Mogas/Autogas Considerations

[noema]

I have found a ton of information on engine consideration but not so much regarding the fuel system.

What makes the fuel system most compatible with autogas? Are there any optimizations that are advisable for the standard gravity-fed system and an O-360?

Looking at the Petersen Aviation website it mentions the following:

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Homebuilt Airplanes

Improve the fuel system of your airplane to reduce the likelihood of vapor lock by doing the following:

• Incorporate NASA vents into the vent system help provide positive pressure.
• Install fuel pumps in the fuel tanks, or as close to the fuel tank as possible. The tanks should have the maximum pressure and flow rating allowable for the carburetor.
• Replace any 90° fittings with 45° fittings or tubing with very smooth gradual bends.
• Make fittings as tight as possible to prevent air from entering the line.
• Secure lines to prevent vibration and harmonics.
• Insulate fuel lines in the engine compartment to prevent heat from soaking through to the fuel.
• Avoid locating fuel lines in close proximity to hot spots in the engine compartment.

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Most of it is a given but the advise about gradual bends and 45deg fittings might be worth looking into. What about 1/2" line downstream from the fuel selector? I have seen some of you do that. Is an increased diameter fuel line beneficial for autogas and robustness against vapor locks?

Curious what you have to say.

[davzLSA]

mogas is usually lower octane than avgas so that means your compression ratio in your engine will be lower so you will be running lower horse power. Otherwise your engine will suffer detonation. And as you pointed out you also run the risk of vapor lock.

[whee]

Petersen’s recommendations pretty much cover it. Bob’s fuel system meets the recommendations and works as is with mogas on a carbureted engine. Fuel pumps in the wings aren’t required because of the ram air vented fuel caps.

1/2” fuel lines won’t do anything to help a carbureted engine.

[zkelley2]

The 1/2 lines are much more forgiving for getting the minimum flow required for gravity feed.

[Bentmettle]

This is a bit interesting -

The head loss due to the tubing size picks up a few factors that are mentioned as things to manage- namely bend radius and inner diameter.

This is a decent enough primer https://www.pipeflow.com/pipe-pressu...ss-calculation

I'll just steal their equation
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Fluid head loss through a fitting can be calculated by the following equation:

h = K x v² / 2g

where
h = pressure loss in terms of fluid head, i.e. fluid head loss
K = manufacturer's published 'K' factor for the fitting
v = velocity of fluid
g = acceleration due to gravity

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To get the same total amount of fuel in the smaller lines, we need to flow at a higher velocity, which is going to have a Squared effect on pressure drop.

So smaller lines hurt on pressure drop. K factor for the 90 degree fittings or tubing bends is usually higher than a 45 or a gradual bend in the tubing. Multiple aspects of the systems suggestions appear aimed at giving the best odds of getting fuel at a sufficient rate to the carb.

This is sort of a me-too with extra math to the first comment

[whee]

I’m pretty familiar with Bernoulli.

The question was specific to mogas and a carbureted 360; 1/2” fuel lines aren’t going to do anything helpful in terms of mogas compatibility. The fuel system works perfectly fine with mogas and a carbureted 360.

Less than a handful of BHs are flying with 1/2” lines. They simply aren’t necessary. I put 1/2” lines in mine for very specific reasons; mogas was not among them.

[zkelley2]

But it does. While one should very much be able to get O-360 fuel flow numbers with 3/8 lines(I did), the mogas compatibility comes with the engine driven fuel pump. Mogas is not compatible with that due to the pump getting very warm and vapor lock. So you have to be able to gravity feed.

[zkelley2]

An alternative to running low compression pistons is to run a custom timing curve on electronic ignition. That's what I did. I run 90 octane(highest pump gas here) and have 2 maps. One for 100LL if I ever run it and want the bit of power back, and one for 90. The difference is 2 degrees retarded at WOT and 1 degree with the power back. It's a lot less HP loss than going to 8 or 7.5:1 pistons. And it's still a HP gain over stock at low power settings.
But our parallel valve 540s were certified on 91/96 to begin with.

[whee]

Zkelley, Why is a fuel pump required on a Lyc running mogas? If that is they case then I agree, 1/2” lines would help relieve that need. I haven’t paid close attention but it seems like the guys needing the engine pump are those with fuel flow transducers that are restrictive.

I have the same ignition system as you and planned to set my ignition maps the same way. However, I didn’t find a need to restart the timing for mogas. In fact I bumped up the timing significantly but never noticed a power difference. To be conservative I reset the timing to factory. Are you able to notice an actual difference?

[zkelley2]

Zkelley, Why is a fuel pump required on a Lyc running mogas? If that is they case then I agree, 1/2” lines would help relieve that need. I haven’t paid close attention but it seems like the guys needing the engine pump are those with fuel flow transducers that are restrictive.

I have the same ignition system as you and planned to set my ignition maps the same way. However, I didn’t find a need to restart the timing for mogas. In fact I bumped up the timing significantly but never noticed a power difference. To be conservative I reset the timing to factory. Are you able to notice an actual difference?

A fuel pump is required if you can't flow enough fuel from gravity to the carb. If I remember correctly, the formula is .55lbs/hr * HP and then you need 150% of that. If you can't get that number, you need a fuel pump. Lycoming engine driven fuel pumps get rather hot since they are attached to the accessory case, which can cause fuel vaporization upstream of the pump at the inlet.
So the importance of fuel line size and mogas is just that you can flow enough to not need that pump. You should be able to easily achieve that on an o-360 with 3/8" lines.

Regarding the SDS ignition, you shouldn't see a HP increase at max power. On 100LL you'd still have it set to factory at whatever max MAP and RPM is for your engine. The advance only comes in at below 65% power where the risk of detonation is more or less gone. You might see a knot or two more.(which checks with the math on HP required for a speed increase.) And if you're doing climbs where full power is now under 65%, you should see a increase in fpm. There'd be a greater increase in climb rate than there would be in airspeed since airspeed isn't really about power.
As far as retarding the timing for mogas, it'd be engine dependant, but with my engine wanting a bit more octane than I'm giving it, talking to Ross, he had me retard the timing slightly for the lower octane. We don't have knock sensors, so we can't go test to detonation. We have to stay far from it. He even gave me a map for 87, and it's a lot lower timing. Running any gasoline possibly available was a feature I wanted since not all of our airports in the bush have avgas available. They all have a pump with car gas. This way I don't have to run my 260hp engine at 230hp all the time and have wildly reduced high altitude performance like low compression pistons cause. It's only that low when I need it to be.

[whee]

Thanks zkelley.

There is a thread on VAF about the CPI and a guy messing with the timing. Seems like he got a few knots when he advanced the timing. I never noticed an increase but my airplane is more draggy than his RV. That’s why I was wondering if you noticed any difference.

Ross wasn’t able to help me much with my timing since it is engine dependent and he doesn’t have any experience with the Continental IO360. My curve is just flat 20deg. I played with it a bit and had it bumped to a flat 25deg but I moved to back after not finding any difference.

I’d like a curved map but I haven’t looked into how to properly develop one. I’m not sure where to start since at the elevations around here WOT only gives me 22” MP and so I run WOT all the time. I’m probably overthinking it but that’s kinda my thing. Haha.

My engine was certified for 100 octane (8.5:1 compression) but seems to be getting along fine with 91.

[zkelley2]

You should be able to get some more HP for takeoff and climb, by advancing the timing at those lower MAPs. If the 540 safe numbers can be brought over, you can probably get up to 4-5 degrees advanced at right around 22". Tapering back off to 0 by 26".

[svyolo]

If you are going to change the ingition advance, it is good to read a bunch of info that is out there. If you have injection that you balanced really well, you can run more advance safely. Without that balance, you can get detonation on the more lean cylinder(s). I think SDS and EFII say about an additional 5 degrees. But I think they both recommending running 3-5 degrees less advance for Mogas.


Sharp corners reduce flow, which seems obvious, but they also create turbulence, which can cause fuel vapors to come out of suspension. I think a fuel return is also preferred to purge any vapors that are created.

[noema]

Thanks for all your input. I take it an O-360 Bearhawk with standard fuel system should be fine on autogas.

If anybody has done some optimization to cover edge cases (say hot day, inadvertently got some fuel with ethanol into the tank) I would be interested to hear about it.

[Scheineman]

Hey Stefan I built and fly my O-360 8.5 CR Bearhawk four place with its designed 3/8 fuel lines gravity fed system with no problems. You can verify by a simple flow test. I also mix mogas 87 octane non ethanol with 100LL to raise the octane a little to meet Bob,s recommendation of minimum octane of 91/93 octane. He mixed mogas himself for years until he got a bad batch of gas. It really depends on your source. A chemical engineer once said that a little lead in gas, a 1/50 ratio can have significant effects on octane. Soo not willing to experiment with that little amount i discovered that even at a 20% mix in the winter the engine pistons are getting coated with to much lead. On another plane i owned i ran a tank of 93 octane 10 % ethanol gas and found the engine ran cooler , probably because of lower BTU output and had lower idle wanting to die out on the grd. Right now on my Bearhawk i,m working on reducing drag and getting closer to 150 mph TAS at 8K ft. I,m thinking it will go faster. Good luck, Wayne Scheineman

[svyolo]

I am not sure where you are located. In the states, different states have different blends of fuels, and they can vary quite a bit depending on summer/winter. In general winter blends are more volatile. Winter blend fuel, hot and high, are the worst combination if you are using fuel pumps. There are too many variables to be more exact, unless you go to the trouble of finding out the exact specs of the fuel you are using.

Gravity feed helps out. Ethanol is only bad if something in your fuel system has materials that are not compatible. It will run OK, but over time it will degrade some materials. It degrades some plastics, and I believe brass, but I can't remember for sure. I think it eats carbs.

[PaulSA]

Unfortunately, incompatible fuel system components is not the only issue with ethanol. If you get some water in the system with straight gas, it will mix quite happily but then some comes out of solution. This will settle to the low point and either eventually get drained off or a "plug" of water will go through the engine and in all probability just cause a bit of a cough.

With an ethanol mix, the water "prefers" going into solution with the ethanol rather than the gas - something to do with the triple-mix point that I used to know about 45 years ago at university! The upshot of this is that if it the water drops out of solution, it takes the ethanol with it - and ethanol is your anti-knock agent in blended fuels. Now you risk serious detonation issues as you have effectively reduced your octane rating.

[JimParker256]

I run Mogas (10% ethanol 93 octane) in my Rotax. Because of what Paul mentioned, if I were to see ANY visible water in a fuel sample, I would consider that entire fuel load contaminated, and would drain the tanks. (Of course, I would save the gas to use in my low-compression mower, because I'm a "typical" cheap pilot.) Thus far, I've seen zero water in my fuel samples. But then I fly often, and the fuel doesn't sit for long...