porcelain,  and  a  side  electrode  connected  to  the shell.   The   side   electrode   is   adjusted   so   that there  is  a  small  space  (gap)  between  it  and the  center  electrode.  This  gap  varies  depending on  the  engine.  When  the  plug  fires,  an  electric spark   jumps   across   the   gap   between   the electrodes. When  the  engine  is  running,  the  electric current  in  the  primary  circuit  flows  from  the battery through the switch, the primary winding in the ignition coil, the breaker points, and then back to the battery. The high voltage is produced in  the  secondary  winding  in  the  ignition  coil,  then flows  through  the  distributor  to  the  individual spark plugs and back to the ignition coil through the engine frame. It is interesting to note that the high voltage that jumps the gap in the spark plugs does not come from the battery but is produced in  the  ignition  coil. The  ignition  coil  and  the  condenser  are  the only parts of the ignition system that require an explanation. The soft iron core and the primary windings  function  as  an  electromagnet.  The current  flowing  through  the  primary  windings magnetizes  the  core.  The  same  core  and  the secondary  windings  function  as  a  transformer. Variations  in  the  primary  current  change  the magnetism  of  the  core,  which  in  turn  produces high  voltage  in  the  secondary  windings. With  the  engine  running  and  the  breaker points closed, low-voltage current flows through the  primary  circuit.  When  the  breaker  points open, this current is interrupted and produces high voltage in the secondary circuit. The electricity, which  would  otherwise  arc  across  the  breaker points as they are separating, now flows into the condenser. The principal purpose of the condenser is to protect  the  breaker  points  from  being  burned. The  condenser  also  aids  in  obtaining  a  hotter spark. The contact-point ignition system is an older type. The electronic ignition system is of the newer type.  The  basic  difference  between  the  contact- point and the electronic ignition systems is in the primary circuit. The primary circuit in the contact- point  system  is  opened  and  closed  by  contact points.  In  the  electronic  system,  the  primary circuit  is  opened  and  closed  by  the  electronic control  unit. The secondary circuits are practically the same for  the  two  systems.  The  difference  is  that  the distributor,  ignition  coil,  and  wiring  are  altered to  handle  the  higher  voltage  that  the  electronic ignition  system  produces. One  advantage  of  this  higher  voltage  of approximately 47,000 volts is that spark plugs with wider gaps can be used. This results in a longer spark, which can ignite leaner fuel-air mixtures. As a result, engines can run on leaner mixtures for  better  fuel  economy  and  lower  emissions. Another  difference  is  that  some  electronic ignition  systems  have  no  mechanical  advance mechanisms—centrifugal  or  vacuum.  Instead,  the spark  timing  is  adjusted  electronically. The starting system of the gasoline engine is basically the same as that of the diesel engine. The generator  keeps  the  battery  charged  and  provides the  current  to  operate  the  lights  and  other electrical  equipment.  The  starter  motor  draws current from the battery and rotates the flywheel and  crankshaft  for  starting. SUMMARY This chapter was designed to give you a brief understanding  of  diesel  and  gasoline  internal- combustion engines. You will find these engines on all ships in the Navy. It will be of great value to you to learn more about them by reading the referenced material given throughout this chapter. 7-14