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All the rotary engines mentioned in this paper operated on the four-stroke Otto cycle, viz, Induction, Compression, Ignition and Exhaust. However, the manner in which they precisely achieved the cycle of events varied considerably from engine to engine, particularly with regard to the induction stroke. The fact that the whole engine rotated had no relevance on the fundamental cycle of operation. The vast majority of the French and British rotaries were manufactured under wartime conditions, but despite this they were all extremely well made using first rate materials.

The rotary aero engine, initially in the form of the French manufactured Gnome of the early twentieth century, was a phenomenon which broke upon the embryonic aviation scene in 1908. Coming seemingly from nowhere, the rotary engine held uncontested sway for just about a decade and was then relegated, almost as quickly, to relative obscurity and obsolescence.

The idea of an internal combustion engine rotating about its own fixed crankshaft axis was not exactly new in 1908. At the Paris Universal Exposition of 1889, Felix-Theodore Millet (1844-1929) exhibited a patented five cylinder rotary engine built into a cycle wheel . Millet's rotary powered wheel was taken up commercially by the Darracq Company in 1900, and they enjoyed a limited amount of success manufacturing and marketing this invention. Although there are a few other possible claimants, it does appear that Felix Millet may genuinely have originated the rotary engine concept.

Early water cooled aero engines were developed from their automobile equivalents but they tended to be heavy and their power to weight ratio was relatively poor. Although light in weight, air cooled engines were unreliable and prone to seizing due to unequal thermal expansion in their cylinders. Into this arena came the 7 cylinder Gnome 50 h.p. Omega rotary engine, which was first exhibited at the Paris Automobile Show of 1908.

Societe Des Moteurs Gnome of 49 Rue Laffitte, Paris, was formed by Louis Seguin in 1906 to initially manufacture stationary industrial engines. When Laurent, the younger brother of Louis, joined the company they set out to design and manufacture a lightweight rotary aero engine. Undoubtedly the Seguin brothers had seen or heard about Millet's rotary engine and perhaps believing that cylinder cooling would be significantly improved by having the entire engine rotate, they embarked upon the design of the Gnome.

At about this time nickel-chrome alloy steels were becoming available, with tensile strengths of 50/60 tons/ (UTS). These are roughly equivalent to present day high tensile steels. The Seguin brothers took the chance to incorporate these alloy steels into their new engine. The cylinder wall thickness of all rotaries was very thin, in the region of only 1.5 mm (0.060”) in the Gnome, and nickel-chrome steels provided more than sufficient tensile strength to cope with the combined loads imposed by combustion pressures and centrifugal forces acting upon the rotating cylinders. The Gnome engine did not use shrunk-in cast iron cylinder liners, although most subsequent rotaries did, thus endowing them with improved working surfaces for their pistons.


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