NAVWEPS 00-801-80 
AIRPLANE PRRFORMANCE 
inlet pressure, throttle position, and super- 
charger or impeller pressure ratio. Of course, 
the throttle is the principal control of mani- 
fold pressure and the throttling action controls 
the pressure of the fuel-air mixture delivered 
to the supercharger inlet. The pressure re- 
ceived by the supercharger is magnified by 
the supercharger in some proportion depend- 
ing on impeller speed. Then the high pressure 
mixture is delivered to the manifold. 
Of course, the engine airflow is a function of 
RPM for two reasons. A higher engine speed 
increases the pumping rate and the volume flow 
through the engine. Also, with the engine 
driven supercharger or impeller, an increase in 
engine speed increases the supercharger pres- 
sure ratio. With the exception of near closed 
throttle position, an increase in engine speed 
will produce an increase in manifold pressure. 
The many variables affecting the character 
,.F the romL.,*r;nn :...^---” “1 L..,, c YYU”Cl”Y process a:e an I.n~“Lrant 
subject of reciprocating engine operation. 
Uniform mixtures of fuel and air will support 
combustion between fuel-air ratios of approxi- 
mately 0.04 and 0.20. The chemically correct 
proportions of air and hydrocarbon fuel would 
be 15 lbs. of air for each lb. of fuel, or a fuel- 
air ratio of 0.067. This chemically correct, or 
“stoichiometric,” fuel-air ratio would provide 
the proportions of fuel and air to produce 
maximum release of heat during combustion of 
a grven weight of mixture. If the fuel-air 
ratio were leaner than stoichiometric, the ex- 
cess of air and deficiency of fuel would produce 
lower combustion temperatures and reduced 
heat release for a given weight of charge. If 
the fuel-air ratio were richer than stoichio- 
metric, the excess of fuel and deficiency of air 
would produce lower combustion temperatures 
and reduced heat release for a given weight of 
charge. 
The stoichiometric conditions would pro- 
duce maximum heat release for ideal conditions 
of combustion and may apply quite closely for 
the individual cylinders of the low speed re- 
ciprocating engine. Because of the effects of 
flame propagation speed, fuel distribution, 
temperature variation, etc., the maximum 
power obtained with a fixed airflow occurs at 
fuel-air ratios of approximately 0.07 to 0.08. 
The first graph of figure 2.16 shows the varia- 
tion of output power with fuel-air ratio for a 
a constant engine airflow, i.e., constant RPM, 
MAP, and CAT (carburetor air temperature); 
Combustion can be supported by fuel-air ratios 
just greater than .0.04 but the energy released 
is insufficient to overcome pumping losses and 
engine mechanical friction. Essentially, the 
same result is obtained for the rich fuel-air 
ratios just below 0.20. Fuel-air ratios be- 
tween these limits produce varying amounts of 
output power and the maximum power output 
generally occurs at fuel-air ratios of approxi- 
mately 0.07 to 0.08. Thus, this range of fuel- 
air ratios which produces maximum power for 
a given airflow is termed ,the “best power” 
range. At jo,me lower range of f-ue;-air rariop, 
a maximum of power per fuel-air ratio is ob- 
tained and this the “best economy” range. 
The best economy range generally occurs be- 
tween fuel-air ratios of 0.05 and 0.07. When 
maximum engine power is required for take- 
off, fuel-air ratios greater than 0.08 are neces- 
sary to suppress detonation. Hence, fuel-air 
ratios of 0.09 to 0.11 are typical during this 
operation. 
The pattern of combustion in the cylinder is 
best illustrated by the second graph of figure 
2.16. The normal combustion process begins 
by spark ignition toward the end of the com- 
pression stroke. The electric spark provides 
the beginning of combustion and a flame front 
is propagated smoothly through the com- 
pressed mixture. Such normal combustion is 
shown by the plot of cylinder pressure versus 
piston travel. Spark ignition begins a smooth