ALTHOUGH the author and his associates have designed, built and tested a Diesel airplane-engine, a description of the mechanical details is omitted because the engine is still in the experimental stage. The general subject of Diesel engines for aircraft is therefore presented in its broader aspects.
Typical indicator-diagrams of a gasoline engine and of a Deisel engine are compared as a means of ascertaining whether the pessimistic attitude that the Diesel engine cannot be made light enough for aircraft-propulsion purposes is justified. These considerations lead to the statement that, since a practicable Diesel aircraft-engine must run at speeds five or six times as fast as the stationary or marine-type of Diesel powerplants, whereas the ignition time-lag is substantially the same, it can be seen that the high-speed engine demands a different type of combustion than does the low-speed Diesel.
Following considerations relating to large low-speed Diesel engines and the differences between low-speed and high-speed characteristics, the author discusses the very high cylinder-pressures that become necessary, which may be a maximum of 1200 lb. per sq. in. Such pressures may be thought to militate against sufficiently light engine-construction, but this fear is unfounded, he asserts, as is proved by the fact that the Packard Diesel aircraft-engine weighs less than 3 lb. per hp. and has been subjected to both flight testing and ground testing. Its capability of withstanding cylinder pressures well in excess of 1200 lb. per sq. in. has been accomplished without having recourse to excessively heavy construction.
Diesel aircraft-engine advantages are listed, an important one being the virtual elimination of the fire hazard; and the subject of economical fuel consumption is treated. Mention is made that the Diesel aircraft-engine interferes less with radio operation than does the Otto-cycle engine, because it has no electric-ignition system, and that the use of compression ignition increases safety.
In conclusion, the probable effects of the Diesel engine on airplane design are forecast, and the reliability of the Diesel engine is compared with that of the gasoline engine, to the advantage of the former.