Your source for information and technical support for the

Ellison Throttle Body Injector

Gravity Feed Fuel Systems

Gravity feed may be used with many high wing installations and some installations which have a fuselage tank. The fuel pressure requirements of each unit are as follows:

Model Min (PSI) Max (PSI)
EFS-2 2 6
EFS-3A 0.5 6
EFS-4 2 6
EFS-4-5 2 6
EFS-5 2 6
EFS-10 12 15

Gravity feed will provide about 1.0 PSI for every 30 inches of head. For example: The installation of an EFS-2 requires that the bottom of the tank must be at least 60 inches vertical distance (plus the head loss of the valves, fittings, lines and filters) above the Throttle Body to provide the minimum required pressure. Remember that this height must be measured in the steepest climb attitude for aircraft with engines in front, and in the steepest decent attitude for aircraft with pusher engines.  In this worst case attitude, the fuel system should be flow tested with fuel flowing through the TBI fuel inlet finger screen, and demonstrate a flow rate 150% of the fuel flow required by the engine at it's maximum power output, when operating on the last gallon in the fuel tank.

A common installation error made by our customers who wish to install the Throttle Body, but don't want to install a fuel pump, is to try to design a gravity feed fuel system where only minimum head in available. This is a dangerous situation because the Throttle Body will meter fuel adequately at low power settings with fuel pressure well below the specified minimum, but at high power settings the Throttle Body is unable to meter sufficient fuel.  As a result, the engine will run lean at full throttle.

It is common aircraft practice to design a fuel system which has both an engine driven pump and a backup pump such as an electrical boost pump, or mechanical wobble pump. Systems which rely completely on electrical pumps without any backup system should be avoided.

An excellent article about fuel system design by Lyle Powell may be accessed by clicking on Fuel Systems for Homebuilts.  

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