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Abstract

Measurements of the reduction in the octane number of leaded fuels by various sulphur compounds were made using a Cooperative Fuel Research Engine. The effects of seven different sulphur compounds were studied and three fuel blends were used, Attempts were then made to correlate the gaseous oxidation characteristics of these sulphur compounds under laboratory conditions with their observed effects on combustion in an engine. Studies were also made of the spontaneous ignition of isooctane-oxygen mixtures in uncoated and lead oxide-coated vessels and of the effects of small additions of three sulphur compounds, namely sulphur dioxide, thiophene and thiophenol, on the ignition limits. Similar measurements were made with a leaded isooctane blend in an uncoated vessel. Finally lead oxide-coated vessels were pre-treated with these three gascous sulphur compounds in turn and studies were then made of the ignition limits of iscoctane-oxygen mixtures in these vessels. Electron diffraction and other related techniques were used to examine the lead oxide coatings before and after treatment with the sulphur compounds.

The Introduction (Section 1) outlines the problems associated with the presence of sulphur compounds in motor gasolines. "Knock" in internal combustion engines and means of suppressing it are also reviewed. The relation of "knock" to the preflame combustion reacticns of hydrocarbons and the mode of action of lead-based additives in its suppress-
ion are also discussed.

The Experimental Section (Section 2) describes the Cooperative Fuel Research Engine and its use for the measurement of octane numbers. The vacuum apparatus constructed and used for laboratory studies of fuel combustion and the accessory equipment are also described. The procedure for coating the inner surface of the reaction vessels with lead oxide and the subsequent treatment of these vessels with sulphur compounds are outlined. A brief account is given too of the electron diffraction techniques used to study surface properties of the lead oxide coatings and the effects on them of exposure to gaseous sulphur compounds.

The Results Section (Section 3) gives detailed values of the increase in octane number caused by the addition to the fuel blends of tetraethyl and tetramethyl lead, and of the effects of 0.05 and 0.10 wt. % S when introduced in the form of various sulphur compounds, into the leaded fuels. Studies of the gaseous oxidation of four of the sulphur compounds, viz. thiophene, dipropyl sulphide, dimethyl disulphide and thiophenol, showed that the order of lead antagonism runs parallel with the rates of oxidation and the tendency to form sulphur dioxide. Lead oxide inhibited combustion of the hydrocarbon fuels and ignition studies showed that the sulphur compounds also inhibited isooctane combustion in an uncoated vessel, but promoted it in a lead oxide-coated vessel. Results obtained with leaded isooctane also showed the inhibiting effect of dissolved lead. After pre-treatment with sulphur compounds, the chain-terminating efficiency of the lead oxide appeared to be much reduced and was indeed even lower than that of an un-coatad vessel. The order of effectiveness of the sulphur compounds in reducing the inhibiting influence of the lead oxide coating was: sulphur dioxide < thiophene < thiophenol., Electron diffraction studies of the lead oxide before and after treatment with the gaseous sulphur compounds revealed changes in its surface properties, the magnitude of which varied with the nature of the sulphur compound in the above order,

Finally in the Discussion (Section 4), an attempt is made to correlate the engine and
laboratory results obtained in this work. It appears to be the chemical nature of the organic sulphur compounds and their initial decomposition and/or oxidation products rather than the sulphur dioxide formed from them, which affect the properties of the lead oxide
particles generated during the combustion of the leaded hydrocarbon fuels.

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

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