1. FUNCTION:
The spark plug conducts the insulated ignition current into the combustion
chamber of the engine. The sparks jumping the electrodes of the spark plug
ignite the compressed fuel/air mixture at a specified piston position.
2. DESIGN:
The main parts of the spark plug are: Terminal bolt made of steel center
electrode made of special alloy. Insulator made of the approved Pyranit ceramic.
Plug shell made of steel.
The terminal bolt and center electrode are connected inside the insulator by
a special conducting fused mass and securely anchored. The spark plug is thereby
made gas-tight over the entire heat range. The insulator together with a gasket
seal and pressure ring is crimped into the plug shell by means of a special
process under high pressure. The ground electrode made of a special alloy is
welded onto the plug shell. The spark plugs carry a permanent gasket and are
screwed into the engine cylinder
The spark plug is very highly stressed electrically, mechanically, chemically
and particularly thermally. The gas temperature during one cycle can change
between 600 and 30000 C (1400 and 5400 F) in the
combustion chamber at the tip of the spark plug; the pressure in the combustion
chamber runs from below 14 psia up to 710 psia. The insulator must at the same
time withstand an ignition voltage of 10 to 20' kV.
Lead and sulfur compounds which result from the combustion of impure fuel and
fuel containing anti-knock substances tend to attach chemically that part of the
spark plug exposed in the combustion chamber and lead to oxidation. Although the
high temperatures in the combustion chamber favor this oxidation, the spark plug
electrodes made of a special alloy are substantially immune to such chemical
attacks.
4.
HEAT VALUE:
It is not possible to use the same standard spark plug in all engines. By
means of thorough tests, engine manufacturers therefore ascertain which spark
plug shows the most favorable heat behavior for the respective engine.
For normal engine operation, the correctly chosen spark plug assumes a
certain operating temperature which must lie within a definite heat range. Above
this heat range, preignition occurs; below, the spark plug tends to foul. A
spark plug with a high heat value tolerates a higher heat load without
preignition occurring. Thus, it is called a "cold" spark plug. A spark
plug with a low heat value is susceptible to overheating, but is less inclined
to fouling. Thus, it is called a "hot" spark plug.
5. THE SPARK PLUG TIP:
Poor engine condition, unfavorable operating conditions, extreme driving
method, and above all, poor fuel can lead to troubles which show up on the spark
plug but are not the fault of the spark plug. Therefore, it is good to check
from time to time that part of the spark plug exposed in the combustion chamber,
the '1spark plug tip1'. Conclusions concerning the causes of the troubles can be
drawn from the appearance of this spark plug tip.
