Spark Plugs

Chap

ter 2

Spa

rk P

lugs

14

Standard tightening torque

Center shaft connecting the terminal andthe center electrode.This shaft is made of steel and has the roleof allowing high-voltage current to flow fromthe terminal to the center electrode withoutloss.

Center shaft (stem)44The housing forms an outer shell thatsurrounds the insulator, supports theinsulator, and installs the spark plug in theengine.At the bottom the ground electrode islocated, so current can flow through theengine itself to the center electrode over thegap.

Housing55Mounted between the center shaft andinsulator to maintain the airtightness.DENSO uses the glass seal method.A special mixture of glass powder andcopper powder is charged in the installationsection for the insulator and center shaftand center electrode and melted at hightemperature. This bonds the center shaftand the center electrode and fuses theinsulator and the metal.The sealing for both is good and the thermalratio of expansion is appropriate, so evenunder harsh conditions gaps do not occurand good airtightness can be secured.

Glass seal66

Makes the housing and the engine fit tightlyto each other and maintains the airtightnessof the combustion chamber.There is a procedure for tightening and theappropriate tightening margin must besecured.

Gasket77Special nickel alloy is used for the centerelectrode to reduce electrode wear.Copper is sealed into the center section toimprove thermal conductivity.

Electrode with copper88

A new iridium alloy tip with a diameter of 0.4mm is laser welded to the tip of the centerelectrode to make the center electrodeultra-fine. This lowers the required voltage, securereliable spark, reduces the quenchingeffect, and improves ignition performance.Iridium, like platinum, is a precious metaland has extraordinarily superior propertiesfor a spark plug electrode, for example high-temperature withstand, high strength, andlow resistance. In order to further improveoxidation resistance at high temperatures,DENSO developed a unique new iridiumalloy containing Rhodium.

0.4-mm iridium centerelectrode1010

The ground electrode has a shape in whichthe electrode tip is cut to a finely taperedshape.This reduces the quenching effect, so itimproves ignition performance.

Taper cut ground electrode1212

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 theground electrode and various measures aretaken with the shape to improve ignitionperformance. 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 andlarge ignition energy can be obtained.DENSO obtained patents for spark plug U-grooves from 1975 to 1992.

U-groove ground electrode1111