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The general method of procedure taken by the Air Service before beginning the actual design and construction of the necessary types of aircraft engine is outlined and the four steps of the development subsequent to a very complete study of existing domestic and foreign engines are stated. After checking over the layouts, if all the details are agreed upon by both the designer and the Engineering Division, the contract is placed, usually for two experimental engines, and the construction work is begun. Acceptance tests are made to demonstrate that the engine is capable of running at normal speed and firing on all cylinders. These are followed by the standard performance test made on the dynamometer at McCook Field. The results of the latter test determine whether the engine can enter the 50-hr. endurance test. The engine is then torn-down and inspected for wear. Suggested modifications are embodied in reconstructed engines which eventually fulfill the requirements.

Ignition is discussed in a broad and non-technical way. The definition of the word ignition should be broad enough to include the complete functioning of the ignition apparatus, beginning from the point where mechanical energy is absorbed to generate current and ending with the completion of the working stroke of the engine. The ignition system includes the mechanical drive to the magneto or generator and the task imposed on the system is by no means completed when a spark has passed over the gap of the spark-plug. Ignition means the complete burning of the charge of gas in the cylinder at top dead-center, at the time the working stroke of the piston commences. The means employed to accomplish this result is the ignition system. In the present-day type of gasoline engine a spark produced by high-voltage electricity is almost universally used for ignition. This high-voltage electricity is produced by a transformer.