Fuel Vapor Prevention
It is very important to design any aircraft fuel system to prevent the formation of
vapor. A diaphragm controlled fuel metering device such as the Ellison Throttle Body
Injector ("TBI"), Bendix RSA-5, or PS-5 pressure carburetor is more susceptible
to operational difficulties due to vapor formation than a conventional float carburetor.
In any of these units, a bubble of vapor or air in the fuel line will cause a momentary
power loss. With float carburetors on the other hand, the float bowl acts as a vapor
separator. Large vapor bubbles can travel down the fuel supply line and be vented out the
float bowl vent while the engine runs without interruption. If a carburetor equipped
aircraft is being updated to the TBI, the fuel system which worked well with a float
carburetor, may need some minor modifications to avoid having vapor problems with the TBI.
Regardless of whether an engine is to be carbureted or injected, it is good practice to
design against vapor formation.
To reduce the possibility of vapor formation the engine driven fuel pump should be
shrouded and blast cooled, with all fuel lines insulated and routed well clear of exhaust
pipes. Locating the gascolator, filter and boost pump outside of the engine compartment is
very helpful in reducing fuel heating. If these components must be in the engine
compartment, they too should be shrouded and blast cooled.
Although the above design precautions will usually prevent vapor formation, some
builders, after hearing our precautions about this subject, have gone to the unnecessary
extreme of installing vapor return lines in their fuel systems. Although they can
effectively control vapor, other complexities are introduced that can be more dangerous
than the problems solved. Click on Recirculating Fuel
Systems for specific information relating to vapor return lines. An excellent
article about fuel system design by Lyle Powell, may be accessed by clicking on
Fuel Systems for Homebuilt