Monday, Mar 27, 2006 at 19:59
Cetane rating for the big three:
Caltex - 46.
BP - 51.
Shell - 49.
Cetane 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'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.
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
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AnswerID:
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