Fig. 79.—Pressure Feed Oil-Supply System of Airplane Power Plants has Many Good Features.
A simple system of this nature is shown graphically in a phantom view of the crank-case at [Fig. 79], in which the oil passages are made specially prominent. The oil is taken from a reservoir at the bottom of the engine base by the usual form of gear oil pump and is supplied to a main feed manifold which extends the length of the crank-case. Individual conductors lead to the five main bearings, which in turn supply the crank-pins by passages drilled through the crank-shaft web. In this power plant the connecting rods are hollow section bronze castings and the passage through the center of the connecting rod serves to convey the lubricant from the crank-pins to the wrist-pins. The cylinder walls are oiled by the spray of lubricant thrown off the revolving crank-shaft by centrifugal force. Oil projection by the dippers on the connecting rod ends from constant level troughs is unequal upon the cylinder walls of the two-cylinder blocks of an eight- or twelve-cylinder V engine. This gives rise, on one side of the engine, to under-lubrication, and, on the other side, to over-lubrication, as shown at [Fig. 80], A. This applies to all modifications of splash lubricating systems.
Fig. 80.—Why Pressure Feed System is Best for Eight-Cylinder Vee Airplane Engines.
When a force-feed lubricating system is used, the oil, escaping past the cheeks of both ends of the crank-pin bearings, is thrown off at a tangent to the crank-pin circle in all directions, supplying the cylinders on both sides with an equal quantity of oil, as at [Fig. 80], B.
WHY COOLING SYSTEMS ARE NECESSARY
The reader should understand from preceding chapters that the power of an internal-combustion motor is obtained by the rapid combustion and consequent expansion of some inflammable gas. The operation in brief is that when air or any other gas or vapor is heated, it will expand and that if this gas is confined in a space which will not permit expansion, pressure will be exerted against all sides of the containing chamber. The more a gas is heated, the more pressure it will exert upon the walls of the combustion chamber it confines. Pressure in a gas may be created by increasing its temperature and inversely heat may be created by pressure. When a gas is compressed its total volume is reduced and the temperature is augmented.
The efficiency of any form of heat engine is determined by the power obtained from a certain fuel consumption. A definite amount of energy will be liberated in the form of heat when a pound of any fuel is burned. The efficiency of any heat engine is proportional to the power developed from a definite quantity of fuel with the least loss of thermal units. If the greater proportion of the heat units derived by burning the explosive mixture could be utilized in doing useful work, the efficiency of the gasoline engine would be greater than that of any other form of energizing power. There is a great loss of heat from various causes, among which can be cited the reduction of pressure through cooling the motor and the loss of heat through the exhaust valves when the burned gases are expelled from the cylinder.