High Cetane Index! What to do?



Here is an interesting story – though it is technical,  sure this will be a worthy read.

One of the newly joined technical superintendents received a fuel oil analysis report for a distillate fuel (LSMGO).  To his surprise, the cetane index was high and this triggered a shiver in him.  He wrote a mail to one of the fuel testing labs asking for clarification.  He was so furious and angry that the testing lab did not raise a flag on high cetane index in the fuel.  The tested result showed 48, whereas the permissible limit was 40.

The fuel testing lab received the email asking for a clarification and the lab manager responded to the email immediately.  The lab manager, who was an ex-marine engineer, wrote to the technical superintendent as below –

“Please note that the analysis results shows cetane index is 48 and this is well within the specification limit.  The specification limit for cetane index is MINIMUM 40 as per ISO 8217:2005”.

The Superintendent got the above reply within few minutes and was happy.  However, he was hesitant to ask one more question which bothered him.  Finally, he decided and wrote an email back to the marine fuel testing lab with the question.

The Question was –

“What happens if the result is lower than the specified limit ?”

The Lab Manager received the mail and he wrote a reply within few minutes, highlighting all important points, here is the reply…

“What is Cetane Number?”

Similar to the octane number seen on a retail gasoline dispenser, a cetane number rates a diesel fuel’s quality of ignition.  But that’s where the similarities end.  The octane number of a gasoline measures its ability to resist auto-ignition commonly referred to as pre ignition, knocking or pinging.  A diesel fuel’s cetane number, however, is actually a measure of the fuel’s ignition delay; the time period between the start of the injection of the fuel and the start of the combustion of the fuel (commonly known as ignition).  In general, a higher cetane fuel will have a shorter ignition delay period than a lower cetane fuel.

What does a high cetane number mean to a diesel fuel user?

Since diesels rely on compression ignition, the fuel must be able to readily auto-ignite and quicker is better.  A higher cetane number, indicating a shorter ignition delay time, usually means more complete combustion of the fuel.

This translates into:

  1. Quicker starting
  2. Quieter operation with less diesel chatter
  3. Improved fuel efficiency
  4. A reduction of harmful emissions
  5. Less wear and tear on the starter and batteries
  6. Quicker pumping of protective lubricating fluids throughout the system

Ignition quality is indicated by cetane number.  The lower the cetane number of a fuel, the greater the ignition delay and the longer the period of time between fuel injection and the beginning of the rapid pressure rise associated with fuel ignition and combustion.  The cetane number of a fuel is dependent on the nature of the hydrocarbon from which the fuel is refined and the extent of refinery processing on the crude oil.  As crude oils are refined more intensely, the fuel oils possess a greater aromaticity, which can increase the ignition delay, and can result in hard knocking or noisy engine running, which is undesirable over long periods of time.  The result could be poor fuel economy, loss of power and may even result in engine damage.

If the fuel oil requires blending for viscosity reduction, the lower viscosity cutter stock will have to contain a higher aromaticity to prevent incompatibility problems after blending.  This higher aromaticity in the cutter stock can result in a similar increase in ignition delay, similar hard engine operation and similar end results.  This can cause serious operational limitations in medium- and high-speed diesel engines, which are sensitive to the ignition quality of the fuel provided.  Diesel engines operating at a lower speed, ie., less than 400 rpm, will be much less sensitive to fuel ignition quality.

Difficulties in diesel fuel oil combustion are largely related to the time required to burn the fuel.  This time includes ignition delay, or the time required to start combustion and the complete combustion time or burning period.  Basically, ignition delay and burning duration increase with increased viscosity and aromatic content of the fuel and such fuels contain a higher percentage of ‘high boiling point’ hydrocarbons.  Both factors tend to lower the cetane number.  Physical factors which influence ignition and burning time are the speed with which fuel droplets are atomized, vaporized and thermally cracked to form a combustible mixture. Ignition and burning time can be improved by decreasing the fuel droplet size and/or increasing swirl.  The past experience also indicates that raising inlet air temperature can reduce the cetane sensitivity of high speed diesel engines.

Cetane number is normally quoted for distillate fuels only.

Now, we are sure that you liked the technical explanation and the story too.

Service That Inspires:

The marine fuel testing lab was none other than Viswa Lab – which claims that their technical advisory is always FREE and anyone can write to them with a query.

Viswa Lab not only replied to such technical queries but also ensures that their replies are prompt and time bound.

On a concluding note, the technical superintendent wrote an email back to Viswa Lab team stating that he was very happy about the prompt and sincere service provided by the team with detailed reports and suggestions.

Viswa Lab says that it is not possible for the ship managers to remember every fuel grade and their specification limits given the amount of workload and stress they go through.  They are welcome to just drop a mail to Viswa Lab support, whenever they have any queries pertaining to fuels, lubes, cylinder oils and other shipboard problems.


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