Gnome Monosoupape - Description

Description

In 1913, Louis Seguin and his brother Laurent (engineers who founded the Société Des Moteurs Gnome 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. Beginning with the power stroke, the four-stroke engine operated normally until the piston was just about to reach the bottom of its stroke (bottom dead center, or BDC), when the exhaust valve was opened "early". This let the still-hot burnt combustion gases "pop" out of the engine while the piston was still moving down, relieving exhaust pressure and preventing exhaust gases from entering the crankcase. After a small additional amount of travel, the piston uncovered 36 small ports around the base of the cylinder, leading to the crankcase which held additional fuel–air mixture (the charge). No transfer took place at this point since there was no pressure differential; the cylinder was still open to the air and thus at ambient pressure. The overhead valve exhausted directly into the slipstream since there was no exhaust manifold in order to save weight.

During the exhaust stroke, total scavenging occurred as the air moving past the cylinder exterior lowered the pressure inside due to the direct exposure of the exhaust port to the slipstream. The piston continued its exhaust stroke until top dead center (TDC) was reached, but the valve did not close. The piston began to move down on its intake stroke with the valve still open, pulling fresh (presumably un-filtered) air into the cylinder. 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. When the piston uncovered the transfer ports again, the low pressure in the cylinder drew in the balance of the charge.

The charge was an overly rich mixture of air, which was acquired through the hollow crankshaft, and fuel that was continuously injected by a fuel nozzle on the end of a fuel line, entering the crankcase through the hollow crankshaft. The nozzle was in the proximity of, and aimed at, the inside base of the cylinder where the transfer ports were located. The fuel nozzle was stationary with the crankshaft, and the cylinders rotated into position in turn. The compression stroke was conventional.

The spark plug was installed horizontally into the rear of the cylinder at the top but had no connecting high-voltage wire. An internal-tooth ring gear mounted on the engine drove a stationary magneto mounted on the firewall, whose high-voltage output terminal was in close proximity to the spark plug terminals as they passed by. This arrangement eliminated the need for points, distributor, high-voltage wiring and capacitors found in conventional mechanically timed ignition systems. This ring gear also drove the oil pump, which supplied oil to all bearings, and through hollow pushrods to the rockers and valves and also drove an air pump which pressurized the fuel tank. The later Gnome 9N engines had dual ignition systems for safety, with twin spark plugs per cylinder which were electrically wired, with the wires routed onto the crankcase and a central pair of magnetos driven by the spinning engine crankcase.