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 Aircraft.