Let's go back to the middle of the first decade in the twentieth century.  In Western Europe, the quest to fly is enticing men to disasters as a flame draws moths to a fiery rendezvous.  Accomplishing the elusive task of flight is so close.  The Wright brothers did it in 1903 in the United States, followed by Glenn Curtiss and the AEA (Aerial Experiment Association) in 1908.  Following Curtiss, the Wright brothers show the rest of the world how to fly at the Rheims, France air meet of 1908.  One thing in particular has been a constant hindrance to progress, the lack of lightweight, high powered, and reliable engines.  On July 25, 1909 Louis Blériot barely makes it from France to England with his 24 hp., air-cooled Anzani powered Blériot due to pending heat seizure. 

In early 1909, there are both water-cooled and air-cooled engines to select from.  Air-cooled units are notoriously low powered and often suffer from overheating.  Automotive, water-cooled engines are typically heavy and require radiators and piping.  A 30 hp. auto engine weighs about 200 pounds.  The typical early Euro-planes are frail, and too light for such heavy engines.  It is no little wonder, when the Seguin brothers of Paris, France introduce their revolutionary "rotary" engine at the 1909 Rheims air meet, it will accelerate Europe's aviation evolution. 

Just what is a "rotary" engine?  This class of engine, unlike rotary engines in late twentieth century automobiles, is unique in its design and operation.  It is a gasoline, four stroke (four cycle) engine of the radial design.  In 1908, seven cylinders arranged radially around the block is a new concept.  But, what sheer madness it must have seemed, to see that engine spinning around with the propeller at 1,200 rpm on a stationary crankshaft. The prop is fitted to the front crankcase cover and rotates in unison with the engine.  This 168 pound air-cooled engine produces 50 hp. at 1200 rpm with the reliability of an automobile engine.  It is exactly what the European aviators have been awaiting.
 
 

Some time before 1908, Louis and Laurent Seguin of France, decided to undertake the task of manufacturing an engine to fill the needs of the French aviators.  The design they used originated in the United States around 1906 where it apparently was ignored for unknown reasons.  Perhaps the designer (currently unknown) suffered neglect from the American aviation industry, and thus decided to sell his design to the Seguins.  Coming from a family with a long history of building locomotives and heavy machinery, these brothers had the knowledge, capital, and facilities to successfully produce rotary engines.  The first engines are produced in 1908.

The Seguins name their engine the, "Gnome," after the rugged little workers of French folklore.  The unusual snarl of its exhaust commands center stage in the skies of the 1909 Rheims air meet, its official introduction to the public.  It soon after becomes a familiar sound in the air over Europe as the Gnome leads the sales of aviation engines.  Now, the European aero designers and manufacturers can truly reach for the sky.  In 1912, a Deperdussin aeroplane exceeds the 100 mph. mark with a rotary engine.  In 1913 another flown by Prévost exceeds 126 mph.  Pilots race from Paris to Cairo, Rome, Madrid and other distant destinations. It is indeed an exciting period for aviation.

Soon competition comes from others who rush to produce rotaries; LeRhone, Clerget, and Oberursel (German).  With the coming of W.W.I, the French makes are also manufactured under license in England.  The Gnome is manufactured under license in the United States as well.  Because rotary engines are light, powerful, reliable and available from many sources, they become the number one choice by most fighter groups at the beginning of W.W.I.  In view of this popularity, it is necessary to mention that some very formidable fighters, such as the Fokker D-VII, Albatros D.Va, British SE-5,  and British/American DH-4, were powered by conventional water-cooled engines.  Most of these entered service in 1917 and 1918 as aviation designs and engine technology advanced.

Gasoline and part of the air for combustion is supplied to the engine through the hollow crankshaft.  Gnome engines use a spray nozzle to atomize the fuel instead of a carburetor.  Then the air and fuel enters the crankcase of the engine where it is thoroughly mixed and blended with the lubricating oil.  Castor oil is the preferred lubrication since Europe's crude oil of this period is of poor quality.  A unique feature about Castor oil is it doesn't degrade when mixed with gasoline.  The elementary design of the fuel system made it impossible to control engine speed.  Once started the Gnome will run wide open at 1,200 rpm.  Slowing the engine speed is accomplished by cycling the ignition on and off by means of a, "Blip switch," attached to the control stick.  Whenever the blip switch is depressed, shorting the magneto (stopping ignition), the unburned fuel and oil is expelled from the exhaust valves.  On front engine planes, the wind stream catches these and carries them back to the pilot.  Pilots who flew rotary powered aircraft typically wore a long scarf for wiping the oil off their goggles.  A shroud around the engine helps minimize the problem by deflecting most of the exhaust towards the bottom of the fuselage.  It is said that a pilot who spends many hours breathing unburned Castor oil never suffers from constipation.

FIRST GENERATION . . .
The earliest Gnomes have seven cylinders producing 50 - 70 hp..  These engines use an outboard support made of steel.  They have two valves, one intake and one exhaust.  The intake valve is a complicated, counter-weighted assembly built into the head of the piston, allowing the air/fuel mixture to enter the cylinder like a two stroke engine, via the crankcase and then through the piston.  The exhaust valve is mounted in the top of the cylinder and operated by an internal cam with external push rod and rocker.  Later models of 80 to 160 hp. engines have nine cylinders.

SECOND GENERATION . . .
In 1913 The Gnome "Monosoupape," started production.  The French word means, "Single valve."  The piston valve (intake) of the original design was discarded.  Fuel intake into the combustion chamber, now takes place through a series of ports at the base of the cylinder sleeves. The pistons, now with solid domes, allow the fuel mixture in by uncovering these ports near the bottom of its intake stroke.  Valve timing likewise is not conventional due to this mixed breed aspirating system.  Remember, the Gnome is a four stroke engine.  The exhaust valve remains open during a major portion of the intake stroke in order to prevent an unacceptably high vacuum from occurring and severely hindering the efficiency of the engine.  The resulting charge of fresh air helps control cylinder temperature but requires the amount of fuel/air mixture admitted through the intake ports to be rich enough to support combustion.  Conversely, during the combustion stroke, the exhaust valve is required to open prematurely before the bottom of the power stroke.  If it didn't, the nearly spent but still expanding gases of combustion would blow back into the crankcase as the intake ports are uncovered by the piston.

THE CAMSHAFT . . .
This complicated valve timing is regulated by a three lobe camshaft.  Each lobe, by means of cam followers, operates the exhaust valves of three cylinders, each 120 degrees apart.  The lobes are mounted one behind the other on a hollow shaft with ring gear.  The crankshaft only supports the camshaft.  The engine rotates twice as fast as the cam and in the same direction.  The camshaft ring gear is driven by three planetary gears on the inside of the front crankcase cover.  Those three gears, because they move in a circle with the engine, are spun into motion by a stationary gear on the crank.