Comments:
A personal project, and to be honest a labour of love. I have had a fascination with
rotary aero engines as long as I can remember. My first sight was as a young lad
with my late father at the London Science Museum, and I was enthralled as he explained
how the entire engine rotated around the stationary crankshaft. They have held my
attention ever since, and I have even made a few examples.
History
Louis Verdet designed a small experimental rotary engine in 1910. Then, in
1912 he developed a larger 7-cylinder design, the 7C, which developed 70 hp with
an engine weight of 90 kg. This proved much more popular. Louis formed the Société
des Moteurs Le Rhône later that year. He quickly followed the 7C with the larger
Le Rhône 9C (as illustrated), a 9-cylinder design delivering 80 hp (60 kW). Compared
with the competitive Gnome engines, the Le Rhône 9C was considerably more conventional,
using copper pipes to bring the fuel to the top of the engine, along with intake
and exhaust valves.
As with the Gnome, the Le Rhône designs were widely licensed, and in the case of
the 9C was produced in Germany (by Oberursel, whose Le Rhone engine copies received
an ‘UR’ prefix), Austria, Britain and Sweden.
After several years of fierce competition, Gnome and Le Rhône finally decided to
merge. Negotiations started in 1914, and on January 12, 1915, Gnome bought out Le
Rhône to form Société des Moteurs Gnome et Rhône. Developments of the 9C continued
to be their primary product, improving in power to about 110 hp (80 kW) in the 9J
by the end of the war.
The 9-series was the primary engine for most early-war designs, both in French and
English service, as well as in Germany where, perhaps somewhat ironically, Oberursel
had taken out a license just before the war.
Le Rhône rotary engines were produced in large scale numbers in France, UK and Sweden.
The Union Switch and Signal Company in Swissvale, Pennsylvania also manufactured
these types. The Le Rhône used less fuel and oil than the contemporary Gnome and
was cheaper, lighter and could idle smoothly at low R.P.M.s
Operation
The Le Rhône had a conventional induction system and an ingenious method
of connecting the piston link rods to the master rod. In the Le Rhône, the fuel mixture
went first to an annular chamber at the back of the crankcase, and then by swept
copper pipes, to conventional intake ports and cam-operated valves in the cylinder
heads. The Le Rhône used a sophisticated slipper bearing system. The master rod had
three concentric grooves to take slipper bearings from all the other cylinders. The
master rod was a split-type to allow assembly of the connecting rods. The remaining
rods carried bronze shoes, shaped to fit in the grooves, at their inner ends. Counting
the master rod as no. 1, the shoes of no's. 2, 5, and 8 rode in the outer groove,
3, 6, and 9 in the middle groove, and 4 and 7 in the innermost groove. Although complex
the LeRhône worked very well.
Le Rhône’s employed an unusual method of valve actuation. A single rocker arm, pivoted
near its centre, was made to operate both the exhaust valve and the intake valve.
Pulled down, it opened the intake valve; pushed up, it opened the exhaust. To do
this, the rocker had to be actuated by a push-pull rod instead of by the usual pushrod.
This, in turn, meant that the cam followers had to have a positive action and a system
of links and levers accomplished this. This system works well enough-some makers
used it up to the late twenties-but its use makes overlap of valve openings impossible.
In an engine designed for high power and speed, the intake valve begins to open before
the exhaust valve is quite closed, but on the Le Rhône, the rocker arm must clear
the exhaust before it can contact the intake. While this puts a limit on power output,
it is not necessarily a fault. As it was, most Le Rhône models produced all the power
that their structural strength and cooling arrangements could cope with.
Slipper bearing system
Although remarkable, the Le Rhône used a sophisticated slipper
bearing system. Its master rod had three concentric grooves to take slipper bearings
from the remaining eight connecting rods. The master rod was a split-type to aid
assembly. The remaining rods carried bronze shoes, shaped to fit in the concentric
grooves, at their inner ends. Counting the master rod as no. 1, the shoes of no's.
2, 5, and 8 rode in the outer groove, 3, 6, and 9 in the middle groove, and 4 and
7 in the innermost one. In spite of their complexity, thousands of Le Rhône’s were
built and proved extremely reliable. Even today, it is possible to turn one of these
90 plus year-old engines over by hand and feel it moving as smooth as silk.
Valve actuation
Le Rhônes engines employed an unusual method of valve actuation. A
single rocker arm, pivoted near its centre, was made to operate both the exhaust
and intake valves. In the ‘down’ position, it opened the intake valve; the ‘up’ position
opened the exhaust. To achieve this, the rocker arm had to be actuated by a push-pull
rod. This meant that the cam followers had to have a positive action, and a system
of links and levers to accomplish this. This system proved popular up to the late
1920’s. But its use makes overlap of valve openings impossible. In an engine designed
for high power and speed, the intake valve begins to open before the exhaust valve
is quite closed, but on the Le Rhône, the rocker arm must clear the exhaust before
it can contact the intake. While this puts a limit on power output, it is not necessarily
a fault. As it was, most Le Rhône models produced all the power that their structural
strength and cooling arrangements could cope with.
Le Rhône 80-hp models were made under license in the United States by a Pennsylvania
firm, Union Switch and Signal. Oberursel made the 110-hp model, supposedly without
authorization in Germany. The Oberursel U.R. II was a straight copy of the Le Rhône
but the Le Rhône was preferred over the Oberursel due to the superior materials used
over the home product. However, there were reports in July, 1918 that there was a
shortage of Castor Oil which the rotaries required. A new Voltol-based lubricant
was substituted and was blamed for a rash of engine failures on Fokker E.V using
the Oberursel U.R. II rotary engine. It has been suggested that without the proper
lubricants, the Le Rhône rotary would have been equally failure prone.
