The specific fuel consumption of the turbo- 
prop powerplant is defined as follows : 
specific fuel consumption= 
engine fuel flow 
equivalent shaft horsepower 
c=lbs. per hr. 
ESHP 
Typical values for specific fuel consumption, c, 
range from 0.5 to 0.8 lbs. per hr. per ESHP. 
The variation of specific fuel consumption with 
operating conditions is similar to that of the 
turbojet engine. The minimum specific fuel 
consumption is obtained at relatively high 
power setting and high altitudes. The low 
inlet air temperature reduces the specific fuel 
consumption and the lowest values of c are ob- 
tained near altitudes of 25,ooO to 3900 ft. 
Thus; the turboprop as well as the turbojet has 
a preference for high altitude operation. 
THE RECRIPROCATING ENGINE 
The reciprocating engine is one of the most 
efficient powerplants used for aircraft power. 
The combination of the reciprocating engine 
and propeller is one of the most efficient means 
of converting the chemical energy of fuel into 
flying time or distance. Because of the in- 
herent high efficiency, the reciprocating engine 
is an important type of aircraft powerplant. 
OPERATING CHARACTERISTICS. The 
function of the typical reciprocating engine in- 
volves four strokes of the piston to complete 
one operating cycle. This principal operating 
cycle is illustrated in figure 2.15 by the varia- 
tion of pressure and volume within the cylin- 
der. The first stroke of the operating cycle is 
the downstroke of the piston with the intake 
valve open. This stroke draws in a charge of 
fuel-air mixture along AB of the pressure- 
volume diagram. The second stroke accom- 
plishes compression of the fuel-air mixture 
along line EC. Combustion is initiated by a 
spark ignition apparatus and combustion takes 
place in essentially a constant volume. The 
combustion of the fuel-air mixture liberates 
NAVWEPS 00-8OT-80 
AlR,Pl.ANE PERFORMANCE 
heat and causes the rise of pressure along line 
CD. The power stroke utilizes the increased 
pressure through the expansion along line DE. 
Then the exhaust begins by the initial rejection 
along line EB and is completed by the upstroke 
along line BA. 
The net work produced by the cycle of opera- 
tion is idealized by the area BCDE on the 
pressure-volume diagram of figure 2.15. Dur- 
ing the actual rather than ideal cycle of op- 
eration, the intake pressure is lower than the 
exhaust pressure and the negative work repre- 
sents a pumping loss. The incomplete expan- 
sion during the power stroke represents a basic 
loss in the operating cycle because of the re- 
jection of combustion products along line EB. 
The area EFB represents a basic loss in the 
operating cycle because of the rejection of 
combustion products along line EB. The area 
EFB represents a certain amount of energy of 
the exhaust gases, a part of which can be ex- 
tracted by exhaust turbines as additional shaft 
power to be coupled to the crankshaft (turbo- 
compound engine) or to be used in operating a 
supercharger (turbosupercharger). In addi- 
tion, the exhaust gas energy may be utilized to 
augment engine cooling flow (ejector exhaust) 
and reduce cowl drag. 
Since the net work produced during the op- 
erating cycle is represented by the enclosed area 
of pressure-volume diagram, the output of the 
engine is affected by any factor which influences 
this area. The weight of fuel-air mixture will 
determine the energy released by combustion 
and the weight of charge can be altered by 
altitude,supercharging,etc. Mixturestrength, 
preignition, spark timing, etc., can affect the 
energy release of a given airflow and alter the 
work produced during the operating cycle. 
The mechanical work accomplished during 
the power stroke is the result of the gas pres- 
sure sustained on the piston. The linkage of 
the piston to a crankshaft by the connecting 
rod applies torque to the output shaft. During 
this conversion of pressure energy to mechani-