Fuel Cetane Engine Number Testing

In a diesel engine, air is compressed to pressures of 1000 psi or greater. Upon compression, the air temperature increases to temperatures in the range of 600°F to 800°F (315.6°C to 426.7°C). At close to maximum compression, diesel fuel is injected into this hot, compressed air. Ignition of fuel within this environment is initiated whenever fuel components with the lowest autoignition temperature begin to combust. 

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. 

Ignition Quality of Diesel Fuels by the Cetane Method 

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

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Unfortunately, cetane engine number values for kerosene vary from below 40 to about 50. For this reason, cetane engine testing is always recommended when kerosene is blended into diesel fuel. Hydrotreated kerosene will probably have a higher cetane number than more aromatic kerosene. 

The cetane engine is a variable compression single cylinder engine very much like the octane engine. The engine is mn at 900 rpm and injection is timed to start at 13° before top dead center (BTDC). The compression ratio is adjusted so that the test fuel starts to ignite at exacdy top dead center (TDC), for an ignition delay of 13° or 2.4 ms. Reference fuels are chosen which bracket the sample and the cetane number of the sample is estimated by interpolation between the two reference fuels. 

Although cetane engine testing has been used for many years to evaluate diesel fuels for their cetane number, confusion still exists with regard to this evaluation method. Problems associated with cetane number testing are usually attributed to the following ... 

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. 

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]. 

Van Gerpen, J. H. 1996. Cetane Number Testing of Biodiesel. In Cundiff, J. S., Gavett, E. E., Hansen, C., Peterson, C., Sanderson, M. A., Shapouri, H., and Van Dyne, D. L. (Eds.), Proc., Third Liquid Fuel Conference Liquid Fuel and Industrial Products from Renewable Resources (pp. 197-206). St. Joseph MI American Society of Agricultural Engineers. 

Cetane Number—The cetane number (C.N.) of a fuel is the percentage by volume of normal cetane in a mixture of cetane and alpha-methylnaphthalene which matches the unknown fuel in ignition quality when compared with a standard diesel engine under specified conditions. The C.N. scale ranges from 0 to 100 C.N. for fuels equivalent in ignition quality to alpha-methylnaphthalene and cetane, respectively. For routine-testing, secondary reference fuels having cetane values of about 25 and 74 are blended in any desired proportion. 

The engine used was a one-cylinder direct-injection test engine at Saab-Scania, Sodertalje, Sweden. During the tests, maximum pressure, fuel consumption, exhaust temperature, CO, CO2, NO, NO2, O2, HC, and smoke were registered at varied injection timings, loads, and speeds. The performance of three microemulsion fuels was compared with a reference fuel of pure diesel oil with the cetane number of 43. The data of the microemulsion fuels are given in Table II. 

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. 

As for all fuels, the properties of a product define the ability to serve a stated purpose. Once the required properties are determined, they are controlled by appropriate tests and analyses. The quality criteria and methods for testing fuels for land and marine diesel engines, such as the cetane number, apply to both fuels. 

To determine the cetane number of any fuel, its ignition delay is compared in a standard test engine with a blend of reference fuels (ASTM D-613, IP 41). 

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. 

The low-density products manufactured in the SMDS process are predominantly paraffinic and free from impurities such as nitrogen and sulphur. Both the kerosine and gas oil have excellent combustion properties (smoke point and cetane number), and their cold-flow characteristics meet all relevant specifications - even the stringent freezing point requirements of aviation turbine kerosine. They also make excellent blending components for upgrading low-quality stock that would otherwise have to be used in fuel oil. The excellent quality of the products was proved in extensive engine tests. 

As mentioned above, high-quality diesel can be obtained via FT processing. High cetane numbers of > 70 are reached and the fuel is almost free of sulphur, nitrogen or aromatic contaminants. Furthermore, engine tests revealed low NOx, CO, hydrocarbons and particulate emissions. Compared to fossil diesel, FT diesel shows superior characteristics and can be employed instead of, or in blends with, fossil diesel without modification of engines and gas station systems. 

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