Rating of CI Engine Fuels:

In compression-ignition engines, the knock resistance depends on chemical characteristics as well as as on the operating and design conditions of the CI engine. Therefore, the knock rating of a diesel fuel is found by comparing the fuel under prescribed conditions of operation in a special engine with primary reference fuels. The reference fuels are normal cetane (C₁₆H₃₄), which is arbitrarily assigned a cetane number of 100 and α-methyl napthalene (C₁₁H₁₀) with an assigned cetane number of 0. Cetane number of a fuel is defined as the percentage by volume of normal cetane in a mixture of normal cetane and α-methyl napthalene which has the same ignition characteristics (ignition delay) as the test fuel when combustion is carried out in a standard engine under specified operating conditions. Since ignition delay is the primary factor in controlling the initial auto ignition in CI engine, it is reasonable to conclude that knock should be directly related to the ignition delay of the fuel. Knock resistance property of diesel oil can be improved by adding small quantities of compounds like amyl nitrate, ethyl nitrate or ether.

Labroratory Method: The test is carried out in a standard single cylinder engine like the CFR diesel engine or Ricardo single cylinder variable compression engine under the operating shown in the below table. The test fuel is first used in the engine operating at the specified conditions. The fuel pump delivery is adjustable to give a particular fuel-air ratio. The injection timing is also adjusted to give an injection advance of 13 degrees. By varying the compression ratio the ignition delay can be reduced or increased until a position is found where combustion begins at Top Dead Center (TDC). When this position is found, the test fuel undergoes a 13 degree ignition delay.

Conditions for ignition quality test on diesel fuels

The cetane number of the unknown fuel can be estimated by nothing the compression ratio of 13 degree and then referring to a prepared chart shown the relationship between cetane number and combustion ratio. However, for accuracy two reference fuel blends differing by not more than 5 cetane numbers are selected to bracket the unknown sample. The compression ratio is varied for each reference blend to reach the standard ignition delay (13 degrees) and by interpolation of the compression ratios, the cetane rating of the unknown fuel is determined.