Al,
I would think that a well-designed fuel system with the pumps located as low as possible, in a cool area, and a return system should not be troubled with vapor locking. Although I guess it could happen in Death Valley in August if the a/c is left out in the sun for 8 hours, and running auto gas. So far, I haven't had any problem here in Texas (temps 105* today).
My purpose for installing the bypass circuit was more critical to safe flight. (The solution was the result of a forced landing, or two, in an Eggenfellner Subaru installation.) During ground runs, using a 5-gallon can, I found that the efi fuel pumps could not restore pressure once the tank had been allowed to run dry. When this occurred, the EFI pump would suck a big slug of air into the inlet side of the pump, loose pressure, and being unable to build psi equal to the pressure regulator setting, it would stop pumping fuel (vapor lock). The only way to restore operation was to crack open the system downstream of the pump until the slug of air could be passed on through the fuel pump. Not sure this meets your definiton of "vapor lock", but I think it does, but for a different reason than fuel vapor pressure.
I have tested this system on the ground and it works as advertized. If the tank runs dry, switched to a full tank, the efi pump will restore pressure to the system within a few seconds. I urge all builders to run this test on their fuel system to determine whether or not their pumps are able to restore pressure after running a tank dry. Or, just don't ever run a tank dry.
Mark S.
On Wed, Aug 17, 2011 at 10:22 AM, Ernest Christley <echristley@att.net> wrote:
Al Wick wrote:
I'm really concerned for some of these fuel designs. The fuel bleed has nothing to do with vapor lock. Virtually no effect at all.
I have a strong, positive head pressure going into my pumps. They, and the regulator, are about 8" directly below the tank. Excess fuel goes back to the opposite side of the tank from the pickup, and a single line goes forward to feed the injectors. The fuel lines are arranged such that heat soaking the lines to the point of boiling the gas will push liquid fuel down hill and behind the firewall, isolating the gaseous gas with its heat at the top of the line. Turning the pumps on will pressurize the line to 55psi, returning most of the gaseous fuel back to a liquid state. The ECM is programmed for a longer clearing pulse on hot start.
The point of the bleed is to allow fuel to move back to the tank. I had the issue of a the pressurized lines being perfectly sealed. The pressurized fuel was finding the path of least resistance out, which just happened to be out the injector and into the intake manifold where it sat as a little puddle. Heat soaking the lines would not push liquid fuel downhill and back behind the firewall. It would push more fuel into the manifold. A puddle of gas sitting in a composite manifold, just above a hot exhaust stack is just bad mojo. A poorly sealed regulator allows the pressure to bleed off in about 5 seconds (give or take), isolating the hot fuel in front of the firewall, and keeping the rest cool and out of the intake manifold.
Got nuthin' to do with vapor lock.
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