There are four main reasons why the Packard diesel was not successful. First the Packard Motor Car Company put the engine into production a brief three years after it was created. The only successful airplane diesel, the German Junkers “Jumo,” was in development more than three times as long (1912-1929). The following tests indicate that the Packard diesel was not ready for production, and hence was unreliable.
Packard Motor Car Company 50-Hour Test (Feb. 15-18, 1930): This test was identical to the standard Army 50-hour test which was used for the granting of the Approved Type Certificate. The engine tested was numbered 100, and was the first to be made with production tools (approximately half a dozen engines had been handmade previously). It had to be stopped three times, twice due to failure of the fuel pump plunger springs and once due to the loosening of the oil connection ring. These failures were attributed to manufacturing discrepancies. In addition, 4 out of a total of 103 valve springs broke.[29]
U.S. Navy 50-Hour Test (Jan. 22, 1931, to March 15, 1931): The engine used in the Navy test was numbered 120. (Apparently only 20 production engines had been built during the preceding 12 months; Dorner in a letter of March 3, 1962, states that the total number of Packard diesels produced was approximately 25.) The engine had to be stopped three times, twice due to valve-spring collar failures and once due to a valve head breaking. Because of these failures this test was not completed. The following significant quotations have been extracted from the test: “The engine is not recommended for service use.... Flight tests, until the durability of the engine is improved, be limited to a determination of the critical engine speeds, and to short hops in seaplanes.... It is believed that this size engine should be made suitable for service use before this type in a larger class is attempted.” This latter statement probably refers to the 400-hp model.
A year had passed between the making of engine 100 and 120, yet the reliability had not improved. Although unreliability was the immediate cause of failure, there were two design defects which would have doomed the engine even if it had been reliable. All the Packard diesels were of the 4-stroke cycle unblown type, yet the most successful airplane diesels were of the 2-stroke cycle blown type.[30] The advantages of the latter type for aeronautical use are that it is of a more compact engine, of lower weight and greater efficiency.[31] The engine was therefore built around the wrong cycle.
The Packard diesel of 1928 was designed to compete with the Wright J-5 “Whirlwind” which powered Lindbergh’s “Spirit of St. Louis” in 1927.[32] The specifications were within two percent of each other. The diesel engine’s fuel consumption was far less although its price was considerably higher.
| Packard Diesel DR-980 | Wright J-5 “Whirlwind” | |||
| Diameter (in.) | 4511⁄16 | 45 | ||
| Horsepower | 225 | 225 | ||
| Weight (lb) | 510 | 510 | ||
| Weight-horsepower ratio | 2.26 | 2.26 | ||
| Fuel consumption (lb per hp/hr at cruising). | 0.40 | 0.60 | ||
| Cost | $4025 | $3000 |
The advantages of lower fuel cost and greater cruising range offered by the diesel engine would be relatively unimportant to a private pilot flying for pleasure, but would be vital to the commercial operator using airplanes powered by engines having several times the horsepower of the Packard diesel. Its size, moreover, was too small for the technology of fuel injectors.[33] The Packard Company realized that the production engine was too small.[34] In 1930 a 400-hp version was built but was not put into production, probably because of the unreliability of the 225-hp model.
The fourth principal reason why the engine failed is explained by the following quotation from The Propulsion of Aircraft, by M. J. B. Davy (published in 1936 by His Majesty’s Stationery Office, London):
Although the development and adoption for transport purposes of the relatively high-speed compression ignition engine has been rapid during the last few years, there has been no corresponding advance in its adoption for aircraft propulsion. A reason for this is the recent great advance in “take-off” power in the petrol (gasoline) engine due to the introduction of 87 octane fuel (which permits higher compression ratios) and the strong probability of 100 octane fuels in the near future, still further increasing this power. The need for increased take-off power results from the higher wing loading necessitated by the modern demand for commercial aircraft with higher cruising speeds with reasonable power expenditure.
Production of the Packard diesel ceased in 1933. During that same year the Pratt & Whitney Aircraft Company and the Wright Aeronautical Corporation specified 87-octane fuel for certain of their engines. Less than 10 years later octane ratings had increased to over 100, putting the diesel at a further disadvantage.[35]