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Abstract

One of the most frequently encountered problems in the general area of pollution is that of diesel exhaust odour, particularly in cities, where the use of diesel-powered buses and lorries provide the inhabitants with much cause for complaint. Despite this, the diesel engine has much to offer in terms of reliability and economy. Thus, if the widespread use of diesel engines is to be extended to include light transport vehicles, such as passenger cars, then the diesel exhaust odour problem must be considerably reduced, In order to contribute to this objective, the present work has attempted to establish the mechanism by which odorants are formed during the combustion process and to elucidate the conditions which influence the processes of formation.

Section 1 describes briefly general aspects of hydrocarbon oxidation, the mechanism of which has been deduced largely from laboratory studies using static vacuum and flow systems. The observations emanating from these studies are relevant to the combustion of diesel fuel in an engine. Thus the four stages of combustion which have been defined are described with particular reference to the ignition delay period during which many of the odorants are formed. In order to provide background information relating to 'odour-science', the various aspects of odorant perception and their measurement are described. Finally, a description is given of the two main diesel exhaust odour research programmes, one of which resulted in the development of an objective odour assessment method.

An account of the apparatus and techniques used during the course of this work is given in Section 2. Objective methods of odour assessment using electronic analogues of the olfactory sensory process were briefly investigated and the problems of temperature and water vapour associated with the use of such devices for the measurement of diesel exhaust odour were considered, The main experimental programme concerned the controlled oxidation of diesel and pure hydrocarbon fuels in a stainless—steel reactor incorporated into a static vacuum system. Investigation of the combustion phenomena taking place during diesel fuel oxidation under various conditions of stoichiometry, pressure and temperature revealed that the kinetic behaviour of this fuel was very similar to that observed for a high molecular weight, straight-chain alkane fuel. For comparison, oxidation experiments using hexadecane, 1-decene and butylbenzene were performed under cool-flame and hot-ignition conditions. Analysis of the reaction products was carried out at various stages during the reaction using liquid and gas-chromatography.

The results obtained during the course of this work are presented in Section 3. The complex nature of diesel fuel was revealed by the gas-chromatographic analyses performed, the chromatograms obtained serving to ‘finger print’ the fuel used. Kinetic data for the oxidation of various results derived from the gas- and liquid-chromatographic analyses of the oxidation products. The liquid-chromatographic data are most important, since the results provide a quantitative measurement of odour intensity. The gas-chromatographic analyses were used to assess the efficiency of the 'odour-trapping’ procedure and to monitor any composition changes which occurred during the oxidation process. Identification of the individual products formed was precluded by the complexity of the chromatograms and the practical limitations of the system.

In Section 4, the analytical and kinetic results are rationalised on the basis of a chain-thermal oxidation mechanism which is well established for many hydrocarbon-oxidant systems. A description is given of the complex interactions between the oxidation processes involving the various types of chemical compound present in diesel fuel, represented by pure alkanes, alkenes and aromatic compounds. The kinetic data recorded, indicated that fuel combustion is initiated in the aliphatic fraction of the fuel, while the analytical results showed that for all of the fuels tested, odorants were formed in the highest concentration under cool-flame conditions. The wide range of products formed is explained in terms of a mechanism involving alkylperoxy radical isomerisation and decomposition. Aromatic molecules present in the fuel have a high resistance to oxidation and therefore contribute extensively to the partially oxidised aromatic fraction of the exhaust. However, it has been shown that aromatic molecules can be formed from straight chain aliphatic molecules, e.g.. hexadecane, provided that the carbon chains are fairly long. During hot ignition, the odorants produced during the first-stage reactions are almost completely destroyed, The heterogeneous distribution of the fuel-oxidant mixture in a diesel results in stoichiometries existing in some regions of the combustion chamber which are too lean or too rich to support combustion, while other regions act as cool-flame quench zones. The formation of partial oxidation products, and therefore odorants, in these regions is expected to be very high. The results obtained have been incorporated into a model of diesel fuel spray combustion, which affords a qualitative description of the odorant formation process.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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