Cetane Number Explained: Why 51 Is Critical

The cetane number (CN) measures the auto-ignition quality of diesel fuel used in compression-ignition engines. It indicates how quickly the fuel ignites after being injected into the combustion chamber.

In simple terms:

• Higher cetane number → shorter ignition delay

• Lower cetane number → longer ignition delay

Ignition delay is the time between fuel injection and the actual start of combustion. This parameter directly influences engine smoothness, efficiency, and emissions.

Diesel engines can operate on:

• Conventional diesel fuel

• Biodiesel blends (B20, B100)

Regardless of the fuel type, cetane number remains a critical combustion parameter.

Why Cetane Number Matters in Diesel Engines

Diesel engines rely entirely on compression for ignition. Unlike gasoline engines, there is no spark plug. If ignition delay becomes too long, excessive fuel accumulates before combustion starts. This leads to:

• Harsh combustion (diesel knock)

• Increased mechanical stress

• Higher NOx and particulate emissions

• Rough engine operation

Modern common-rail systems operate at extremely high injection pressures. Precise combustion timing becomes even more critical in these engines.

A balanced cetane number ensures:

• Smooth cold starts

• Stable idle

• Reduced combustion noise

• Lower emissions

• Optimized fuel consumption

European Standard Requirement – EN 590

In Europe, diesel fuel quality is regulated by EN 590.

This standard defines multiple parameters:

• Density

• Viscosity

• Sulfur content

• Flash point

• Water contamination

• Lubricity

• Cetane number

The minimum required cetane number under EN 590 is 51.

All fuel suppliers within the European Union must comply with this regulation.

Modern diesel engines sold in the EU are engineered specifically for fuel with a cetane number between 51 and 55.

What Happens If Cetane Number Is Too Low?

Using diesel fuel below the recommended cetane value may cause:

1. Difficult Starting

Especially noticeable in:

• Cold climates

• High altitude environments

2. Increased Engine Noise

Longer ignition delay causes rapid pressure rise once combustion starts, leading to knocking.

3. Higher Fuel Consumption

Incomplete or delayed combustion reduces thermal efficiency.

4. Higher Emissions

• Increased NOx

• Higher particulate matter

• Visible smoke

5. Oil Dilution

Unburned fuel may wash cylinder walls, contaminating engine oil.

6. Premature Wear

Increased mechanical stress affects pistons, bearings, and injectors.

Can Cetane Number Be Too High?

Yes.

Fuel with a cetane number above 60 may ignite too quickly. When ignition occurs too early:

• Air-fuel mixing is incomplete

• Combustion duration changes

• Efficiency may drop

• Smoke and emissions can increase

Modern diesel engines are calibrated for a specific ignition delay window. Exceeding that window may disturb optimal combustion phasing.

The ideal operational range remains 51–55 for most European diesel engines.

How Cetane Number Is Measured

Cetane number is determined using a standardized laboratory procedure involving a special single-cylinder test engine.

Reference Fuels

The test compares the fuel sample against two reference substances:

1. Cetane (C16H34)

• Straight-chain hydrocarbon

• Excellent auto-ignition properties

• Assigned cetane number 100

2. Alpha-methylnaphthalene (C11H10)

• Aromatic hydrocarbon

• High resistance to ignition

• Assigned cetane number 0

Due to cost and stability issues, alpha-methylnaphthalene has largely been replaced by:

• Heptamethylnonane (isocetane), assigned cetane number 15

Critical Compression Ratio Method

Testing is performed on a CFR (Cooperative Fuel Research) engine under controlled conditions:

• Engine speed: 900 rpm

• Intake air temperature: ~65.6°C

• Variable compression ratio

Procedure:

1. Adjust compression ratio until ignition occurs.

2. Compare with reference fuel blends.

3. Calculate cetane number through linear interpolation.

The final cetane value depends on the chemical composition of the tested fuel.

Cetane Number in the United States

In the United States, diesel fuel standards differ.

Typical cetane values:

• Minimum: ~40

• Common range: 40–45

This is significantly lower than the European requirement of 51.

To compensate, fuel suppliers often use cetane improver additives, typically alkyl nitrate-based compounds.

These additives:

• Decompose quickly

• Release oxygen

• Improve ignition quality

However, potential downsides include:

• Increased oxidation

• Deposit formation

• Water generation

• Long-term injector contamination

Excessive use of nitrate-based additives is generally discouraged.

Biodiesel and Cetane Number

Biodiesel often has a naturally higher cetane number compared to petroleum diesel. However:

• Combustion characteristics differ

• Energy density is lower

• Cold flow properties may be reduced

Blends such as B20 typically maintain acceptable cetane performance within EN 590 limits.

Key Takeaways

• Cetane number measures diesel fuel ignition quality.

• Higher cetane number = shorter ignition delay.

• European standard EN 590 requires a minimum of 51.

• Optimal range for modern EU engines: 51–55.

• Too low → rough operation and higher emissions.

• Too high → altered combustion efficiency.

Engines are calibrated for a specific ignition window. Fuel quality must match engine design parameters for optimal performance and durability.