This  shows  a  gain  for  the  Diesel  engine  of  only  1.5  per  cent,  but
it  should  be  pointed  out  that  a  compression  pressure  of  200
pounds  in  an  Otto  cycle  can  only  be  reached  with  extremely  lean
fuel  gases,  or  with  separate  fuel  and  air  compression.  However
that  may  be,  the  advantage  of  the  constant-pressure  cycle  over
the  constant-volume  cycle,  presupposing  equal  maximum  pres-
sures in  each,  is  comparatively  small,  and  hence  the  undoubted
gain  that  the  Diesel  engine  shows  in  practice  over  the  average
constant-volume  engine  is  by  some  writers  attributed  not  so
much  to  the  cycle  as  to  the  greater  perfection  of  combustion.
That  this  is  approximately  true  has  been  proven  by  Giildner  who
constructed  engines  operating  on  the  Otto  cycle  upon  the  most
advanced  ideas,  and  obtained  efficiencies  fully  as  good  as  those
obtained  by  Diesel.
