13 - 13 E ffect of Dissociation on C ombustion Pa r a m...

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13 Effect of Dissociation on Combustion Pa ra meters Dissociation is an equilibrium process by which the products of combustion achieve the minimum Gibbs function for the mixture (see Chapter 12). The effect is to cause the products that would be obtained from complete combustion to break down partially into the original reactants, and other compounds or radicals. This can be depicted on a U-T diagram as shown in Fig 13.1. - ,. ,' . I. Products ' ' Products (with dissociation) (with complete combustion) T, for issociation T, for complete combustion 7 Temperature, T Fig. 13.1 Internal energy-temperature diagram for combustion both with and without dissociation It can be seen from Fig 13.1 that the effect of dissociation is to reduce the temperature of the products after combustion. This, in turn, reduces the amount of energy that can be drawn from the combustion process and reduces the work output of engines. The other effect of dissociation is to form pollutants. This is particularly true in combustion in engines, when carbon monoxide (CO) can be formed even when the mixture is weak - that is, there is more than sufficient oxygen in the air to oxidise completely both the carbon and hydrogen in the fuel. If chemical kinetics are considered (see Chapter 14), i.e. account is taken of the finite rate by which equilibrium is achieved, then it is possible to show how some of the major pollutants are formed in the quantities measured at the exit from engines. Chemical kinetic effects and dissociation are the sole
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266 Effect of dissociation on combustion parameters causes of the production of NO, from fuels which do not contain nitrogen (through combining of the nitrogen and oxygen in the air), and they also increase the amount of carbon monoxide (CO) during weak and slightly rich combustion. Processes which include chemical kinetics are not equilibrium processes, but they attempt to reach equilibrium if there is sufficient time. The effect of chemical, or rate, kinetics can be assessed by considering a simple example. Imagine a spark ignition engine in which the mixture is compressed prior to ignition by a spark. If the compression temperature is not too high it can be assumed that the reactants do not react before ignition. After ignition the reactants bum to form products at a high temperature, and these are initially compressed further (as the piston continues to rise to top dead centre [tdc], and the combustion process continues) before being expanded when the piston moves down and extracts work from the gases. As the pressure increases, the temperature of the gas also increases and the products tend to dissociate: they attempt to achieve an equilibrium composition but the speed of the engine is too rapid for this. The effect can be seen in Fig 13.2, which depicts the way in which the actual level of pollutants attempts to follow the equilibrium level, but lags the equilibrium values. This lag means that the maximum level of pollutant (e.g. NO,) formed does not achieve the
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This note was uploaded on 03/09/2010 for the course MECHANICAL ME9802701 taught by Professor Prof.william during the Spring '10 term at Institut Teknologi Bandung.

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13 - 13 E ffect of Dissociation on C ombustion Pa r a m...

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