Katerina Swanson
Cetane number (CN) is a critical indicator of diesel fuel quality, playing a similar role to the octane rating in gasoline. It reflects the fuel's combustion speed and compression needed for ignition. While it is a significant factor, it is not the sole determinant, as other factors like energy content, density, lubricity, and sulfur content also come into play. The CN is determined by blending cetane and isocetane to match ignition delays, with cetane rated at 100 and isocetane at 15. Testing methods for CN include the Fuel Ignition Tester (FIT) and the standard single-cylinder variable compression ratio diesel engine test. The FIT method involves injecting fuel into a combustion chamber at approximately 575 °C, measuring the ignition delay, and calculating the CN using an empirical inverse relationship. The test engine method, ASTM D613, uses a single-cylinder, four-stroke cycle, variable compression ratio engine to determine CN. The cetane index (CI) is another important statistic, calculated from fuel properties like density and distillation range, providing a measure of ignition quality without the costly CN test.
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Cetane number
The cetane number (or cetane rating) is an indicator of the combustion speed of diesel fuel and the compression needed for ignition. It is an important factor in determining the quality of diesel fuel. The higher the cetane number, the better the potential for efficient combustion and power output.
The cetane number is defined by finding a blend of cetane and isocetane with the same ignition delay. Cetane has a cetane number of 100, while isocetane's number is 15. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number.
The cetane number test uses a standard single-cylinder variable compression ratio diesel engine. Fuel is injected into a constant-volume combustion chamber at a temperature of 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.
The cetane index (CI) is another statistic used to control fuel quality. It is a calculated number based on the density and distillation range of the fuel. The cetane index can be used to measure fuel ignition quality without the need to run the costly cetane number test.
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Fuel ignition tester
The Fuel Ignition Tester (FIT) is an instrument used to measure the cetane number of diesel fuel. The cetane number is an important factor in determining the quality of diesel fuel, as it indicates the combustion speed and efficiency of the fuel. A higher cetane number generally indicates a higher quality fuel that will lead to maximum engine power and efficiency.
To use the Fuel Ignition Tester, fuel is injected into a constant volume combustion chamber, which is maintained at a temperature of approximately 575 °C. The fuel then 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 by measuring the time elapsed between the start of fuel injection and the start of combustion.
The cetane number of the fuel can be calculated using an empirical inverse relationship to the ignition delay. A shorter ignition delay corresponds to a higher cetane number, indicating a higher quality fuel. The cetane number is also related to the compression needed for ignition, with higher cetane fuels requiring less compression to ignite.
The Fuel Ignition Tester provides a simpler and more robust approach to measuring the cetane number compared to other methods, such as the CFR method. It is important to note that the cetane number is not the only factor determining the quality of diesel fuel. Other measurements, such as energy content, density, lubricity, cold-flow properties, and sulfur content, also play a significant role in fuel quality.
Additionally, the ignition delay of diesel fuel is influenced by factors such as the temperature and pressure of the environment into which the fuel is injected. Therefore, tests using the Fuel Ignition Tester must be carried out under carefully controlled conditions to ensure accurate results.
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Cetane index
The cetane index (CI) is a calculated value that provides a measure of a diesel fuel's ignition quality without the need to run the costly cetane number test. It is derived from easily measured fuel properties such as density, volatility, and distillation range. The cetane index can be calculated using either metric or Imperial units, and the number of distillation points used can vary.
The cetane number is the most important and universally accepted test for ignition quality. It is determined by finding a blend of cetane and isocetane with the same ignition delay and then calculating the volume-weighted average of their respective cetane numbers (100 for cetane and 15 for isocetane). However, the cetane number test requires the use of a specialised engine, such as a standard single-cylinder variable compression ratio diesel engine.
The cetane index, on the other hand, can be calculated from simple fuel property measurements. Ignition delay, which is the time between the start of ignition and the first identifiable pressure increase during combustion, can be measured using the constant volume combustion chamber method. This method involves injecting fuel into a constant volume combustion chamber at a temperature of approximately 575 °C. The high rate of pressure change within the chamber defines the start of combustion, and the ignition delay can be calculated as the time elapsed between the start of fuel injection and combustion.
While the cetane index is a useful tool for measuring ignition quality, it is important to note that cetane improver additives can affect the cetane number but not the cetane index. This is because cetane improvers do not change the fuel density or alter its distillation temperatures, which are the properties used to calculate the cetane index. Therefore, when choosing a diesel fuel, it is important to consider both the cetane number and the cetane index to ensure optimal engine performance.
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Cetane rating
The cetane rating, also known as the cetane number (CN), is an indicator of the combustion speed of diesel fuel and the compression needed for ignition. It is an important factor in determining the quality of diesel fuel, but not the only one. Other measurements include energy content, density, lubricity, cold-flow properties, and sulfur content.
The cetane number of a fuel is defined by finding a blend of cetane and isocetane with the same ignition delay. Cetane has a cetane number of 100, while isocetane's is 15. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number.
The cetane number is an inverse function of a fuel's ignition delay, which is the time period between the start of ignition and the first identifiable pressure increase during combustion. In a particular diesel engine, higher cetane fuels will have shorter ignition delays than lower cetane fuels. This means that higher speed diesel engines operate more effectively with higher cetane number fuels.
There are several methods to test the cetane rating of diesel fuel. One reliable method is the Fuel Ignition Tester (FIT), which involves injecting fuel into a constant volume combustion chamber at an ambient temperature of 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, and the fuel's derived cetane number can be determined using an empirical inverse relationship to ignition delay.
Another method is the ASTM D613 standard test, which 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: n-hexadecane (also called cetane, n-C16H34), which has very good ignition quality, was assigned the cetane number of 100; 1-methylnaphthalene, which has poor ignition quality, was assigned a cetane number of zero.
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Cetane improvers
The use of cetane improvers offers a cost-effective and convenient method to improve engine performance. They are commonly employed by refiners in premium diesel fuels to enhance their performance. Cetane improvers can be particularly beneficial when the required cetane number for an engine is not readily available, or when seeking to optimise the performance of an engine that already uses diesel fuel meeting the necessary requirements.
It is important to note that cetane improvers do not affect the Cetane Index, which is a calculated value based on fuel properties such as density and distillation range. Instead, they directly influence the Cetane Number, which is a key indicator of the fuel's aptitude to self-ignite in the combustion chamber.
Examples of cetane improver additives include nitrates, nitroalkanes, nitrocarbonates, and peroxides. One of the most commonly used additives is 2-ethylhexyl nitrate, which has a decomposition temperature of 130 °C. This additive undergoes exothermic decomposition, initiating a series of fuel reactions that result in the start of combustion at lower temperatures. The effectiveness of cetane improvers can vary depending on the fuel type and its crude oil quality and refinement method.
Products such as Super-Tane Cetane Improver claim to increase the cetane rating of diesel fuel, resulting in quieter engine operation, improved cold starts, and better overall engine performance. It is recommended to follow the engine manufacturer's guidelines and use diesel fuel that meets the specified requirements to ensure optimal performance and avoid unnecessary costs.
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Frequently asked questions
Cetane number (cetane rating) (CN) is an indicator of the combustion speed of diesel fuel and compression needed for ignition. It is an important factor in determining the quality of diesel fuel.
The cetane number is calculated as a volume-weighted average, rounded to the nearest whole number, of cetane's 100 and isocetane's 15.
A better cetane rating in diesel means the fuel ignites faster in the cylinder. In contrast, a better octane rating in gasoline means that gas ignites more slowly.
In North America, the minimum cetane number is set at 40, with typical values in the 42-45 range. In Europe, diesel cetane numbers were set at a minimum of 38 in 1994, 40 in 2000, and 51 in 2001.
The cetane number test uses a standard single-cylinder variable compression ratio diesel engine. 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.
