Page 248 - Advanced thermodynamics for engineers

CHAPTER
11
CHEMISTRY OF COMBUSTION

The thermodynamics of combustion were considered in Chapter 10, and it was stated that adiabatic
combustion could be achieved. The concept of adiabatic combustion runs counter to the experience of
many engineers, who tend to relate combustion to heat addition or heat release processes. This
approach is encouraged in mechanical engineering by the application of the air standard cycle to
engines to enable them to be treated as heat engines. In reality combustion is not a process of energy
transfer but one of energy transformation. The energy released by combustion in a spark ignition
(petrol) engine is all contained in the mixture prior to combustion, and it is released by the spark. It will
be shown that the energy which causes the temperature rise in a combustion process is obtained by
breaking the bonds which hold the fuel atoms together.

11.1 BOND ENERGIES AND HEAT OF FORMATION
Heats (enthalpies and internal energies) of formation can be evaluated empirically by ‘burning’ the
fuel. They can also be evaluated by consideration of the chemical structure of the compound. Each
compound consists of a number of elements held together by certain types of bond. The bond energy is
the amount of energy required to separate a molecule into atoms; the energy of a particular type of
bond is similar irrespective of the actual structure of the molecule.
This concept was introduced in Chapter 10, and heats of formation were used to evaluate heats of
reaction (Hess’ Law). The process of breaking the chemical bonds during the combustion process
can be depicted by a diagram such as Fig. 11.1. It is assumed that element molecules can be atomised
(in a constant pressure process) by the addition of energy equal to DH a . If these atoms are then
brought together they would combine, releasing dissociation energy of SDH(X Y) R to form the
reactants. The sum of the dissociation and atomisation energies (taking account of the signs) results in
the enthalpy of formation of the reactants. In a similar way, the enthalpy of formation of the products
can be evaluated. Using Hess’ Law, the enthalpy of reaction of the fuel can be evaluated as the
difference between the enthalpies of formation of the products and the reactants. These energies are
essentially the bond energies of the various molecules and some of these energies are listed in
Table 11.1.
Figure 11.2 shows how the energy required to separate two atoms varies with distance: the bond
energy is defined as the minimum potential energy relative to that at infinity. The point of minimum
energy indicates that the molecule is in equilibrium.

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