horse-power  Crossley  engine,  determined  a  fluid  friction  loss  of
15  horse-power  at  full  load,  which  is  -j^^  =  3.8  per  cent.  When
the  engine  was  taking  in  air  only,  this  loss  was  33  horse-power,
but  even  assuming  that  there  was  no  increased  loss  at  say  half
load,  the  fluid  friction  would  thus  have  been  ^%  =  7.5  per  cent.
It  is  found  in  general  practice  that  fluid  friction  in  hit-and-miss
engines  represents  from  4-10  per  cent  of  the  engine  power  at  full
load,  depending  upon  proper  design.
3.  The  two-stroke  Otto  Cycle.  Fundamentally  there  is  no
difference  between  the  compression,  combustion  and  expansion
lines  of  the  four-cycle  and  two-cycle  types  of  engines,  whether
they  operate  on  the  constant-volume  or  constant-pressure  com-
bustion principle.  The  difference  between  the  methods  of  opera-
tion is  principally  that  the  exhaust  and  charging  actions  of  the
four-cycle  type  are  done  in  another  way  in  the  two-cycle,  and  that
therefore  the  crank  receives  an  impulse  every  revolution  instead
of  every  other.     Fig.  4-16  shows  an  idealized  two-cycle  diagram.
