Cetane number is a measure of diesel fuelCompression ignition engineImportant index of medium firing performance.The higher the cetane number, thediesel oilGood ignition performanceIgnition delayShort, uniform combustion, stable engine start.Low cetane number indicates difficult combustion and long ignition delay,Engine operating statusRude.However, if the cetane number is too high, a small amount of black smoke will be produced due to partial incomplete combustion.
And gasolineOctane numberSimilarly, the cetane number of diesel fuel is also used for standard test under specified operating conditionsSingle cylinder diesel engine(GB/T 386-2010).The standard fuels used are n-hexadecane and heptamethylnonane.N-hexadecane has a very short ignition delay period,Spontaneous combustionIts performance is very good, so its cetane number is 100.However, heptamethylnonane has a long ignition delay period and poor spontaneous combustion performance, and its cetane number is specified as 15.By mixing these two compounds in different proportions, various standard fuels with different cetane numbers can be prepared.Compare the measured fuel with the standard fuel, and the cetane number of the standard fuel with the same ignition performance is the cetane number of the measured fuel.Cetane number of standard fuel is calculated as follows:
The cetane number of vehicle diesel (GB 19147-2009) is not less than 45 according to the national petroleum product standard.
If there is no condition to directly measure the cetane number of fuel, the following empirical formula can be used to correlate the combustion performance of diesel fuel from its physical and chemical properties.[1]
Cetane index
According to the standard method of GB/T 11139-1989, the cetane index of distillate oil can be calculated by the following formula according to the density and middle average boiling point of distillate oil:
Where ρ20—Density of distillate oil at 20 ℃, g/cmthree；
t50—The middle average boiling point of the distillate oil is the corresponding temperature when distilling 50% of the oil volume with the method of GB/T 6536-1997, ℃.[1]
Diesel index
Diesel index=
In the formula, A represents itsAniline point(℃), the aniline point is the lowest temperature at which aniline can be completely miscible when mixed with equal volume oil sample.The diesel index and cetane number of the same diesel oil sample are not equal, but their values are relatively close.[1]
Calculation formula
According to a large number of measured data of diesel oil properties in China, the petroleum commercial department in China regressed the following correlation between relative density and cetane number:
Cetane number=442.8-462.9
The average deviation of this equation is
3.5。[1]
influence factor
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Cetane number, chemical composition andfractionComposition related.Among various diesel oil components with the same carbon atom number,N-alkaneHas the highest cetane number,Polycyclic aromatic hydrocarbonHas the lowest cetane number,olefin、Naphthenic hydrocarbonBetween alkanes andaromatic hydrocarbonbetween.HydrocarbonisomerismThe higher the degree, the more the ring number, the lower the cetane number;The cetane number of cycloalkanes and aromatic hydrocarbons increases with the increase of the side chain length, while the cetane number decreases with the increase of the side chain branches.To makeDiesel engineThe straight run diesel oil with more alkanes and high cetane number is usually used for working balance.However, the higher the cetane number is, the better it is. The higher the cetane number is, the more alkanes there are.But diesel oil with more alkanesFreezing pointIt also increases. In addition, the thermal stability of alkanes is poor. At the initial stage of combustion, some alkanes are thermally decomposed in the combustion chamber, and a large amount of carbon is decomposed. The combustion of these carbon takes a long time. The carbon that is too late to burn is discharged from the exhaust pipe in the form of smoke, polluting the atmosphere and increasing fuel consumption.Therefore, the higher the cetane number of diesel, the better. According to the results of engine test and driving test, to ensure the uniform combustion of diesel and not cause unnecessary increase in fuel consumption, the cetane number of diesel generally reaches 40~50, which can ensure the normal operation of diesel engine.[2]
Determination method
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The cetane number of diesel oil is determined by comparing the ignition property with that of standard fuel with known cetane number under standard operating conditions.The method is to adjust the engineCompression ratio(indicated by the handwheel reading) to obtain the "ignition lag period" determined by the tested sample, that is, the time between the start of fuel injection and the start of combustion (indicated by the crankshaft angle).According to the compression ratio of the engine obtained during the test sample, select two standard fuels with a difference of no more than 5 cetane number units, and use the same method to obtain the "ignition delay".When the compression ratio of the sample is at the compression ratio of the two standard fuels selected, useInterpolation methodCalculate the cetane number of the sample with the symbol XXX/CN means, for example 50.6/CN.[2]
There are many types of cetane number improvers. The main types of cetane number improvers are nitrate compoundsOrganic peroxide、Organic sulfur compoundsDinitro compounds, ethers, fatty acid derivatives, etc.[3]
Performance
The cetane number of diesel can be increased by adding proper cetane number improver, but different oil components have different sensitivity to additives.Moreover, the sensitivity of various diesel fuels to cetane number improver is not linear with the amount added, but tends to zero gradually when the amount added reaches a certain level.The addition of additives in diesel fuel can affect the flash point of diesel fuel, and the cetane number of diesel fuel with additives will decline after storage for a period of time.Therefore, it is recommended that when using cetane number improver to blend oil products, the cetane number should be above 46 as far as possible and the storage period should be minimized.[4]
Development trend
With the improvement of requirements for locomotive fuel oil and the restriction of economic benefits of existing fuel oil, the market has an urgent demand for new, safe and cost-effective cetane number improvers.Generally speaking, cetane number improvers have lower viscosity, lower freezing point and similar flash point than diesel fuel.The selection of cetane number improver can not only improve the cetane number of diesel, but also have good stability.In addition, other factors should be considered, such as high corrosivity to metals and solubility with diesel oil. These factors are very important for selecting appropriate functional groups of cetane number improvers.[4]
