The  steam  pressure  is  exerted  on  the  piston  for  a  much  larger
portion  of  the  stroke  than  an  explosion  in  a  gas  engine  piston,
and  as  a  consequence  the  inertia  of  moving  parts  is  depended
upon  less  for  uniform  speed  than  in  the  gas  engine,  as  will  be
afterwards  shown.
In  the  past  the  steam  engine  has  had  a  great  advantage  over
the  gas  engine,  due  to  the  fact  that  it  has  been  more  reliable  in
operation  and  could  produce  a  more  nearly  uniform  motion.
The  development  of  the  gas  engine  has,  however,  removed  in  a
large  measure  such  defects,  and  at  the  present  time  there  is  no
great  difference  in  respect  to  reliability  and  regulation  on  the
part  of  these  two  classes  of  engines.  The  use  of  a  producer  in
which  gas  can  readily  be  made  from  solid  fuels  also  equalizes  any
advantages  which  the    steam  engine  has  had  from  that   source.
22


INTERNAL  COMBUSTION  ENGINES


■  14.  Hot  Air  Engines.  —  The  hot  air  engine  is  principally
of  importance  to-day  for  its  scientific  value.  Its  actual  com-
mercial use  is  confined  to  pumping  small  quantities  of  water
under  favorable  conditions.  It  is  of  scientific  interest  because
it  is  the  only  heat  engine  yet  produced  which  represents  almost
perfectly  the  standard  ideal  cycle  of  Carnot,  with  which  the
operation  of  nearly  all  heat  engines  is  compared.
In  the  hot  air  engine  a  mass  of  air  is  successively  heated  and
cooled;  during  the  time  that  it  is  heated  either  its  volume  or
pressure  increases,  and  during  the  time  it  is  cooled  the  reverse
operation  takes  place.  Mechanical  work  is  performed  by  the
change  of  volume  or  pressure  which  is  utilized  for  moving  a  piston.
