Electrical characteristics of spark generators for automotive ignition Page: 21 of 30
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ELECTRICAL CHARACTERISTICS OF SPARK GENERATORS FOR AUTOMOTIVE IGNITION 287
If the empirical equation (4) really represented the facts over the whole range, then the
intercept on the axis of S/E , of the graphs of Si/E, vs. S should have the value of - This
relation was found to obtain in many of the cases tested. In other cases Tm was 20 to 30 per
cent less than this relation would indicate. This is presumably the result of an appreciable
damping of the oscillation; still other cases showed values of Em higher than the theoretical.
These can only be accounted for by stating that in these generators.the equation does not fit
the facts closely enough to justify extrapolation to extremely low values of S.
The slope of the line obtained by plotting S/ vs. S has the value - and hence the
quotient of twice the intercept divided by the slope of the line equals Z, and should theoretically
be equal to n Zm or F Z,.
This relation was found to be only approximately true. The points marked by open
circles in Figure 42 were located by using the value of b obtained by the oscillograph and the
n-
value of Zm obtained as the mean ratio of crest voltage to current at break at the lower speeds.
In the cases plotted and in a few others the agreement between these two sets of data obtained
from independent observations is quite satisfactory. On the other hand, in several cases the
directly observed values of Z~ were nearly 60 per cent in excess of those deduced from the
S/J, vs. S lines. The disagreement in the intercept values in no case exceeded 30 per cent.
In general, with any given system, a change such as adding capacitance in parallel with the
secondary which decreases Z is also found to decrease the slope of the SIT,, vs. S line but to
leave its intercept unchanged. A change such as speed which changes I, is found to markedly
change the intercept of the line, but to affect only slightly the quotient of slope to intercept.
The relation between crest voltage and capacitance added in parallel with the secondary
is indicated by the equation
Emn V + (5)
This is the same as equation (3) except that the total effective capacitance now consists of two
parts, Co, which represents that inherent in the spark generator; and C, which may be added
in the form of the capacitance of the spark plug cables or of condensers. If E, were zero and F
constant, then a plot of 1 as ordinate against C as abscissa would yield a straight line.
The intercept of this line on the axis of abscissae would give C, and the slope of the line
would be F I ' Unfortunately, F varies when different condensers Ca are connected, because
the frequencies of the resulting oscillations are changed. Also in most magnetos the E. M. F. of
rotation E, is not entirely negligible. It happens, however, that in all the systems tested the
effects of F and E, were such that the plot of 1/V,2 vs. Ca was almost a straight line, though
slightly convex upward. This empirical straight line relation can therefore be used with some
assurance to predict the variation of VT with C, in other systems. (See figs. 20 to 30.) The
intercept of the plotted lines, however, was in all but one case decidedly greater than the value
of Co obtained by other methods, and was often two or three times that value.
While the preceding discussion has shown certain relations between performance and other
measurements on a given spark generator, a correlation of the performance data with the energy
and current data given in columns 8 to 12 of Table 3, for the different generators, yields consid-
erable additional information as regards two of the desirable properties of a spark generator,
viz, (1) its ability to produce a reasonably high voltage on open circuit under adverse conditions,
such as low speed when the plug is fouled with oil; and (2) its ability to fire a plug which is fouled
with carbon. For the former property we may consider that the maximum secondary voltage
developed at 50 R. P. M., as given in column 3 of Table 5, is representative. Combining this
value with the total effective capacitance of the system (referred to the secondary side) which
is given in column 7 of Table 4, we get the amount of energy (given in column 6 of Table 5)
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Brode, R. B.; Randolph, D. W. & Silsbee, F. B. Electrical characteristics of spark generators for automotive ignition, report, 1927; (https://digital.library.unt.edu/ark:/67531/metadc65894/m1/21/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.