Keeping Oil Out of Combustion Chambers
An examination of the engine design that is economical in oil consumption discloses the use of tight piston rings, large centrifugal rings on the crank-shaft where it passes through the case, ample cooling fins in the pistons, vents between the crank-case chamber and the valve enclosures, etc. Briefly put, cooling of the oil in this engine has been properly cared for and leakage reduced to a minimum. To be specific regarding details of design: Oil surplus can be kept out of the explosion chambers by leaving the lower edge of the piston skirt sharp and by the use of a shallow groove (C), Fig. 126, just below the lower piston ring. Small holes are bored through the piston walls at the base of this groove and communicate with the crank-case. The similarity of the sharp edges of piston skirt (D) and piston ring to a carpenter’s plane bit, makes their operation plain.
The cooling of oil in the sump (A) can be accomplished most effectively by radiating fins on its outer surface. The lower crank-case should be fully exposed to the outer air. A settling basin for sediment (B) should be provided having a cubic content not less than one-tenth of the total oil capacity as outlined at Fig. 126. The depth of this basin should be at least 21⁄2 inches, and its walls vertical, as shown, to reduce the mixing of sediment with the oil in circulation. The inlet opening to the oil pump should be near the top of the sediment basin in order to prevent the entrance into the pump with the oil of any solid matter or water condensed from the products of combustion. This sediment basin should be drained after every five to seven hours air service of an airplane engine. Concerning filtering screens there is little to be said, save that their areas should be ample and the mesh coarse enough (one-sixteenth of an inch) to offer no serious resistance to the free flow of cold or heavy oil through them; otherwise the oil in the crank-case may build up above them to an undesirable level. The necessary frequency of draining and flushing out the oil sump differs greatly with the age (condition) of the engine and the suitability of the oil used. In broad terms, the oil sump of a new engine should be thoroughly drained and flushed with kerosene at the end of the first 200 miles, next at the end of 500 miles and thereafter every 1,000 miles. While these instructions apply specifically to automobile motors, it is very good practice to change the oil in airplane engines frequently. In many cases, the best results have been secured when the oil supply is completely replenished every five hours that the engine is in operation.