A compression of 115 pounds per square inch (commonly used) is difficult to handle with a light construction, but this pressure must be obtained if the output is to be kept within practical limits. Engines having a compression ratio of as high as 6 are running satisfactorily at sea level, this ratio giving a mean effective working pressure of 134 pounds per square inch. With this ratio the engine cannot be used with full open throttle at sea level for more than 10 or 15 minutes without causing damage to the shaft, bearing and valves. At about 10,000 feet the compression is normal.
At great altitudes carburetion has become a great problem, and as aerial battles have already taken place at elevations of 20,000 feet, it is quite possible that future motors will be equipped with some device that will force a measured fuel charge into the cylinders. The air necessary for the combustion will also have to be pumped in by some means.
Weight Per Horsepower. The weight per horsepower of the engine is a very loose term since so much depends upon the equipment included in the weight. As many as 20 items may be considered as being in the doubtful list, and among these are the radiator water, piping, mounting, propeller hub, oil in sump, wiring, self-starter, etc. The only true unit weight is that obtained by taking the plant complete (ready to run), with the cooling system, gasoline for an hour's flight, and the oil. The weight of the bare engine signifies nothing. The weights of the various items used on well known motors are given in a table under the chapter "Weight Calculations." While the bare weight of a certain engine may be very low per brake horsepower, an excessive fuel consumption will often run the effective weight up and over that of a type in which the bare weight is far greater. The weight of the engine per horsepower, including the magneto and carbureter, will run from 2.2 to 5.0 pounds, according to the type.
Two Examples of Cowls Used Over Rotary Cylinder Motors (Air Cooled).
Fuel Consumption. The fuel consumption of water-cooled motors varies from 0.48 to 0.65 pounds per horsepower hour, an average of 0.6 being safe. The fuel consumption of a rotary air-cooled motor will range from 0.6 to 0.75. The oil consumption varies from 0.18 gallons per horsepower in the air-cooled type to 0.035 with the water-cooled stationary motor.
Radiators. Owing to extremes in the temperature of the air at different altitudes, the radiating surface should be divided into sections so that a constant cooling effect can be obtained by varying the effective surface of the radiator. The temperature can also be controlled by an automatically regulated by-pass which short circuits a part of the radiator water at low temperatures. Constant water temperature has much to do with the efficiency and general operation of the motor, and there will be only one temperature at which the best results can be obtained.
Typical Radiators. A) Side or Top Type.
