Spark plugs are the essential part of a vehicle. Without a spark, a vehicle will not start and that is why it is so important. But in a Diesel Engine, there is an absence of a spark plug.
Let’s see why is that the case?
A spark plug renders electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while bearing combustion pressure within the engine. It has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto.
The spark plug’s metal shell is screwed into the engine’s cylinder head and thus electrically grounded. The central electrode sticks out through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell and designated the side, earth, or ground electrode(s)
Diesel is used in a high-compression engine. Air is compressed until it is heated above the auto-ignition temperature of diesel. Then the fuel is injected with a high-pressure spray. There is no ignition source. As a result, diesel is required to have a high flash point and a low auto-ignition temperature.
The flash-point of a fuel is the lowest temperature at which it can form an ignitable mix with air. The high flash point in diesel fuel means that it does not burn as easily as gasoline, which is a safety factor. Too low of a flash point is a fire hazard because ignition may continue and lead to an explosion. While in compression stroke the temperate achieves to self-ignite the diesel for the power stroke.
Why is a spark plug not used in Diesel Engines?