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National Advisory Committee for Aeronautics, Technical Notes - Compression Ignition Engine Tests of Several Fuels

Much interest has been shown in the development of
fuels for internal—combustion engines and improvement in _
engine performance has been a direct result of the improve—
ment of fuels. The evaluation of the results of efforts to
improve fuels requires that there be a suitable standard
set up for the comparison of the operating characteristics
of the different fuels involved. The standard known as oc—
tane number has been adepted for fuels for spark~ignition
engines,and is apparently quite satisfactory. In the rat—
ing of fuels for compression—ignition engines much work has
been done without arriving at an adequate standard although
arbitrary ratings have been develOpeddggiih give a good com-
parative ranking to a series of fuels. The complex inter-
relation of the variables affecting combustion in a com—
pressidn—ignition, fuel-injection engine makes it difficult
to set up a satisfactory standard of engine characteristics
and test conditions without'which it will be much more dif—
ficult to erect the standard for the fuel.

The characteristic of a fuel that is conceded to be of
utmost importance in determining its usefulness is its ig—
nitibility. The usual method of evaluating this ignitibil—
ity is by measurement of the'ignition lag. For these tests
ignition lag is considered to be the.interval between the
start of the injection of fuel and the point at which the
pressure in the cylinder becomes greater than the compres-
sion (or expansion) pressure would be if no ignition oc-
curred.‘ This definition of ignition lag is not dependent
uponu_the experimenter's conception of what constitut‘es ig—
nition but is considerably dependent upon the apparatus
used for. establishing the end- points. especially the indi-
cating apparatus with whic-h the pressure—time or pressure—
volume cards are taken."

The engine-tests reperted herein were started with the
idea-of deveIOping a method'of rating fuels on a performr
ance basis but it was soon realized that the fuels used for
the tests were included within a small part of the range. of
possible fuels for fuel—injection engines and that no great
difference in performance could be expected when engine
tests were made under strictly comparable'Operating condi—
tiOns. It was, therefore, decided to try to establish tests
which would give a satisfactory differentiation between
several fuels and which would give same indication of the
operating conditions under which a particular fuel might be
used in an individual engine.