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Just tried MOGAS, didn't go well

Elwell81

Member
After getting the first 90 hours on my -9A, I decided to tried the 91 Octane autogas that is sold at my home airport. Being overly cautious, I completely drained the left tank of 100LL and filled it with autogas. On the next flight, I took-off on the right tank (100LL) and once leveled off and with everything stabilized, I switched over to the left tank. About 30-45 seconds later, I felt a noticeable loss of power and the engine began surging from cruise power to lower power. I had the engine page up on the PFD during the tank switch just in case and the fuel pressure was dropping, accompanying the power surge. My IO-320's fuel pressure usually runs at a steady 18 psi, but it was dropping to 8-9 psi for a second, then back up to normal, then down again (again with a power loss).

Flipping on the boost pump (Andair) instantly brought the fuel pressure up to 29-30 psi and the engine ran normally. Turning it off, same symptoms returned. Going back to the right tank, after the 91 octane got run out of the lines, fuel pressure stayed at 18 psi like normal so it would seem the 91 octane was the cause? To make sure it wasn't some problem I induced while draining the tank, I ran on the left (91 octane) tank with the boost pump on, way ROP to burn as much as I could on this leg so I could refill with 100LL.

Upon refilling tank with 100LL, I had approx. 1/4 tank of 91 left so it was much more 100LL than 91 octane. On the next leg, I performed the same test. The fuel pressure still fluctuated, but nowhere near as much. I've since refilled 2 more time so it should be back to nothing but 100LL. Now the fuel pressure is running 18 psi on both tanks, just like before this experiment.

I'm just wondering what is going on with this? I've read countless posts about people running auto gas with no problems at all. What could be causing a problem specific to this plane?
 
Being overly cautious, I completely drained the left tank of 100LL and filled it with autogas. On the next flight, I took-off on the right tank (100LL) and once leveled off and with everything stabilized, I switched over to the left tank.

^ Excellent risk mitigation in evidence here.
 
Auto Gas Issues

After getting the first 90 hours on my -9A, I decided to tried the 91 Octane autogas that is sold at my home airport. Being overly cautious, I completely drained the left tank of 100LL and filled it with autogas. On the next flight, I took-off on the right tank (100LL) and once leveled off and with everything stabilized, I switched over to the left tank. About 30-45 seconds later, I felt a noticeable loss of power and the engine began surging from cruise power to lower power. I had the engine page up on the PFD during the tank switch just in case and the fuel pressure was dropping, accompanying the power surge. My IO-320's fuel pressure usually runs at a steady 18 psi, but it was dropping to 8-9 psi for a second, then back up to normal, then down again (again with a power loss).

Flipping on the boost pump (Andair) instantly brought the fuel pressure up to 29-30 psi and the engine ran normally. Turning it off, same symptoms returned. Going back to the right tank, after the 91 octane got run out of the lines, fuel pressure stayed at 18 psi like normal so it would seem the 91 octane was the cause? To make sure it wasn't some problem I induced while draining the tank, I ran on the left (91 octane) tank with the boost pump on, way ROP to burn as much as I could on this leg so I could refill with 100LL.

Upon refilling tank with 100LL, I had approx. 1/4 tank of 91 left so it was much more 100LL than 91 octane. On the next leg, I performed the same test. The fuel pressure still fluctuated, but nowhere near as much. I've since refilled 2 more time so it should be back to nothing but 100LL. Now the fuel pressure is running 18 psi on both tanks, just like before this experiment.

I'm just wondering what is going on with this? I've read countless posts about people running auto gas with no problems at all. What could be causing a problem specific to this plane?

Some things to look for:

1. Do not use 90* fittings on the INPUT side to the engine mounted fuel pump. If the fuel flow turns suddenly, you will get cavitation (air bubbles) which cause loss of mechanical fuel pump pressure. Same issue with the output of a firewall mounted gascolator, no 90* fittings. All fuel line runs inside the engine compartment hot area should only have gradual turns (i.e., radiused runs). This is not an issue of fuel line runs between the mechanical fuel pump and the fuel body.

2. Consider a cooling showd over the mechanical fuel pump with cooling air.

3. Be sure that you DO NOT have anything in the fuel lines between both tanks and the mechanical fuel pump that causes any excessive fuel pressure drop. Sources of these pressure drops include paper fuel filters, too small or restrictive fuel lines (for the required fuel flow), and in some cases, the AUX fuel pump itself. The more suction that the mechanical pump has to pull to get the required fuel flow, the more the possibility of fuel vaporization in that line. You stated that turning ON the AUX pump appeared to solve the issue. This leads me to believe you have something in the fuel line between that pump and the mechanical engine mounted pump that represents a pressure drop.

I've been running 93 OCT E10 autogas with no issues for over three years....
 
Some things to look for:

1. Do not use 90* fittings on the INPUT side to the engine mounted fuel pump. If the fuel flow turns suddenly, you will get cavitation (air bubbles) which cause loss of mechanical fuel pump pressure. Same issue with the output of a firewall mounted gascolator, no 90* fittings. All fuel line runs inside the engine compartment hot area should only have gradual turns (i.e., radiused runs). This is not an issue of fuel line runs between the mechanical fuel pump and the fuel body.

How can a 90 turn cause air bubbles? Are they somehow coming out of solution?

-Dan
 
I'm ready to learn on this one...

The specific gravity of AVGAS is right in the middle of the 100LL specific gravity range.

Why would any pump behave differently with a fluid of same density?
 
I use 93 E10 in my IO-320 (AFM FM 100 system) about 70% of the time and there are several known issues with using auto fuel in fuel injected engines on acft that have to be taken into account before using it. Auto fuel typically will "boil" at lower temperature and pressures that 100LL, meaning that instead of running through the fuel lines as a liquid, it may actually begin to boil and produce bubbles fuel, which causes fuel pressure fluctuations, extreme cases vapor lock. At ambient temps about 90 F, I have to run my aux pump continuously on the ground to avoid pressure fluctuations and a very rough idle. Once in the air and things have cooled off, I typically shut off the aux pump with no pressure fluctuations. I see 23-25 psi from my mech fuel pump and 28 to 35 psi with the aux pump on, so 18 psi seems a bit low, which may be part of the issue. In order to avoid boiling the fuel, it needs to be pressurized and kept cool. Suggest looking at the fuel lines from the tank to the pump to ensure that there are no leaks, which in a suction type system will draw air in, causing aeration. Also, for a fuel injected engine, I would definitely look at why you have 18 psi instead of something closer to 23 to 25 psi or whatever the Mfg requires.
 
The pump is not behaving differently - the vapor pressure of the fuel is changing. 100LL has a lower vapor pressure than mogas, meaning that mogas will boil easier. Boiling occurs not only at elevated temperature, but also at reduced pressure. If you have restrictions of ANY kind in your fuel line, that will create a region of lowered pressure as the fuel flows through that restriction. If the fuel is warm enough, it can create its own vapor bubbles in the fuel, and the fuel pump does not do well at all with vapor. The fuel pump itself is bolted to that great big heating element up front, and the fuel will pick up heat in the pump itself as well, making the problem worse.

Keeping your fuel line routing as clean as possible with cooling air on your fuel pump will help, but it's going to be trial and error to eliminate the cause. You'll notice a difference between summer and winter fuel blends also, the winter blend will be more likely to cause the problem you are seeing.
 
Thanks for the suggestions

Some things to look for:

1. Do not use 90* fittings on the INPUT side to the engine mounted fuel pump. If the fuel flow turns suddenly, you will get cavitation (air bubbles) which cause loss of mechanical fuel pump pressure. Same issue with the output of a firewall mounted gascolator, no 90* fittings. All fuel line runs inside the engine compartment hot area should only have gradual turns (i.e., radiused runs). This is not an issue of fuel line runs between the mechanical fuel pump and the fuel body.

2. Consider a cooling showd over the mechanical fuel pump with cooling air.

3. Be sure that you DO NOT have anything in the fuel lines between both tanks and the mechanical fuel pump that causes any excessive fuel pressure drop. Sources of these pressure drops include paper fuel filters, too small or restrictive fuel lines (for the required fuel flow), and in some cases, the AUX fuel pump itself. The more suction that the mechanical pump has to pull to get the required fuel flow, the more the possibility of fuel vaporization in that line. You stated that turning ON the AUX pump appeared to solve the issue. This leads me to believe you have something in the fuel line between that pump and the mechanical engine mounted pump that represents a pressure drop.

I've been running 93 OCT E10 autogas with no issues for over three years....


There are no 90 degree fittings in the engine compartment. There is only a about a 2' run from the AUX pump to the mechanical pump with nothing in between.

I understand the fact that excess restriction can cause issues and ran my fuel lines accordingly. The only thing different in my fuel system is an Andair gascolator (for easily servicable filter) between the tank selector valve and Aux. pump.

Temperature also seems like it wouldn't be an issue because OAT was 25 degrees. Running on 100LL this summer with 90 plus degrees OAT, I never had any hint of an issue.

Is the difference between auto gas and 100LL that substantial, that I have no issues 100LL but can hardly pump 91 octane?
 
If your auto gas was bought locally in Green Bay, you probably got winter gas. Auto gas is seasonally adjusted to make it easier to vaporize in cold winter conditions - just as you experienced.
 
I know it's unlikely, but any chance you got 'old' gas? How often does the mogas supply cycle at your field? I've run 91 octane for many years (carb'd engine), & the only glitch I've had was when I was (stupidly) running winter blend gas on a 100+ degree day, after the engine/cowl heat soaked on a ramp.

Charlie
 
Auto Gas and 90* fittings

How can a 90 turn cause air bubbles? Are they somehow coming out of solution?

-Dan

Auto gas, especially winter blended fuel, has a lower vapor level rating. The sudden turn in flow will cause the fuel to "boil", especially at a low fuel pressure like you have before the mechanical fuel pump. ANY type of restriction to flow between the tank and the mechanical pump will make the situation worse because the fuel pressure will be lower.
 
Winter Gas

Years ago I had a Piper Cherokee with an auto gas STC. On a warm day one February I had some "stumbles" climbing out and during taxi after landing I could hear the electric fuel pump running furiously and not pumping up pressure as prior flights (no fuel PSI gauge so this was subjective). I attribute this to "winter gas" and was the last time I used auto gas in the plane. A carbureted engine but still a data point. You said that the OAT was 25 C. Pretty warm for this time of year.
 
Like Bob said, this is purely a function of the Reid Vapor Pressure (RVP) of your winter-blend mogas. This means at any given temperature and pressure, it will be more susceptible than avgas to boil in the fuel pump and/or fuel lines and cause the symptoms you described. In other words, the gas boils at a lower temperature than Avgas.

Avgas has a tightly controlled (and relatively high) RVP. Mogas, even that which is ASTM-spec, has a wider tolerance band for RVP, the center of which changes by season and location. There used to be a commonly available RVP measurement device one could purchase, similar to a brake bleeder, to measure what came out of the local gas pump.

Anecdotally, I ran mogas in my RV9A for the first 20-30 hours. One day, after about an hour break on the ground, the outside temperature had climbed a bit and I hopped back in the airplane to head for home. About 600 feet off the runway the engine started doing exactly what you described. Quite the attention-getter when you're close to the ground. Lowering the nose a bit and pulling some power out reduced the under-cowl heat enough to get things working well again. That event sparked my research into RVP and my subsequent decision to only fly 100LL in this airplane.

Research the issue, keep testing SAFELY, and you'll find what works for you. My guess is you'll have a much different (better) experience with summer blend mogas.
 
Option

Instead of putting 100% mogas in one side, try a 50/50 mix or maybe 75/25 mix for the mogas side. It doesn't take much avgas to "help" mogas burn well.

cj
 
If the gascolator is forward of the firewall as most are, it's a pre-warmer for the fuel. It soaks up heat, then heats the fuel, which increases the potential for the fuel boiling in the lines. The presence of a gascolator may be at least contributory to the symptoms reported.
 
Auto Gas

There are no 90 degree fittings in the engine compartment. There is only a about a 2' run from the AUX pump to the mechanical pump with nothing in between.

I understand the fact that excess restriction can cause issues and ran my fuel lines accordingly. The only thing different in my fuel system is an Andair gascolator (for easily servicable filter) between the tank selector valve and Aux. pump.

Temperature also seems like it wouldn't be an issue because OAT was 25 degrees. Running on 100LL this summer with 90 plus degrees OAT, I never had any hint of an issue.

Is the difference between auto gas and 100LL that substantial, that I have no issues 100LL but can hardly pump 91 octane?

So measure the fuel pressure (which will be negative) and fuel temps at the input to the mechanical fuel pump. That will be the indicator to what is causing your issue...
 
How can a 90 turn cause air bubbles? Are they somehow coming out of solution?

-Dan

To answer this specific question--

As others have mentioned, it is a function of pressure drops in the system and the vapor pressure of the fluid being moved. The bubbles in the system caused by this mechanism are not air, but fuel being vaporized due to the low pressure at the discharge of the fitting.

Cavitation can be a serious problem in any kind of fluid system--in my professional experience it has been seen mostly in water pump systems with disastrous results for pump impellers and other components. Here's an example of what cavitation can do:
fig604.jpg
This is a result of tiny bubbles forming and collapsing at the impeller surface due to the local static pressure being below the fluid's vapor pressure at the operating temperature. When the microbubbles collapse they can create very powerful shock waves that can damage metal surfaces. The picture is of the rotor of a water turbine on the discharge (low pressure) side, but the same type of damage happens to pumps on the suction (low pressure) side.

Here's a link to a brief description of cavitation.
http://www.engineeringtoolbox.com/cavitation-d_407.html

And another (the source of the above linked image):
http://authors.library.caltech.edu/25019/1/chap6.htm

Cavitation in our fuel lines isn't likely to result in terrible damage like this (unless it happens in the fuel pump itself, perhaps), but the same principle is behind vapor bubbles forming in fuel lines near restrictions that induce pressure drops.
 
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quote:If the gascolator is forward of the firewall as most are, it's a pre-warmer for the fuel. It soaks up heat, then heats the fuel, which increases the potential for the fuel boiling in the lines. The presence of a gascolator may be at least contributory to the symptoms reported.
unquote

And, it's an extra restriction, even though it should be a small one. And, from the previous description, it's prior to the boost pump; does that mean that the boost pump is FWF, as well? That would be an *additional* restriction in a heat soaked area when it's not running, and another source of heat to the fuel (the pump gets heat soaked, too).

Some alt engine guys are now putting all the pumping operations (electric pumps only; no mech pump) on the cool side of the firewall, so only pressurized fuel is exposed to engine heat.

To me, it still sounds like a problem who's cause is more than simply the vapor pressure of mogas. There are too many successful systems flying for it to be that simple. Bad gas, restriction in the line, gascolator, configuration, etc etc. Yes, mogas vapor pressure is higher, but not that much higher. My problem happened with winter blend in OAT of 100+ F, and only after flying, landing, and heat soaking.

Charlie
 
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Some alt engine guys are now putting all the pumping operations (electric pumps only; no mech pump) on the cool side of the firewall, so only pressurized fuel is exposed to engine heat.
Charlie

Not just the alt engine guys - many of us with Lycosaurs (myself included) have done this.
 
Isn't 18psi low to begin with? Isn't 23-27 most normal?

This got me thinking. I went back through all of my G3X touch data-logs since first flight (April 2015) and noticed a trend. The fuel pressure started at 24 psi in cruise, then in the early August time frame began to drop slowly (slowly enough I never noticed a difference) to 18 psi, where it is now. I have changed the gascolator filter since then so I'm confident its not a filter issue but I'm going to check it again. I'm also going to make up a test hose to bypass the gascolator and AUX pump to make sure I'm not getting to much restriction through them. I have doubts that this is the case but have to check to be sure. But definitely need to find out what happened to 6 psi of fuel pressure. The autogas just magnified the issue.

If the gascolator is forward of the firewall as most are, it's a pre-warmer for the fuel. It soaks up heat, then heats the fuel, which increases the potential for the fuel boiling in the lines. The presence of a gascolator may be at least contributory to the symptoms reported.

I installed my gascolator in the cabin, under the tank selector valve for that very reason.
 
This got me thinking. I went back through all of my G3X touch data-logs since first flight (April 2015) and noticed a trend. The fuel pressure started at 24 psi in cruise, then in the early August time frame began to drop slowly (slowly enough I never noticed a difference) to 18 psi, where it is now. I have changed the gascolator filter since then so I'm confident its not a filter issue but I'm going to check it again. I'm also going to make up a test hose to bypass the gascolator and AUX pump to make sure I'm not getting to much restriction through them. I have doubts that this is the case but have to check to be sure. But definitely need to find out what happened to 6 psi of fuel pressure. The autogas just magnified the issue.

Fill a tank with 100LL and see if the pressure returns when running on that tank.
 
Good Test Protocol!

You might get a sample of the fuel and test it for boiling point. A hand vacuum pump from HF and the adapter can do this. You must know the temperature of the fuel, but put a sample in the small clear container and reduce pressure with the vacuum pump until it "boils". Do the same with the 100LL for reference. Let us know the results.

I don't use mogas (yet) but the above is a comment from another VAFer that does and he tests each batch to ensure it is suitable for use. It eliminates guessing and hoping.

Lycoming recommends a maximum of 2 psi pressure drop for fuel into the mechanical pump. It would have to be measured with appropriate flow. I think they say 16 gph for the 180 hp 360.

EDIT: Go to the December 2013 issue and read our PeteHowell's writeup on mogas.
 
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A few different ideas

Most of the suggestions here are right, such as:

1) your mechanical pump pressure is a little low to begin with -- mine runs 24 psi. So might be a marginal mechanical pump

2) you probably got winter gas, which does have high "front-end volatility" and will easily vaporize/cavitate

But here is an experience I had a few weeks ago that rings similar.
During condition inspection, I removed the Airflow Performance fuel filter for inspection and cleaning. I did not turn the fuel selector off first, as I probably should have, so with the line open at the filter, the fuel lines drained back to the tanks. After reassembly, and doing some other inspections, I did a test run.
I found that the mechanical pump was only providing 10 psi and the engine wouldn't run smoothly above idle. I turned the boost pump on, and the pressure came right up to 25 psi (normal). I turned the boost pump off, pressure dropped back to 10-11 psi. Boost pump on, pressure came right up. Boost pump off, pressure dropped again. Then, I put the boost pump on, increased engine RPM to 1900, let it run there for a minute, then turned the boost pump off. Pressure held normal. Dropped RPM to idle, pressure normal (on mechanical pump still).
At first I was really puzzled. Seems like even with the lines drained, turning the boost pump on was able to 'prime' the mechanical pump, so why did pressure fall again when the boost pump was turned off?
My belief is that there was a large slug of air in the fuel filter (not fuel vapor, but air). The boost pump was able to suck fuel past the large air bubble in the filter and achieve normal pressure, but the mechanical pump could not, it essentially lost prime because of the large air bubble. When I increased fuel flow by increasing RPM, plus just more running time, I eventually sucked that air bubble out, got the lines full, and everything is normal.
Just to be sure it wasn't a coincidental failure of the mechanical pump, I have been watching it like a hawk. About 5 hours of flight time, and it is acting normally.

So, consider that by draining your tank, you got a bunch of air in your line from the fuel selector to the tank. Then, same as me, running on that tank un-primed the mechanical pump. Like me, it may be that your boost pump was able to pull enough fuel past the air bubble to maintain pressure, but your mechanical pump couldn't. (it would have pulled the bubble through the small line, into the filter or gascolator)
It may be that by the time you ran off 3/4 of a tank with the boost pump on, that bubble was gone, and if you had turned the boost pump off at that point, the mechanical pump may have worked fine with the mogas.

Then, you mention seeing similar symptoms when you put 100LL back in. Hmmm, but you didn't drain the tank again, did you? Just topped it up with AVGAS? So this may kill my theory --suggests that it was fuel cavitation somewhere, just less so because of the avgas mix reducing the volatility some. But something to think about.

PS I will continue watching my pump pressure closely
 
I also tried mo gas and read everything I wanted to read - all positive. Only ran auto gas at cruise. And guess what it did nothing good. Power surges, and finally a power outage when I switched tanks.

It's not worth it. I could have died.
 
Running fine

My 92 octane Marine fule runs fine. No detectable difference. NONE! I have one tank with avgas and the other with 92 octane. NOTHING DIFFERENT! NO 90 bends, no sharp turns in the fuel lines and yes it does cavitate if you have them. Just put a piece of clear plastic tubing on the end of the fuel line before it goes in the throttle body/ carb and you will see them!
 
This got me thinking. I went back through all of my G3X touch data-logs since first flight (April 2015) and noticed a trend. The fuel pressure started at 24 psi in cruise, then in the early August time frame began to drop slowly (slowly enough I never noticed a difference) to 18 psi, where it is now. I have changed the gascolator filter since then so I'm confident its not a filter issue but I'm going to check it again. I'm also going to make up a test hose to bypass the gascolator and AUX pump to make sure I'm not getting to much restriction through them. I have doubts that this is the case but have to check to be sure. But definitely need to find out what happened to 6 psi of fuel pressure. The autogas just magnified the issue.



I installed my gascolator in the cabin, under the tank selector valve for that very reason.

I have no gascolator, don't believe it is necessary with Fi. A large filter, yes, but no gascolator.

Your experience with MOGAS is unusual. It should be ok with winter temperatures, lots of guys use it all year round. You may have a pump pressure problem, I see 30 psi with electric on, 28 with it off. 18 psi is very low for Lycoming FI.
 
Some thoughts - not always directly related, but hopefully worth knowing.

1) Did you know that you can cheaply & easily test fuels for ethanol content yourself ?

- Get a tall, thin graduated glass container. Half fill with water, then top up to full with your chosen fuel. Mark the intersection line of the 2 fluids with a sharpie pen. Leave overnight and check for a movement of the intersection against the line. If movement is observed it is because the Ethanol in the fuel has absorbed some of the water. More movement = more ethanol.

2) The key problem with Mogas is its lower vapour pressure. In practice this means that you are MUCH more at risk of fuel vaporisation than with 100LL. Heating the fuel is the enemy and should be controlled at all costs.

3) Ethanol in motor fuels absorbs water (see "1"). Water is corrosive, Vans RV's have aluminium fuel tanks and fuels lines.

4) When you leave your aeroplane in the sun the fuel in the tanks is being warmed. Solar Absorptivity for Black = 0.94, Pure White = 0.12

5) Back to Vapour Pressure. If you dilute 100LL with mogas by even 5% the vapour pressure of the mixture becomes virtually the same as pure mogas.

(Apologies to the many on here who probably know all this)

My personal experience is that my RV was specified and built from the outset for (currently illegal) running on Mogas. For me this meant

- Use of ethanol tollerant components (In particular the Polymer fuel system parts)
- Purchase of a 160hp engine with an 8.5:1 compression ratio and latest generation cylinders
- A "2 tank" strategy - 1 always pure 100LL, 1 sometimes Mogas / Mogas Mix
- Always use "Shell V Power" Mogas (a UK brand) - Shell spec is nil or max 5% ethanol, 99 RON octane. It lacks the additives found in 100LL.
- Andair electric boost pump & Lyco Mech Pump, F.I. & no gascalator.
- Never run @ > 24" manifold or > 1250f EGT on mogas
- Never Run Mogas @ less than 1500' AGL
- Never take off or Land on Mogas
- Always run at least 5 mins on 100LL every time out (Valve stem lube in 100LL)
- I have minimised the amount of fuel "in the system" to make changeover of fuel types as fast as possible - I estimate that switching tanks takes about 20 seconds (0.3l system volume @ 45 LPH flow rate) to be effective ... provided the engine keeps running of course !

By and large this has worked well for me for about 60 hours. However, I have experienced fuel vapourisation problems on hot (at least by our standards) days.

MOGAS is in my opinion not without risk - you should only take risks that you fully understand and can afford to loose. (for instance - flying over water, adverse terrain, high ambient temps, high altitudes etc)
 
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You may want to experiment with some mogas from a different source just to eliminate a fuel problem as a potential issue.

I got a tank of gas in 2013 that caused what felt exactly like vapor lock symptoms. After draining the fuel and running the engine long enough to clear the bad gas out of the system the engine performed normally, and has for years. I've never had a repeat of this problem.

I filled my riding lawn mower with some of the fuel I had drained out and mowed about 100' before the engine quit like I had turned the ignition switch off. I assume it took that long for the existing gas in the lines to be burned up and the drained fuel to reach the engine.

As a test I put some of this fuel in my truck and the engine ran OK, but the truck shook like a dog at idle. I added as much high octane fuel as I could to the tank and the truck idled normally after that as soon as the mixture sloshed around a bit.

I spoke with a testing lab about having the fuel I drained out tested, but $1000 was more than I wanted to spend to satisfy my curiosity.

This was summer blend and tests for water in the fuel were negative.
 
Gascolators

Good discussion here. I enjoyed the thoughtful post. I have the Doll's G3X redline warning for fuel pressure set a 18 psi. For me, that is an absolute minimum. So that needs to be addressed.

The grandfather worship for the use of gascolators in RV's has always baffled me. They are great for gravity feed systems like C-150's and Cubs, but gascolators are to be placed at the low point in the system so any water can be stopped there. The low point in RVs are the tank drains. That is especially true for taildragers. When mounted on the firewall, gascolators serve as a poor filter and a great heat sink, and IMHO, are just inviting trouble.

Certified Grumman low wing singles do not have gascolators because, like RVs, the low point in the system are the fuel tank drains. If I were you, I'd remove the gascolator and have just a fire shielded fuel line between the firewall fitting and the engine driven fuel pump.
 
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To answer this specific question--

As others have mentioned, it is a function of pressure drops in the system and the vapor pressure of the fluid being moved. The bubbles in the system caused by this mechanism are not air, but fuel being vaporized due to the low pressure at the discharge of the fitting.

Cavitation can be a serious problem in any kind of fluid system--in my professional experience it has been seen mostly in water pump systems with disastrous results for pump impellers and other components. Here's an example of what cavitation can do:
fig604.jpg
This is a result of tiny bubbles forming and collapsing at the impeller surface due to the local static pressure being below the fluid's vapor pressure at the operating temperature. When the microbubbles collapse they can create very powerful shock waves that can damage metal surfaces. The picture is of the rotor of a water turbine on the discharge (low pressure) side, but the same type of damage happens to pumps on the suction (low pressure) side.

Here's a link to a brief description of cavitation.
http://www.engineeringtoolbox.com/cavitation-d_407.html

And another (the source of the above linked image):
http://authors.library.caltech.edu/25019/1/chap6.htm

Cavitation in our fuel lines isn't likely to result in terrible damage like this (unless it happens in the fuel pump itself, perhaps), but the same principle is behind vapor bubbles forming in fuel lines near restrictions that induce pressure drops.

Thanks, that is a great explanation!

KBVI? I've flown my Cardinal into KBVI a few times. Really nice airport. I have family in East Palestine. Coming up for Christmas (flying united this time).

-Dan
 
I appreciate all of the input

The only reason I installed the gascolator is I like the serviceable aspect of it. With the Andair gascolator, I can remove/clean/replace the filter element without undoing any fuel fittings. It gives me the opportunity to know what I have as far as contamination, and a simple inline replaceable filter doesn't do that.

As I said in previous post, I installed the gascolator in the cabin, under the fuel selector valve to prevent a heating issue.

Autogas is on the back burner now (no pun intended). I need to get my fuel pressure sorted out. I think trying the autogas just magnified an issue that I was already experiencing.

I ordered a new mechanical fuel pump to try out. Best case I get my fuel pressure back, worst case is I have a spare fuel pump. I'm finding it hard to believe there is a restriction problem upstream of the mechanical fuel pump causing this, but stranger things have happened. I'll update when I get the new pump installed.
 
Certified Grumman low wing singles do not have gascolators because, like RVs, the low point in the system are the fuel tank drains. If I were you, I'd remove the gascolator and have just a fire shielded fuel line between the firewall fitting and the engine driven fuel pump.

Just another data point.......

Certified Pipers (the low wing kind) with Carb'ed engines do have gascolators, even though the apparent lowest point on those is also the sump drains at the tanks.

I don't disagree that the gascolator can be a source of fuel heating, but thousands of RV-'s flying successfully with them proves it is not a bad design (I flew one in the heat of Phx AZ for 5 years).

In this case the OP says the gascolator is aft of the firewall, so it is not likely contributing to the problem.
 
.....
Certified Grumman low wing singles do not have gascolators because, like RVs, the low point in the system are the fuel tank drains. If I were you, I'd remove the gascolator and have just a fire shielded fuel line between the firewall fitting and the engine driven fuel pump.

They sort of have a gascolator...:)

There is a similar sized filter than needs service every inspection integral to the firewall mounted electric fuel boost pump. Facet rates it as a 74 micron filter.

1132.JPG


As you mention, it is not the lowest point in the fuel system, but it is the equivalent of a gascolator filter after the fuel pump.

I'm assuming that the "gascolator" functions both as a water collection point and as a filter.
 
The ethanol test can be done in less than 2 minutes, and more reliably, if you have a capped container. I use an olive jar, ~1.5" dia ~6" tall. ~1" of water, up to my permanent mark on the jar. Fill with gas. Shake vigorously; let it sit for ~30 seconds & watch the water settle out. E-gas will show ~1/8-1/4" rise in the 'water' level at the bottom, depending on how much ethanol is in the gas.

IMO, the whole vapor pressure issue is a *hardware system* issue; not a fuel issue, as was hinted at in a recent post about planes like the RV-12, which were designed for auto e-gas. Since the advent of fuel injection in cars, is there anyone reading this who can say that they've ever had vapor lock (edit: in their fuel injected car)? I'm old enough to remember carb'd cars having the problem fairly often, and guess how a typical Lycoming engine delivers its fuel. (I'll make it easy: it uses an ancient auto style diaphragm pump mounted on the engine (where it's nice & HOT) to *suck* fuel *up* a long skinny line from the fuel source.) Supply fuel under at least minimal positive head to the pump, make sure that all fuel forward of the firewall is under pressure, and no fuel injected engine will ever experience vapor lock as long as it isn't on fire. The above would *not* apply to carb'd engines; you can't pressurize the float bowl.

Is there anyone who's running one of the auto style injection systems (using only redundant electric fuel pumps mounted in the cabin) who's ever experienced vapor lock? I consider this a valid point, because even 100LL can, and does, vapor lock in some situations (see above).

Charlie
 
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Do you have a source for this? Sounds like an interesting read.

I don't think that's true. I had heard this before and set about to test various ratios. The full thread is here. My tests would conclude that the above statement is not correct.

Summary: Adding a little 100LL to mogas disproportionally raises the vapor pressure. Pure mogas for this test tested at 16", 100LL tested at 21", and a ratio of 50/50 tested at about 20.2".

AvgasMixGraph.png


There's more to it, so look at the linked thread if you're so inclined.
 
I don't think that's true. I had heard this before and set about to test various ratios. The full thread is here. My tests would conclude that the above statement is not correct.

That was my understanding too, Brad. I was hoping to read the other side of the coin if one exists.

Edit: Found some factual references to compare RVP between mogas and 100LL.

Shell (here) lists RVP for 100LL at 38-49 kpa (5.5-7.1 psi) at 38C (100F).

The ASTM spec for auto fuel costs $$ to purchase, but in this paper by CARB, the ASTM values for certain areas of California are published. (The numbers are also in psi at 100F.) You can see how they vary by month and area, and how in general are higher than 100LL (especially in winter).
 
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Some thoughts - not always directly related, but hopefully worth knowing.

1) Did you know that you can cheaply & easily test fuels for ethanol content yourself ?

- Get a tall, thin graduated glass container. Half fill with water, then top up to full with your chosen fuel. Mark the intersection line of the 2 fluids with a sharpie pen. Leave overnight and check for a movement of the intersection against the line. If movement is observed it is because the Ethanol in the fuel has absorbed some of the water. More movement = more ethanol.

2) The key problem with Mogas is its lower vapour pressure. In practice this means that you are MUCH more at risk of fuel vaporisation than with 100LL. Heating the fuel is the enemy and should be controlled at all costs.

3) Ethanol in motor fuels absorbs water (see "1"). Water is corrosive, Vans RV's have aluminium fuel tanks and fuels lines.

4) When you leave your aeroplane in the sun the fuel in the tanks is being warmed. Solar Absorptivity for Black = 0.94, Pure White = 0.12

5) Back to Vapour Pressure. If you dilute 100LL with mogas by even 5% the vapour pressure of the mixture becomes virtually the same as pure mogas.

(Apologies to the many on here who probably know all this)

My personal experience is that my RV was specified and built from the outset for (currently illegal) running on Mogas. For me this meant

- Use of ethanol tollerant components (In particular the Polymer fuel system parts)
- Purchase of a 160hp engine with an 8.5:1 compression ratio and latest generation cylinders
- A "2 tank" strategy - 1 always pure 100LL, 1 sometimes Mogas / Mogas Mix
- Always use "Shell V Power" Mogas (a UK brand) - Shell spec is nil or max 5% ethanol, 99 RON octane. It lacks the additives found in 100LL.
- Andair electric boost pump & Lyco Mech Pump, F.I. & no gascalator.
- Never run @ > 24" manifold or > 1250f EGT on mogas
- Never Run Mogas @ less than 1500' AGL
- Never take off or Land on Mogas
- Always run at least 5 mins on 100LL every time out (Valve stem lube in 100LL)
- I have minimised the amount of fuel "in the system" to make changeover of fuel types as fast as possible - I estimate that switching tanks takes about 20 seconds (0.3l system volume @ 45 LPH flow rate) to be effective ... provided the engine keeps running of course !

By and large this has worked well for me for about 60 hours. However, I have experienced fuel vapourisation problems on hot (at least by our standards) days.

MOGAS is in my opinion not without risk - you should only take risks that you fully understand and can afford to loose. (for instance - flying over water, adverse terrain, high ambient temps, high altitudes etc)

Time to kill some OWT's before anyone gets too carried away.

Items in RED

Avgas is a different fuel to mogas, it is not some special additive apart from TEL which dramatically improves the detonation characteristics (and nothing else). The rest of the fuel is a higher spec base alkalyte and it has different requirements for vapour pressure.

There is no valve guide lube?..biggest of the OWT's that get around. As for MP and EGT limits, those are a mystery.

Bottom line is ask yourself why the STC's for mogas are all carby engines?.as best I recall Petersens have never had one for an IO engine for good reason.

Stick to Avgas. Agressively lean on the ground and run WOTLOP whenever you can. :)
 
The pump is not behaving differently - the vapor pressure of the fuel is changing. 100LL has a lower vapor pressure than mogas, meaning that mogas will boil easier. Boiling occurs not only at elevated temperature, but also at reduced pressure. If you have restrictions of ANY kind in your fuel line, that will create a region of lowered pressure as the fuel flows through that restriction. If the fuel is warm enough, it can create its own vapor bubbles in the fuel, and the fuel pump does not do well at all with vapor. The fuel pump itself is bolted to that great big heating element up front, and the fuel will pick up heat in the pump itself as well, making the problem worse.

Keeping your fuel line routing as clean as possible with cooling air on your fuel pump will help, but it's going to be trial and error to eliminate the cause. You'll notice a difference between summer and winter fuel blends also, the winter blend will be more likely to cause the problem you are seeing.

Pretty good summation. ;)

And as you climb past 7000' or there a bouts the issues of hot engine bay, low wings and sucking fuel up hill help promote vapour bubbles!
 
Bottom line is ask yourself why the STC's for mogas are all carby engines?.as best I recall Petersens have never had one for an IO engine for good reason.

Stick to Avgas. Agressively lean on the ground and run WOTLOP whenever you can. :)

Yep, out with the old wives tales and in with some new ones.

Peterson does in fact have an STC for an IO engine - the IO-470 in a Bonanza/Debonair.. I would not be surprised, however, to find out that such has more to do with the airframe and fuel system than it does the engine of course.

There is certainly a fair amount of anecdotal evidence and field testing among the RV crowd to suggest that there isn't anything inherently wrong with mogas and an IO engine; in fact, Superior and other engine builders specifically call it out (not just any car gas, of course - octane and no-ethanal restrictions apply).
 
87, 88 Whatever it takes...

I use 87 from here in Texas with ethanol in it all the time. I had a rough engine once but determined that it was water in the fuel. I have had no problems aside from that 5 second burp, ever. By the way, I mix 87 and 100LL, use straight 87, pure 100LL, whatever I want. My engine runs like a sewing machine. A $30K ECI Titan sewing machine but still. Mine is injected BTW. Not sure if that matters.
 
I have been running one tank with 100LL and in the other 90 octane ethanol free "recreation fuel" I pick up at a local gas station as soon as I was out of Phase 1. I have found absolutely no difference between the two when I am cruising along and switch tanks. I planned to do this during the build so I put a shroud and blast tube on the mechanical fuel pump and doubled up on fire sleeve on the fuel supply hose from the pump to the servo, then wrapped that in reflective aluminum tape because it runs near the exhaust. I take off and land on the 100LL tank just to be safe. Lately I have been mixing the 90 rec fuel with 93 Premium auto fuel laced with 10% ethanol. That's required by law here in Michigan. Again no issues so far but that was a about a 30% 93 mix. I intend to increase that ratio a little at a time to experiment. I have done landings with the Recreation fuel and taxied back to the hanger on it. It does seem to boil out though and make a hot restart more difficult.
 
Time to kill some OWT's before anyone gets too carried away.

Items in RED

Avgas is a different fuel to mogas, it is not some special additive apart from TEL which dramatically improves the detonation characteristics (and nothing else). The rest of the fuel is a higher spec base alkalyte and it has different requirements for vapour pressure.

There is no valve guide lube…..biggest of the OWT's that get around. As for MP and EGT limits, those are a mystery.

Bottom line is ask yourself why the STC's for mogas are all carby engines….as best I recall Petersens have never had one for an IO engine for good reason.

Stick to Avgas. Agressively lean on the ground and run WOTLOP whenever you can. :)

Hi David.

Here's an extract from the UK Light Aircraft Associations technical guidelines on fuel.

"With these engines, many of which were originally produced many decades ago, there have
been a variety of different valve seat and valve materials used over the years, and there is a
possibility that some combinations in the field might suffer problems with valve seat recession if
deprived of the dry-lubricating effect of tetraethyl lead in leaded fuels. To guard against this
possibility it is recommended that all users of Continental and Lycoming engines cleared for
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unleaded Mogas use should either use a fuel mixture with 10% 100LL in it or run a tankful of
100LL through the engine at least every 75 running hours to lubricate the valves and valve
seats."

The 24" max manifold effectively means that I cannot operate @ more than 80% of rated power on mogas. The max EGT is a cross check on not getting too lean (provided of course you are not already LOP). These measures are aimed at reducing the chance of detonation.

Regarding the F.I. setup, My company designs and makes automotive F.I. systems. We have also supplied aviation F.I. component parts and ignition systems for 40 years - This puts me in a good place to assess the suitability of component parts for ethanol bearing fuels. In my experience, a LOT of legacy aviation fuel systems are not ethanol fuel tollerant.

Just to be clear - My original post never said that "this would work" or suggested that others did the same, just describing what I do and why then sharing the results - "warts and all"

Regards,

Richard
 
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The only reason I installed the gascolator is I like the serviceable aspect of it. With the Andair gascolator, I can remove/clean/replace the filter element without undoing any fuel fittings. It gives me the opportunity to know what I have as far as contamination, and a simple inline replaceable filter doesn't do that.

As I said in previous post, I installed the gascolator in the cabin, under the fuel selector valve to prevent a heating issue.

Autogas is on the back burner now (no pun intended). I need to get my fuel pressure sorted out. I think trying the autogas just magnified an issue that I was already experiencing.

I ordered a new mechanical fuel pump to try out. Best case I get my fuel pressure back, worst case is I have a spare fuel pump. I'm finding it hard to believe there is a restriction problem upstream of the mechanical fuel pump causing this, but stranger things have happened. I'll update when I get the new pump installed.

I'm not convinced that your gascolator is not the problem even though it is on the cool side of the firewall. Unlike an in-line filter gascolators churn up the fuel just like 90-degree bends in fuel fittings. I replaced all 90-degree fittings in my fuel delivery system, have one in-line filter and a blast tube cooling my mechanical fuel pump. My injected Lycoming has been running on 100% 91 octane non-ethanol mogas for over 1200 hours. I occasionally get low fuel pressure during a long hot climb but turning on the boost pump brings the pressure back to 24 psi. (Many certified aircraft manufacturers recommend that the boost pump remain on during climb!)

IMHO gascolators should be accorded the same disposition as vacuum pumps and 8-track tape players. :D
 
I remember times when flying one tank avgas and one tank auto gas in the past, I would enrich the mixture, switch tanks, then re-lean. I definitely had a different mixture setting for smooth operation with each type of fuel. If I just switched from one to the other without changing the mixture, I would usually get roughness until I fixed the mixture.

Not saying this is necessarily the cause of what you experienced, but maybe a factor to consider/eliminate as you sort it out.
 
RV-12

It probably isn't relevant, but some of the issues discussed seem related to the RV-12 design with it's always on boost pump and it's preference for high octane auto fuel.
 
Time to kill some OWT's before anyone gets too carried away.

Items in RED

Avgas is a different fuel to mogas, it is not some special additive apart from TEL which dramatically improves the detonation characteristics (and nothing else). The rest of the fuel is a higher spec base alkalyte and it has different requirements for vapour pressure.

There is no valve guide lube…..biggest of the OWT's that get around. As for MP and EGT limits, those are a mystery.

Bottom line is ask yourself why the STC's for mogas are all carby engines….as best I recall Petersens have never had one for an IO engine for good reason.

Stick to Avgas. Agressively lean on the ground and run WOTLOP whenever you can. :)



I would suspect the valve related OWT came from the lead issue. Pre-70's all engines, Lyc included, used regular steel valve seats. The lead from the burned gas would coat the valves and seats and provide insulation from the heat. One of the first thing the auto mfg's had to do in order to support lead free gas was to move to hardened valve seats to avoid cracked seats. There were many problems with older cars using unleaded gas once it became available and they were advised to use leaded gas (back when both were still being sold). This probably morphed into the above OWT.

Not sure what Lycoming has done about this, but many experts still recommend initial engine runs on 100LL to coat things as well as using 100LL every 3-4 tanks. Assuming they are in the know, Lyc still isn't using hardened seats, but I have no idea.

This is still a potential issue with Lycomings.
Larry
 
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