CETANE NUMBER

Cetane number is the measure of combustion quality of diesel oil or it is the measure of the ignition delay, the higher the cetane number of diesel the shorter the ignition delay, and the greater the fuel quality, and vice versa. A short delay affords more time for diesel fuel to burn completely.

Benefits of a High Cetane Rating

A higher cetane number resulting in quicker ignition of the fuel leads to less non-ignited fuels building up inside the combustion chamber, as well as more complete fuel combustion. Better fuel combustion and quick ignition leads to quicker starting for vehicles, as well as an engine that operates more quietly as only misted fuel is ignited and build up inside the engine is minimized. Moreover, fuel efficiency improves with more complete combustion and harmful emissions are reduced.

Cetane is the hydrocarbon with chemical formulas C16H34 and CH3(CH2)14CH3. Named n-hexadecane, it is an unbranched saturated alkane. Cetane ignites with a short delay under compression, and is assigned a cetane number of 100. Alpha-methylnaphthalene, which has a long delay period, was assigned a cetane number of 0, but has been replaced as a reference fuel by 2,3,4,5,6,7,8-heptamethylnonane, whose cetane number is 15.All other hydrocarbons in diesel fuel are indexed to cetane as to how rapidly they ignite under compression, i.e., diesel engine conditions. Since hundreds of components in comprise diesel fuel, the overall cetane number of that fuel is the average cetane quality of all the components. High-cetane components have a disproportionate influence, hence the use of high-cetane additives.

Cetane rating, also known as cetane number is a measurement of the quality or performance of diesel fuel. The higher the number, the better the fuel burns within the engine of a vehicle. The cetane number is similar to the octane rating in that it is a rating assigned to a fuel to rate the quality of its combustion. The difference is that octane rating rates gasoline whereas cetane rates diesel. Just as higher performance gasoline vehicles require fuels with a higher octane rating, high performance diesel vehicles require fuel with a higher cetane rating.

The cetane rating of a specific diesel mixture is based off of how much cetane—a clear, colourless hydrocarbon that ignites under high pressures—is in some fuel. Pure cetane represents the highest purity of diesel fuel possible, and thus has a cetane rating of 100.

The main difference between cetane and octane ratings is that the octane rating represents how well a fuel can resist pre-ignition due to compression—ensuring the fuel ignites only from a spark from the spark plug. However, the cetane number measures the delay in the ignition time of the fuel. In other words, it is how minimized the delay is between when the fuel is injected into the chamber and when the combustion begins. Unlike gasoline engines which attempt to resist any ignition due to compression, diesel engines rely on this compression ignition and thus no spark is involved. A higher cetane number simply means the time between when the fuel is injected into the combustion chamber and when the fuel ignites is minimized. This means the fuel has the ability to ignite more easily and readily due to compression. This shorter delay time results in more complete fuel combustion.

Generally, diesel engines operate well with a CN from 48 to 50. Fuels with lower cetane number have longer ignition delays, requiring more time for the fuel combustion process to be completed. Hence, higher speed diesel engines operate more effectively with higher cetane number fuels.

Additives

Alkyl nitrates (principally 2-ethylhexyl nitrate & di-tert-butyl peroxide ) are used as additives to raise the cetane number.

Alternative fuels

Biodiesel from vegetable oil sources have been recorded as having a cetane number range of 46 to 52, and animal-fat based biodiesels cetane numbers range from 56 to 60. Dimethyl ether is a potential diesel fuel as it has a high cetane rating (55-60) and can be produced as a biofuel. Most simple ethers, including liquid ones, such as diethyl ether can be used as diesel fuels, although the lubricity can be of concern.

Measuring cetane number

Cetane numbers are rather difficult to measure accurately, as it requires a special diesel engine called a Cooperative Fuel Research (CFR) engine. Under standard test conditions, the operator of the CFR engine uses a manual-wheel to increase the compression ratio (and therefore the peak pressure within the cylinder) of the engine until the time between fuel injection and ignition is 2.407 ms. The resulting cetane number is then calculated by determining which mixture of cetane (hexadecane) and isocetane (2,2,4,4,6,8,8-heptamethylnonane) will result in the same ignition delay.

Ignition Quality Tester (IQT)

Another reliable method of measuring the derived cetane number (DCN) of diesel fuel is the Ignition Quality Tester (IQT). This instrument applies a simpler, more robust approach to CN measurement than the CFR. Fuel is injected into a constant volume combustion chamber at approximately 575 °C and 310 psi (21 bar). The time between the start of injection and the recovery of the combustion chamber pressure to 310 psi (21 bar) is defined as the ignition delay. This measured ignition delay is then used to calculate the DCN of the fuel. The fuel's DCN is then calculated using an empirical inverse relationship to ignition delay. Because of the reproducibility, material cost, and speed of the IQT, this has been the definitive source for DCN measurements of fuels since the late 2000s.

Fuel ignition tester

Another reliable method of measuring the derived cetane number of diesel fuel is the Fuel Ignition Tester (FIT). This instrument applies a simpler, more robust approach to CN measurement than the CFR. Fuel is injected into a constant volume combustion chamber in which the ambient temperature is approximately 575 °C. The fuel combusts, and the high rate of pressure change within the chamber defines the start of combustion. The ignition delay of the fuel can then be calculated as the time elapsed between the start of fuel injection and the start of combustion. The fuel's derived cetane number can then be calculated using an empirical inverse relationship to ignition delay.

Cetane index

Another method that fuel-users control quality is by using the cetane index (CI), which is a calculated number based on the density and a distillation range of the fuel. There are various versions of this, depending on whether metric or Imperial units are used, and the number of distillation points are used. These days most oil companies use the '4-point method', ASTM D4737, based on density, 10% 50% and 90% recovery temperatures. The '2-point method' is defined in ASTM D976 and uses just density and the 50% recovery temperature. This 2-point method tends to overestimate the cetane index and is not recommended. Cetane index calculations can not account for cetane improver additives and therefore do not measure the total cetane number for additized diesel fuels. Diesel engine operation is primarily related to the actual cetane number and the cetane index is simply an estimation of the base (unadditized) cetane number.

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