Fig. 85.—Anzani Testing His Five-Cylinder Air Cooled Aviation Motor Installed in Bleriot Monoplane. Note Exposure of Flanged Cylinders to Propeller Slip Stream.
An air-cooled engine must be placed in the fuselage, as at [Fig. 85], in such a way that there will be a positive circulation of air around it all the time that it is in operation. The air current may be produced by the tractor screw at the front end of the motor, or by a suction or blower fan attached to the crank-shaft as in the Renault engine or by rotating the cylinders as in the Le Rhone and Gnome motors. Greater care is required in lubrication of the air-cooled cylinders and only the best quality of oil should be used to insure satisfactory oiling.
The combustion chambers must be proportioned so that distribution of metal is as uniform as possible in order to prevent uneven expansion during increase in temperature and uneven contraction when the cylinder is cooled. It is essential that the inside walls of the combustion chamber be as smooth as possible because any sharp angle or projection may absorb sufficient heat to remain incandescent and cause trouble by igniting the mixture before the proper time. The best grades of cast iron or steel should be used in the cylinder and piston and the machine work must be done very accurately so the piston will operate with minimum friction in the cylinder. The cylinder bore should not exceed 41⁄2 or 5 inches and the compression pressure should never exceed 75 pounds absolute, or about five atmospheres, or serious overheating will result.
As an example of the care taken in disposing of the exhaust gases in order to obtain practical air-cooling, some cylinders are provided with a series of auxiliary exhaust ports uncovered by the piston when it reaches the end of its power stroke. The auxiliary exhaust ports open just as soon as the full force of the explosion has been spent and a portion of the flaming gases is discharged through the ports in the bottom of the cylinder. Less of the exhaust gases remains to be discharged through the regular exhaust member in the cylinder-head and this will not heat the walls of the cylinder nearly as much as the larger quantity of hot gas would. That the auxiliary exhaust port is of considerable value is conceded by many designers of fixed and fan-shaped air-cooled motors for airplanes.
Among the advantages stated for direct air cooling, the greatest is the elimination of cooling water and its cooling auxiliaries, which is a factor of some moment, as it permits considerable reduction in horse-power-weight ratio of the engine, something very much to be desired. In the temperate zone, where the majority of airplanes are used, the weather conditions change in a very few months from the warm summer to the extreme cold winter, and when water-cooled systems are employed it is necessary to add some chemical substance to the water to prevent it from freezing. The substances commonly employed are glycerine, wood alcohol, or a saturated solution of calcium chloride. Alcohol has the disadvantage in that it vaporizes readily and must be often renewed. Glycerine affects the rubber hose, while the calcium chloride solution crystallizes and deposits salt in the radiator and water pipes.
One of the disadvantages of an air-cooling method, as stated by those who do not favor this system, is that engines cooled by air cannot be operated for extended periods under constant load or at very high speed without heating up to such a point that premature ignition of the charge may result. The water-cooling systems, at the other hand, maintain the temperature of the engine more nearly constant than is possible with an air-cooled motor, and an engine cooled by water can be operated under conditions of inferior lubrication or poor mixture adjustment that would seriously interfere with proper and efficient cooling by air.
Air-cooled motors, as a rule, use less fuel than water-cooled engines, because the higher temperature of the cylinder does not permit of a full charge of gas being inspired on the intake stroke. As special care is needed in operating an air-cooled engine to obtain satisfactory results and because of the greater difficulty which obtains in providing proper lubrication and fuel mixtures which will not produce undue heating, the air-cooled system has but few adherents at the present time, and practically all airplanes, with but very few exceptions, are provided with water-cooled power plants. Those fitted with air-cooled engines are usually short-flight types where maximum lightness is desired in order to obtain high speed and quick climb. The water-cooled engines are best suited for airplanes intended for long flights. The Gnome, Le Rhone and Clerget engines are thoroughly practical and have been widely used in France and England. These are rotary radial cylinder types. The Anzani is a fixed cylinder engine used on training machines, while the Renault is a V-type engine made in eight- and twelve-cylinder V forms that has been used on reconnaissance and bombing airplanes with success. These types will be fully considered in proper sequence.