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From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Subject: Cetane number of Disel
Date: Sun, 02 Feb 1997 06:55:58 GMT

The following post was initially cross-posted to a range of
group. however it didn't make rec.autos.tech because of a typo
in the newsgroups line. I've added a couple of lines of extra
info, and deleted some stuff...

...
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 comporession ratios that bracket tthe sample.
The cetane number is then determined by interpolation, and the higher 
the Cetane Number, the shorter the delay between injection and ignition.
Now, if the fuel is pure hydrocarbons ( does not contain cetane number
improving agents like alkyl or amyl nitrates ) then the cetace 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 autoignition, whereas for diesel, the
desirable property is to autoignite. 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 propeties.
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 propety 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 vapours 
to ignite when a flame is applied. In all cases it's usually well above 
ambient ( 60C+, kerosine is 37C+, whereas gasoline  is typically below 
-30C ), and anybody mixing a lowert 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 kerosine, 
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 availble, 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 kerosine 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 lowerr flash
fuels. Some countries, like NZ, avoid this by having diesel cheaper
than gasoline at the service station, and imposing taxes based on 
distance travelled (  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 )

                   Bruce Hamilton

 
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