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Boiling fuel/vapor lock..an engineering question

Mark33

Well Known Member
So as we all know, boiling fuel, caused by either heat or decreased vapor pressure at higher altitudes is not a good thing and causes vapor lock. I know that different types of fuel (Avgas VS. Mogas) have different temperatures and vapor pressures at which it boils, but for this discussion let's take that variable out of the equation and pretend that regardless of the type, that it all boils at the same temperature and vapor pressure. So:

1. Does anyone know how much pressure is required to overcome this boiling point and to keep the fuel in its liquid state?

2. As the temperature or altitude increases, does it take additional pressure to keep the fuel in its liquid state? If so, is there a simple formula to calculate that variable?

Thanks,
Mark
 
I found this with a quick Google search.
It appears to have come from a discussion on the Sonex site.

WXLeemC.png


I believe that at the temperature we deal with it is not the pressure that we are concerned with but the lack of. i.e. negative or suction.
I doubt that we have an issue on the downstream side of the pump only the upstream side. Usually between the filter (restriction) and the pump.
 
In General

In general, Sir. Vapor pressure lock isn't an issue if you've done three things:
- Fuel line routing for temperature exposure. Keep the fuel lines away from heat sources
- Keep restrictions out of fuel lines (causes a low pressure area just down stream) e.g valving, undersized filters, excessive fittings, etc.
- Don't "draw" fuel through lines (to excess). The effects of negative relative pressure are obvious.

Look at the link previously posted (two up from here). It doesn't take much absolute positive pressure (above zero/can be below atmospheric pressure) at most conditions. Just be very aware of the differences between MoGas and AvGas; lots of other discussions regarding this. Some, a bit too cavalier for my taste.

Some will disagree but a boost pump is always the right thing to have. Doesn't have to be continuous duty. Without one, it's game over for fuel feed/thrust if hit the wrong fuel conditions and vapor lock. Fly safe
 
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Thank you gentlemen for your input. As I stated in my opening question/statement, I'm aware that different types of fuel will boil at various temperatures and different vapor pressures. ie. Avgas VS. Mogas, VS. fuel with or without ethanol, summer blends VS. winter blends, etc., etc. along with the preventive measures to take to help avoid vapor lock. I'm also aware that it's primarily heat issues that we deal with that causes our vapor lock problems rather than altitude and loss of vapor pressure, but when you throw in the other variables listed above, not to mention those different fuels all have different pressures that they require to keep them in a liquid state, it certainly becomes something that we should always be aware of and take proper measures to prevent it. However, for this discussion, with all things being equal, I'm just trying to get an idea as to how much pressure is required to overcome all of the above listed variables and to keep the fuel in a liquid state. Like I said, I know there are a lot of variables to consider, but I guess I'm primary interested in a "rule of thumb". So, just for arguments sake, let's say you're at 15k', burning winter blend Mogas, and the temperature of the fuel in the fuel line was 200*F. In those extreme conditions the fuel would certainly boil and cause vapor lock. So the question is, how much pressure would it take in this scenario to overcome all of those vapor lock causing issues and keep the fuel in a liquid state and prevent it from boiling? Once again, I know there are a lot of variables and these conditions are hypothetical and extreme, but there must be a fuel pressure at which we could overcome the boiling fuel so that vapor lock would never be an issue regardless of what temperatures, altitudes, or fuels that we use in the typical conditions that we fly in.
 
So I guess the crux of my curiosity is based around the electric fuel ejection systems that are now available to us. These systems typically run at 45-50 psi line pressure. So if you were to take out all of the other variables...(fuel type, temperature, altitude, etc., etc.)....because those variables can change, and only look at the line pressures that EFI systems operate at, is there any way we could still experience vapor lock in the typical environment that we operate our RV's at?....or will the fact that those high line pressures will overwhelm any of the other variables and prevent the fuel from boiling and avoid a vapor lock situation by keeping the fuel in a liquid state?

Mark
 
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So I guess the crux of my curiosity is based around the electric fuel ejection systems that are now available to us. These systems typically run at 45-50 psi line pressure. So if you were to take out all of the other variables...(fuel type, temperature, altitude, etc., etc.)....because those variables can change, and only look at the line pressures that EFI systems operate at, is there any way we could still experience vapor lock in the typical environment that we operate our RV's at?....or will the fact that those high line pressures will overwhelm any of the other variables and prevent the fuel from boiling and avoid a vapor lock situation by keeping the fuel in a liquid state?

Mark

Once the fuel passes the pump and is under pressure, It's almost impossible to have 100LL boil downstream. As has been discussed before in other similar threads, it's still possible to have vapor lock at the pump inlet using mogas, hot and high with poor plumbing practices or using winter blend fuel on warm days.

As far as hot start issues go, using 100LL, that should be a thing of the past. Dave Anders tested hot starting with SDS at 118F ambient after letting the engine sit for 5-10 minutes after a hot shutdown. Fired right up and idled smoothly at 600 rpm.
 
Once the fuel passes the pump and is under pressure, It's almost impossible to have 100LL boil downstream. As has been discussed before in other similar threads, it's still possible to have vapor lock at the pump inlet using mogas, hot and high with poor plumbing practices or using winter blend fuel on warm days.

But the injector lines are not pressurized, so fuel can and does boil there, at least with an AFP type injection system, correct? At least that is my understanding of why I sometimes have a stumble in my idle after a flight.

Erich
 
But the injector lines are not pressurized, so fuel can and does boil there, at least with an AFP type injection system, correct? At least that is my understanding of why I sometimes have a stumble in my idle after a flight.

Erich

On an EFI type of system the entire fuel rail....from the fuel pressure regulator to the electronic injectors, are kept at a constant 45-50 psi. This is why vapor lock isn’t (for the most part) an issue with these types of systems. However, this is also some of the clarification I’m trying to get to my question regarding fuel pressure alone and it’s ability to keep the fuel in a liquid state regardless of other variables.

Mark
 
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But the injector lines are not pressurized, so fuel can and does boil there, at least with an AFP type injection system, correct? At least that is my understanding of why I sometimes have a stumble in my idle after a flight.

Erich

The poster referenced EFI which has the injectors and lines under pressure all the time the pump is running.
 
Ross,

Assuming no issues such as pump inlet suction loss or any other plumbing or mechanical issues and also assuming that everything is working as efficiently as it possibly can, but taking into account any and all other variable that may be introduced...such as (fuel type, temperature, blends, altitudes, etc. etc.) which will change,....will the simple fact the these EFI systems operate at such a high line pressures (45-50psi) overwhelm any of those variables that can and do change and by that alone keep the fuel in a liquid state and prevent it from boiling and causing vapor lock? I'm sure there's some point and some pressure that will keep the fuel in a liquid state...regardless of any variable that might be thrown at it. What that pressure is....I don't know. Is it 20psi? 30psi? 45-50psi??

Thanks,
Mark
 
Ross,

Assuming no issues such as pump inlet suction loss or any other plumbing or mechanical issues and also assuming that everything is working as efficiently as it possibly can, but taking into account any and all other variable that may be introduced...such as (fuel type, temperature, blends, altitudes, etc. etc.) which will change,....will the simple fact the these EFI systems operate at such a high line pressures (45-50psi) overwhelm any of those variables that can and do change and by that alone keep the fuel in a liquid state and prevent it from boiling and causing vapor lock? I'm sure there's some point and some pressure that will keep the fuel in a liquid state...regardless of any variable that might be thrown at it. What that pressure is....I don't know. Is it 20psi? 30psi? 45-50psi??

Thanks,
Mark

Im not Ross, but I think you are asking two questions at the same time. One part of your question has been answered about as absolutely as possible without committing to an "absolute": the pressure side of EFI will not vapor lock- due primarily to high system pressure. However, the high pressure attribute will not prevent vapor lock on the suction side, so "the system" is capable of failure due to vapor lock. Unfortunately, the suction side is where there is the least standardization among homebuilts. Type and placement of the pump, length of lines, number of fittings and filter type all play a role in determining the susceptibility of cavitation/vapor lock. And on top of that, you have the added variables of fuel composition, altitude, and temp.

The entire fuel system needs to be designed from the outset to accommodate the worst case scenario. Tried and true low pressure systems using Avgas exclusively can tolerate a lot more "less than ideal" hardware integration decisions than an edge case like using mogas while high and hot.
 
gasoline vapor pressure

This diagram might help you.

Vapour-pressure-of-isooctane-14.png


Can't vouch for its accuracy, but looks reasonable.

The way automobiles deal with vapor lock seems to be working quite well. They climb rather high mountains on hot days with summer blend and it's quite rare to hear about vapor lock issues. They pressurize the fuel from the tank all the way through the system. Seems like something we should think about doing as well.
 
Ross,

Assuming no issues such as pump inlet suction loss or any other plumbing or mechanical issues and also assuming that everything is working as efficiently as it possibly can, but taking into account any and all other variable that may be introduced...such as (fuel type, temperature, blends, altitudes, etc. etc.) which will change,....will the simple fact the these EFI systems operate at such a high line pressures (45-50psi) overwhelm any of those variables that can and do change and by that alone keep the fuel in a liquid state and prevent it from boiling and causing vapor lock? I'm sure there's some point and some pressure that will keep the fuel in a liquid state...regardless of any variable that might be thrown at it. What that pressure is....I don't know. Is it 20psi? 30psi? 45-50psi??

Thanks,
Mark

Ignoring anything happening before the pump, we have never seen vapor lock issues with avgas downstream of the pump so the answer is basically yes, the typical 45-55 psi that our EFI system runs prevents any vapor lock issues from occurring at the injectors under any conditions we've seen (up to 118F anyway).

I believe, from looking at the boiling points of winter mogas, you could still have hot start issues with winter mogas on a hot summer day on a hot start.
You should NEVER run winter mogas in the summer in your aircraft IMO because it's quite likely, you could get vapor at the pump inlet under hot and high conditions.

Given high enough temps and low enough pressure, any fuel will boil.

Once in flight, with the top mount injectors, everything runs very cool due to the high airflow over the injectors and the high fuel flow through them. I'd say zero chance of boiling under those conditions with any fuel that got you to altitude.
 
Im not Ross, but I think you are asking two questions at the same time. One part of your question has been answered about as absolutely as possible without committing to an "absolute": the pressure side of EFI will not vapor lock- due primarily to high system pressure. However, the high pressure attribute will not prevent vapor lock on the suction side, so "the system" is capable of failure due to vapor lock. Unfortunately, the suction side is where there is the least standardization among homebuilts. Type and placement of the pump, length of lines, number of fittings and filter type all play a role in determining the susceptibility of cavitation/vapor lock. And on top of that, you have the added variables of fuel composition, altitude, and temp.

The entire fuel system needs to be designed from the outset to accommodate the worst case scenario. Tried and true low pressure systems using Avgas exclusively can tolerate a lot more "less than ideal" hardware integration decisions than an edge case like using mogas while high and hot.

Michael,

I totally agree....the entire fuel system needs to be designed and laid out properly all the way through. I think I have mine laid out about the best that I can considering the mechanical variables. My pumps are mounted directly on the floorboard in the tunnel of the -7 so they're at the lowest and coolest point in the airplane and only a few inches forward of the fuel selector, so they're also as close to the fuel tanks as I can get them. Hopefully with this layout there won't be any (or much) suction loss or cavitation. Mostly all of the work they'll be doing will be "pushing" the fuel, not "pulling" it. The filters, both pre and post pumps, are mounted directly to the fuel pump manifold. From that point the fuel will be pushed forward and up. The fuel lines themselves will be running up the firewall on the inside of the cockpit and exit up at the top of the firewall just as high up as possible and go into the fuel distribution block which will also have the fuel pressure regulator mounted directly to it. The fuel will then return directly from there and get recycled back to the tanks. So the only time the fuel will really be subjected to any heat will be from the fuel distribution block forward to the engine/fuel rail. I'm designing a dead-head rail rather than a full flow-around rail so that's the only fuel that I'm concerned about regarding any heat soak. Even though the rail will have a continuous 45-50 psi on it controlled by the fuel pressure regulator, I was still curious about the potential of vapor lock occurring in that small part of the system and if that 45-50 psi pressure alone would able to overcome the potential for vapor lock due to my dead-head design. Once again, the rail from the distribution block forward to the injectors will have a continuous 45-50 psi on it so hopefully that'll be enough pressure to prevent vapor lock regardless of the type of fuel I'm running or the temperature of the fuel in that part of the system. The rest of the system I'm really not that concerned with because that fuel will stay nice and cool and flowing in the best way possible as described above.

Mark
 
Ignoring anything happening before the pump, we have never seen vapor lock issues with avgas downstream of the pump so the answer is basically yes, the typical 45-55 psi that our EFI system runs prevents any vapor lock issues from occurring at the injectors under any conditions we've seen (up to 118F anyway).

I believe, from looking at the boiling points of winter mogas, you could still have hot start issues with winter mogas on a hot summer day on a hot start.
You should NEVER run winter mogas in the summer in your aircraft IMO because it's quite likely, you could get vapor at the pump inlet under hot and high conditions.

Given high enough temps and low enough pressure, any fuel will boil.

Once in flight, with the top mount injectors, everything runs very cool due to the high airflow over the injectors and the high fuel flow through them. I'd say zero chance of boiling under those conditions with any fuel that got you to altitude.

Thanks Ross, I feel confident that the system will work well.
 
This diagram might help you.

Vapour-pressure-of-isooctane-14.png


Can't vouch for its accuracy, but looks reasonable.

The way automobiles deal with vapor lock seems to be working quite well. They climb rather high mountains on hot days with summer blend and it's quite rare to hear about vapor lock issues. They pressurize the fuel from the tank all the way through the system. Seems like something we should think about doing as well.

Mickey, thanks a lot for that chart...that provides a nice visual depiction.

Mark
 
I probably posted this in another thread, but it might be interesting to some. Sorry about the poor quality - it is a scan of a fax!

UvsDpA4zSHjuLALSWbETj9UwdrN1BL-G-oDp-cxh897U2k6Xz_lpL1Bn2ZSt6swYRs_IVouE7p1-7yx77TCUxUD1NKadhj5Cj9rhrQ0ofTi5aalZ9v6tY0UGLMchnjH_jtiZO1dD82QPMt42-X1aJwF_huSrXOsvpKXpgmZucH-7GILIstfex8Vhi3VZ1g_RrGVxBOgYZlGur7AXpsNBr6jirYXbyaJ4PrXGM1XhjCtiehK6vSvsId_sGQzLLpWUp722X6zTeDA9yXqTCGPB7TkdSO4pkHZi_x5hN1w8_YACc_8wtq_XIEKQgUeTjY7DpivFL5EAfJs-1OGVtX8pbAYesOP3BZgU8umKtDjrEA0r7cZFIoTK0Q1JU26McxwJoMHN29JmPuONMW1StPBjvDl0S38z3wZaCw1nxf1r2Abd1WXtb3a25My1p7mFmhibVjeXazCV9p5Ehvq1-QL7A0G9dp4FBsEMJJ7FZx563g1QOouCzkgVJ0QTYVPZ8vfSNkZ9EIIWockt5GkEKrtIcD5PCbhbG77QwOTxyiTNrpHDd9NO6KtX4vxQZXUWKy-kh2DuiwpXza3g-R8sxPt1XTHYA13H2RitDEK-zhBlcpkVyqCR5wy_KCAkS1ky5y2nBhvTN3J0Qmo2eH5t38GXoE4ezw=w713-h978-no


As an example, at 40C, it takes about 45 KPa of pressure absolute (~110F, 6.5 psi) to prevent avgas from boiling (it is the top curve). At idle, a standard aviation fuel injection system might only be delivering a couple psi gauge downstream of the servo, so perhaps a total of 17 psi absolute if at sea level. This corresponds to about 60C or 140F. After landing, when little air is flowing through the cowl, it is easy to imagine the lines downstream of the servo can exceed this temperature, causing percolation of fuel into the cylinders. The classic barfing and puking that some injected engines do at hot idle.
 
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