Fuel Filtering
The Ellison Throttle body Injector (TBI) requires clean fuel for proper operation. A 70
micron filter is required to keep contaminates out of the TBI. The TBI does have a last
chance fuel strainer installed on the fuel inlet fitting. This strainer is there only to
trap large particles in the event of a primary filter failure. The strainer is fine enough
to reduce the possibility of particles plugging the fuel metering tube but particles which
can pass through the fuel inlet screen can keep the valve in the fuel regulator from
closing completely. This can result in an erratic idle and fuel leakage after engine
shutdown. Many throttle body installations use a standard gascolater with screen
before the fuel pumps to prevent large particles from damaging the fuel pumps and to
provide a low point water drain, but then after the engine driven pump, a 70 micron filter
or finer must be installed.
This filter must be carefully selected. Some paper filters will not pass water
and could cause fuel system blockage if saturated with water. A drain must be provided
and it must be drained before each flight. If this filter is installed on the
pressure side of the boost pump it must have sufficient strength to handle pump pressure
without bursting or leaking. The Glassair III uses a Fram HPG 1 with a low point water
drain added which is a very good filter housing and perfectly capable of operating with
pressurized fuel. As with all fuel system components, the filter must be located
well away from sources of heat, preferably outside of the engine compartment. If
located in the engine compartment it must be blast cooled to prevent vapor formation.
An excellent article about fuel system design by Lyle Powell, may be accessed by
clicking on Fuel Systems for
Homebuilt Aircraft.
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