The sparking voltage of spark plugs Page: 3 of 16
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REPORT No. 202
THE SPARKING VOLTAGE OF SPARK PLUGS
By FRANCIS B. SILSBEE
I. INTRODUCTION
The study of this subject was originally undertaken at the request of the Army Air Service.
In view of the intimate connection between this and other phases of the ignition problem studied
under the auspices of the National Advisory Committee for Aeronautics, it is published as a
technical report of the latter body to put on record the present state of knowledge of the volt-
age required to produce a spark across the gap of a spark plug in the cylinder of an internal
combustion engine. This voltage is affected by a considerable number of conditions, some of
which are little understood at present. In the following discussion the approximate range of
each of these conditions will be indicated and at least the order of magnitude of its effect on
the sparking voltage.
The importance of having an approximate knowledge of this voltage lies in the fact that
it constitutes a sort of common meeting point in the performance of the engine, the spark
plug, and the magneto (or coil). The voltage attained by the spark coil, and hence the stress
to which all parts of the electrical system are subjected, is fixed solely by the sparking voltage.
Thus any change in, for example, the compression ratio of the engine affects the operation of
the magneto, or the most desirable setting of the spark plug points only in so far as it changes
the sparking voltage. Information on sparking voltages is of importance also in connection
with the safety gaps of magnetos, and to an even greater extent with the various forms of
standard test gaps used in testing the performance of ignition apparatus.
A further point is the possible direct relation between the initial voltage of a spark and
its igniting power. Work at the British (69)1 National Physical Laboratory has shown quite
definitely that of two very weak sparks of equal energy content, the one which has the greater
sparking voltage is capable of igniting a less inflammable mixture. Certain writers (11), (48)
have concluded from this that a high sparking voltage is therefore always desirable inca spark
plug. It would seem, however, that the increased difficulty of producing any spark at all
with such a plug would more than offset the benefit from the increased igniting power when a
spark is produced. The experiments referred to in (69) were made with well-carburetted
gaseous mixtures, and more data are needed when conditions are otherwise.
The range of variables covered by the present report will be only sufficient to cover ade-
quately the limits met with in automotive practice. These are, roughly, gap lengths from 0.1
mm. to 6 mm., gas pressures from one-half atmosphere to 10 atmospheres, temperatures up to
7000 C., and voltages up to 15,000 volts. For pressures or gap lengths materially outside this
range the character of the electric discharge may become very different from those described
in this report.
In the present report, after a brief account of the present accepted theory of the spark
discharge, there will be given a tabulation of the principal variables affecting the sparking
voltage and a detailed discussion of each. The report will then treat of the application of
these facts to standard test gaps, auxiliary series gaps, and safety gaps, and will close with a
general summary of the contents of the earlier sections.
The literature relating to spark gaps in general is very voluminous. The bibliography
appended to this report contains such references as seem to have even a remote bearing on
ignition work, arranged chronologically under a number of principal topics. The numbers in
parentheses scattered through the text of the report refer to references in the bibliography
covering the particular phase under discussion.
SReference Is made by number (italic) to " Bibliography of Sparking Voltages."
357
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Silsbee, F. B. The sparking voltage of spark plugs, report, 1925; (https://digital.library.unt.edu/ark:/67531/metadc65853/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.