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National Advisory Committee for Aeronautics, Technical Notes - Engine Performance and Knock Rating of Fuels for High Output Aircraft Engines

Data are presented to show the effects of inlet—air
pressure, inlet—air temperature, and compression ratio on
the maximum permissible performance obtained on a single-
cylinder test engine with aircraft—engine fuels varying
from a fuel of 87 octane number to one of 100 octane num—
ber plus 1 ml of tetrasthyl_ lead per gallon. The data
were obtained on a 5— inch by 5. 75- inch liquid—cooled en—
gine- operating at 2, 500 r.p.m. The compression ratio was
varied from 6.50 to 8.75. The inlet-air temperature Was
varied from 120° F. to 280° 3'. and the inlet—air pressure,
from 50 inches of mercury absolute to the highest permis-
sible.

The limiting factor for the increase in compres—
sion ratio and in inlet—air pressure was the occurrence
of either audible or incipient knock. The data are corre-
lated to show that, for any one fuel, there is a definte
relationship between the limiting conditions of inlet—air
temperature and density at any compression ratio. This
relationship is dependent on the combustion-gas tempera-
ture and density relationship that causes knock. The re-
port presents a suggested method of rating aircraft—engine
fuels based on this relationship. It is concluded that
aircraftuengine fuels cannot be_§atisfactorily rated by
any single factor, such as octane number, highest useful
compression ratio, or allowable boost pressure. The fuels
should be rated by a curve that expresses the limitations
of the fuel over a variety of engine conditions.

The performance obtained from a spark—ignition engine
with a given fuel is limited by the severity of the engine
operating conditions to which the fuel can be subjected
without knocking. The major engine variables that must be
controlled to prevent combustion knock are the inlet-air
pressure, the inlet=air temperature, the_compression ratio,
the engine temperature, the spark timing, and the engine
speed. For aircraft engines. high inlet-air pressure is
desirable for take—off conditions when maximum power is
needed. A high compression ratio is desirable chiefly
from considerations of fuel economy, although an increase
in compression ratio is accompanied by an increase in
power. A high inlet-air temperature is undesirable be-
cause it decreases the mass of air inducted into the en—
gino. The use of a superchargerI however. results in
increased inlet-air temperatures unless an intercooler is
provided.