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National Advisory Committee for Aeronautics, Technical Notes - A Comparison of Ignition Characteristics of Diesel Fuels as Determined in Engines and in a Constant Volume Bomb

Ignition—lag data have been obtained for seven fuels
injected into heated, compressed air under conditions
simulating those in a compression—ignition engine. The
results of the bomb tests have been compared with similar
engine data, and the differences between the two sets of
results are explained in terms of the response of each
fuel to variations in air density and temperature.

Earlier tests with the N.A.C.A. high—temperature
bomb (reference 1) have shown that the ignition lag at
the highest bomb temperature is roughly twice that for
the same fuel in an N.A.C.A. high—speed Diesel engine hav-
ing a comparable air density at top center. The minimum
engine ignition lags reported by Schweitzer (reference 2)
are in substantial agreement With those obtained with the
bomb.

It has been reported (reference 3) that the G.F.R.
engine fuel ratings are in substantially the same order
when either the critical»oompression—ratio (C.G.R.) or
the ignition—delay method is used. The fact that the
G.C.R. method involves longer ignition lags than the de-
lay method indicates that the bomb would also give com—‘;'
parable ratings provided that the greater air turbulence
in the engine was not an influential factor. On this
basis, tests were made in the bomb with fuels of differ—
ent ignition qualities, or different cetane numbers, to
determine the limitations of this apparatus for rating
fuels. The test conditions included bomb temperatures of
870° and 1.1550 F. and air densities of 0.59, 0.89, and
1.18 pounds per cubic foot.

The engine data with which these results are compared r
were obtained with an N.A.G.A. dieplacer—type compression—
ignition engine at 2,000 r.p.mt and a compression ratio of
14.5 (reference 4).

Seven fuels have been tested in the bomb, three of
which were obtained by adding an ignition accelerator to
a single base fuel designated L1 Diesel in the table and
the figures. Ethyl nitrate was used for two of these
fuels and a commercial Diesel dope for the third fuel.
The four other fuels used in the bomb were selected from
among eight fuels tested by this laboratory in a compres—
sion—ignition engine. Thekproperties of the eight previ—
ously tested fuels are listed in table I; the properties
of the three fuels with added ignition accelerator are ‘
not available. With the exception of the N.A.G.A. engine '
data (the ignition lags} the maximum pressures, and the
maximum rates of pressure rise), all the data in this ta—
ble were furnished through the courtesy of the United
States Naval Engineering Experiment Station, Annapolis,
Md. The cetane ratings were obtained by the procedure
recommended in reference 3, using the constant ignition-
delay method with a modified magnetic pick-up.