Why There Is No Spark Plug In Diesel Engine?

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.

A Spark Plug is used in vehicles that run on petrol but the ones with a diesel engine do not use it. Petrol and diesel have dissimilar properties, one of which is auto-ignition. Diesel has low auto-ignition temperature. The Auto-ignition temperature means that a fuel which starts burns automatically at a particular temperature.Because of low auto-ignition temperature, diesel can easily ignite.

In a diesel engine, the highly compressed air (compression ratios of 18 to 25) is introduced into the combustion chamber, this compressed air increases the temperature of diesel fumes or vapors to kindling point, and thus the mixture self-ignites.But, to introduce such high compression ratios, the engine cost drastically increases, thus to keep engine cost economical, we use the auxiliary assistance of glow plugs. They help in reaching the kindling point of diesel fume air mixture. They also come handy, during cold start, and when there is heat leak from the cylinder block and cylinder head.Diesel has low volatility and thus it cannot form the desired air-fuel mixture to be ignited by spark plugs.

Relevant Facts:

1.) The diesel engine (also known as a compression-ignition or CI engine) is named after Rudolf Diesel.

2.) Diesel engines can operate at high compression ratios.

3.) The ignition temperature of diesel ranges around 473 K.

4.) The diesel engine has the highest thermal efficiency (engine efficiency) of any practical internal or external combustion engine due to its very high expansion ratio and inherent lean burn which enables heat dissipation by the excess air.