Background
Earlier Gnome (as opposed to Le Rhône) designs used a unique arrangement of valves in order to avoid needing pushrods and other complex devices that operated during the inlet phase of the combustion cycle on more conventional engines. Instead, a single exhaust valve on the cylinder head was operated by a pushrod that opened the valve when the pressure dropped at the end of the power stroke. The inlet valve, which was operated by a counterweight, 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 both in order to perform maintenance of the intake valve, which could easily become jammed, and in order to adjust the timing and pressures correctly for the rod-less operation; and the Gnomes exhibited even poorer fuel economy than other rotaries because the inlet valves opened at times that were not efficient.
Read more about this topic: Gnome Monosoupape
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