----- Original Message -----

From: Jim Sower

To: Rotary motors in aircraft

Sent: Tuesday, February 15, 2005 12:28 AM

Subject: [FlyRotary] Re: Bad day at the airport

<... I was using auto fuel (regular). I have a fuel return line and it goes ...>

DAMN.  I hadn't heard about that.  Paul, that's a really important factor.  Maybe I lost it in all the chatter going on.  Auto fuel can be pretty close to avgas in volatility (Vapor Pressure) but for the most part it's anyone's guess.  Avgas is regulated at max~ 6 psi Vapor Pressure.  Mogas used to get as high as 16-20 psi VP.  That high a pressure can petty much guarantee vapor lock under high under hood / cowl conditions.  As time went on and more and more cars got MPFI and environmental regulations progressed, Mogas vapor pressures came down some.  IIRC Calif has a 7-8 psi max vapor pressure, and places with strict environmental regulations (like cities who aren't making their air quality numbers) are more likely to require lower VPs.  Trouble is, VP can vary widely across various sources of Mogas.

I would guess that anywhere that requires gas pump nozzles that capture vapor as you fill your tank will have lower VPs, but sadly, they are also most likely to have alcohol added to mogas.  Alcohol is a major NO NO for aircraft.  Mogas fuel systems should be very carefully designed to prevent vapor lock or you are apt to get into some VERY SERIOUS SHIT!!.  Like there have been maybe 80,000 Mogas STCs  issued to Lycs and Conts.  NONE to ANY certified engine that's injected.

Which is not to say that you can't make a system that runs reliably with higher VP Mogas, but you need to be very VERY attentive to the issue, both in design and operation.  You also have to figure out a way to find out what local VPs are.  Alcohol is easy to detect and distributors know how much they're adding to the fuel.  Good info on VP is more difficult to come by and it might vary some from one distributor to another.

I've been researching Mogas and recently gave a little presentation to my EAA chapter and a couple of flying clubs.  It's wonderful stuff for your engine (lead is really terrible) but you need to be very careful to not allow ANY alcohol into your airplane and be very careful about checking and monitoring VP.

Unleaded Avgas is coming, but VP will be an issue with it ... Jim S.


Mark R Steitle wrote:

Paul,

It looks like the group has focused in on the fuel system, or more specifically the sump tank as the likely culprit.  I suspected you might have been running mogas.  Its my understanding that auto fuel has a much lower vapor pressure than avgas and is therefore more likely to vapor lock. 

Just a SWAG, but I think the engine and related systems got hot enough to heat the fuel in the fuel rails so that once it passed the pressure regulator (pressure drop) it started to boil, similar to when you open a hot soda.  With the return fuel going to an unvented tank, the boiling fuel expanded enough to cause pressure in the header tank so as to prevent fresh fuel from flowing into the tank.  Eventually, the header tank ran dry and the engine quit.  You sure handled the situation like a pro.  Hope I never have to face that scenario.

Mark S.

 

Hi, Mark...my humblest appologies for taking so long to respond.  I am whittling down the 200 plus messages in my inbox.  I was using auto fuel (regular). I have a fuel return line and it goes to the top of my aluminum sump tank that is located on the cool side of the firewall (inside the cabin, behind the rear seat). The return line is -8 in size.  Hope this helps.  Paul Conner

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