(95) High Tension Spark Plug.
The high tension spark plug is a device that introduces the spark gap and spark into the combustion chamber, and at the same time insulates the current carrying conductor from the cylinder walls. Since the voltage of the jump spark current is very high it is evident that the insulation of the plugs must be of a very high order and that this insulation must be capable of withstanding the high temperature of the combustion chamber. A cross-section of a typical plug is shown by Fig. 98, together with its connections and the course of the current, the latter being shown by the arrow heads.
The electrode B through which the current enters the cylinder is thoroughly insulated from the walls by the porcelain rod C.
The porcelain forms a gas tight joint with the threaded metal bushing F at the point P, the tension caused by the electrode B and the nut I holds the porcelain firmly on its seat at P.
The nut is supported by the porcelain shell H which rests in the top of the metal bushing F. A washer L is inserted between H and F to insure against the leakage of gas from the plug should a leak develop at P. L being a soft washer (usually asbestos) allows the porcelains C and H to expand and contract without breaking. A packing washer or gasket is also placed at the point where the electrode B passes through the porcelain H. This is the washer Q, held in position by the nut I. This washer is elastic and reduces strain on porcelain caused by the expansion.
The cylinder wall G has a threaded opening R into which the plug is screwed, the threads of the opening corresponding with the threads on the metal sleeve E. The plug may be removed from the cylinder for examination without disturbing the adjustment of the electrode and porcelains by unscrewing it at R.
Allowing the current to jump from the electrode to the cylinder wall via the metal sleeve saves one wire and connection, the cylinder and the frame of the engine serving as a return path for the current. This simplifies the wiring and minimizes the danger of high tension short circuits.
Fig. 98. Cross-Section of Typical Spark Plug.
By unscrewing the threaded metal bushing F it is possible to examine the condition of the porcelain rod C at the point where it is exposed to the heat of the cylinder. This inspection can be made without disturbing the packed joints at L or Q.