DIESEL FUELS

Diesel fuel is a mixture of a few hundred hydrocarbons derived from refining of crude petroleum. When petroleum prices were low, the diesel fuels were produced mostly by blending various refinery streams from the atmospheric distillation unit of petroleum refineries. To meet the increasing demand of the diesel fuels, products of secondary refinery processes like thermal and catalytic cracking, hydro-cracking, vis-breaking etc., also are used as blending components of the current diesel fuels.  The diesel fuel streams evaporate generally in the temperature range of 150- 390º C. The key properties of the diesel fuel have already been given in Table 8.1. Besides these, other significant properties include cold flow characteristics at low ambient temperatures, water and sediment content etc. The important diesel fuel quality parameters are discussed below.

Ignition Quality

Ignition quality is a measure of ease of self-ignition of diesel fuel when the fuel is injected in hot compressed air in the engine cylinder. Cetane number (CN) is the most widely accepted measure of ignition quality as it is measured by a test on the engine. The cetane number scale is defined in terms of blends of two pure hydrocarbons used as reference fuels;

• A high ignition quality hydrocarbon: n- Hexadecane or Cetane ( n-C₁₆H₃₄) given CN =100
• Another hydrocarbon with poor ignition quality: Hepta-methyl nonane (HMN) assigned CN =15.

The cetane number scale is given by:

Cetane number is measured in a standard single cylinder, variable compression ratio CFR engine according to ASTM D613 method. The test engine is a prechamber diesel engine. The test conditions are:

(i) Intake air temperature = 65.6° C,

(ii) Coolant temperature = 100° C,

(iii) Engine speed = 900 rpm,

(iv) Injection advance = 13° btdc.