Rajan Wilcox
The cetane number, or cetane rating, is a measure of the quality and performance of diesel fuel. It is an indicator of the combustion speed of diesel fuel and the compression needed for ignition. 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, but while a good octane rating means a fuel doesn't ignite too soon, a good cetane number means a fuel ignites as soon as possible. This is because diesel engines rely on compression ignition, so a higher cetane number means a shorter delay between the fuel being injected into the chamber and combustion beginning, resulting in more complete fuel combustion.
| Characteristics | Values |
|---|---|
| Definition | An indicator of the combustion speed of diesel fuel and compression needed for ignition. |
| Scale | Ranges from 0 to 100, with higher numbers indicating better ignition quality. |
| Measurement | Determined by burning the fuel in a Cooperative Fuel Research (CFR) engine under standard test conditions and measuring the ignition delay. |
| Importance | Affects engine performance, fuel efficiency, emissions, power output, and combustion stability. |
| Cetane Index (CI) | A calculated number based on the density and distillation range of the fuel, used as an estimation of the base cetane number. |
| Common Cetane Numbers | Regular no. 2 diesel: 40-42, Cenex Premium Diesel: 47-52, Modern highway diesel engines: 45-55. |
| Additives | Cetane improvers can increase the cetane number but may involve guesswork and risk. |
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What You'll Learn
Cetane number measures combustion efficiency
The cetane number, or cetane rating, is a measure of combustion efficiency in diesel fuel. It indicates the fuel's ignition behaviour, measuring the delay in the ignition time of the fuel. In other words, it is how minimised the delay is between when the fuel is injected into the chamber and when combustion begins.
The cetane number scale is based on the ignition characteristics of two hydrocarbons: n-hexadecane (cetane) and 2,3,4,5,6,7,8-heptamethylnonane. Cetane, which ignites with a short delay under compression, has a cetane number of 100, while heptamethylnonane, which has a long delay period, has a cetane number of 15. The cetane number of a fuel is defined by finding a blend of cetane and heptamethylnonane with the same ignition delay.
The higher the cetane number, the better the combustion efficiency. This is because a higher cetane number results in quicker ignition of the fuel, leading to less non-ignited fuel building up inside the combustion chamber, and more complete fuel combustion. This, in turn, leads to improved fuel efficiency, better power output, and reduced harmful emissions.
The cetane number is an important factor in determining the quality of diesel fuel, but it is not the only one. Other measurements of diesel fuel quality include energy content, density, lubricity, cold-flow properties, and sulfur content. The cetane index (CI) is another statistic used by fuel users to control quality, which is a calculated number based on the density and distillation range of the fuel.
There are several methods for measuring the cetane number of diesel fuel. One widely used method is based on the calculated Cetane Index formula, which represents a method for estimating the cetane number of distillate fuels from API gravity and mid-boiling point. Another method, considered the definitive source for Derived Cetane Number (DCN) measurements since the late 2000s, is the Ignition Quality Tester (IQT). This instrument uses a simpler, more robust approach than the CFR engine, injecting fuel 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.
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It indicates the ignition quality of diesel fuels
The cetane number is a critical indicator of the combustion speed and ignition quality of diesel fuel. It is a measure of how efficiently the fuel burns and how quickly it ignites under compression. The higher the cetane number, the better the fuel's ignition quality, resulting in improved engine performance, increased fuel economy, reduced emissions, and enhanced power output.
The cetane number is calculated by measuring the ignition delay, which is the time between fuel injection and ignition. This delay is influenced by the percentage of cetane, a clear, colourless hydrocarbon, in the fuel. Pure cetane, with its excellent ignition qualities, is assigned a cetane number of 100, while fuels with lower cetane numbers experience longer ignition delays.
The cetane index (CI) is another important statistic used to assess the quality of diesel fuel. It is calculated based on the fuel's density and distillation range, providing an estimation of the base cetane number. However, it does not account for cetane improver additives, which are commonly used to enhance the combustion speed and overall performance of diesel fuel.
The cetane number plays a similar role for diesel fuel as the octane rating does for gasoline. While a high octane rating indicates resistance to ignition, a high cetane number signifies the fuel's ability to ignite easily and efficiently. This distinction arises from the fundamental differences in the combustion processes of gasoline and diesel engines.
To measure the cetane number accurately, specialised equipment such as the Cooperative Fuel Research (CFR) engine or the Ignition Quality Tester (IQT) is employed. These instruments help determine the ignition delay and calculate the corresponding cetane number, ensuring the quality and performance of diesel fuel meet the required standards.
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It's used to evaluate the performance of diesel engines
The cetane number is an important metric for evaluating the performance of diesel engines. It is a measure of the quality and performance of diesel fuel, indicating how efficiently it burns within the engine of a vehicle. The higher the cetane number, the better the fuel burns, leading to improved engine performance.
The cetane number measures the ignition delay of the fuel, or how quickly it ignites under compression. Diesel engines rely on compression ignition, so a shorter delay time results in more complete fuel combustion. This, in turn, leads to quicker starting for vehicles, improved fuel efficiency, and reduced harmful emissions. Additionally, the engine operates more quietly as only misted fuel is ignited, minimizing buildup inside the engine.
The cetane number is also used to determine the appropriate fuel for a diesel engine. Modern highway diesel engines typically require fuel with a cetane number ranging from 45 to 55. Regular no. 2 diesel has a lower cetane number, usually between 40 and 42. Engine manufacturers recommend specific cetane ratings to ensure optimal engine performance.
The cetane number is calculated using ASTM D613, a standard test method. A prototype CFR single-cylinder diesel engine is used, and the fuel is rated against mixtures of standard hydrocarbons with known compositions, specifically n-hexadecane (cetane) and isocetane (2,2,4,4,6,8,8-heptamethylnonane). The resulting cetane number is determined by finding the blend of cetane and isocetane that matches the ignition delay of the test fuel.
Another method for measuring the cetane number is the Ignition Quality Tester (IQT), which offers a simpler and more robust approach. Fuel is injected into a constant volume combustion chamber at approximately 575 °C, and the ignition delay is measured. This method has been widely adopted due to its reproducibility, material cost, and speed.
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It's similar to octane rating, but for diesel fuel
The cetane number of diesel fuel measures its combustion speed and compression needed for ignition. It is similar to the octane rating, which is a measure of fuel stability for gasoline. The octane rating indicates how resistant the fuel is to premature ignition, or "knocking", when being compressed. Similarly, the cetane number measures the delay in the ignition time of diesel fuel.
Both ratings are used to rate the quality of combustion of the respective fuels. A higher cetane number means a more efficient diesel engine, just as a higher octane rating means a more stable gasoline fuel.
The cetane number is calculated by determining the blend of cetane and isocetane that will result in the same ignition delay as the fuel being tested. Cetane has a cetane number of 100, while isocetane has a cetane number of 15. The resulting cetane number is then calculated as a volume-weighted average of the two.
The octane rating is calculated using two different methods: the research octane rating (RON) and the motor octane rating (MOR). The octane number is the simple average of these two methods.
It is important to note that the cetane number is not published at the pump, so diesel owners typically rely on the green-handled ULSD pump to fill up their tanks. Similarly, in some countries, gasoline is offered in different grades based on the octane level, such as unleaded, super, or super premium, which correspond to the octane rating.
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Cetane number is calculated using ASTM D613
The cetane number of diesel fuel is a measure of its combustion quality or combustion efficiency. It is similar to the octane rating in that it is a rating assigned to a fuel to rate the quality of its combustion. However, octane rates gasoline, while cetane rates diesel. A higher cetane number indicates that the time between when the fuel is injected into the combustion chamber and when the fuel ignites is minimized. This means the fuel ignites more easily due to compression. This results in more complete fuel combustion, leading to quicker starting for vehicles, a more quietly operating engine, improved fuel efficiency, and reduced harmful emissions.
The cetane number is calculated by finding a blend of cetane and isocetane with the same ignition delay. Cetane has a cetane number of 100, while isocetane's measured cetane number is 15. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number, of cetane's 100 and isocetane's 15.
One method to calculate the cetane number is ASTM D613. This method involves using a Cooperative Fuel Research (CFR) engine. Under standard test conditions, the operator of the CFR engine increases 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.
Another method to calculate the cetane number 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. The fuel's derived cetane number (DCN) is then calculated using an empirical inverse relationship to this ignition delay.
In cases where a test engine is not available, the Calculated Cetane Index by Four Variable Equation can be used to estimate the ASTM cetane number. This method is based on density and distillation recovery temperature measurements. It is a supplementary tool for estimating the cetane number when a result by Test Method D613 is not available and if a cetane improver is not used.
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Frequently asked questions
Cetane number, also known as cetane rating, is a measurement of the quality or performance of diesel fuel. It measures the delay in the ignition time of the fuel.
The cetane number of diesel fuels is measured according to ASTM D 613—Standard Test Method for Cetane Number of Diesel Fuel Oil. A prototype CFR single-cylinder diesel engine is used, running under variable compression ratio until maximum knock is measured.
A higher cetane number is better as it indicates that the fuel ignites more easily and readily due to compression. Modern highway diesel engines tend to require fuel with a cetane number ranging from 45 to 55.
