Spark Plugs
Chapter 2 Spark Plugs
14
Standard tightening torque
Center shaft connecting the terminal and
the center electrode.
This shaft is made of steel and has the role
of allowing high-voltage current to flow from
the terminal to the center electrode without
loss.
Center shaft (stem)
4
4
The housing forms an outer shell that
surrounds the insulator, supports the
insulator, and installs the spark plug in the
engine.
At the bottom the ground electrode is
located, so current can flow through the
engine itself to the center electrode over the
gap.
Housing
5
5
Mounted between the center shaft and
insulator to maintain the airtightness.
DENSO uses the glass seal method.
A special mixture of glass powder and
copper powder is charged in the installation
section for the insulator and center shaft
and center electrode and melted at high
temperature. This bonds the center shaft
and the center electrode and fuses the
insulator and the metal.
The sealing for both is good and the thermal
ratio of expansion is appropriate, so even
under harsh conditions gaps do not occur
and good airtightness can be secured.
Glass seal
6
6
Makes the housing and the engine fit tightly
to each other and maintains the airtightness
of the combustion chamber.
There is a procedure for tightening and the
appropriate tightening margin must be
secured.
Gasket
7
7
Special nickel alloy is used for the center
electrode to reduce electrode wear.
Copper is sealed into the center section to
improve thermal conductivity.
Electrode with copper
8
8
A new iridium alloy tip with a diameter of 0.4
mm is laser welded to the tip of the center
electrode to make the center electrode
ultra-fine.
This lowers the required voltage, secure
reliable spark, reduces the quenching
effect, and improves ignition performance.
Iridium, like platinum, is a precious metal
and has extraordinarily superior properties
for a spark plug electrode, for example high-
temperature withstand, high strength, and
low resistance. In order to further improve
oxidation resistance at high temperatures,
DENSO developed a unique new iridium
alloy containing Rhodium.
0.4-mm iridium center
electrode
10
10
The ground electrode has a shape in which
the electrode tip is cut to a finely tapered
shape.
This reduces the quenching effect, so it
improves ignition performance.
Taper cut ground electrode
12
12
Thread diameter
(mm)
Tightening torque
(Nm)
8
10
12
14
18
8
〜
10
10
〜
15
15
〜
20
20
〜
30
30
〜
40
Nickel chrome material is used for the
ground electrode and various measures are
taken with the shape to improve ignition
performance.
One of these measures is the U- groove.
1 The surface contacted by the air-fuel
mixture is large,
2 There is much edge section, and sparks
occur easily.
3 The flame core (flame size) widens easily.
There are many other feaures as well and
large ignition energy can be obtained.
DENSO obtained patents for spark plug U-
grooves from 1975 to 1992.
U-groove ground electrode
11
11
