Brake Horsepower Brake  horsepower  (bhp),  sometimes  called shaft   horsepower,   is   the   amount   of   power available for useful work. Bhp is less than ihp because of the various power losses which occur during engine operation. To determine the brake or shaft horsepower that is delivered as useful work by an engine, the sum   total   of   all   mechanical   losses   must   be deducted from the total ihp. CYLINDER PERFORMANCE LIMITATIONS The factors which limit the power that a given cylinder can develop are the piston speed and the mep. The piston speed, as stated before, is limited by the inertia forces set up by the moving parts and by frictional heat. In the case of the mep, the limiting factors are as follows: 1.  Heat  losses  and  efficiency  of  combustion. 2. Volumetric efficiency (the amount of air charged into the cylinder and the degree of scavenging). 3. Mixing of the fuel and air. The limiting meps, both bmep and imep, are prescribed  by  the  manufacturer  or  NAVSEA. They should never be exceeded. In a direct-drive ship, the meps developed are determined by the rpm of the power shaft. In electric-drive ships, the horsepower and bmep are determined by a computation  based  on  readings  from  electrical  in- struments and from generator efficiency. CYLINDER LOAD BALANCE In  order  to  ensure  a  balanced,  smooth- operating  engine,  the  general  mechanical  condi- tion of the engine must be properly maintained so   that   the   power   output   of   the   individual cylinders is within the prescribed limits at all loads and speeds. In order to have a balanced load on the  engine,  each  cylinder  must  produce  its  share of  the  total  power  developed.  If  the  engine  is developing its rated full power, or nearly so, and one cylinder or more is producing less than its share, the remainder of the cylinders will become overloaded. Using the rated speed and bhp, it is possible to determine for each INDIVIDUAL CYLINDER a rated bmep which may not be exceeded without overloading  the  cylinder.  If  the  ENGINE  rpm drops below the rated speed, then the cylinder bmep generally drops to a lower value. The bmep should  never  exceed  the  normal  mep  at  lower engine speed. Usually, it should be somewhat lower if the engine speed is decreased. Some engine manufacturers design the fuel systems so that it is impossible to exceed the rated bmep. This is done by installing a positive stop to limit the maximum throttle or fuel control. This positive stop regulates the maximum amount of fuel  that  can  enter  the  cylinder  and  limits  the maximum load of the cylinder. In order to meet emergency situations, engines used by the Navy are generally rated lower than those designed for industrial use. The economical speed for most of the Navy’s diesel engines is approximately  90%  of  the  rated  speed.  For  such speed,  the  best  load  conditions  have  been  found to be from 70% to 80% of the rated load or out- put. When an engine is operated at an 80-90 com- bination (80% of rated load at 90% rated speed) the  parts  last  longer  and  the  engine  remains cleaner and in better operating condition. Diesel  engines  do  not  operate  well  at  ex- ceedingly  low  bmep  such  as  that  occurring  at idling  speeds.  You  are  well  aware  that  idling  an engine tends to gum up parts associated with the combustion spaces. Operating an engine at idling speeds   for   long   periods   will   result   in   the necessity  for  cleaning  and  overhauling  the  engine much sooner than when operating at 50 to 100% of load. Symptoms of Unbalance Evidence  of  an  unbalanced  condition  between the cylinders of an engine may be indicated by the  following  symptoms: 1. Black exhaust smoke. When this occurs, it is not always possible to determine immediately whether  the  entire  engine  or  just  one  of  the cylinders  is  overloaded.  To  determine  which cylinder  is  overloaded,  you  must  open  the ENGINEMAN  1  &  C 5-4