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Fuel System
Fuel Pump
Fuel enters the fuel pump at the swirl well of the
vapor separator. Here, vapor is separated by a
swirling motion so that only liquid fuel is fed to
the fuel pump. The vapor is drawn from the top
center of the swirl well by a small pressure jet of
fuel and is fed into the vapor return line. This line
carries the vapor back to the fuel tank. There are
no moving parts in the vapor separator, and the
only restrictive passage is used in connection with
vapor removal. Thus, there is no restriction of main
fuel flow.
Ignoring the effect of altitude or ambient air
conditions for the moment, the use of a positive
displacement, engine-driven pump means that
changes in engine speed affect total pump flow
proportionally. The fuel pump provides greater
capacity than is required by the engine. Thus, a
recirculation path is provided.
The fuel pump incorporates a mixture control
valve that routes fuel, through the recirculation
path, back to the vapor separator swirl chamber
when the mixture control lever is placed in the idle
cutoff position. In the idle cutoff position, fuel is
not allowed to exit the pump outlet. The
recirculation path is blocked in the full rich
position. When the mixture control is placed in the
9-4
full rich position, fuel exits the fuel pump outlet. If
the mixture control is placed in an intermediate
position, fuel flows through the pump outlet and
the recirculation path.
A relief valve and variable orifice regulate fuel
pump outlet pressure in proportion to engine speed
and turbocharger compressor discharge pressure
(upper deck pressure).
The fuel pump uses an aneroid and rod assembly
that assists in controlling fuel flow. The aneroid
housing is referenced to deck pressure. The aneroid
bellows expands when the air pressure surrounding
it decreases. As the aneroid expands, it moves the
rod increasing the size of the orifice opening. An
increased amount of fuel flows through the orifice
to a recirculation path. This decreases fuel flow
from the fuel pump to the fuel metering unit.
A check valve is provided so that boost pressure to
the system can bypass the engine driven fuel pump
during engine starting. This feature also aids in the
suppression of vapor formation during high
ambient temperature conditions. The check valve
permits use of the auxiliary airframe fuel boost
pump in the unlikely event of an engine driven fuel
pump malfunction.
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