The loss of water by evaporization is from .3 to .6 of a gallon per horsepower hour; that is, for a 5 hp. engine the loss would be from 1.5 to 3 gals. for every hour that the engine was operated under full load.
The cylinder and the water jacket are cast in one integral piece, with no joints of any kind in either the combustion chamber or in the water jacket.
Fig. 124. Air Cooled “Grey Eagle” Aeronautical Motor. Note the Depth of Cooling Ribs.
A system of cooling by which the heat of the walls is radiated to the air directly without the medium of water is often used on small high speed engines, and is known as “AIR COOLING.”
This type of cylinder is surrounded with radiating ribs or spires which increases the radiating surface of the cylinder to the extent that the required amount of heat is lost to allow of economical lubrication. This system is desirable where the weight of radiators and water would be a drawback, where it would be inconvenient to obtain water, or where there would be trouble from freezing. An air cooled motor generally is provided with a fan that increases the efficiency of the radiating surface by changing the air between the ribs. With aeronautical motors such as the Gnome, and Gray Eagle, shown by Fig. 124, the circulation of the air due to the propeller and the rush of the aeroplane is sufficient to thoroughly cool the machine.
As a rule, the air cooled motor is made more efficient in fuel consumption than the water cooled type because of the high temperature of the cylinder walls. In fact all engines are air cooled eventually, whether the heat is radiated at a high temperature by the fires, or at a lower temperature through the circulating water and radiator.
When the engines are of the portable type, and likely to be used out of convenient reach of water, the hopper or EVAPORATOR TANK system is used, the tank system being used for the larger engines. In effect, the tank system is the same as the hopper cooler, the heat being dissipated principally by evaporation, although some heat is radiated from the surface of the tank itself. The difference between the two systems is merely one of size, the tank offering a greater area for the emission of heat than the hopper.
A tank-cooled engine has one pipe running from the top of the cylinder to a point near the top of the tank, the bottoms of the cylinder and tank being connected together by another pipe.
When the water becomes heated in the cylinder, it expands and becomes lighter than the cold water in the tank and consequently rises to the surface of the water in the tank through the upper pipe. As the warm water flows into the tank, it is immediately replaced by the heavier cold water that flows into the cylinder from the bottom of the tank through the lower pipe. This successive discharge of the heated water from the cylinder to the tank sets up a continuous flow of water through the water jacket of the cylinder, which transfers the excess heat of the cylinder to the tank where it is dissipated to the atmosphere by evaporation and radiation.