Page 221 - Advanced Thermodynamics for Engineers, Second Edition

10.1 SIMPLE CHEMISTRY 209

diatomic gases, whereas CO 2 is a triatomic gas. Equation (10.1) also indicates that two molecules of
CO 2 will always break down into two molecules of CO and one molecule of O 2 : this is signified by the
symbol 5 which indicates that the processes can go in both directions. It is conventional to refer to the
components to the left of the arrow as the reactants and that to the right as the products: this is because
exothermic combustion (i.e. in which energy is released by the process) would require CO and O 2 to
combine to give CO 2 . Not all reactions are exothermic and the formation of NO during dissociation
occurring in an internal combustion (i.c.) engine is actually endothermic.
It should be noted from the combustion Eqn (10.1) that three molecules of reactants combine to
produce two molecules of products, hence there is not necessarily a balance in the number of mole-
cules on either side of a chemical reaction. However, there is a balance in the number of atoms of each
constituent in the equation and so mass is conserved.

10.1.1 FUELS

In 1973, the price of crude oil rapidly increased to three to four times its pre-1973 cost. This major
price change had a marked impact on the economy of the industrial nations. It also provided a sharp
reminder of their vulnerability to the pricing and conservation policies of the oil-producing countries,
and of the finite nature of the world’s petroleum supplies. This problem was ameliorated during the
1980s with the discovery and capture of alternative supplies in other parts of the world. At present, in
2014, the situation is changing again with the development of fracking to access gases trapped in the
rock strata. However, no matter how effective man becomes at obtaining fossil fuels, in the end they
will prove to be a finite resource, and their price will rise. The downside of accessing more sources of
fossil fuels is that their combustion will form more carbon dioxide, carbon monoxide and nitrogen
oxides to be produced, with the possibility of further global warming. It is possible that this problem
can be reduced by carbon sequestration, but at what energy cost – and where will it come from? The
most dominant fuel issues of today are those of cost and availability.
Crude oil is known to exist at various depths beneath land and sea in most parts of the world, but
commercial quantities are produced mainly in four areas – North America, the Caribbean, the Middle
East, and the Soviet Union. Crude oils are extremely complex mixtures of gases, liquids and dissolved
solids that always consist mainly of hydrocarbons, with small amounts of nitrogenous substances and
organic sulphur compounds. A typical barrel of crude oil contains a range of hydrocarbons, and these
are separated at a refinery; the oil produces the constituents shown broadly in Fig. 10.1. Hydrocarbon
fuels are rarely single component in nature due to the methods of formation of the raw material and its
extraction from the ground. Invariably they also contain traces of various metallic constituents. It is
usual to classify the hydrocarbons present in petroleum fuel into four main groups: paraffinic, olefinic,
naphthenic and aromatic. The proportions in which these groups are present largely define the char-
acter of the fuel. The chemical structure of fuels will be discussed in some detail in Chapter 11.
Fuels for spark-ignition engines require an ability to resist spontaneous ignition at the high pres-
sures and temperatures achieved after compression, and in the end gas during combustion: this
characteristic is defined by the octane number of the fuel. A fuel with a high octane number will be
more resistant to spontaneous ignition than one with a low octane number, and can run in an engine
with a higher compression ratio. Spontaneous combustion results in detonation, which can be an
audible ‘tink-tink’ at low speed; at high speed the combustion ‘knocks’ causing major damage to
combustion chamber components. Knock is much less prevalent in engines nowadays because ‘knock’