Boiling fuel/vapor lock..an engineering question

[Mark33]

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

[snopercod]

Some formulas and information here: http://www.etc-cte.ec.gc.ca/database...line_110LL.pdf As you know, fuel vaporizes (boils) over a range of temperatures. I think they call that the "initial point" and "endpoint". What strikes me is that the initial point for 100LL is only 104F.

[MarkW]

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



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.

[Freemasm]

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

[Mark33]

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.

[DanH]

Boiling point is the temperature at which vapor pressure exceeds local pressure.

Here you're asking for local pressure, so you need to know vapor pressure of the fuel at a particular temperature.

Try Figure 7.1-14a and the equations on the following page:

https://www3.epa.gov/ttnchie1/ap42/c...nal/c07s01.pdf

[Mark33]

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

[rv6ejguy]

Originally Posted by Mark33

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.

[erich weaver]

Originally Posted by rv6ejguy

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

[Mark33]

Originally Posted by erich weaver

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