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The ignition quality of diesel fuel is linked to the ignition delay time, the time between the start of injection and the start of combustion. Fuels of high ignition quality are characterized by short ignition delay and vice versa, fuels of poor ignition quality produce long ignition delay. The ignition delay was discussed in more detail in the paper on diesel combustion.
While ignition quality is a property of the fuel itself and is determined by the molecular composition of the fuel, the ignition delay time used to characterize it is not. Ignition delay time is also strongly affected by conditions such as the temperature and pressure of the environment into which the fuel is injected. Therefore, tests developed to measure ignition quality of a particular fuel have to be carried out under carefully controlled test conditions to ensure that only fuel effects are being measured.
A number of different tests have evolved for the quantification of ignition quality, which can be grouped into three types of methods:
Cetane numbers are measured using a method developed in the 1930s by the Cooperative Fuel Research (CFR) Committee, and later standardized as ASTM D613. The test involves running the fuel in a single cylinder, continuously variable compression ratio CFR Cetane Engine. Two primary reference fuels (hydrocarbons) define the cetane number scale:
In 1962, the low cetane number reference fuel was replaced with 2,2,4,4,6,8,8-heptamethylnonane (also called isocetane or HMN), which had better oxidation stability and was easier to use in the CFR engine. When measured against the two original standards, 2,2,4,4,6,8,8-heptamethylnonane has a cetane number of 15. When a fuel has the same ignition delay period as a mixture of the two primary reference fuels, its cetane number is derived from the volume percent of cetane and heptamethylnonane, as follows:
(1)Cetane number = % cetane + 0.15 (% heptamethylnonane)
In routine operations, the two primary reference fuels are replaced by two secondary reference fuels: T-fuel and U-fuel. These fuels are calibrated against the primary reference fuels and made available to testing labs by Chevron Phillips, the sole source supplier. The fuel supplier provides blend ratio instructions to achieve cetane numbers bounded by the values for the U and T fuels. Table 1 provides an example of T and U reference fuels, as well as their specifications.
|Property||U-16||U fuel specification||T-23||T fuel specification|
|Specific gravity at 15.6°C (60°F)||0.7840||0.7830 - 0.7883||0.7920||0.7900 - 0.7975|
|API gravity||48.9||48.0 - 49.2||47.1||45.9 - 47.6|
|Flash point, °C||30.6||26.7 - 54.4||70||61 - 74|
|Sulfur, ppm||1||10 max||127||120 - 150|
|Viscosity at 40°C, cSt||1.1||1.0 - 2.0||2.1||2.1 - 2.3|
|Cetane number||19||18 - 20||75||74 - 77|
|Cetane index||40.1||38.8 - 41.4||64.8||63.1 - 66.6|
|Distillation range at 760 mm Hg, °C|
|Initial boiling point||158||149 - 163||186||182 - 193|
|10%||166||160 - 171||218||216 - 227|
|50%||177||174 - 185||250||246 - 260|
|90%||226||216 - 227||273||271 - 288|
|End point||280||271 - 288||311||304 - 332|
|Hydrocarbon type, vol%|
|Aromatics||21.2||20.0 - 23.0||6.7||5.0 - 8.0|
|Olefins||0.8||0.0 - 2.0||2.4||1.0 - 3.0|
|Source: Chevron Phillips Chemical Company LP|