The problem of deposits in automotive induction systems arising from gasoline, while not new, has received renewed emphasis in recent years. This is because the demands for greater efficiency and output that have been made on automotive engines have resulted in more rigid requirements being imposed on the gasoline used. This problem was studied in a simple laboratory apparatus which gave results correlative with field experience. Our results showed that the formation of such deposits was due mainly to the thermal condensation or polymerization of oxygenated precursors formed during previous storage. This was demonstrated by the fact that tests with air and nitrogen atmospheres gave almost identical deposits, both in oxygen content and weight.
As the thermal environment becomes more severe, the amount of insoluble deposit generally increases, but to a varying degree with different gasolines. This is dependent to some extent upon the conversion, by heating, of some of the gasoline-soluble portion of the deposits to material insoluble in gasoline. Since prior oxidation is involved, the amount and type of storage inhibitor used exerts a considerable effect on the amount and type of deposits formed. Thus the use of a trialkylphenol rather than the phenylenediamine type antioxidant reduces the deposit tendency by about 40 per cent.
Some data suggest that vapor-phase oxidation does occur to a slight extent in the manifold. However, the main reactions leading to increased deposition are those which occur in the liquid film in the manifold consequent to prior oxidation of the fuel.