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Low fuel pressure on climb out


Scrapman1959

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In the climb out at wide-open throttle our CTLS 2011 model has fuel pressure drop into the yellow after about 3000 feet of hard climb. Not wanting to let it go any further we lower the nose and reduce the power and the fuel pressure goes back into the normal range after 10 or 15 seconds. There’s good fuel flow to the gascalator when checking for water in the fuel. Any ideas on what might be causing this? 

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Interesting new note (P.2-4) in recent update (1/1/23) of Rotax 912 Series Operators Manual:

NOTE Low fuel pressure indications are possible and allowed. But the pressure must stabilize to the operating limit within 10 seconds. If not, the cause should be determined and rectified. Due to the technical design and installation conditions (construction of the return line, etc.) pressure fluctuations, at the fuel pump are possible. These pressure fluctuations within the specified operating limits are not considered a problem.

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During the warming season, there is still a lot of winter fuel blends out there. Winter blends have a lower vapor pressure.

I am dealing with the issue right now that I flew from Ohio to SC. I had to stay near the airport because fuel pressure and fuel flow were wildly erratic. As a precaution I am investigating the system, but I will probably solve the issue adding 2-3 gallons a side of avgas.

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Corey - I believe that winter blend mogas has a higher vapor pressure than summer blend mogas.  Higher vapor pressure fuel evaporates (at any given temperature) more readily than lower vapor pressure fuel.  In the winter, given lower temperatures, higher vapor pressure fuel will more readily vaporize and contribute to combustion than lower vapor pressure fuel.  Of course, the problem is that winter blend fuel, in hot weather, is more likely to vaporize in the fuel line (or carb bowl) and cause vapor lock.

I think that the practice of adding some 100LL to high vapor pressure winter blend mogas is not a solution to the problem of vapor lock when winter mogas is used in warm weather.  Vapor lock results from vaporization of the lowest boiling point component of the fuel (ie, the fuel component with the highest vapor pressure).  A relatively small proportion of winter blend fuel will still have enough high vapor pressure ingredient(s) to cause vapor lock (this is because a very large amount of vapor is liberated when a relatively small amount of liquid evaporates).  So, as long as there is some winter blend fuel in the tank, no matter what other fuel is mixed with it, vapor lock will occur just as it would have if the winter blend fuel wasn't mixed with any other fuel.  

So, if the problem is vapor lock, then the vapor pressure of the most volatile component of the fuel is the culprit.  Adding some 100LL doesn't make that component any less volatile.  

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You're right i got it backwards. Avgas has a very low vapor pressure.

As for the rest: intermolecular interactions are far too complex to just say "the lowest components will just vaporize first". Gasoline has many components to it, many in common between winter and summer blends, and there's a lot of stuff out there on the subject.

Please see this thread for some of the research someone tested and confirmed a SIGNIFICANT change in vapor pressure even at 8:1 mogas to avgas.

https://vansairforce.net/community/showthread.php?t=101954

In my experience, I have solved minor alarms and annoyances before by using a little avgas. This was the first time I had constant alarms and wild fuel flow readings. I flew dowm from ohio to SC, where it is 30-40 degrees hotter. That was the only change, I filled tanks to full before coming here. After adding 20% avgas, absolute silence, not even the usual temporary fuel pressure drop during takeoff, solid, stable fuel pressure and fuel flow.

I need to find a little electric boost pump for more testing...

Edit: https://www.eia.gov/todayinenergy/detail.php?id=11031 mentions at the end both blends have butane, but summer blends have less butane because it makes the blend too volatile. As does this https://blog.amsoil.com/the-difference-between-winter-and-summer-blend-gas/amp/

And https://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/summer-fuel1.htm says butane's vapor pressure is 52 psi, which I can believe since it's in your little BIC lighters.

Finally, ethanol, despite having a reid vapor pressure of about 2psi... Actually INCREASES the RVP of a gasoline blend until around 50%. https://www.card.iastate.edu/products/publications/pdf/18pb21.pdf

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You are correct.  Otherwise, a mixture of water and antifreeze (ethylene glycol) would still boil at 212 degrees F, which, obviously, it does not.  

The relationship between vapor pressures of components of a mixture are described by Raoult's Law, see: https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/13%3A_Solutions_and_their_Physical_Properties/13.06%3A_Vapor_Pressures_of_Solutions

The law applies only to ideal solutions.  As you noted, intermolecular forces can result in some deviation from that predicted by Raoult's law.  

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You know more of chemistry than I do. I did run across a post that I have been unable to source (as expected, fuel blends are proprietary) that avgas is highly parrafin based. Let me know if that sounds possible. I don't know what the parrafin molecules are called so I can't find RVP values.

Years and years ago I had run into issues that I thought might be vapor related, and that's how I came across research and posts regarding mixing fuels. I too doubted that just adding a little avgas would make much of a difference, but anecdotal evidence suggested otherwise.

This is the first time I actually was able to properly test this and having the only variable change was temperature with the same tank of gas. That's when I remembered the avgas trick, tried it, and it worked, better than I ever expected.

I really want to pipe a return line back to my fuel tank. I believe this would enable a much faster vapor purge than just relying on the carbs. With the restrictor it won't allow much flow back (i think it's a half gallon an hour?) so it won't do anything for temperature, but air will pass through the restrictor much faster than any fuel would.

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"Parrafin" isn't a single compound.  It is a generic term that has slightly different meanings depending on context.  Liquid paraffin is a mixture of straight-chain hydrocarbon molecules (generically called alkanes) with molecular weights low enough to be liquid at room temperature (as opposed to a solid at room temperature, like paraffin candle wax, which could have a formula of C20H42 and a melting point of about 98 degrees F). Because paraffin is the term for a family of compounds, it does not have a unique vapor pressure.

A few examples: C8H18 is the alkane known as n-octane (yep, that octane).  The vapor pressure (at 20 degC) is 11 mmHg.  The next straight chain hydrocarbon in the sequence, C9H20 (called n-nonane), has a vapor pressure of 3mmHg.  So, the larger molecule is less volatile (and has a lower the vapor pressure).  As you noted, butane (C4H10) is added to winter blend gasoline because it is very volatile and increase the vapor pressure of winter fuel.  It also has relatively lower energy value so winter fuel must be consumed at a greater rate to produce the same energy (ie, poorer MPG in your car or greater fuel burn per hour at a given airspeed in your airplane).

Purging vapor back to the fuel tank (as opposed to the gascolator) might reduce the tendency for vapor lock.  Still, without (i) reducing the temperature of the fuel in the fuel line (or carburetor bowl), (ii) reducing the vapor pressure of the fuel, or (iii) increasing the pressure of fuel in the fuel line, the production of vapor will continue, even if it is vented.  In fact, to the extent that venting decreases the fuel pressure, vapor will be produced at an even higher rate. 

 

 

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The reason I want to purge the vapor is to try to reduce the time spent with the warnings on climbout. During taxi and runup, the fuel is of course being "cooked" with the warming engine on longer taxis and runups. I habe noted that being an issue before as well and even tested it in the same day by letting that engine get REAL hot in the runup. I started getting alarms just on the roll.

I am going to try a thicker firesleeve at next hose change too. But in the end, I might have to just find a small boost pump.

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I’ve been getting more and more low fuel pressure alarms on climb out over the last few months. 2008 CTLS with new engine, has 105 hrs on it.  Recently also getting hard to start with full choke. Start and die a few times. Then the engine stumbled many times on full power climb out.
Scared the wife this time! 
Of course I tested the flow out of the gascolator, 16-18 gallons per hour gravity flow each wing.
So, I installed a new pump yesterday and all the problems went away, with the same 93 mogas fuel in the tanks.
Startup perfect, runup great 5.6 psi. Climb out great, 4.9psi at 7gph. Tested the hell out of it in flight, all great. Back to normal.
I say bad pump. I know of 2 others in my aviation community. Bad pump at less than 200 hours. replaced and all back to normal.

 

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You can't do the flow test with the gascolator drain valve, it's too much of a restriction.

You need to do it with the gascolator bowl removed. The purpose of the flow test is to verify there aren't any blockages in the line. By the time it shows up with the drain valve, you can have a significant blockage upstream.

When troubleshooting, you want to remove the connection to the fuel pump and test there. There is a screen that the fuel has to flow through at the gascolator bowl and you're trying to test the system as installed.

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I don't mean the flow test at the gascolator doesn't have value, but it doesn't give a complete picture. Also when doing the test for flow at the condition inspection pinch off the line on each side and check flow individually. It will be a couple gallons less typically for one side alone. Also if you have a test with low flow rate at the gascolator the first thing to check is the quick drain valve. The only time I have had one with low flow rate it was the valve, due to a swollen seal. When troubleshooting a issue with a reduction in RPM there is fuel system beyond that gascolator that can cause issues.

 

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I never liked the small diameter fuel hose diameters used in the CT, it goes against all fuel system protocols for aircraft. I changed all my hoses to 3/8 hoses up to the gascolator (except for wing tank hoses). The fuel pressure drop on 1/4" is a multiple of 10 and for 5/16" is about 4 , and 3/8 is almost negligible all based on 10 feet of hose length with no fittings. I was going to change all fittings to AN anyway so it was an easy choice of hose size.

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