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. 
BHP=G 
or 
TN 
BHP= 5255 
where 
BHP= brake horsepower 
T=output torque, ft.-lbs. 
N=output shaft speed, RPM 
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. 
BHP=@MEP)(D)(N) 792,m 
where 
BHP= brake horsepower 
BMEP= brake mean effective pressure, psi 
D=engine displacement, cu. in. 
N= engine speed, RPM 
The BMEP is not actual pressure within the 
cylinder, but an effective pressure representing 
the mean gas load acting on the piston during 
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. 
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. 
BHP = @M.W(DXN) 
792,000 
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 
137