AEROPLANE ENGINES. There are three prime requisites for a good aeroplane en gine — reliability, small weight per horse-power and low fuel and oil consumption. Since these requirements are more or less conflicting, a i compromise is necessary and the designer is confronted by the delicatequestion: Which of these is the most important? German design ers, with their customary thoroughness, have produced more reliable, but, generally speak ing, heavier engines than those of England and France. For military work this has proved a weak point, because the lighter, and hence faster, aeroplanes can easily secure positions of advantage over their enemies, which usually makes it possible to bring them down. For non-military service, however, except racing, reliability is unquestionably the fundamental requirement. The balance between weight and fuel economy will be fixed by the particular service required, for the matter of importance is not the engine weight 'only, but the total weight of the engine and fuel. This is easily appreciated when one realizes that the fuel required for a flight of six or seven hours weighs as much as the engine itself. For short flights during which but little fuel is used, an exceptionally light engine is desirable. for the poor economy usually characteristic of such an engine is not very important. How ever as the length of flight increases, economy becomes more and more important. Other requisites which must not be overlooked. though they are by no means as important as those just mentioned, are compactness, ac cessibility, freedom from vibration, flexibility, silence and reasonable cost. Little need be said in regard to these as they are almost self evident to any engineer.
The reliability of an engine is the final proof of the skill of its designer in mastering every little detail. This is particularly true of an aeroplane engine because its service is so much more severe than that of an automobile engine that any weakness will show itself much sooner. Since weight is of such great import ance, every member of an engine will probably be designed so as to be stressed to the maxi mum safe limit. For this reason any cast metal should be avoided as far as possible be cause of its uncertain strength. In nine en gines a certain cast member will be strong enough, yet in the tenth some imperfection in the casting may cause a failure at a critical moment. Forged or rolled steel is one of the most dependable materials known to-day and as it is also the lightest for a given strength, it is ideal for aeroplane engines. Consequently
it is standard practice in a large percentage of the successful aeroplane power plants to-day. Since fhe greatest strength of steel is utilized when the stress acting upon it is one of tension it is advisable to design the engine so that parts which sustain the greatest strains shall meet with purely tensional stresses as far as possible. Lubrication is another factor on which the re liability of an engine is dependent and is re ceiving more and more attention from design ers. It is probably the one point on which the success or failure of a heavily-worked engine hinges more than anything else. One of the improvements in lubricating systems which is gaining favor at present is that of cooling the oil after it has been used once and before using it a second time. The reason for this is that oil apparently deteriorates upon pro longed exposure to heat, causing a falling off of the power of an engine because of inferior Vol. i — 7 lubrication. An aeroplane engine must, of course, be as economical as possible of both fuel and oil. Fuel economy implies a high compression, since efficiency rises with the compression ratio; and careful workmanship is an essential, so as to eliminate any unneces sary friction.
For any particular type of engine there are three places to look for a possibility of im proved efficiency—the carburetor, gaspas sages and ignition system. These are facts well known to automobile engineers and have been the cause of much research. For very high engine speeds it has been proved that the number and location of the spark plugs has an appreciable effect upon efficiency. When two point ignition is used, failure of one of the plugs must be guarded against because it will not cause missing and may not be detected. Since two-point ignition requires a smaller angle of advance than the single point, the failure of one plug will cause late ignition in its cylinder with its resultant heating and lowered efficiency.
At present the magneto reigns supreme for the ignition of aeroplane engines, but it may be replaced by some battery systems because of the increasing necessity for an electric gen erator for wireless outfits, searchlights, stabil izers, etc.
It almost goes without saying that any un necessary weight on an aeroplane engine must be eliminated and the greater use of aluminum and the higher grades of steel will play an im portant part in having decreased weight go hand in hand with all round efficiency.