MECHANICAL FRICTION AS AFFECTED BY THE LUBRICANT 240009
Very few data seem to be available on the frictional losses in automobile engines caused by the failure of the oil to perform its function as a lubricant. The researches of the Lubrication Inquiry Committee in England indicate that the friction of a flooded bearing is proportional to the speed of the engine, the area of the bearing and the viscosity of the lubricant and is independent of the pressure and of the materials of which the opposing surfaces are composed.
The principal sources of friction in an engine are the crankshaft, the camshaft and the connecting-rod bearings, which rotate; the pistons and the valves, which slide; and the auxiliaries, such as the generator, the pump and the distributor. Experiments by the author led to the conclusion that a variation in the viscosity of the lubricant can easily account for reduced mechanical efficiency and consequently for the increased consumption of gasoline, and that the friction-loss is proportional to the cubic capacity of the engine, whereas the brake mean effective pressure for the same horsepower is inversely proportional to the capacity. Tables are given showing the variation of the friction mean effective pressure at various speeds and at various temperatures of the water and the oil, the fuel consumption of the engine at part throttle and at varying oil and water temperatures and a method of calculating the reduction of the gasoline consumption, assuming a constant indicated thermal efficiency. The identity of the results, as determined from the friction curves and from actual tests, is said to suggest that the thermal efficiency is affected but little by the water-jacket temperatures and that the saving on account of the thermostats and the radiator shutters is caused almost entirely by the reduction in the viscosity of the oil. Because of thermostatic control the water temperature rises very rapidly to 120 deg. fahr. in short runs and to about 170 deg. fahr. after 15 or 20 min., but the oil temperature, even for periods of 30 min. of city driving, does not rise more than 40 deg. fahr. above atmospheric temperature, and after prolonged fast driving the rise amounts to about 90 deg. fahr. On the assumption that the temperature-difference between the oil and the atmosphere is independent of the atmospheric temperature, it is shown that to obtain a mean oil and water temperature of 170 deg. fahr., the condition of maximum efficiency, with water at 170 deg. fahr., the atmospheric temperature must be about 80 deg. fahr. if the car is driven at approximately 40 m.p.h., while for ordinary city driving the conditions of maximum efficiency are not realized unless the atmospheric temperature is about 130 deg. fahr. The suggested use of thin oil might not avail because in winter very severe conditions of load and speed are frequently encountered and a slight difference in the viscosity might be of great importance.
In making a study of methods of keeping dirt out of an automobile-engine crankcase, it was found that certain oils that worked well under normal conditions became too viscous in cold weather. This was remedied by making the troughlike conduit to the pump suction of wire gauze through which the oil when sufficiently hot drops into the main body of oil in the oil-pan. The property of oiliness increases in importance as the bearing surfaces more nearly touch each other, and especially when the engine is starting from rest.