II.  Variation  of  the  Ratio  of  Fuel  to  Air  with  Change  in
Load;  Quality  Governing.
III.  Variation  of  the  Quantity  of  the  Charge  to  suit  the  Load.
Ratio  Fuel  to  Air  remaining  constant;  Quantity  Governing.
IV.  Combination  Systems.
V.  Governing  by  Varying  Time  of  Ignition.
I.  The  Hit-and-Miss  System
This  system  effects  speed  regulation  by  cutting  out  explosions
altogether,  depending  on  the  load.  Thus,  for  instance,  if  the
engine  is  running  at  full  load,  the  explosions  or  cycles  will  follow
each  other  in  regular  order  until  the  speed  has  increased  enough
above  the  mean  to  cause  the  governor  to  act,  preventing  the  draw-
ing in  of  the  next  charge,  thus  causing  a  "miss."  This  in  turn
causes  the  speed  to  fall  sufficiently  below  the  mean  to  make  the
governor  act  the  opposite  way,  causing  the  explosions  to  recur.
At  any  other  load  less  than  the  full  load  the  governor  action  is
the  same,  except  that  as  we  go  down  in  the  scale  the  proportion
of  "misses"  to  "hits"  constantly  increases.  This  system  may
be  operated  in  any  of  the  following  ways :


REGULATION    OF   GAS   ENGINES  443

(a)  By  keeping  the  fuel  valve  closed,  so  that  the  engine  draws
only  air  for  the  miss  cycle.
(6)  By  keeping  the  inlet  valve  closed,  thus  preventing  the
admission  of  both  fuel  and  air.
(c)  By  keeping  the  exhaust  valve  open.  In  this  case  the
admission  valve  is  usually  automatic,  and  its  opening  is  prevented
by  the  fact  that  on  the  next  stroke  no  vacuum  is  formed,  the
exhaust  gases  being  sucked  back  into  the  cylinder.
