Piston friction is much the largest item of mechanical loss in an engine, amounting to fully one-half the indicated horsepower at light loads. Although opinions differ as to the most desirable temperature of the jacket-water for full-load operation, no question has arisen as to that for part load. It should be as high as possible, in order that piston friction can be reduced by keeping down the viscosity of the oil on the bearing surfaces, and that complete vaporization of the fuel may be secured. By reducing the friction of the piston and improving the vaporization, steam-cooling increases economy, which, on a number of cars of different makes, has been found to average 20 per cent more miles per gallon.
Water is practically a non-conductor of heat. Boiling water, or a mixture of water and steam, is far more effective for cooling than is water that is not boiling. Contrary to the almost universal belief, the extreme turbulence of boiling water enables it to absorb heat from three to five times as quickly as when cold. The failure of a water cooling-system, when boiling, is not caused by the inability of the boiling water to carry off heat, but by the fact that when steam has been formed it has no place to go and forces water out through the overflow.
In the steam cooling-system, the cylinder-block is merely a steam boiler that does not require forced circulation, will not burn out, so long as feed-water is supplied to replace that lost by evaporation, and remains at constant temperature. As dry steam is not essential, no dome or separating chamber is necessary. A working model of a steam cooling-system made possible an observation by the author of what goes on inside the jacket and the radiator while the engine is running.
Tests showed that crankcase-oil dilution can be reduced to any amount thought desirable by heating the crankcase, practically no dilution occurring if the cylinder jacket is maintained at 212 deg. fahr. Freezing can be prevented by the addition of alcohol, the percentage of which remains constant all winter because it is condensed and returned in the same manner as is the water. Other advantages of steam-cooling are said to include the use of a heater, for warming the car body, and freedom from odor, noise and fire hazards.