ARMY TM 9-2815-257-24 AIR FORCE TO 38G1-128-2 MARINE CORPS TM 10155A/2815-24/3 2-2 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 solenoid’s  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 engine’s fuel injector just before the piston reaches its high point.  The compressed air and fuel mist mixture combusts in the chamber.