Fig. 4. Automatic Electrical Shut-Down System
Incorporating Hot Wire Relay
Some engines are equipped with an electrically
Fig. 3. Automatic Electrical Shut-Down System
incorporates a hot wire relay (Fig. 4).
over the power element of the valve and should the
Since the fuel pressure builds up rapidly, the fuel oil
water temperature exceed approximately 203 F., the
pressure switch could close before the lubricating oil
valve will close the contacts in the micro-switch and
pressure switch opens, and effect a shut-down of the
energize the shut-down circuit. If a loss of water occurs,
engine. The hot wire relay, however, delays the closing
the heat of the exhaust gases will be transmitted through
of the fuel oil pressure switch for several seconds to
the copper plug to the temperature-sensing valve and
enable the lubricating oil pressure to build up and open
cause the shut-down circuit to be activated.
the oil pressure switch contacts.
If the engine speed exceeds the high speed setting of
When the lubricating oil pressure falls below 10 2 psi,
the overspeed governor, the governor switch will close
the contacts in the oil pressure switch used in this
and activate the shut-down mechanism.
system will close and current will flow through the hot
wire relay to the solenoid. The few seconds required to
When the engine is shut-down, the decrease in speed
heat the hot wire relay provides sufficient delay to avoid
will open the governor switch, and the decrease in oil
an engine shut-down when low oil pressure is caused by
and fuel pressures will close the oil pressure switch and
a temporary condition such as an air bubble or a
open the fuel pressure switch, thus de-energizing the
temporary overlap in the operation of the oil pressure
switch and the fuel oil pressure switch when starting or
stopping the engine.
The cause of the abnormal conditions must then be
determined and corrected before the engine is started
The water temperature switch, which remains open
again. Also, the air shut-off valve must be manually
during normal engine operation, is installed in the side
reset in the open position before the engine can be
of the thermostat housing. The switch contacts close
when the water temperature reaches approximately
205 F. and activate the shut-down solenoid.