Wednesday, August 14, 2013

Supercharger Development in the U.S. During the Inter-War Period Part 3 - Developments in the U.S.

Developments in the U.S.
Fig. 3.  Miller supercharger designed by Dr. Moss. An unusual feature is the small impeller installed in the intake nozzle. (Courtesy Miller/Offenhauser Society)
Dr. Sanford A. Moss (1872 – 1946) made the turbocharger practical, advanced the cause of gas turbines, and ended his long career by pressurizing civilian airliners. Earning a doctorate from Cornell in 1903, he was immediately hired by General Electric to head up their turbine research facility at West Lynn, Massachusetts, where he would remain until retirement in 1938. Some idea of his thinking comes about from his habit of asking prospective employees if, as a child, they had ever taken a clock apart to see how it works. For “A young fellow who never took a clock apart can never become a mechanical engineer.”

According to Dr. Moss, around 1900 the French engineer Auguste Rateau (1862-1930) was the first to compress air with a centrifugal pump. Prior to that time centrifugal pumps were limited to moving water and other incompressible fluids. Rateau made the discharge nozzle convergent so that the air stream accelerated upon discharge and would subsequently gain pressure through expansion.
As part of its work on gas turbines, General Electric made the same discovery in 1904 and almost immediately began manufacture centrifugal air and gas compressors. GE may have been the first to incorporate a vaned diffuser to assist in expansion. The first compressor was installed at the Lynn Works in 1906 and remained in service until at least 1940. GE went on to build a variety of single and multi-stage compressors with tip velocities of from 500 to 1,500 feet per second (fps) and rotational speeds of 3,600 to 20,000 rpm. Moss would apply what was learned at Lynn to superchargers.
The first geared centrifugal blower for aircraft built in the United States was intended for use on inline engines. In an attempt to mitigate the effects of torsional vibration, the blower took power off the crankshaft through a small-diameter flexible shaft. Ground tests on a Liberty engine were conducted between 1918 and 1920.
Moss, as representative of the leading U.S. manufacturer of centrifugal pumps, may have been personally involved in this project. While his work on turbochargers would earn him a Collier Trophy, Moss had a deep affection for gear-driven superchargers. In a 1941 Popular Science profile, the scientist was described “as a disappointed man.”[3] Too much publicity had been given to his work on turbos. The writer continues, obviously paraphrasing Moss: His more important contribution had been in the area of gear-driven superchargers “of value in improving power on the take-off, as well as its main purpose of maintaining power at altitude…” While the turbocharger gives better performance at high altitude, “few people wanted to fly above 20,000 ft…[which] was both uncomfortable and dangerous for the aviator. Use of the turbo-supercharger has been limited to a few experimental ships and the most advanced Army planes. And though the Air Corps engineers worked eagerly with Dr. Moss to develop the turbo-supercharger, it never seems to him that the tactical units made adequate use of its possibilities.”
All this was changing as the interview was published. Moss collaborated with the Duesenberg brothers on the first geared supercharger used on an American racing car. This machine won the Indianapolis race in 1924. The compressor mounted near the center of the straight-eight engine and took power through a shaft from the accessory drive section at the front of the block. The shaft acted as a torsion bar to cushion the drive gears. Harry Miller also took advantage of Dr. Moss’s services and had him design a two-stage centrifugal compressor for his 91 CID front-wheel-drive cars (Fig. 3).

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