Fuel for CI engines should have certain qualities to be the ideal fuel for these engines. Diesel is used as the fuel in CI engines because it possesses the qualities that are desired from the fuel. Some of the desired characteristics of these fuels are:
1) Knocking characteristics: In case of the CI engine the burning of fuel occurs due to compression of air. It is desired that as soon as the fuel is injected into the cylinder, it starts burning, but in practical situations this never happens as there is always a time lag between the injection of the fuel and burning of the fuel. As the duration of ignition lag increases, more and more amounts of fuel get accumulated in the cylinder head. When the fuel is finally burnt, excessively large amounts of energy is released, which produces extremely high pressure inside the engine. This causes the knocking sound inside the engine, which can be clearly heard. Thus the engines should have a short ignition lag so that the energy is produced uniformly inside the engine and there is no abnormal sound. The ignition of the fuel also affects starting, warming, and production of exhaust gases in the engine.
The knocking capacity of the fuel is measured in terms of cetane rating of the fuel. The fuel you are using for your CI engine should have a cetane number high enough to avoid knocking of engine.
2) Volatility of the fuel: Thorough mixing of the fuel and air when fuel is injected in the cylinder head ensures uniform burning of the fuel. The fuel should be volatile in nature within the operating temperature range of the cylinder head so that it gets converted into a gaseous state and mixes thoroughly with compressed air.
3) Starting characteristics of the fuel: The smooth starting of the vehicle depends greatly on the fuel used for the vehicle. For easy starting of the vehicle it is important that the fuel has good volatility so that it mixes with the air uniformly and it readily forms into the combustible mixture. The high cetane number of the fuel ensures that the ignition of the fuel will be fast, which in turn will lead to faster starting of the vehicle.
4) Smoke produced by the fuel and its odor: The exhaust gases produced from the fuel should not have too much smoke and odor.
5) Viscosity of the fuel: The fuel should have a viscosity low enough so that it can easily flow through the fuel system and the strainer at the lowest working temperatures.
6) Corrosion and wear: The fuel used for the CI engine should not cause corrosion of any components of the engine before or after combustion.
7) Easy to handle: Large quantities of fuel for a CI engine have to be transported from one place to the other, hence it should be easy to handle and transport. The fuel should have a high flash point and high fire point to avoid it catching fire during transport.