Cetane number determination

Occasionally, cetane improvers are added to distillate fuel and are capable of increasing the cetane number of diesel fuel from 1 to 14 numbers. The reason for this range of response is due to the differences in the chemical nature of the fuel. Fuel containing a higher level of aromatic compounds will typically respond less effectively to cetane improver treatment than fuels with fewer aromatic compounds. Fuel cetane number determination is evaluated by a standardized engine method ASTMD-613. 

C. T. O Connor, R. D. Forrester and M. S. Scurrellt, Cetane number determination of synthetic diesel fuels. Fuel 71, 1323-1327 (1992). 

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 procedure for determining the cetane number in the CFR engine is not extremely widespread because of its complexity and the cost of carrying it out. There also exist several methods to estimate the cetane number of diesel fuels starting from their physical characteristics or their chemical structure. 

However, this conventional method presents a certain number of limitations. In the first place, the traditional end-use property itself can be difficult to determine. Consider the cetane number for example is it a good characterization of diesel fuel with respect to its behavior in commercial diesel engines In the second place, concern for protecting the environment imposes new specifications which are often specifications linked to the composition of products very low content of certain contaminants, reduced levels of certain families of compounds, or even a specific compound as already discussed. 

LCO is highly aromatic (50 wt% to 75 wt%) and has a low cetane index (20-30). The cetane number and sulfur content determine the amount of LCO that can be blended into the diesel or heating oil pool. 

Most recent scientific applications involve the determination of direct relationships between input parameters and a known target response. For example, Santana and co-workers have used ANNs to relate the structure of a hydrocarbon to its cetane number,4 while Berdnik s group used a theoretical model of light scattering to train a network that was then tested on flow cytometry data.5... 

LCO quality was determined by two different methods. Before 2-D chromatography was available, liquid product aromatics were measured for all samples using a variation of the ASTMD 5186-96 method. A selected number of samples were cut between 221°C and 343°C using a TBP column to produce an LCO fraction for direct ASTMD 3747 cetane index determination. The cetane values of the LCO cuts were then correlated to the total liquid product aromatics (Figure 2.1) and the correlation was used to estimate the LCO quality of the other samples. 

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). 

D. DIESEL FUEL CETANE NUMBER AND CETANE INDEX DETERMINATIONS... 

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. 

The cetane number, however, is an engine measurement that requires a skilled operator and a well performing engine to determine. Because of the variability among engines and the differences in operator skill, engine number determinations can differ. 

Cetane engine number determinations between operators in different laboratories do vary. If the variance is within the limits shown above, the variance is acceptable and should not be regarded as erroneous. If the difference is outside of the reported limits, then the results can be fairly questioned. 

Either primary or secondary reference fuels can be utilized in the cetane engine when determining the cetane number of distillate fuel. Primary reference fuels are n-cetane and heptamethylnonane. Secondary reference fuels are identified as T Fuel and U Fuel. The characteristics of each of these fuels are outlined in TABLE 4-7. 

The cetane number of a diesel fuel is determined by comparing its ignition quality with those for blends of reference fuels of known... 

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 Number—The ignition quality of a diesel fuel measured using an engine test specified in ASTM Method D 613. Cetane number is determined using two pure hydrocarbon reference fuels cetane, which has a cetane rating of 100 and heptamethylnonane (also called isocetane), which has a cetane rating of IS [2.2]. 

Anon. 2007b. D 6890 Standard Test Method for Determination of Ignition Delay and Derived Cetane Number (DCN) of Diesel Fuels Oils by Combustion in a Constant Volume Chamber. In Annual Book of ASTM Standards (05.05). West Conshohocken PA ASTM International. 

Freedman, B., Bagby, M. O., Callahan, T. J., and Ryan, T. W. III. 1990. Cetane Numbers of Fatty Esters, Fatty Alcohols and Triglycerides Determined in a Constant Volume Combustion Bomb. In SAE Techn. Pap. Ser.. Warrendale PA Society of Automotive Engineers (Paper No. 900343). 

H. Yang, Z. Ring, Y. Briker, N. McLean, W. Friesen and C. Fairbridge, Neural Network prediction of cetane number and density of fuel from its chemical composition determined by FC and GC-MS, Fuel 81, 65-74 (2002). 

Ignition delays similar to those observed for the emulsion fuels may be experienced with microemulsion fuels as illustrated by Table I, which shows cetane numbers of microemulsion fuels determined on a CFR engine. 

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