on its way to the starting air system, must bypass through  a  pressure-reducing  valve,  which  reduces the  higher  pressure  to  the  operating  pressure required to start a particular engine. A relief valve is installed in the line between the reducing valve and the starting system. The relief valve is normally set to open at 12 percent above the required starting air pressure. If the air pressure leaving the reducing valve is too high, the relief valve will protect the system by releasing air in excess of a preset value and permit air only at  safe  pressure  to  reach  the  starting  system  of the engine. START   AIR   MOTOR   SYSTEM.—   Some engines, usually gas turbine types, are designed to  crank  over  by  starting  motors  that  use compressed  air.  Air-starting  motors  are  usually driven by air pressures varying from 90 to 200 psi. COMPRESSED AIR ADMISSION SYSTEM.— Most  large  diesel  engines  are  started  when compressed air is admitted directly into the engine cylinders.  Compressed  air  at  approximately  200 to 300 psi is directed into the cylinders to force the piston down and thereby, turn the crankshaft of the engine. This air admission process continues until  the  pistons  are  able  to  build  up  sufficient heat  from  compression  to  cause  combustion  to start the engine. GASOLINE  ENGINES The main parts of the gasoline engine are quite similar  to  those  of  the  diesel  engine.  The  two engines  differ  principally  in  that  the  gasoline engine has a carburetor and an electrical ignition system. The  induction  system  of  a  gasoline  engine draws  gasoline  from  the  fuel  tank  and  air  from the  atmosphere,  mixes  them,  and  delivers  the mixture  to  the  cylinders.  The  induction  system consists  of  the  fuel  tank,  the  fuel  pump,  the carburetor, and the necessary fuel lines and air passages.  Flexible  tubing  carries  the  fuel  from  the tank to the carburetor, while the intake manifold carries  the  fuel-air  mixture  from  there  to  the individual cylinders. The fuel-air mixture is ignited by  an  electric  spark. The carburetor is a device used to send a fine spray  of  fuel  into  a  moving  stream  of  air  as  it moves to the intake valves of the cylinders. The spray is swept along, vaporized, and mixed with the  moving  air.  The  carburetor  is  designed  to maintain the same mixture ratio over a wide range of  engine  speeds.  The  mixture  ratio  is  the  number of  pounds  of  air  mixed  with  each  pound  of gasoline vapor. A rich mixture is one in which the percentage of gasoline vapor is high, while a lean mixture  contains  a  low  percentage  of  gasoline vapor. The  electrical  ignition  system  is  designed  to deliver a spark in the combustion chamber of each cylinder  at  a  specific  point  in  that  cylinder’s cycle   of   operation.   A   typical   ignition   system includes a storage battery, an ignition coil, breaker points,  a  condenser,  a  distributor,  a  spark  plug in  each  cylinder,  a  switch,  and  the  necessary wiring. There are two distinct circuits in the ignition system—the   primary   and   the   secondary.   The primary circuit carries a low-voltage current. The secondary circuit is high voltage. The battery, the ignition switch, the ignition coil, and the breaker points are connected in the primary circuit. The secondary  circuit,  also  connected  to  the  ignition coil, includes the distributor and the spark plugs. The storage battery is usually 6, 12, or 24 volts. One  terminal  is  grounded  to  the  engine  frame, while the other is connected to the ignition system. The ignition coil, in many respects, is similar to  an  electromagnet.  It  consists  of  an  iron  core surrounded by primary and secondary coils. The primary coil is made up of a few turns of heavy wire, while the secondary coil has a great many turns  of  fine  wire.  In  both  coils,  the  wire  is insulated and the coils are entirely separate from each  other. The breaker points form a mechanical switch connected  to  the  primary  circuit.  They  are  opened by a cam that is timed to break the circuit at the exact instant that each cylinder is due to fire. A condenser is connected across the breaker points to  prevent  arcing  and  to  provide  a  better  high- voltage  spark. The  distributor,  connected  to  the  secondary or high-voltage circuit, serves as a selector switch that  channels  electric  current  to  the  individual cylinders.   Although   the   breaker   points   are connected  in  the  primary  circuit,  they  are  often located  in  the  distributor  case.  The  same  drive shaft  operates  both  the  breaker  points  and  the distributor. The  spark  plugs,    which  extend  into  the combustion  chambers  of  the  cylinders,  are connected  by  heavily  insulated  wires  to  the distributor.  A  spark  plug  consists  essentially  of a metal shell that screws into the spark plug hole in  the  cylinder,  a  center  electrode  embedded  in 7-13