Part 3 (1/2)

[Ill.u.s.tration: Figure 34.--Modified pistons after endurance run. U.S.

Navy test, 1931. (Smithsonian photo A48325D.)]

Combustion Chamber: In 1931 the contour of the cylinder head was changed slightly. This improved the combustion efficiency to the extent that the stroke of the fuel pumps could be decreased about 15 percent. The specific fuel consumption then decreased about 10 percent. In addition the compression ratio was reduced from 16:1 to 14:1.[20]

These changes were designed to eliminate smoke from the exhaust at cruising speed, and to reduce it at wide-open throttle.

Valves: A two-valve-per-cylinder model was built, but not put into production. It featured more horsepower (300), a higher rate of revolutions per minute (2000), and a better specific fuel consumption (about .35 lb/hp/hr).[21]

Capt. Woolson designed the production model with a single large valve for each cylinder. This was done in order to shorten the development period, for it is easier to design a single valve which serves both the intake and exhaust functions than one valve for each function. Not only are there fewer parts, but more important, there are no heat-dissipating problems. Although the single valve is heated when it releases the exhaust gases, it is immediately cooled by the incoming air of the next cycle. This cooling advantage is not shared by a valve which only pa.s.ses exhaust gases.[22]

Cylinder Head: Ribs were added to increase its rigidity (compare fig. 32 with fig. 33).

Engine Size: A 400-hp model was developed in 1930. It was not put into production.[23]

Comments

Comments of Aeronautical Engineers: These comments appeared in _Aviation_ for February 15, 1930, just a month before the Packard diesel received its approved-type certificate. They were in answer to the question, ”What is your opinion of the probable early future of the compression ignition type of engine in aircraft powerplants?” Most of the engineers were enthusiastic about the diesel engine's future in aviation; however, neither George J. Mead nor C. Fayette Taylor shared their colleagues' opinions. Mead's prophesy was accurate except for his discounting the diesel's role in lighter-than-air craft. Taylor was correct in implying that there was a future for the diesel in powering airs.h.i.+ps.

George J. Mead (vice president and technical director, Pratt & Whitney Aircraft Company):

Compared with the present Otto cycle engine, the Diesel powerplant weight, including fuel for a long-distance flight, would apparently be less. It is doubtful whether there would be any saving if the orthodox engine were operated on a more suitable fuel. Inherently the Diesel engine must stand higher pressures and therefore is heavier per horsepower. A partial solution of this difficulty is the two-cycle operation, which seems almost a requirement if the Diesel cycle is to be considered at all for aircraft. For any normal commercial operation in the United States there seems to be little or no improvement to be had from the Diesel. After all, it is not entirely a question of fuel cost but payloads carried for a given horsepower. It seemed at one time as though the Diesel was particularly desirable for Zeppelin work. Now that blau gas has been introduced, which obviates the need of valving precious lifting gas, the Diesel cycle seems much less interesting for this purpose. There may be a reduction in fire hazard and radio interference with the Diesel cycle, but it is doubtful whether it will be used in view of these considerations alone.

C. Fayette Taylor (professor of aeronautical engineering, Ma.s.sachusetts Inst.i.tute of Technology): ”I believe that the compression ignition engine will continue to remain in the experimental stage during the year 1930. I should expect its first really practical installation to be in lighter-than-air craft.”

P. B. Taylor (acting chief engineer, Wright Aeronautical Corporation): ”I believe the compression ignition engine is probably the type which will eventually supersede the present electric ignition units. This development will come slowly and will not be a solid injection engine.”

Henry M. Mullinnix (former chief of powerplant section, Navy Bureau of Aeronautics):

The advantages of compression-ignition, including reduced fire hazard, more efficient cycle, elimination of electrical apparatus and hence of radio interference, elimination of carburetion problems, and other benefits less evident, would seem to outweigh the difficulties encountered in metering and injecting minute quant.i.ties of fuel at the proper instant. Although the Diesel engine suffers upon comparison with the Otto cycle engine in flexibility there seems to be a definite field for employment of Diesels and a gradual extension of their use may be predicted.

John H. Geisse (chief engineer, Comet Engine Corporation): ”I am firmly convinced that the Diesel engine in the future will not only maintain the advantages of Diesel engines as they are now known, but will also be lighter in pounds per horsepower than the present Otto engines.”

Lt. Cdr. C. G. McCord (U.S. Navy, Naval Aircraft Factory): ”The use of compression ignition in due time appears to be a.s.sured; but increase in weights above those of present Otto cycle engines, to insure reliability, must be expected.”

L. M. Woolson (aeronautical engineer, Packard Motor Car Company): ”There is no question that the compression ignition aircraft engine will in time offer severe compet.i.tion to the gasoline engine. There are, however, many basic problems to be solved for the solution of which there exists no precedent.”

N. N. Tilley (chief engineer, Kinner Airplane and Motor Corp.):

Considerable development of the compression ignition type of engine for aircraft will be required before it is commonly available. It is believed that the weight per horsepower must be equal to, or less than, that of the present type of engines, in order to interest the public, since rapid take-off, rate of climb, and speed are desired, rather than low fuel consumption or high mileage. Most flights are of few hours duration. It is believed that flights must be of over five or six hours duration in order to show any advantage of Diesel engines (with low fuel consumption) if appreciably heavier than present engines. Also the difference between Otto cycle and Diesel becomes slight as the compression ratios come closer together.

Comments of Flight Crews: The preceding comments were made by engineers thinking primarily of the commercial possibilities of the diesel.

Following are comments by flight crewmembers about the operating characteristics of the Packard diesel. The former were largely optimistic. Most of them were only familiar with the aeronautical diesel as a design project and therefore did not have the practical experience necessary to understand all of its limitations. The latter were pessimistic, as they knew firsthand various shortcomings of the engine which only became apparent when it was operated.

Clarence D. Chamberlin, pioneer pilot: