MAINTENANCE OF THE M88A1 - OD1672 - LESSON 3/TASK 1
2.
Construction
a.
General. The auxiliary power unit consists of an overhead valve, two-
cylinder, four-cycle, air-cooled diesel engine connected directly to a 300
ampere, 28 volt, shunt-wound, direct current generator (which is also used
as a starter), and an auxiliary hydraulic system pump.
b.
Starting and Generating Systems. The 300 ampere, 28 volt generator is
also used as a starter motor.
This is accomplished by first feeding 24
volts direct current to the field coil of the generator from the preheat
relay when the preheat switch is activated.
Twenty-four volts direct
current is applied to the generator armature through a starting relay when
the START switch is activated.
This motorizes the generator, which is
directly coupled to the engine by a drive chain. When the engine starts,
the current in the generator armature reverses direction, and the system
automatically changes from a motorizing mode to a generating mode.
After
the engine starts, the START and PREHEAT switches must be released to
prevent loading down the engine, since the voltage regulator is bypassed
during the start cycle and full voltage is being applied to the field.
After the engine has started, and the PREHEAT and START switches are
released, the generator output is controlled by a solid state voltage
regulator and a current limiting device.
c.
delivers it through a replaceable oil filter to the engine.
Normal oil
pressure should be 25 psi or higher under normal operating conditions.
A
crankcase oil drain valve, with operating handle, is provided to drain the
lubricating oil without removing the APU from the vehicle.
d.
Governor. The governor is set for the engine to maintain a constant
speed at 2000 100 rpm (revolutions per minute). The governor uses a ball
and cup mechanism on the camshaft gear as the sensing device.
A yoke,
resting on the cup, connects to an arm and spring mechanism controlling the
throttle lever. Any change in engine speed is transmitted from the cup to
the yoke and to the throttle. The engine speed may be adjusted by changing
the spring tension.
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