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National Advisory Committee for Aeronautics, Technical Notes - A Comparison of Several Methods of Measuring Ignition Lag in a Compression Ignition Engine

The ignition lag of a fuel oil in the combustion cham—
ber of a high-speed compression—ignition engine was meas-
ured by three different methods. The start of injection
of the fuel as observed with a Stroborama was taken as the
start of the period of ignition lag in all cases. The end
of the period of ignition lag was determined by observa-
tion of the appearance of incandescence in the combustion
chamber, by inspection of a pressure—time card for evi—
dense of pressure rise, and by analysis of the indicator
card for evidence of the combustion of a small but defi~
nite quantity of fuel.

A comparison of the values for ignition lags obtained
by these three methods indicates that the appearance of
incandescence is later than other evidences of the start
of combustion, that visual inspection of a pressure—time
diagram gives consistent and usable values with a minimum
requirement of time and,/or apparatus, and that analysis of
the indicator card is not worth while for ignition lag
alone.

At the'preSent stage of the development of the high—
speed compression-ignition engine considerable attention
is being given to methods of rating fuel oils as to their
suitability for use in such engines (references l»5). it
is generally recognized that there is a definite connection
between the ignitibility of fuel oil and its satisfactory.
operation in an engine. As a measure of ignitibility the
length of the period of delay between the injection and
the ignition of the fuel has been used, but no method as
yet devised for the measurement of this period can be con~ l
sidered as universally satisfactory and acceptable.

Ignition lag in a compression—ignition engine is most
s.atisfactorily defined as the interval between the start
of injection of fuel and- t? e point at which the pressure
in the engine cylinder mee.surably exceeds the compression
or expansion pressure -eithout ignition. Ignition lag may
thus be measured in time units or it may be more convenient~
ly ompressed in degrees of crankshaft rotation.