If  the  amount  of  air  is  determined  for  a  gas  engine,  together
with  the  necessary  temperatures,  so  that  the  volume  and  the
temperature  of  the  air  entering  the  cylinder  per  stroke,  and  that
of  the  gas  are  known,  we  may,  by  combining  this  with  the  other
data,  compute  the  temperature  for  a  point  in  the  compression
curve.  In  this  computation  we  must  allow  for  the  volume  of
the  exhaust  gases  remaining  in  the  cylinder  at  the  end  of  the
stroke.  The  temperature  at  the  point  in  the  compression  curve
where  it  meets  or  crosses  the  atmospheric  line  will  be  given  by
the  formula:

4Q1  4  V

where  V  is  the  total  volume  corresponding  to  the  point  where
the  compression  curve  meets  or  crosses  the  atmospheric  line;  V"


504  INTERNAL  COMBUSTION  ENGINES

the  volume  of  the  air  at  atmospheric  pressure  entering  the  cylin-
der during  each  working  cycle,  reduced  to  the  equivalent  volume
at  32  degrees  Fahrenheit;  V"  the  volume  of  the  gas  consumed
per  cycle  reduced  to  the  equivalent  at  atmospheric  pressure  and
32  degrees  Fahrenheit;  and  F""  the  volume  of  the  exhaust
gases  retained  in  the  cylinder  reduced  to  the  same  basis.  To
reduce  the  actual  volumes  to  those  at  32  degrees  Fahrenheit,
multiply  by  the  ratios  of  491.4^  {T'  +  459.4),  where  T'  is  the
observed  temperature  of  the  air  and  of  the  gas  used  as  fuel.  For
the  exhaust  gases  retained  in  the  cylinder  at  the  end  of  the  stroke
T'  may  be  taken  as  the  temperature  of  the  exhaust  gases  leaving
the  engine,  provided  the  engine  is  not  of  the  "scavenging"  type.
