing issue in their manual. They recommend the the timing be set to 22 BTDC instead of the normal 25 BTDC for extensive use of auto fuel. The logic is that auto fuel burns faster than 100LL so less advance is needed. The problem is that I see myself using both fuels, so I'm not planing on adjusting the advance at this time. It will be several days before I have the time to fly again, but when I do I will try to record and post more information.

Mark

MOGAS procedure

Hi fellas:

SAFETY. It's kinda important....

I have used MOGAS in more than a few Rockets -- they will all run on the stuff, with a few caveats. I have never purchased premium or mid-grade -- only regular -- and this is OK with 9.5:1 compression and cold air induction w/electronic ignition.

The rough idle is genuine -- you should be running on 100LL during that phase, and others.

My personal process:
Keep a min 50% mix of 100LL/MOGAS in one tank (maybe 75% in the winter), and whatever you end up with in the other tank (call this second tank your MOGAS tank). Do not switch sides for this procedure, once you start it. We don't need any extra confusion -- we have plenty to start with.

Use 100LL on the ground, taxi, takeoff, climb, descent, pattern, and landing.

On your long X-CYs, run the mogas tank dry, so you have plenty of 100LL left for the landing phase.

The ship with carbs handle the MOGAS easier than the FI equipped ships -- especially if you have a vapor return set up at the carb inlet. I don't recommend such a setup with FI.

Remember: MOGAS has a higher vapor pressure in the winter (more susceptible to vapor lock); this allows your car to start easier. You can use this to your advantage when starting and warming up your Lycosaur, but remember to switch to 100LL before taxi.

I have experimented with ethanol mixtures up to 30% (in summer), and found no difference in performance. Do not take this as a recommendation to ignore warnings against using MOGAS with ethanol in it -- the problem is corrosion associated.

Your spark plugs will last longer with this procedure, as the engine doesn't run on a constant diet of leaded fuel.

My 'toy' airplane burns 40GPH in cruise; 2.5x that on takeoff -- you can bet the farm I use MOGAS whenever possible, but I keep the mains with a min of 25% 100LL (to manage the vapor pressure), and follow the procedure lined out above. With 6:1 compression (supercharged to 37"MP on takeoff), she doesn't really care about octane -- the engines only require 78 octane.

Carry on!
Mark

Hey Mark,
Thanks for the thoughts on the mogas. Depending on the outcome of the rest of my testing, I'm kind of leaning toward the separate tanks concept too. I'll stay with 91 octane mogas with no ethanol IAW the superior manual and keep that in the right tank, with the left tank being 100LL. I will experiment around with the mogas idle some more, and may increase my idle speed up to closer to 750-800 RPM. If I'm not happy with the smoothness there, I'll probably just stick to 100LL for ground ops, takeoff, and landing as you recommend.

Mark

I have a carbed O-360, with a Bendix mag and an Electroair and it idles like a sewing machine on 87 pump gas. My idle is set to 550-600 RPM. Its been a few months since I've had to put 100LL in it, all of my flying has been local.

I'm curious as to why an FI engine will run rougher at idle on MOGAS. It might be a vapour pressure issue, but normally increasing vapour pressure improves combustion stability... One would assume that the vapour pressure of MOGAS is normally greater than AVGAS, especially in winter.

Does the rough running include a lower idle speed? If so, it could be a vapour-displacement effect dropping the airflow - effectively the same as closing the throttle a little further.

It might also be worth testing how an engine runs on MOGAS from cold, where the actual vapour pressure in the manifold will be significantly less.

FWIW

A

Typically, a FI engine heats up the fuel on top of the engine via the injection lines and may boil quicker when the engine is hot. The uneven fuel distribution is what causes the roughness. As soon as you hit the throttle it usually stops due to cool fuel replacing the hot fuel. A good run up prior to take off is a good idea.

The vapor pressure is higher in auto fuel, but this means it is more volatile.

I've switched to mogas years ago with no major problems.

Just remember not to store your plane with mogas. If you are not flying alot or put your plane away for the winter you should have 100LL in it. JMHO.

EDIT: Post corrected. LAG.

Mogas

With an IO360 and the pumps in the wingroots I very rarely get a rough idle..maybe a slight stumble in really hot wX when taxiing.

I run electronic ignition with a max advance of about 38 degress i think.

Take off is perfect runs the same as 100LL

CR is 8.5:1

Frank

With an IO360 and the pumps in the wingroots I very rarely get a rough idle..maybe a slight stumble in really hot wX when taxiing.
I run electronic ignition with a max advance of about 38 degress i think.

Take off is perfect runs the same as 100LL

CR is 8.5:1

Click to expand...

Frank, What RPM do you have your idle set to?

Mark

Umm

I dunno..depends on how far i got the mixture pulled out...about 550 if I pull the throttle out hard.

I really don't use the idle stop so it seems a bit irrelavent to me.

Gets a little lumpy at 540 RPM..

Frank

Geico266 said:

The vapor pressure is lower in auto fuel.

Click to expand...

This is not normally the case. As far as I understand the RVP of AVGAS is around 7-7.5 PSI, whereas summer Californian RFG has been legislated down to a max of about 7.0 PSI. Some winter grade fuels can go as high as 15 PSI, so MOGAS is normally quite a bit higher than AVGAS.

What

So at 15psi RVP...Mogas is boiling at sea level at 100F...Wow...sounds awfully high but if thats the case I'm sure glad I got my fuel pumps in the wingroots.

Of course having a winter grade fuel in your airplane on a 100F day is very unlikely..

Frank

Andy_RR said:

This is not normally the case. As far as I understand the RVP of AVGAS is around 7-7.5 PSI, whereas summer Californian RFG has been legislated down to a max of about 7.0 PSI. Some winter grade fuels can go as high as 15 PSI, so MOGAS is normally quite a bit higher than AVGAS.

Click to expand...

You are right. The mogas has a higher vapor pressure, but that means it is more volatile not less volatial. More prone to vapor lock under certain circumstances.

Andy_RR said:

This is not normally the case. As far as I understand the RVP of AVGAS is around 7-7.5 PSI, whereas summer Californian RFG has been legislated down to a max of about 7.0 PSI. Some winter grade fuels can go as high as 15 PSI, so MOGAS is normally quite a bit higher than AVGAS.

Click to expand...

Andy,

It depends on how VP is measured. I use the Hodges system and it tests the fuel sample in a vacuum and reports a kPa number. This is part of the STC requirement with certified airplanes and mogas. The test kit is available from Petersen Aviation.

The bottom number on the scale is 36kPa. The instructions say do not fly with a reading lower than that. kPa can be converted to an altitude with a chart available on the internet and 36 is good up to about 8000' at whatever temperature the fuel is tested at. If it cools, it is good for a higher altitude.

100LL usually tests around 62kPa, good for up to about 22000'. Most of the mogas I burned with the Subby came in at 48 to 52. I tested some E85 and it came in at 60, almost as good as 100LL. The temperature of the fuel of course is a factor, the warmer the fuel the lower it will test. Octane and ethanol have no affect on the vapor pressure test. Whatever it reads, that's it.

Mogas is most critical in spring when winter fuel blends are still on the market and the OAT hits 90. Winter fuel has tested in the low 30's in such a circumstance and I had to drain it.

Yes but

RVP is "reid vapour pressure" which is the measured VP at 100F...I guess they use 100F to provide a baseline.

So a sample of fuel RVP of 14.7psi will be boiling at sea level on a 100F day...depends on exactly where the altimeter is set of course...And this is with no sucking on the fuel at all.

At altitude the fuel would be boiling out of the tanks depending on temperature..scary..


Frank

Ethanol in Mogas??

Just curious.... is this Mogas from local airports or are you guys finding it somewhere locally without Ethanol in it?

Ethanol

Mogas without ethanol is available at lebanon airport in Western Oregon.

Personally I have been running 10% ethanol in my IO360 for about 6 months from the local gas station.

Frank

satisfied mogas customer

been running mogas for years with no problem. 50/50 mix in right side take off and land tank. 5/1 mix in left side cruize tank. i switch to landing tank about 5 minutes out from landing, and switch to cruize tank after reaching pattern altitude. have not found any ethanol in pump gas yet in south florida. i use regular sunoco.

Differnece in price of Mogas/Avgas = price of Airplane! (@ 2000 hrs)

Guys,

There is something I do not understand! All this vapor pressure and what have ye? Many times I have driven my car up the mountains in Austria, Switzerland, or France, in the spring or the summer, up to 3.000 m or even more (10.000 ft). Outside temperatures will have been 30?C (86?F) and more. My cars never had any problem with vapor lock.

The only vapor lock event I ever came across, was on holiday in Egypt. Temperatures were around 50?C (122?F). The old Peugeot 504 Taxi, we were in, stalled after sitting in a traffic jam for about half an hour. Temperatures under the black bonnet must have been double the outside temperature. After pouring a bottle of water over the carb, fuel pump and fuel lines, it started again and took us straight to the hotel.

Although stil building, I to, intend to fly on Mogas (EUR 1,08/ltr = approx. USD 5,70/gal), because Avgas is now still at EUR 2,30/ltr = approx. USD 12,20/gal.

My setup is as follows: OX-320, 8,5-1 compression ratio, with dual Lightspeed Plasma II El. Ign., Precision fuel injection, with return line to the tank (Andair double stack valve), electric boost pump (Vans setup) and a cooling shroud on the engine driven fuel pump. Of course all fuel lines will be insulated with fire sleaves. This should make sure that fuel temps never get as high as in the Peugeot!

The engine builder (Dachsel Flugmoteoren in Munich) ran the engine on the test stand, for several, hours with the return line installed and everything worked fine. I am looking foreward to the first flight, to test it myself (maybe this year?). First 25 hours have to use Avgas though,till the engine is broken in.

Regards, Tonny.

Exactly

But your car is not using 1930's technology mechanical fuel pump that is sucking on the fuel.

Use a higher VP gas, put it through a hot pump and you may well get vapour lock. Seems to happens all the time...But not to me.

Now the boost pump (still not ideal but in a cooler place at least) can be flipped on to prevent VL.

Now that means that under (admittedly) rare circumstances you are effectively relying on a single electric fuel pump because the mechanical one won't work.

hence the dual electric pump in the wingroots method..I've never seen vapour lock in close to 900 hours over two airplanes with this system and never tested the vapour pressure of any fuel..Although I will admit to caution if planning to fly up high on possibly Winter grade gas in the Summer.

As the gas station gets weekly deliveries thats pretty unlikely however.

Frank

I'm with Frank.... dual batteries, dual electronic ignitions and dual electric fuel pumps. No mechanical pump whatsever in my Rocket. I'm confident with this setup I'll be able to run ethanol-blend mogas.

Auto fuel & Hi comp pistons question

Not on topic but related.... I know all the gurus say don't burn auto fuel with hi comp pistons, however I have to believe that some are and I would like to know what their experience has been. I have 9.2-1 pistons in my carburetor 0-360 and access to auto fuel without ethanol and have thought about giving it a try....

Pilottonny said:

Guys,
There is something I do not understand! All this vapor pressure and what have ye? Many times I have driven my car up the mountains in Austria, Switzerland, or France, in the spring or the summer, up to 3.000 m or even more (10.000 ft). Outside temperatures will have been 30°C (86°F) and more. My cars never had any problem with vapor lock.

Click to expand...

The reason why modern cars don't vapour lock is mainly due to the high pressure fuel systems. Most port fuel injected engines run at least 3.0Bar (~45PSI) so fuel temperature has to get pretty high to cause vapour locking.

Many cars these days have strategies to deal with hot soak re-start conditions, including starting richer than normal and elevating the fuel pressure if it's controlled by the ECU. On development testing I've been involved in, I've seen fuel temperatures of above 100°C (212°F) and only at these points do you begin to see vapour formation issues when running high volatility fuels (RVP~15PSI)

Of course, the next question is why don't aeroplanes use this technology? I think mostly it surrounds the issue of electrical redundancy. Once your electric pump stops for whatever reason, so does your engine!

well..

638RS said:

Not on topic but related.... I know all the gurus say don't burn auto fuel with hi comp pistons, however I have to believe that some are and I would like to know what their experience has been. I have 9.2-1 pistons in my carburetor 0-360 and access to auto fuel without ethanol and have thought about giving it a try....

Click to expand...

Like most things in aviation things happen very slowly because we are a risk adverse bunch and quite rightly so. I honestly don't know the answer but I suspect it would be OK. The reason being is that RocketBob has been using 87 Octane in what I believe is a 8.5:1 CR engine without problem.

So it stands to reason that s lightly higher CR will be OK with 91 Octane fuel.

Now detonation is the thing we are trying to avoid. Detonation is a function of fuel octane, timing advance and temperature of the fuel-air mixture.

In other words, when the spark happens you want it to be lighting a bonfire from one side (or in this case two sides)..I.e the flame front progresses evenly across the mixture which gradually raises the cylinder pressure.

If the mixture is too hot, the the timing too far advanced or the fuel octane too low, the mixture will explode in a flash..i.e it all burns simultaneously..hence the cylinder pressure goes way high and possibly before the crank reaches TDC...Not good.

So what you could do is move your advance at full power back to say 20 degrees and run it like that until you have gained confidence..All the time carefully watching for runaway CHT's or EGT's.

This is not an OK to go do this..If there are others with direct experience to say its ok that would be best..but I suspect there won't be many examples.

FWIW

Frank

Andy_RR said:

Of course, the next question is why don't aeroplanes use this technology? I think mostly it surrounds the issue of electrical redundancy. Once your electric pump stops for whatever reason, so does your engine!

Click to expand...

The limitation is the mechanical fuel pump. If you eliminate that its just a matter of running higher pressures and smaller diameter injectors. The other benefit in that is now there are higher pressures in the metered side from the servo, which would help eliminate vapor lock there. I believe the AFP system can run upwards of 90psi. 90psi is nothing compared to my diesel car engine, which runs at 20,000psi.

Exactly

Andy_RR said:

The reason why modern cars don't vapour lock is mainly due to the high pressure fuel systems. Most port fuel injected engines run at 3.0Bar (~45PSI) so fuel temperature has to get pretty high to cause vapour locking.

Many cars these days have strategies to deal with hot soak re-start conditions, including starting richer than normal and elevating the fuel pressure if it's controlled by the ECU. On development testing I've been involved in, I've seen fuel temperatures of above 100?C (212?F) and only at these points do you begin to see vapour formation issues when running high volatility fuels (RVP~15PSI)

Of course, the next question is why don't aeroplanes use this technology? I think mostly it surrounds the issue of electrical redundancy. Once your electric pump stops for whatever reason, so does your engine!

Click to expand...

And we can emulate this as Rocketbob describes. If you use the AFP purge valve you'll never suffer from a hot start issue either..Well at least I have't so far..

Frank..IO360 Wingroot pumps only running 45psi at takeoff.

Mogas numbers

Well I had a chance to fly again today and get some better numbers to compare my 100LL performance to 91 octane mogas. I flew multiple climbs, and compared cruise numbers as well. For those that don't want to dig through the numbers, I'll cut to the chase: I don't see any performance difference between the two that would fall outside the margin of error.

7,500 feet 65% cruise 30 degrees LOP:

100LL: 7.8 gal/hr, 1448 highest EGT, 334 highest CHT, 160 KTAS.
91 mogas: 7.8 gal/hr, 1444 highest EGT, 326 highest CHT, 161 KTAS.

2,000 foot timed climb from 6-8K. 100 KIAS, WOT, 2600 RPM, mixture full rich, 1676 lbs gross weight:

100LL: 1395 ft/min, 1298 highest EGT, 362 highest CHT.
91 mogas: 1429 ft/min, 1341 highest EGT, 348 highest CHT.

Better performance!

Papa,

Your test shows better performance!

0,6% increese in cruise speed @ 2,4% lower CHT.
2,4% increase in climb @ 3,9 % lower CHT.

And you save money to!

Regards, Tonny.