ARMY TM 9-2815-257-24
AIR FORCE TO 38G1-128-2
MARINE CORPS TM 10155A/2815-24/3
Section I. PRINCIPLES OF OPERATION
2-1. PRINCIPLES OF OPERATION.
The theory behind the operation of the engine assembly is described in the following paragraphs. The
information contained herein will assist unit, direct support, and general support maintenance personnel in
understanding how the engine functions. This knowledge will assist in isolating components which have failed.
2-2. ENGINE STARTING SYSTEM. (Figure 2-1)
a. Normal engine start-up is controlled by the engine starter motor (Figure 2-1). A recoil starter (5,
Figure 2-2) allows the operator to manually crank the engine should the starter motor fail.
b. The starter motor consists of three major components: a solenoid (1, Figure 2-1), continuous current
motor, and pinion (10). An armature (11), brushes (17), and field coil (18) create the magnetic force required
to operate the solenoid plunger. Brush holders (16) keep the brushes in their proper position. A gear case (7),
yoke (12), rear cover (13), and dust covers (2, 3, 14) protect the internal components of the starter motor. A
gear case bearing (9) and rear cover bearing (15) ensure smooth operation.
c. During operation, the solenoid (1) moves a spring loaded shift lever (5) to engage and disengage the
pinion (10). The pinion (10) transfers driving power from the motor to a ring gear. An over-running clutch (6)
is used to prevent damage should the engine run too fast.
d. When the solenoid (1) is energized, magnetic force pulls the solenoid plunger in. As the plunger is
pulled in, the torsion spring (4) on the shift lever (5) moves the lever. The lever pushes the pinion (10) away
from the pinion stopper (8) and out to engage the engine flywheel. When the solenoid plunger is all the way in,
the main contacts close, providing battery power to the motor.
e. The starter motor will operate until the solenoid (1) is de-energized. Once de-energized, the
solenoids magnetic force is removed and the solenoid plunger slides out, opening the main contacts and
removing power from the motor. The shift lever (5) retracts, disengaging the pinion (10) from the engine
flywheel ring gear.
2-3. ENGINE COMBUSTION CYCLE. (Figure 2-2)
a. Combustion Cycle. The diesel engine combustion cycle can be divided in four separate strokes;
compression stroke, power stroke, exhaust stroke, and intake stroke. Thus the term four-stroke engine is
applied to this type of engine.
b. Compression Stroke. During the engine compression stroke, the engine starter motor cranks the
engine flywheel (4, Figure 2-2). The crankshaft (6) turns, forcing the piston (8) to rise to its highest point in
the cylinder. The upward movement of the piston compresses air trapped inside the combustion chamber,
causing temperature to rise in the chamber. A fine mist of fuel is sprayed into the combustion chamber by the
engines fuel injector just before the piston reaches its high point. The compressed air and fuel mist mixture
combusts in the chamber.