Diesel Oil

DIESEL OIL

It is a fraction obtained between 250 - 320°C during fractional distillation of petroleum. It is a mixture of C₁₅H₃₂ to C₁₈H₃₈ hydrocarbons. Its calorific value is about 11000 kcal/kg. It is used as a very good diesel engine fuel.

Causes of knocking in CI engines (Diesel engines)

In a diesel engine, first air is alone compressed. This compression raises the temperature of the cylinder to about 500°C. Then the oil is sprayed into the heated air. This further raises the temperature as well as pressure. The expanding gases push the piston and power stroke begins.

The combustion of a fuel in a diesel engine is not instantaneous and the time between injection of the fuel and its ignition is called Ignition lag or Ignition delay. This delay is due to the time taken for the vapourisation of oil droplets and raising the temperature of vapour to its ignition temperature. Long ignition lags lead to accumulation of more vapours in the cylinder, which undergo explosion during ignition. This is responsible for diesel knock. If the ignition lag is short, diesel knock will not occur.

1. Diesel index

The quality of a diesel oil is indicated by diesel index number using the following formula.

Diesel index number = Specific gravity (API) ×  Aniline point in °F / 100

Aniline point and specific gravity is noted from (American Petroleum Institute) Scale.