Fuel control. The hydromechanical fuel control contains a dual-element fuel pump, gas
producer and power-turbine speed governors, an acceleration and deceleration control, an airbleed signal
mechanism, and a fuel shutoff valve. Functionally, the fuel control unit can be divided into two
sections: the flow control section and the computer section. A flow control schematic is shown in figure
5.13. The flow control section meters engine fuel flow. The computer section schedules the positioning
of the metering valves in the flow control section. The computer section also signals the actuation of
the compressor bleed band and the variable inlet vanes systems.
Liquid-to-liquid cooler. From the fuel control, fuel passes through the cooler mounted
on the engine compressor housing. The cooler uses fuel to aid in cooling the engine oil. The cooler is a
counter-flow type with the fuel passing through small aluminum tubes. The heat from the oil is
transferred to the fuel. This type of cooler serves two purposes, to cool the oil and heat the fuel to aid
in better atomization. A liquid-to-liquid cooler is illustrated in chapter 2, figure 2.12.
Start fuel system. The electrically operated, normally closed igniter fuel solenoid valve
is located on the compressor housing at the 9 o'clock position. The solenoid valve is actuated by a
switch in the cockpit that energizes the valve and allows fuel to flow from the fuel control unit through
the starting manifold and to the start fuel nozzles. On T55-L-5 and T55-L-7 engines, two start nozzles
are connected to the starting fuel manifold at approximately the 3 and 9 o'clock positions. Starting fuel
passes through these nozzles into the combustion chamber where it is ignited by a spark from an igniter
plug adjacent to each start fuel nozzle. On T5 5-L-7C engines, four start fuel nozzles are connected to
the starting fuel manifold at approximately the 1, 4, 7, and 10 o'clock positions.
Main fuel system. The main and start fuel manifold are positioned on the rear surface
of the combustion chamber assembly. The main fuel manifold consists of two manifold halves, with
attaching points for 14 fuel vaporizer tubes. The main fuel manifolds for the T55-L-7 and L-7C/11A
are shown in figure 5.14.
The T55-L-7C and T55-L-11 engines have an improved fuel system. This system
illustrated in figure 5.15 consists of 28 dual-orifice fuel nozzles in a two-section main fuel manifold. A
flow divider was added to meter fuel to the fuel nozzles at 9 to 10 percent N1; as the rpm increases to
32 percent N1, secondary fuel flows to the nozzles.