REACTIONS of unsaturated fuel constituents with oxides of nitrogen, formed during combustion, play an important part in formation of engine deposits. Engine varnish, the organic binder in engine deposits, results in large part from reactions of nitrogen dioxide with gasoline constituents.
Simplified kinetic studies indicate that nitrogen fixation and amounts of nitric oxide present in exhaust gases could be predicted. Tests have demonstrated that only under conditions leading to appreciable nitrogen fixation does heavy engine varnishing occur.
Because commercial engine oils are fairly resistant to oxidation, it is likely that current deposit problems result from the nature of fuel and prevailing operating conditions. Under average driving, low-temperature operation, lean mixtures with consequent high nitrogen oxide content contribute much to varnish formation.