27.2 What are CNG/LPG/gasoline/kerosine/diesel?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
27.2 What are CNG/LPG/gasoline/kerosine/diesel?
Crude oil consists mainly of hydrocarbons with carbon numbers between one and
forty. The petroleum refinery takes this product and refines it into several
fuel fractions that are optimised for their intended application. For spark
ignition engines, the very volatile and branched chain alkane hydrocarbons
have desirable combustion properties, and several fractions are produced.
CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20%
ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to
pentane. The major disadvantage of compressed gaseous fuels is the reduced
range. Vehicles may have between one to three cylinders ( 25 MPa, 90-120
litre capacity), and they usually provide about 50% of the gasoline range.
LPG ( Liquefied Petroleum Gas ) is predominantly propane with iso-butane
and n-butane. It has one major advantage over CNG, the tanks do not have
to be high pressure, and the fuel is stored as a liquid. The fuel offers
most of the environmental benefits of CNG, including high octane - which
means higher compression, more efficient, engines can be used. Approximately
20-25% more fuel than gasoline is required, unless the engine is optimised
( CR 12:1 ) for LPG, in which case there is no decrease in power or any
significant increase in fuel consumption [4,5].
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial boiling
point is increasing, and the final boiling point is decreasing, both
changes are for environmental reasons. A detailed description of the
composition of gasoline, along with the properties and compositions of CNG,
LPG, and oxygenates can be found in the Gasoline FAQ, which is posted monthly
to rec.autos.tech.
Kerosine is a hydrocarbon fraction that typically distils between 170-270C
(narrow cut kerosine, or Jet A1) or 100-250C ( wide cut kerosine, or JP-4 ).
It contains around 20% of aromatics, however the aromatic content will be
reduced for high quality lighting kerosines, as the aromatics reduce the
smoke point. The major use for kerosines today is as aviation turbine (jet)
fuels. Special properties are required for that application, including high
flash point for safe refuelling ( 38C for Jet A1 ), low freezing point for
high altitude flying ( -47C for Jet A1 ), and good water separation
characteristics. Details can be found in any petroleum refining text and
Kirk Othmer.
Diesel is used in compression ignition engines, and is a hydrocarbon fraction
that typically distils between 250-380C. Diesel engines use the Cetane
(n-hexadecane) rating to assess ignition delay. Normal alkanes have a high
cetane rating, ( nC16=100 ) whereas aromatics ( alpha methylnaphthalene = 0 )
and iso-alkanes ( 2,2,4,4,6,8,8-hexamethylnonane = 15 ) have low ratings,
which represent long ignition delays. Because of the size of the hydrocarbons,
the low temperature flow properties control the composition of diesel, and
additives are used to prevent filter blocking in cooler temperatures. There
are usually summer and winter grades. Environmental legislation is reducing
the amount of aromatics and sulfur permitted in diesel, and the emission of
small particulates ( diameters of <10um ) that are considered possibly
carcinogenic, and are known to cause other adverse health effects. Details
can be found in any petroleum refining text and Kirk Othmer.
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