Fuels Rating for SI and CI Engines

Generally fuels rating is given for their antiknock qualities. The rating of fuels is done by defining two parameters called Octane number and Cetane number for gasoline and diesel oil respectively. The rating of Spark-Ignition (SI) engine fuels and rating of Compressed-Ignition (CI) engine fuels are discussed in this article.

Rating of SI Engine Fuels:

Resistance to knocking is an extremely important characteristic of fuel for spark-ignition (SI) engines. These fuels differ widely in their ability to resist knock depending on their chemical composition. A satisfactory rating method for comparing the antiknock qualities of the various fuels has been established. In addition to the chemical characteristics of hydrocarbons in the fuel, other operating parameters such as engine speed, fuel-air ratio, ignition timing, dilution, shape of the combustion chamber, compression ratio, ambient conditions, etc. affect the tendency to knock in the engine cylinder. Therefore, in order to determine the knock resistance characteristic of the fuels, the SI engine and its operating variables must be fixed at standard values.

According to a standard practice, the antiknock value of an SI engine fuel is determined by comparing its antiknock property with a mixture of two reference fuels, normal heptane (C7H18) and iso-octane (C8H18). Iso-octane chemically being a very good antiknock fuel, is arbitrarily assigned a rating of 100 octane number. Normal heptane (C7H16) has very poor antiknock qualities and is a given a rating of 0 octane number. The octane number fuel is defined as the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, which exactly  matches the knocking intensity of the fuel in a standard engine under a set of standard operating conditions. The addition of certain compounds (e.g. tetraethyl lead) to iso-octane produces fuels of greater antiknock quality (above 100 octane number). The antiknock effectiveness of tetraethyl lead, for the same quantity of fuel added, decreases as the total content of the lead in the fuel increases. Further, each octane number at high range of the octane scale will produce greater antiknock effect compared to the same unit at the lower end of the scale. For instance octane number increase from 92 to 93 produce greater antiknock effect than a similar increase from 32 to 33 octane number. because of this non-linear variation, a new scale was derived which expresses the approximate relative engine performance and the units of the scale are known as the Performance Numbers (PN).

Octane number, On above 100 can be computed by using the formula

Formula for calculating octane number for finding out fuels rating
Where ‘A’ is the amount of tetraethyl lead in ml/gal of fuel.

Octane number can be computed with Performance Number (PN) by using the formula

calculation of octane number using performance number of engine
Laboratory Method: SI engine is run at specified conditions with a definite compression ratio and a definite blend of reference fuels. The intensity of knock at these standard conditions is called standard knock. The knock meter is adjusted to give a particular reading under these conditions. The test fuel is now used in the engine and air-fuel ratio is adjusted to give maximum knock intensity. The compression ratio of the engine is gradually changed until the knock meter reading is the same as in the previous run (standard knock). The compressible ratio is now fixed and known blends of reference fuels. are used in the engine. The blend of reference fuels which gives a knock meter reading equal to the standard valve will match the knocking characteristics of the test fuel. Percentage by volume of iso-octane in the particular blend gives the octane number.

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

Engine speed 900 rpm
Jacket water temperature 100° C.
Inlet air temperature. 65.5° C
Injection advance Constant at 13° begins at TDC
Ignition delay 13°

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.

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