NAVWEPS 00401-30  AlRPlANE PERFORMANCE  the power stroke. As such, BMEP is a con-  venient index for a majority of items of recip-  rocating engine output, efficiency, and operat-  ing limitations.  of friction and the mechanical output is less  than the available pressure  energy. The power  output from the engine will be determined by  the magnitude and rate of the power impulses.  In order to determine the power output of the  reciprocating engine, a brake or load device is  attached to the output shaft and the operating  characteristics are determined. Hence, the  term “brake”  horsepower, BHP, is used to  denote the output power of the powerplant.  From the physical definition of “power”  and  the particular unit of “horsepower” (1 h.p. =  33,ooO  ft.-lbs. per min.), the brake horsepower  can be expressed  in the following form.  The actual power output of any reciptocat-  ing engine is a direct function of the combina-  tion of engine torque and rotative speed.  Thus, output brake horsepower can be related  by the combination of BMEP and RPM or  torque prc~surc  and RPM.  No other engine  instruments can provide this immediate indi-  cation of output power.  If all other factors are constant, the engine  power output is directly related to the engine  airflow. Evidence of this fact could be appre-  ciated from the equation for BHP in terms of  BMEP.  or  where  BHP= brake horsepower  T=output torque, ft.-lbs.  N=output shaft speed, RPM  This equation relates that, for a given BMEP,  the BHP is determined by the product of en-  gine RPM, N, and displacement, D.  In a  sense, the reciprocating engine could be con-  sidered primarily as an air pump with the  pump capacity directly affecting the power  output.  Thus, any engine instrumems which  relate factors affecting airflow can provide some  indirect reflection of engine power. The pres-  sure and temperature of the fuel-air mixture  decide the density of the mixture entering the  cylinder. The carburetor air temperature will  provide the temperature of the inlet air at the  carburetor. While this carburetor inlet air  is not the same temperature as the air in the  cylinder inlet manifold, the carburetor inlet  temperature provides a stable indication inde-  pendent of fuel flow and can be used as a stand-  ard of performance. Cylinder inlet manifold  temperature is difficult to determine with the  same degree of accuracy because  of the normal  variation of fuel-air mixture strength.  The  inlet manifold pressure  provides an additional  indication of the density of airflow entering the  combustion chamber. The manifold absolute  pressure, MAP, is affected by the carburetor  BHP= brake horsepower  BMEP= brake mean effective pressure, psi  D=engine displacement, cu. in.  N= engine speed, RPM  where  In this relationship, the output power is ap-  preciated as some direct variable of torque, T,  and RPM.  Of course, the output torque is  some function of the combustion gas pressure  during the power stroke. Thus, it is helpful  to consider the mean effective gas pressure  during the power stroke, the “brake mean  effective pressure” or BMEP.  With use of  this term, the BHP can be expressed in the  following form.  The BMEP is not actual pressure within the  cylinder, but an effective pressure  representing  the mean gas load acting on the piston during  137  BHP=@MEP)(D)(N)  792,m  BHP=G  TN  BHP= 5255  BHP = @M.W(DXN)  792,000 