Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a gas-filled
discharge tube suitable for use in the ignition system with
a series gap of an automotive engine and the like.
The ignition system with a series gap has a
discharge tube having a predetermined capacitance and
connected in series to a spark plug to prevent the soothing
of the spark plug with carbon. Such an ignition system with
a series gap disclosed in Japanese Utility Model Laid-open
(Kokai) No. 63-lnl486 has a circuit configuration. This
known ignition system comprises an ignition coil, a
discharge tube, and a spark plug. Each of the discharge
tube and the spark plus has a capacitance. The electric
potential of the center electrode of the spark plug, i.e.,
the electric potential of a first predetermined point
increases in proportion to the ratio between the respective
capacitances of the discharge tube and the spark plug when
the voltage V applied across the discharge tube, i.e. the
electric potential of another predetermined point, is
increased. Discharge occurs in the discharge tube when the
electric potential of a first point reaches a sufficiently
high level, and then the electric potential of the another
predetermined point drops sharply and the electric potential
of the first predetermined point increases sharply.
If the circumference of the center electrode of
the spark plug is soothed with a carbon layer, it is
possible that a leakage current flows through the carbon
layer to a portion of the center electrode deep from the
extremity. When the center electrode of the spark plug,~3)is
in such a soothed condition, the potential of the deep
portion of the cen-ter electrode, i.e. the potential of a
second predetermined point, increases with the potential of
the center electrode with a delay as the potential of the
center electrode is increased sharply by the discharge of
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the discharge tube. Suppose that the firing potential of
the center electrode is 8 kV. Then, the firing potential of
the deep portion is about 6 kV, because the gap between the
deep portion and the casing is smaller than that between the
center electrode and the opposite electrode. Consequently,
a normal spark discharge is passed between the center elec-
trode and the opposite electrode at the point of intersec-
tion of a line corresponding to 8 kV and a curve repre-
senting the variation of the potential of the center
electrode, and an abnormal spark discharge is passed between
the deep portion and the casing at the point of intersection
of a line corresponding to 6 kV and the curve representing
the variation of the potential of the deep portion.
~ However, the normal spark discharge occurs earlier by a
- 15 predetermined time between the center electrode and the
opposite electrode than the abnormal spark discharge, and
hence the spark plug may not be soothed due to the abnormal
spark discharge.
The conventional discharge tube comprises a tube
formed of an electrically insulating material, such as a
ceramic, and having opposite open ends, a pair of electrode
bases closely fitted respectively in the opposite ends of
the tube, and a pair of discharge electrodes held on the
electrode bases within the tube. The tube is filled with an
inert gas. The sum of the capacitance between the pair of
discharge electrodes and the capacitance between the pair of
electrode bases is the capacitance of the discharge tube
In such an ignition system, it is desired to
employ a compact discharge tube to enable the plug cap
attached to the extremity of the high-tension cable to be
formed in a compact construction. When the length of the
discharge tube of a construction is reduced to increase the
capacitance of the discharge tube and such a discharge tube
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having a comparatively large capacitance is employed in the
ignition system and the same spark plug having the same
capacitance is used, the potential of the center electrode
and the potential of the deep portion of the spark plug
increase according to the increase in the capacitance of the
discharge tube. Consequently, a time interval between the
point of occurrence of the normal spark discharge between
the center electrode and the opposite electrode, and the
point of occurrence of the abnormal spark discharge becomes
very short and hence it is a matter of probability that
which of the normal spark discharge and the abnormal spark
discharge will occur first. Therefore, increase in the
capacitance of the discharge tube entails increase in the
possibility of the abnormal spark discharge soothing the
spark plug.
Accordingly, it is an object of the present
invention to provide a gas-filled discharge tube of a
compact construction, having an appropriate capacitance
which will not cause soothing the spark plug.
According to the present invention, there is
provided a gas-filled discharge tube comprising an
insulating tube unit having a tube formed of an electrically
insulating material and having openings in the opposite ends
thereof, a pair of electrode bases closely attached to the
opposite ends of the tube so as to close the openings,
respectively, a pair of discharge electrodes attached
respectively to opposite inner surfaces of the electrode
bases to form a series gap, and an inert gas filling the
insulating tube unit, wherein the diameter of peripheries of
the openings covered with the electrode bases is smaller
than the inside diameter of the tube.
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Since the diameter of the portions of the opposite
ends of the tube provided with the openings closed by the
electrode bases is smaller
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than that of other portions, the diame~er, hence the area, of the
electrode bases may be comparatively small, so that the capacitance
between the electrode bases, hence the capacitance of the sas-filled
discharge tube, is comparatively small and hence the gas-filled dis-
charge tube can be formed in a compact construction without excessively
increasing the capacitance.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the
present invention will become more apparent from the following descrip-
tion taken in connection with the accompanying drawings, in which:
Figure 1 is a longitudinal sectional view of a gas-filled dis-
charge tube in a first embodiment according to the present invention;
Figure 2 is a longitudinal sectional view of a gas-filled dis-
charge tube in a second embodiment according to the present invention;
Figure 3 is a circuit diagram of an ignition system with a series
gap;
Figure 4 is a graph showing potential variations at different
points in the ignition system of Fig. 3;
Figure 5 is an enlarged fragmentary sectional view of the ex-
tremity of a spark plug; and
Figure 6 is a longitudinal sectional view of a conventional
discharge tube.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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Gas-filled discharge tubes embodying the present invention will
be described hereinafter with reference to Figs. 1 and 2, in which parts
like or corresponding to those of the conventional discharge tube
previously described with reference to Figs. 3 to 6 will be denoted by
the same reference characters.
First Embodiment
Referring to Fig. 1, a gas-filled discharge tube 2 comprises an
insulating tube unit 7 formed of an electrically insulating material,
such as alumina ceramic, steatite or crystallized glass, and provided
with openings 10 and 12 of the same diameter R2 at its opposite ends, a
pair of metallic electrode bases 9 attached to the opposite ends of the
insulating tube unit 7 so as to close the openings, respectively, a pair
of discharge electrodes 8 projecting from the opposite inner surfaces of
the electrode bases 9 so as to form a series gap, and an inert gas, such
as argon gas or an argon-nitrogen mixed gas, filling the insulating tube
unit 7.
The insulating tube unit 7 consists of a tube 11 and an end cap
13 fitted in one end of the tube ll and provided with an opening 12.
The other end of the insulating tube 7 is reduced to form the opening 10
having a diameter R2 smaller than the diameter Rl of the open end of the
tube 11 closed by the end cap 13. The tube ll and the end cap 13 are
joined together with glass frit 14 or the like to form the insulating
tube unit 7 provided with the openings 10 and 12 of the same diameter R2
smaller than the diameter Rl of the open end of the tube 11, i.e., the
inside diameter of the tube ll.
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The pair of discharge electrodes 8 are inserted through the
openings 10 and 12 in the insulating tube unit 7 so as to form a dis-
charge gap between the opposite top surfaces thereof. Each of the
discharge electrodes 8 is of a so-called Rogowskii type having a flat
top surface 8a having a curved periphery 8b, and provided with a number
of small holes. Flanges 8c formed around the base ends of -the discharge
electrodes 8 are seated on the peripheries of the openings 10 and 12,
respectively. The electrode bases 9 have the shape of a cap and are
jointed to the peripheries of the openings 10 and 12 so as to hold the
flanges 8c of the discharge electrodes 8 on the peripheries of the
openings 10 and 12 and to close the openings 10 and 12, respectively.
The openings 10 and 12 may be sealed by soldering the flanges 8c of the
discharge electrodes 8 held by the electrode bases 9 to the metallized
surfaces 15 of the peripheries of the openings 10 and 12, respectively.
A yas-charging pipe 16 connected to the electrode base 9 is sealed with
a sealant after charging the insulating tube unit 7 with the inert gas.
Since the diameters R2 f the openings 10 and 12 formed at the
opposite ends of the insulating tube unit 7 are smaller than the diame-
ter R1 of the other portion of the tube 11, the electrode bases 9 having
a comparatively small diameter, hence a comparatively small area, can be
used for sealing the openings 10 and 12 after inserting the perforated
discharge electrodes 8 of a Rogowskii type through the openings 10 and
12 in the insulating tube unit 7, so that the gas-filled discharge tube
2 can be formed in a comparatively small length without entailing
increase in the capacitance, because the capacitance between the
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electrode bases, hence the capacitance of the gas-filled discharge tube
2, is comparatively small.
The insulating tube unit 7 thus constructed has a comparatively
large surface area so that electric discharge passed along the surface
of the insulating tube unit, i.e., so-called creeping discharge, can be
prevented. An electric discharge can stably be passed between the
discharge electrodes 8 of a Rogowskii type at a comparatively high
firing potential even if the discharge electrodes 8 are disposed with a
small discharge gap.
Second Embodiment
Referring to Fig. 2, a gas-filled discharge tube 2 in a second
embodiment according to the present invention is substantially the same
in construction as the gas-filled discharge tube 2 in the first embodi-
ment, except that the gas-filled discharge tube 2 in the second embodi-
ment employs an insulating tube unit 7 consists of two short tubes 19.
One end of each of the short tubes 19 is reduced to form an opening 17
of a diameter Rz smaller than the inside diameter R1 of the short tube
19. The edge of the fully open end of one of the short tubes 19 is cut
to form an external taper surface 18, and the edge of the fully open end
of the other short tube 19 is cut to form an internal taper surface 18;
the taper surfaces 18 are joined closely when the short tubes 19 are
joined together with glass frit or the like to construct the insulating
tube unit 7.
The effects and advantages of the gas-filled discharge tube 2 in
the second emboaiments are the same as those of the gas-filled discharge
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tube 2 in the first embodiment.
If we now refer to the prior art shown in Figures
3 to 6, we see that an ignition system with a series gap has
a circuit configuration as shown in Fig. 3. This known
ignition system comprises an ignition coil 1, a discharge
tube 2, and a spark plug 3. Indicated at C1 and C2 are the
respective fixed capacitances of the discharge tube 2 and
the spark plug 3. As shown in Fig. 4, the electric
potential v1 of the center electrode of the spark plug, i.e.,
the electric potential of a point B in Fiy. 3, increases in
proportion to the ratio between the respective capacitances
C1 and C2 of the discharge tube 2 and the spark plug 3 when
the voltage V applied across the discharge tube 2, i.e., the
electric potential of a point A in Fig. 3, is increased.
lS Discharge occurs in the discharge tube 2 when the electric
potential of the point A reaches a sufficiently high level,
and then the electric potential of the point A drops sharply
and the electric potential of the point B increases sharply.
Fig. 5 shows a portion of the spark plug 3. If
the circumference of the center electrode 4 of the spark
plug 3 is sooted with a carbon layer 5 as shown in Fig. 5,
it is possible that a leakage current flows through the
carbon layer 5 to a portion 6 of the center electrode 4 deep
from the extremity. When the center electrode 4 of the
spark plug 3 is in such a sooted condition, the potential V2
of the deep portion 6 of the center electrode 4, i.e., the
potential of a point C in Fig. 3, increases with the
potential V1 of the center electrode 4 with a delay as the
potential V1 of the center electrode 4 is increased sharply
by the discharge of the discharge tube 2. Suppose that the
firing potential of the center electrode 4 (point B) is 8
kV. Then, the firi.ng potential of the deep portion 6 (point
C) is about 6 kV, because the gap between the deep portion
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6 and the casing is smaller than that between the center
electrode 4 and the opposite electrode. Consequently, as
shown in Fig. 4, a normal spark discharge s1 is passed
between the center electrode 4 and the opposite electrode at
the point of intersection of a line corresponding to 8 kV
and a curve representing the variation of the potential V1,
and an abnormal spark discharge S2 is passed between the deep
portion 6 and the casing at the point of intersection of a
line corresponding to 6 kV and the curve representing the
variation of the potential V2. However, as is obvious from
Fig. 4, the normal spark discharge S1 occurs earlier by a
time t1 between the center electrode 4 and the opposite
electrode than the abnormal spark discharge S2, and hence ths
~ spark plug may not be sooted due to the abnormal spark
; l5 discharge S2 .
Fig. 6 shows the discharge tube 2 having the
predetermined capacitance C1. The discharge tube 2 comprises
a tube 7 formed of an electrically insulating material, such
as a ceramic, and having opposite open ends, a pair of
electrode bases 9 closely fitted respectively in the
opposite ends of the tube 7, and a pair of discharge
electrodes 8 held on the electrode bases 9 within the tube
7. The tube 2 is filled with an inert gas. The sum of the
capacitance C3 between the pair of discharge electrodes B and
the capacitance C4 between the pair of electrode bases 9 is
the capacitance C1 of the discharge tube 2.
In such an ignition system, it is desired to
employ a compact discharge tube to enable the plug cap
attached to the extremity of the high-tension cable to be
formed in a compact construction. When the length of the
discharge tube 2 of a construction shown in Fig. 6 is
reduced to increase the capacitance C1 of the discharge tube
2 and such a discharge tube having a comparatively large
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capacitance is employed in the ignition system and the ~ame
spark plug 3 having the same capacitance C2 is used, the
potential V1 of the center electrode ~ (point B) and the
potential Vz of the deep portion 6 (point C) of the spark
plug 3 increase according to the increase in the capacitance
of the discharge tube 2 as indicated by broken lines in Fig.
4. Consequently, a time interval t2 between the point of
occurrence of the normal spark discharge S1 (the point of
intersection of the line correpsonding to 8 kV and the curve
representing the variation of the potential V1 of the center
electrode) between the center electrode 4 and the opposite
electrode, and the point of occurrence of the abnormal spark
discharge Sz (the point of intersection of the line
corresponding to 6 kV and a curve representing the variation
of the potential V2 of the deep portion 6) becomes very short
and hence it is a matter of probability that which of the
normal spark discharge S1 and the abnormal spark discharge S2
will occur first. Therefore, increase in the capacitance C1
of the discharge tube 2 entails increase in the possibility
of the abnormal spark discharge S2 sooting th0 spark plug 3.
Although the invention has been described in its
preferred forms with a certain degree of particularity,
obviously many changes and variations are possible therein.
It is therefore to be understood that the present invention
may be practiced otherwise than as specifically described
herein without departing from the scope and spirit thereof.
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