Continued operation of internal combustion engines results in the formation of a thin deposit of carbonaceous material and inorganic lead compounds over most of the exposed surfaces of the combustion chamber. These deposits increase to an equilibrium thickness and cause the octane requirement of the engine to increase 10 to 15 units. Full scale engine studies showed that small amounts of borates, borines, ethyl sulfate, and silicone added to leaded fuels largely repressed this octane requirement increase. These observations are correlated with laboratory studies which showed a difference in the catalytic effect between the oxides of boron and silicon and lead sulfate, and the components of the combustion zone deposits on the oxidation of ra-heptane.The boron compounds and silicone do not provide a complete answer to the problem of engine deposit harm, as they were found to be ineffective in engines already containing deposits. However, the ability of these compounds to affect the nature of these deposits offers encouragement in solving the problem in the future and lends support to a catalytic rather than thermal conductivity theory as an explanation of the cause of octane requirement increase.
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