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Hi! Leon
You make sense as usual, can you see any reason not to use in- tank EFI pumps in a header tank instead of external units (I happen to have 4 in stock).
Georges B.
-------Original Message-------
Date: 06/12/05 14:35:43
Subject: [FlyRotary] EFI Fuel Systems was Re: [FlyRotary] Re: Is common sense dead (rant mode on)
Hi Guys,
Gotta chime in here. This risk analysis stuff is all very interesting, but fails to get to the important points, based on common sense and practical experience.
How the hell can you ever get a vapour lock in an EFI system?? In an EFI system, the fuel is supposed to ALWAYS be at a POSITIVE pressure of at least 30 PSI. At WOT, that pressure climbs to 45 PSI.
Vapour locks normally occur in a mechanical system where a lift pump is "sucking" fuel from the tank and supplying it to a carburettor. Classic case occurs on a V8 where the fuel will boil in the supply line because of the close proximity of the fuel line to an exhaust header. Bad initial design, which can be corrected by either re-routing the fuel line, and / or shielding it and insulating it.
Further, the vapour lock can only occur if the system is at high temp, and the fuel line is subjected to a partial vacuum of a lift pump. So the only place it is possible to get a vapour lock in an EFI system is in the low pressure supply side when using a lift pump. If you are SILLY enough to have your EFI pump higher than the fuel supply, you can also get a vapour lock. EFI pumps are NOT designed to "Suck" anyway - they are blowers or pushers.
Proper fuel system design requires that at all times, the lift pump AND the EFI pump(s) SHOULD ALWAYS have a head of fuel on them so that they are not required to "Suck" fuel, and therefore can't subject the fuel supply line to a partial vacuum.
The lift pump SHOULD supply what is commonly known as a "Surge" tank, or a header tank, or a "make-up" tank. This tank, whatever you want to call it, is used to supply EFI pump(s) with a positive head of fuel. That is the way the system MUST be designed. If you do it any other way, sooner or later, it will bite you on the backside. Guaranteed!
There is another possibility in a badly designed fuel system. AIR lock. Having experienced it myself, an air lock in the supply line is quite capable of reducing flow in the high pressure side of fuel supply. I won't bore you with specific details, but I have had it happen on more than one occasion.
It will normally occur where the fuel line is higher than either the fuel rail or the injector supply. In such cases, it is imperative that a bleed nipple is installed to bleed out any air. This will be most evidenty where there is no fuel return. My personal opinion is that anybody that runs an EFI system without a proper fuel return is just asking for it. A fuel return will assist, but will not necessarily eliminate an air lock in the fuel supply.
There is a minimum system configuration complexity in any EFI system, below which, if you try to simplify it any further, can cause problems under certain conditions. I KNOW that many current model cas have returnless fuel sustems, but the design is quite complex, and unless you understand the differences, you will not be able to implement such a system.
----- Original Message -----
Sent: Sunday, June 12, 2005 4:39 PM
Subject: [FlyRotary] Re: Is common sense dead (rant mode on)
Everyone here is interested in solutions, SPECIFIC solutions.
Specifically, Follow this general principal:
When you have a theory, find way to prove it's true. Use facts.
Example:
I think my fuel system is not going to vapor lock.
Measure it. Place pressure gage on the fuel inlet to the pump. There is direct correlation of INLET pressure to vapor lock risk. Measure the pressure and compare to other aircraft. If you have unusual aircraft, also measure with nose down, nose up.
Get a coffee can of fuel. Have one person in your group measure the pressure change while adjusting the following:
Pump temp.
head pressure.
reduced atmospheric pressure on tank
inlet filter
Fuel type
This info provides perspective as to how significant each is. It can be tested easily with coffee can.
The result: the entire group has lower risk because they can measure how close they are to vapor lock when ground testing.
-al
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