Fuel cetane index and engine performance
When it comes to diesel engines, whether in marine ships, trucks, or generators, fuel quality plays a huge role in determining how smoothly, efficiently, and reliably the engine performs. One of the most important indicators of diesel fuel quality is the Cetane Index (CI). Understanding the cetane index helps engineers and operators determine how well a particular fuel will behave during combustion, and how it will impact the engine's performance, noise, efficiency, and emissions.
1. What Is Cetane Index?
The cetane index is a calculated number that estimates the ignition quality of diesel fuel. It predicts how easily the fuel will ignite in the engine cylinder after injection. The higher the cetane index, the shorter the ignition delay—that is, the fuel ignites faster after it is injected.
It's important not to confuse the cetane index with the cetane number. While both relate to ignition quality:
- Cetane number is determined through engine testing in a standardized test engine.
- Cetane index is a calculated value, derived from properties like fuel density and distillation temperatures, using standard formulas (like the ASTM D976 or D4737 method).
2. Why Is Cetane Index Important in Diesel Engines?
Diesel engines rely on compression ignition. Air is compressed to high pressure and temperature, and then diesel fuel is injected. If the fuel has poor ignition quality (low cetane), it takes longer to ignite, resulting in rough combustion, higher emissions, and possible engine damage.
A higher cetane index means:
- Quicker ignition
- Smoother combustion
- Lower combustion noise
- Reduced knocking
- Better cold start performance
- Improved fuel efficiency
- Lower particulate and NOx emissions
On the other hand, low cetane index fuels can lead to:
- Hard starting
- Engine knocking
- Poor performance
- Increased exhaust smoke
- Higher emissions
- Greater wear and tear
3. Cetane Index and Engine Performance Parameters
Let’s break down how cetane index specifically affects engine performance in various ways:
a. Combustion Timing and Smoothness
Diesel fuel must ignite at the right moment—after being injected and mixing with hot compressed air. A short ignition delay (thanks to high cetane) means combustion starts closer to optimal timing, leading to efficient power generation and minimal pressure spikes. This results in smoother engine operation.
b. Engine Knock
Low-cetane fuels tend to ignite unpredictably, causing diesel knock, which is a sharp, metallic sound due to rapid pressure rise. High cetane index fuels prevent this by igniting predictably and gradually.
c. Cold Weather Performance
In cold environments, fuel atomization and vaporization are slower, increasing ignition delay. High-cetane fuel compensates for this by igniting more easily, aiding cold starts and reducing white smoke during warm-up.
d. Fuel Efficiency
When combustion is more complete and better timed, less unburned fuel goes to waste. High cetane fuels reduce losses due to incomplete combustion, thus improving thermal efficiency and saving fuel over time.
e. Emissions Control
Poor combustion leads to unburned hydrocarbons (HC), carbon monoxide (CO), and soot. High cetane fuels reduce ignition delay and improve combustion uniformity, resulting in lower emissions. This is especially critical for meeting modern environmental regulations (like IMO Tier III for marine engines or EPA standards).
4. Typical Cetane Index Ranges and Standards
Most commercial diesel fuels have a cetane index between 40 and 55. Engine manufacturers often recommend a minimum cetane index of 45 or 47, depending on engine design, age, and application.
In marine engines, for example:
- Heavy Fuel Oils (HFO) often have low cetane index values and require preheating or ignition improvers.
- Marine Gas Oils (MGO) and Diesel Oils used in auxiliary or medium-speed engines usually have higher cetane indices, ensuring easier starts and smoother performance.
5. Cetane Index vs Engine Design
Modern diesel engines are increasingly high-pressure, high-efficiency designs with strict emissions targets. These engines are more sensitive to fuel quality. A fuel with a low cetane index may lead to:
- Higher peak cylinder pressures
- Incomplete combustion
- Long-term wear on pistons, injectors, and valves
On the other hand, older or large low-speed marine engines may be more tolerant but still benefit from optimized fuel quality.
6. Improving or Adjusting Cetane Index
In real-world marine or industrial operations, fuels sometimes need treatment to raise the cetane index. This can be done by:
- Blending with higher cetane fuels
- Using cetane improvers like 2-ethylhexyl nitrate (EHN)
- Optimizing fuel heating and atomization in engines running heavy fuels
- Ensuring proper fuel injection timing and maintenance of the injection system
7. Practical Example
Imagine a ship operating in a colder region like the North Sea. If the diesel used has a cetane index of only 40, the engine may struggle to start, produce black smoke, and knock during early operation. Switching to a higher-cetane index fuel (say, 48–50) immediately improves starting, reduces emissions, and allows the engine to run smoothly without adjustments.
8. Summary: Why Cetane Index Matters
| Aspect | High Cetane Index | Low Cetane Index |
|---|---|---|
| Ignition Delay | Short | Long |
| Engine Noise | Low (smooth combustion) | High (knocking) |
| Cold Start Performance | Easy | Difficult |
| Combustion Quality | Efficient and complete | Incomplete and delayed |
| Emissions | Lower (cleaner burn) | Higher (more soot, CO, HC) |
| Engine Wear | Lower due to smooth operation | Higher due to shock loads |
Conclusion
In diesel engine performance, especially in critical applications like marine engineering, cetane index is a key parameter. It gives insight into how the fuel will behave under compression and whether it will support smooth, efficient, and clean combustion. Choosing the right fuel with an appropriate cetane index—and understanding how it influences your engine's behavior—is essential for long-term reliability, cost savings, and environmental compliance.