Gnome Monosoupape

Two differing nine-cylinder versions were produced, the 100 hp (75 kW) 9B-2 and 160 hp (120 kW) 9N, with differing displacements giving the larger displacement 9N version a nearly-cylindrical shaped crankcase, with the 9N also adopting a dual ignition system for increased flight safety.

A pressure-operated inlet valve, which was balanced by a counterweight to equalize the centrifugal forces, was placed in the centre of the piston crown, where it opened to allow the fuel–air charge to enter from the engine's central crankcase.

Although ingenious, the system had several drawbacks: the cylinder heads had to be removed to perform maintenance of the intake valves, and to adjust the timing correctly.

Fuel economy suffered in comparison to other rotaries because the inlet valves could not be opened and closed at the ideal times.

In 1913, Louis Seguin and his brother Laurent (engineers who founded the Société Des Moteurs Gnome [the Gnome motor company] in 1905) introduced the new Monosoupape series, which eliminated the inlet valve, replacing it with piston-controlled transfer ports similar to those found in a two-stroke engine.

The piston continued its exhaust stroke until top dead center (TDC) was reached, but the valve remained open.

It remained open until it was two-thirds of the way down, at which point the valve closed and the remainder of the intake stroke greatly reduced the air pressure.

This arrangement eliminated the need for distributor and high-voltage wiring found in conventional mechanically timed ignition systems.

The later 160 CV output 9N subtype also featured an unusual method of functioning with its integral dual-ignition setup, that allowed output values of one-half, one-quarter and one-eighth power levels to be achieved through use of the coupe-switch and a special five-position rotary switch that selected which of the trio of alternate power levels would be selected when the coupe-switch was depressed, allowing it to cut out all spark voltage to all nine cylinders, at evenly spaced intervals to achieve the multiple levels of power reduction.

This explains why most rotaries were fitted with cowls, with the lowermost quarter omitted to direct the spray of castor oil away from the pilot.

The Sopwith Tabloid reproduction shows the sheet-metal cowling used to redirect the oil sprayed by the rotating engine.