Cetane number calculation

An important application of this type of analysis is in the determination of the calculated cetane index. The procedure is as follows the cetane number is measured using the standard CFR engine method for a large number of gas oil samples covering a wide range of chemical compositions. It was shown that this measured number is a linear combination of chemical family concentrations as determined by the D 2425 method. An example of the correlation obtained is given in Figure 3.3. 

The differences between the measured cetane numbers and the cetane indices calculated by the above formula are relatively small for cetane indices between 40 and 55. Nevertheless, for diesel fuels containing a pro-cetane additive, the CCl will be much less than the actual value (refer to paragraph d). 

Calculational methods. Associating the analysis, the knowledge of the property-structure relationships, and the calculation methods has made possible the replacement of costly and arduous test methods by quicker tests whose results are linked by calculations to the characteristic under study. Some examples are the cetane number, in some cases, the octane number, or the characteristics of LPG (refer to Chapter 3). 

Cetane number is difficult to measure experimentally. Therefore, various correlation equations have been developed to predict cetane number from fuel properties. One such equation may be found in ASTM D4737 to calculate a cetane index (Cl). ASTM D975 allows use of Cl as an approximation if cetane numbers are not available. 

Cetane number is measured in a single-cylinder laboratory engine, but cetane index (Cl) is more commonly used. Cetane index is a calculated value and correlates adequately with the cetane number. Most refiners use the ASTM equation (Method D-976-80) to calculate the cetane index. The equation uses the 50% boiling point and API gravity (Example 6-1). 

Cetane Number is a numerical indication of a fuel s (kerosene, dic.sel, heating oil) ignition quality. Cetane number is measured in a single-cylinder engine, whereas cetane index is a calculated value. 

Cetane index an approximation of the cetane number (q.v.) calculated from the density (q.v.) and midboiling-point temperature (q.v.). 

It is impractical to determine the cetane number of residual fuels in the ASTM D-613 cetane engine. Because of this, the Calculated Carbon Aromaticity Index and the Calculated Ignition Index were respectively developed by Shell and BR These values can be determined from the following equations where d = Density in kg/m3 59°F (15°C) and v = Viscosity in cSt 122°F (50°C). 

The calculated cetane index methods were developed to help predict the fuel cetane number without extensive engine testing. Since the calculated cetane index is determined from physical property values such as the specific gravity and the 10%, 50%, and 90% distillation points, it is readily measurable from laboratory data. The ASTM methods D-976 and D-4737 are used to determine the diesel fuel cetane index. 

As a general rule, it is usually safe to assume that the calculated cetane index will always be higher than the actual cetane number. On rare occasions, the numbers will be the same, but usually not. Because of this, if a calculated cetane index value is known, it is reasonable to believe that the cetane engine number is usually one to two numbers lower than the calculated cetane index. For this reason, refiners often set cetane index specifications higher than the desired number by one to two numbers or more. 

Cetane improver addition will not change the calculated cetane index only the engine cetane number will change. 

Cetane Index A calculated value which incorporates fuel distillation properties and °API which can be used to approximate the cetane number of a fuel. This method, however, does not provide information on the performance provided by a cetane improver. 

This number is determined from the percentage of n-cetane which must be mixed with heptamethylnonane to give the same ignition performance as the fuel being tested. The cetane number can be obtained by using the following calculation ... 

Cetane index is an estimation of the cetane number, which measures the anti-knoclc properties of the fuel. This index is more used than the cetane number because it can be calculated by a four-variable equation (density, BP 10%, BP 50% and BP 90%) according to ASTM D4737 standard method, instead of the test engine needed to measure cetane number. In our case, the relatively low amount of product obtained in these initial experiments does not allow determination of the cetane number, so we have used the cetane index to evaluate and compare our products, an approximation perfectly valid at this initial stage of investigation. 

The Calculated Cetane Index (CCI), though not an accurate predictor of Cetane Number for biodiesel, since it is based on a calculation using specific gravity and the distillation curve, was used to estimate the Cetane Number of the biodiesel and two diesel fuels. From the results it can be seen that biodiesel has a CCI value higher (55) than that of diesel No.2 (49). This accords the biodiesel with better cold-start properties, minimizes the formation of white smoke (emissions), and leads to less engine noise and hence improved engine durability and reduced fuel consumption. ... 

The cetane number of a diesel fuel is defined as the whole number nearest to the value determined by calculation from the percentage by volume of normal cetane (cetane No. 100) in a blend with heptamethylnonane (cetane No. = 15) which matches the ignition quality of the test fuel when compared by this method. The matching blend percentages to the first decimal are inserted in the following equation to obtain the cetane number ... 

Because the determination of cetane number by engine testing requires special equipment, as well as being time-consuming and costly, alternative methods have been developed for calculating estimates of cetane number. The calculations are based on equations involving values of other known characteristics of the fuel. 

One of the most widely used methods is based on the calculated cetane index formula. This formula represents a method for estimating the cetane number of distillate fuels from API gravity and mid-boiling point. The index value as computed from the formula is designated as a calculated cetane index (ASTM D-976, IP 218). Because the formula is complicated in its manipulation, a nomograph based on the equation has been developed for its solution. 

The aromatic hydrocarbon content of diesel fuel affects the cetane number and exhaust emissions. One test method (ASTM D-5186) is applicable to diesel fuel and is unaffected by fuel coloration. Aromatics concentration in the range 1-75 mass% and polynuclear aromatic hydrocarbons in the range 0.5-50 mass% can be determined by this test method. In the method, a small aliquot of the fuel sample is injected onto a packed silica adsorption column and eluted with supercritical carbon dioxide mobile phase. Mono- and polynuclear aromatics in the sample are separated from nonaromatics and detected with a flame ionization detector. The detector response to hydrocarbons is recorded throughout the analysis time. The chromatographic areas corresponding to the mononuclear aromatic constituents, polynuclear aromatic constituents, and nonaromatic constituents are determined, and the mass-percent content of each of these groups is calculated by area normalization. 

Al-propyl nitrate is a flammable liquid with boiUng point of 110.5 °C and density of 1.063. Its explosive property is not severe, thus that it is difficult to be detonated. The calculated explosion heat is 549 kcal/kg (water vapor). The lead-block value after rectification of the detonator is only 15 cm. The explosion limit of the mixture with air is 2-100 %. Its flash point is 20 °C and the ignition point in air is 176.7 °C. Its vapor, which is heavier than air, can spread around and be ignited by distant fire. Furthermore, the ignition flame would be returned to the -propyl nitrate source. Being heated, M-propyl nitrate would be exploded. A significant detonation could be happened if it mixes with diatomaceous earth with ratio of 71.5/28.50. The lead-block value is 230 cm. A/-propyl nitrate can be used as a sensibilizer in cloud detonation, a rocket propellant, and a promoter of cetane number for diesel oil. 

Cetane index, as defined by ASTM D976, is not a measurement, but the result of a calculation based on density and mid-boiling point. It is nearly the same the as cetane number for diesel fuels comprised mostly of straight-run gas oils blended with lesser amounts of cracked stocks. Diesel index is a ... 

The Calculated Cetane Index is not an optional method for expressing ASTM cetane number. It is a supplementary tool for predicting cetane number when used with due regard for its limitations. 

Note I—This test method is temporarily retained because the proposal to the U.S. EPA to control diesel fuel aromatics concentrations via a 40 Calculated Cetane Index minimum is based on the correlation between Test Method D 976 and aromatics concentration. Test Method D 4737 is the preferred method as estimator of cetane number. Test method D 976 is intended to be letter balloted for withdrawal from the book of standards in 1993. 

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