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What is the Cetane number referring to?

Author : Unknown
Presented by : Martin's Marine Engineering Page -

June 2017

The Cetane number measures the ignition quality of a diesel fuel.

It is the % volume of Cetane (n-hexadecane, Cetane Number = 100) in alpha methyl naphthalene (Cetane Number = 0), that provides the specified standard of 13 degrees (crankshaft angle) ignition delay at the identical compression ratio to that of the fuel sample.

These days, heptamethyl nonane - with a Cetane Number of 15 - is used in place of alpha methyl naphthalene because it is a more stable reference compound. It is measured in special ASTM variable compression ratio test engine that is closely controlled with regard to temperatures (coolant 100C, intake air 65.6C), injection pressure (1500psi), injection timing 13 degrees BTDC, and speed (900rpm).

The compression ratio is adjusted until combustion occurs at TDC (the ignition delay is 13 degrees). The test is then repeated with reference fuels with five Cetane numbers difference, until two of them have compression ratios that bracket the sample. The Cetane number is then determined by interpolation and the higher the Cetane Number, the shorter the delay between injection and ignition.

If the fuel is pure hydrocarbons (does not contain Cetane number improving agents like alkyl or amyl nitrates) then the Cetane number can be predicted fairly well using some physical properties, such as boiling point and aniline point.

It's obvious from the above that the higher the Cetane number (100 = normal alkane, 15 = iso-alkane), then the lower the octane number 100 = iso-alkane, 0 = normal alkane). This is because the desirable property of gasoline to prevent knock is the ability to resist auto ignition, whereas for diesel, the desirable property is to auto ignite.

The octane number of normal alkanes decreases as carbon chain length increases, whereas the Cetane number increases as the carbon chain length increases.

Many other factors also affect the Cetane number, and around 0.5 volume % of Cetane number improvers will increase the Cetane number by 10 units. Cetane number improvers can be alkyl nitrates, primary amyl nitrates, nitrites, or peroxides.

In general, aromatics and alcohols have low Cetane numbers (that's why people using methanol in diesels convert it to dimethyl ether). One of the obvious effects of running on low Cetane number fuel is the increase in engine noise. Typically, engines are designed to use fuels with Cetane Numbers of 40-55, because below 38, a more rapid increase in ignition delay.

The significance of the Cetane number increases with the speed of the engine, and large, low speed diesel engines often only specify viscosity, combustion and contaminant levels, as Cetane Number requirement of the engine is met by most distillate and residual fuels that have the appropriate properties. High speed diesel engines (as in cars and trucks) virtually all are designed to accept fuels around 50 Cetane Numbers, with higher numbers being a waste.

However, Cetane Number is only one important property of diesel fuels, with three of the others being also very important. Firstly, the viscosity is important because many injection systems rely on the lubricity of the fuel for lubrication.

Secondly, the cold weather properties are important, remember that normal alkanes are desirable, but the desirable diesel fraction alkanes have melting points above 0C temperature, so special flow-enhancing additives and changes to the hydrocarbon profiles occur seasonally. That's why it's never a good idea to store diesel from summer for winter use.

Thirdly, diesel in many countries has a legal minimum flash point (the minimum temperature it must attain to produce sufficient vapors to ignite when a flame is applied. In all cases it's usually well above ambient (60C+, kerosene is 37C+, whereas gasoline is typically below -30C), and anybody mixing a lower flash point fraction with diesel will usually void all insurance and warranties on the vehicle.

The recent increase in blending fuels has resulted in significantly more frequent analyses of fuel tank contents from diesel vehicle fires.

From all of the above, you can see some common factors emerging, larger normal alkanes are desirable, and they also burn with a less smoky flame and have higher flash points than gasoline and kerosene, making them also desirable for home heating fuels, however the relatively expensive Cetane Index improvers have no value in heating fuels.

Most engines show an increase in ignition delay when the Cetane number is decreased from around 50 to 40, with an increase of 2 degrees being typical, and minimal advantages accrue of lower CN fuels are used.

Heating oils are often a slightly different fraction, and may have differing additives (for cleaner combustion) to fuels used for high speed diesel engines. For low speed (large, stationary and marine engines), they often use the cheapest residual fuel oil available, as do the larger heating boilers - so there is commonality of fuel as size increases.

Details of the important, specified properties of various grades (1D, 2D, 4D) of diesel fuel oils can be found in the Annual Book of ASTM Standards. ASTM D975-93 "Standard Specification for Diesel Fuel Oils", as can the fuel oil specification for grades 1, 2, 4, 5, and 6 in " Standard Specification for Fuel Oils ASTM D 396-92. Note that ASTN D975-93 actually defines the low temperature requirements by dividing the USA into regions.

It is possible for a fuel to meet both specifications, but the diesel specification may have additional requirements such as Cetane Number and Cloud Point (temperature at which the fuel goes cloudy), whereas the Fuel Oil may have additional limits on the distillation properties, and viscosity at 100C. A fuel has to be tested for all the criteria in each specification grade before it can be said to comply with the relevant grade in each specification.

The interchanging and dilution of fuels is performed by suppliers, taking into account the effect on all of the above, but especially flash point, as that is closely regulated in many countries.

Adding kerosene and gasoline to diesel can have dramatic, adverse effects on the flash point, with minimal gains in the flow properties if the fuel already contains flow-improving additives. Regardless of what other people may advise, check your insurance policies before embarking on experimentation. These days, assessors for both vehicle and insurance companies these days are far more aware of the signs of the dilution of expensive diesel fuel by cheaper lower flash fuels.

Some countries, like NZ, avoid this by having diesel cheaper than gasoline at the service station, and imposing taxes based on distance traveled (as measured by hubometers on vehicle wheels), number and location of axles, axle loads, and gross vehicle weight, as they more accurately indicate road damage potential.

For people that are interested in diesel fuel properties and the effects on engine performance, the following are good sources, especially the first two....

Internal Combustion Engine Fundamentals
John B. Heywood
McGraw Hill ISBN 0-07-100499-8 (1988)

Automotive Fuels Reference Book
Keith Owen, Trevor Coley
SAE ISBN 1-56091-589-7 (1995)

Modern Petroleum Technology
Edited by G.D.Hobson
John Wiley & Sons ISBN 0-471-262498 (1984)

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