Cetane Number (CN) and Diesel Index (DI) explained

 

Cetane Number (CN)

Cetane Number (CN) is a measure of the ignition quality of diesel fuel, indicating how readily the fuel ignites when injected into the combustion chamber of a diesel engine. The cetane number is analogous to the octane number for gasoline but applies to diesel fuels.

Cetane Number (CN) Key Points:

1. Cetane number Scale:

   • The cetane number is expressed on a scale from 0 to 100, where:
     • 0 represents a fuel that is very difficult to ignite.
     • 100 represents a fuel that ignites almost instantaneously when injected into the combustion chamber.

2. Recommended Values of Cetane number:

   • Most modern diesel engines require a cetane number of at least 40-55 for optimal performance. Higher cetane numbers (above 55) are often found in premium diesel fuels and can enhance performance.

3. Effects of High Cetane Numbers:

   • Improved Combustion: Fuels with higher cetane numbers ignite more quickly, resulting in smoother combustion and reduced engine noise.
   • Reduced Emissions: Better ignition quality leads to more complete combustion, reducing the production of particulate matter and other emissions.
   • Enhanced Engine Performance: Increased power output and responsiveness, especially during cold starts.

check this : Ignition delay period & After burning in C.I. Engine

Diesel Index (DI)

The Diesel Index (DI) is an empirical measure that provides an alternative representation of the overall ignition quality of diesel fuel. It takes into account the fuel's density and distillation characteristics.

Key Points:

1. Diesel Index Formula:

1. • The Diesel Index is calculated using the fuel's specific gravity and distillation data. A common formula for the Diesel Index is:

$$\text{DI}=\text{(Cetane Number)}+\frac{(D_{9}5-100)}{10}$$

• Where $D_{95}$ is the distillation temperature at which 95% of the sample evaporates.

2. Diesel Index Relation to Ignition Quality:

   • The Diesel Index provides a good estimate of the ignition quality of diesel fuel and can serve as a pragmatic way to evaluate fuels that may not have a clearly defined cetane number.

3. Diesel Index Usefulness:

• While CN is more widely used in specifications, the Diesel Index helps in assessing fuels based on their physical properties, particularly for fuels that may be blended or have varying distillation ranges.

check this : Octane Number and Engine Knocking

Summary

• Cetane Number (CN) is crucial for determining the ignition properties of diesel fuels, influencing combustion efficiency, engine performance, and emissions.
• Diesel Index (DI) provides an empirical approach to evaluating diesel fuels based on density and distillation characteristics, serving as a complementary measure to cetane number.

Understanding both cetane number and diesel index is essential for selecting the appropriate diesel fuel, ensuring optimal performance and reduced emissions in diesel engines. 

If you have any further questions or need more details on these topics, feel free to ask!