HYDRAMECHANICAL PROTECTIVE SYSTEM
HYDRAULIC CIRCUITS (LATER)
(With Check Valves in Diverter Valve)
Later hydramechanical protective systems
have hydraulic circuits that use check valves to
hold hydraulic pressure on (lock) the fuel shutoff
actuator in the "SHUTOFF" position, after the
engine has been shutdown. In this system, the
start-up override valve must be operated to
release the hydraulic pressure from the fuel
shutoff actuator before the engine can be
started. Also, make sure the air inlet shutoff is in
the open position before the engine is started.
The operation of these hydraulic circuits is -
the same as that of the earlier hydraulic circuits
except for the check valves in the diverter valve
for the fuel shutoff circuit.
Make Reference to Schematic No. 13
When operated, the start-up override valve
connects the fuel shutoff actuator circuit to drain.
This removes any hydraulic pressure on the
actuator so the governor can move the fuel
control linkage and the engine can be started.
Also, on hot restart, after severe operating
conditions, the engine oil pressure can increase
slowly. If the rate of pressure increase is too
slow, the protective system activates actuator (8)
to move the fuel control linkage to the
"SHUTOFF" position be- cause of a low engine
oil pressure fault. Therefore, an override of the
low engine oil pressure circuit is needed in the
An electric solenoid or air operated start-up over-
ride valve (3) is installed in the diverter valve
return line. The valve is normally closed. When
start-up override valve (3) is operated, the outlet
of the di- verter valve is connected to drain. This
maintains a pressure drop across orifice (4) and
does not let the diverter valve shift to the
shutdown position. The fuel shutoff actuator line
is also connected to drain to make sure fuel
shutoff actuator (8) does not hold the governor
shutoff strap in the off position.
When start-up override valve (3) is not in use,
the engine oil circuit is put back into normal
operation as in Schematics No. 2 and No. 4.