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13.8 CONCLUDING REMARKS 305

of about 0.13, and this is equivalent to about 13,000 ppm (parts per million [by mass]): this level is
typical of that which would be achieved at equilibrium conditions in an engine.
Before considering the effect of the rate of reactions on the actual levels of NO achieved, it is
worthwhile seeing the effect of fuel on the level of exhaust constituents. The additional reactions do
not have a major effect on the exhaust composition with octane as a fuel, and the CO 2 level is still
significantly higher than with methane, as would be expected. However, the general level of NO is
higher with octane than methane because of the slightly higher temperatures achieved with this fuel,
and the larger proportion of CO 2 which can be ‘robbed’ for oxygen. The maximum levels of NO again
occur around f ¼ 0.7 to 0.8, and result in about 14,000 ppm.
The levels of NO in the exhaust gas of an engine are significantly lower than those calculated above
for a number of reasons. First, the reactions occurring in the cylinder are not instantaneous, but are
restricted by the rate at which reacting molecules meet: this is known as chemical kinetics or rate
kinetics, and is the subject of Chapter 14. The effect of these kinetics is that the instantaneous level of
NO in the cylinder does not achieve the equilibrium level, and lags it when it is increasing as shown in
Fig. 14.1. When the equilibrium concentration is decreasing the actual level of NO still cannot
maintain pace with it, and again lags the level while it decreases down the expansion stroke of the
engine, as shown qualitatively in Fig. 14.1; in fact these reactions tend to ‘freeze’ at around 2000 K,
and this level then dominates the exhaust value. Second, the combustion process does not take place
instantaneously at tdc as shown here, but is spread over a significant period of the cycle and the
pressures and temperatures are lower than those attained in the ‘Otto’ cycle: this will reduce the peak
equilibrium level of NO.
It is shown on Figs 13.10 and 13.11 that the peak level of NO at equilibrium occurs at around
f ¼ 0.7–0.8, but experience shows that the maximum values of NO in engine exhaust systems (around
2000–5000 ppm) occur at about f ¼ 0.9. This can be explained by considering the effect of rate
kinetics. It will be shown in Chapter 14 that the rate at which a reaction occurs is exponentially related
to temperature. Hence, while the equilibrium level of NO is maximum at f ¼ 0.7–0.8 the lower
temperature at this equivalence ratio limits the amount of NO produced by the reaction. The peak value
of NO occurs at f ¼ 0.9 because the driving force (the equilibrium concentration) and the rate of
reaction combine at this equivalence ratio to maximise production of this pollutant.

13.8 CONCLUDING REMARKS
This chapter has laid the foundation for considering the production of emissions from combustion
systems. It delivers essential information for the evaluation of rate kinetics, which is considered in
Chapter 14.
The chapter has shown that the composition of typical gas mixtures found in engines at tdc is very
dependent on the fuel, equivalence ratio, the temperature and pressure achieved during the process.
The early sections, which neglect NO x , show how the proportions of CO, CO 2 ,H 2 O and H 2 vary with
equivalence ratio, and the particular carbon/hydrogen ratio of the fuel used. The effect of dissociation
on the peak temperatures and pressures achieved is also discussed.
The final section considered the dissociation processes that produced NO, and showed that the
maximum equilibrium concentration of NO was found at relatively weak mixtures, when there was an
abundance of ‘free’ oxygen.