One who is not thoroughly familiar with engine construction will seldom locate troubles by haphazard experimenting and it is only by a systematic search that the cause can be discovered and the defects eliminated. In this chapter the writer proposes to outline some of the most common power-plant troubles and to give sufficient advice to enable those who are not thoroughly informed to locate them by a logical process of elimination. The internal-combustion motor, which is the power plant of all gasoline automobiles as well as airplanes, is composed of a number of distinct groups, which in turn include distinct components. These various appliances are so closely related to each other that defective action of any one may interrupt the operation of the entire power plant. Some of the auxiliary groups are more necessary than others and the power plant will continue to operate for a time even after the failure of some important parts of some of the auxiliary groups. The gasoline engine in itself is a complete mechanism, but it is evident that it cannot deliver any power without some means of supplying gas to the cylinders and igniting the compressed gas charge after it has been compressed in the cylinders. From this it is patent that the ignition and carburetion systems are just as essential parts of the power plant as the piston, connecting rod, or cylinder of the motor. The failure of either the carburetor or igniting means to function properly will be immediately apparent by faulty action of the power plant.

To insure that the motor will continue to operate it is necessary to keep it from overheating by some form of cooling system and to supply oil to the moving parts to reduce friction. The cooling and lubrication groups are not so important as carburetion and ignition, as the engine would run for a limited period of time even should the cooling system fail or the oil supply cease. It would only be a few moments, however, before the engine would overheat if the cooling system was at fault, and the parts seize if the lubricating system should fail. Any derangement in the carburetor or ignition mechanism would manifest itself at once because the engine operation would be affected, but a defect in the cooling or oiling system would not be noticed so readily.

The careful aviator will always inspect the motor mechanism before starting on a trip of any consequence, and if inspection is carefully carried out and loose parts tightened it is seldom that irregular operation will be found due to actual breakage of any of the components of the mechanism. Deterioration due to natural causes matures slowly, and sufficient warning is always given when parts begin to wear so satisfactory repairs may be promptly made before serious derangement or failure is manifested.

A TYPICAL ENGINE STOPPAGE ANALYZED

Before describing the points that may fail in the various auxiliary systems it will be well to assume a typical case of engine failure and show the process of locating the trouble in a systematic manner by indicating the various steps which are in logical order and which could reasonably be followed. In any case of engine failure the ignition system, motor compression, and carburetor should be tested first. If the ignition system is functioning properly one should determine the amount of compression in all cylinders and if this is satisfactory the carbureting group should be tested. If the ignition system is working properly and there is a decided resistance in the cylinders when the propeller is turned, proving that there is good compression, one may suspect the carburetor.

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Fig. 160.—Side Elevation of Thomas 135 Horse-Power Airplane Engine, Giving Important Dimensions.

If the carburetor appears to be in good condition, the trouble may be caused by the ignition being out of time, which condition is possible when the magneto timing gear or coupling is attached to the armature shaft by a taper and nut retention instead of the more positive key or taper-pin fastening. It is possible that the inlet manifold may be broken or perforated, that the exhaust valve is stuck on its seat because of a broken or bent stem, broken or loose cam, or failure of the cam-shaft drive because the teeth are stripped from the engine shaft or cam-shaft gears; or because the key or other fastening on either gear has failed, allowing that member to turn independently of the shaft to which it normally is attached. The gasoline feed pipe may be clogged or broken, the fuel supply may be depleted, or the shut-off cock in the gasoline line may have jarred closed. The gasoline filter may be filled with dirt or water which prevents passage of the fuel.