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Cetane in Diesel Fuel

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The Cetane Number is a measure of the ignition quality of a diesel fuel.

It is often mistaken as a measure of fuel quality.

The Cetane number is actually a measure of a fuel's ignition delay.

This is the time period between the start of injection and start of combustion (ignition) of the fuel.

In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels.

Cetane number should not be considered alone when evaluating diesel fuel quality.

API gravity, BTU content, distillation range, sulfur content, stability and flash point are also very important.

In colder weather, cloud point and low temperature filter plugging point may be critical factors.

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Determining the Cetane Number

The optical method for determining the cetane number is ASTM test D-613.

This method requires the use of an industry standard test engine equipped with accepted instrumentation and operated under specific conditions.

In this test, the engine compression ratio is varied for the test sample and reference fuels of known cetane number to obtain a fixed ignition delay.

The compression ratio of the sample is bracketed by those of two reference fuels.

The cetane number of the sample fuel is determined by estimating between the two reference fuel points.

Because the ASTM D-613 test is time consuming and expensive, calculated cetane index (ASTM D-976 or D-4737 is often substituted for cetane number.

The calculated cetane index is derived from the fuel's density and boiling range.

While useful for estimating the cetane number of distillate fuels, this technique can not be applied to fuels containing additives that raise cetane number.

These additives do not change the fuel density or distillation profile, so they do not alter the calculated cetane index.

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How Does the Cetane Number Affect Engine Operation?

There is no benefit to using a higher cetane number fuel than is specified by the engine's manufacturer.

The ASTM Standard Specification for Diesel Fuel Oils (D-975) states

"The cetane number requirements depend on engine design, size, nature of speed and load variations, and on starting and atmospheric conditions. Increase in cetane number over values actually required does not materially improve engine performance. Accordingly, the cetane number specified should be as low as possible to insure maximum fuel availability."

This quote underscores the importance of matching engine cetane requirements with fuel cetane number.

Diesel fuels with a cetane number lower than minimum engine requirements can cause rough engine operation.

They are more difficult to start, especially in cold weather or at high altitudes.

They accelerate lube oil sludge formation.

Many low cetane fuels increase engine deposits resulting in more smoke, increased exhaust emissions and greater engine wear.

Using fuels which meet engine operating requirements will improve cold starting, reduce smoke during start-up, improve fuel economy, reduce exhaust emissions, improve engine durability and reduce noise and vibration.

These engine fuel requirements are published in the operating manual for each specific engine or vehicle.

Overall fuel quality and performance depend on the ratio of parafinic and aromatic hydrocarbons, the presence of sulfur, water, bacteria, and other contaminants, and the fuel's resistance to oxidation.

The most important measures of fuel quality include API gravity, heat value (BTU content), distillation range and viscosity.

Cleanliness and corrosion resistance are also important.

For use in cold weather, cloud point and low temperature filter plugging point must receive serious consideration.

The Cetane number does not measure any of these characteristics.

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cetane,cetane number,cetane rating,cetane booster,Diesel, biodiesel

cetane,cetane number,cetane rating,cetane booster,Diesel, biodiesel