Carbon-Depositing Tendency of Heavier Motor-Oils 270008

THIS article reports a series of tests on the carbon-depositing tendencies of motor oils, supplementary to those reported at the 1926 Annual Meeting.4 The first investigation was concerned with conditions prevailing in passenger-car engines, the results leading to the formation by the authors of a theory explaining the relationship between certain characteristics of oils and their carbon-depositing tendencies. In the second series of tests the authors controlled the various factors to values commonly found in heavy-duty engines and in their findings confirmed the theory previously set forth.
A tentative explanation offered for certain facts revealed by the tests as to the carbon-depositing tendencies of motor-oils rests on the volatility, in the combustion space of the engine, of the oils used. An approximately straight-line relationship also was established between the amount of carbon deposited per liter of oil consumed and the Conradson carbon-residue test. A tentative explanation of this relationship is offered.
The authors conclude that, from the considerations presented, it appears desirable to use an oil viscous enough to supply the necessary lubrication, but volatile enough to vaporize after it has reached the piston and cylinder-head surfaces, leaving the minimum residue.


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