Like gasoline, diesel is a compound of hydrogen and carbon, extracted from crude oil.
There are different grades of diesel fuel for diesel engines. What is commonly sold in a service station is highly refined, and is suitable for use in high-speed diesel engines, including those in light automotive use.
The cetane rating of a diesel fuel defines how easily the fuel will ignite when it is injected into the cylinder.
The lower a fuel’s cetane rating, the longer it takes to reach ignition point.
Using a fuel with too low a cetane rating will increase the amount of diesel knock in an engine.
When diesel fuel is injected into the cylinder, it does not ignite instantly. It takes time for the heat of the compressed air in the cylinder to heat the fuel sufficiently for it to ignite. This period of time from the start of injection, to the start of combustion is called the delay period. During this delay period, fuel continues to be injected into the cylinder.
When the fuel is heated sufficiently, it erupts into flame. Combustion occurs. The sudden pressure rise sends a shock wave through the combustion chamber, that can be heard outside the engine. This is the sound called diesel knock.
Diesel knock can also be caused by poor atomization of the fuel, which can take too long to reach combustion temperature.
The higher the cetane rating of the fuel, the easier a cold engine will be to start. The engine will produce less smoke and odors, and there will be fewer deposits in the combustion chamber.
Diesel engines are also required to operate in low temperatures. During low temperatures, the fuel becomes thicker. If the temperature is too low, paraffins in the fuel begin to solidify, and form waxes. These waxes can block filters, causing fuel starvation, and low power output.
To help prevent this, filters are fitted close to the engine, and sometimes heaters are used.
Diesel fuel also acts as a lubricant for the fuel system components - provided it is free of water, and abrasive particles.
