Compression  ratio  influences  the  thermal efficiency of an engine. Theoretically, the ther- mal efficiency increases as the compression ratio is increased. The minimum value of a diesel engine compression ratio is determined by the compres- sion required for starting; and this compression is, to a large extent, dependent on the type of fuel used. The maximum value of the compression ratio is not limited by the fuel used, but is limited by  the  strength  of  the  engine  parts  and  the allowable engine weight per bhp output. Mechanical  Efficiency This is the rating that shows how much of the power developed by the expansion of the gases in  the  cylinder  is  actually  delivered  as  useful power. The factor which has the greatest effect on mechanical efficiency is friction within the engine.  The  friction  between  moving  parts  in  an engine remains practically constant throughout the   engine’s   speed   range.   Therefore,   the mechanical efficiency of an engine will be highest when the engine is running at the speed at which maximum  bhp  is  developed.  Since  power  output is bhp, and the maximum horsepower available is ihp, then bhp Mechanical  efficiency  =  ihp  ×  100 During the transmission of ihp through the piston and connecting rod to the crankshaft, the mechanical losses which occur may be due to fric- tion, or they may be due to power absorbed. Fric- tion losses occur because of friction in the various bearings, between piston and piston rings, and between piston rings and the cylinder walls. Power is  absorbed  by  valve  and  injection  mechanisms, and by various auxiliaries, such as the lubricating oil and water circulating pumps and the scavenge and  supercharge  blowers.  As  a  result,  the  power delivered  to  the  crankshaft  and  available  for doing useful work (bhp) is less than indicated power. The mechanical losses which affect the effici- ency  of  an  engine  may  be  called  frictional horsepower (fhp) or the difference between ihp and   bhp.   The   fhp   of   the   engine   used   in the   preceding   examples,   then,   would   be 1343 (ihp) – 900 (bhp) = 443 fhp, or 33% of the ihp developed in the cylinders. Then, using the  expression  for  mechanical  efficiency,  the percentage of power available at the shaft is com- puted as follows: 900 Mechanical  efficiency  =  1343  =  0.67,  or  67% When an engine is operating under part load, it has a lower mechanical efficiency than when operating at full load. The explanation for this is that most mechanical losses are almost indepen- dent   of   the   load,   and   therefore,   when   load decreases, ihp decreases relatively less than bhp. Mechanical  efficiency  becomes  zero  when  an engine operates at no load because then bhp = 0, but ihp is not zero. In fact, if bhp is zero and the expression for fhp is used, ihp is equal to fhp. To show how mechanical efficiency is lower at part load, assume the engine used in preceding examples  is  operating  at  three-fourths  load.  Brake horsepower at three-fourths load is 900 × 0.75 or  675.  Assuming  that  fhp  does  not  change  with load, fhp = 443. The ihp is, by expression, the sum of bhp and fhp. ihp  =  675  +  443  =  1118 Mechanical  efficiency  =  675/1118  =  0.60,  or 60%;  this  is  appreciably  lower  than  the  67% indicated for the engine at full load. Bmep  is  a  useful  concept  when  dealing  with mechanical efficiency. Bmep can be obtained if the  standard  expression  for  computing horsepower (ihp) is applied to bhp instead of ihp and the mean pressure (p) is designated as bmep. bhp  = (bmep)  ×  L  ×  A  ×  N 33,000 or 33,000  ×  bhp bmep  =  L  ×  A  ×  N From the relations between bmep, bhp, ihp, and   mechanical   efficiency,   by   designating ENGINEMAN  1  &  C 5-8