THE present paper, which is an extension of previous work done at the Mellon Institute of Industrial Research in connection with carbon deposits from lubricating oils, deals with the results of tests made on heavy-duty engines in actual service. In making these tests 18 sleeve-valve-engine motorcoaches of the Pittsburgh Motor Coach Co. were used. These were divided into three groups of six vehicles each; the first group was lubricated with a paraffin-base oil of normally high carbon-residue value; the second, with a paraffin-base oil of extremely low carbon-residue value; and the third, with a typical naphthene oil. The fuel was an average grade of motor gasoline. In general, no deviation was made from the regular maintenance practice but all steps of maintenance and servicing were under the supervision of the authors.
Tentative conclusions drawn from the tests are that the carbon-residue test is a fairly reliable index of carbon deposition in a poppet-valve engine and can be used in connection with paraffin-base oils. Protection to the user from excessive carbon-deposits can be secured by placing reasonable limits on carbon-residue values in specifications for oils of the same viscosity class. In the case of a sleeve-valve engine, a paraffin oil of very low carbon-residue value was only slightly better than one having a normal value, and both were decidedly inferior to the naphthene oil. The addition of a provision for a vacuum distillation to the specification is recommended. This is stated tentatively by the authors as requiring a volatility of at least 90 per cent when distilled under a 10-mm. vacuum and the oil should boil between suitable limits of liquid temperature, those for the motorcoach engines used being approximately 550 and 700 deg. fahr.
