Gnome Monosoupape

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Gnôme 9N Monosoupape

Cylinder cross-section

The Monosoupape, French for single-valve, was a particular engine design used by Gnome et Rhône's later rotary engines. It used a clever arrangement of internal ports and a single valve to replace a large number of parts normally found on a conventional arrangement, and made the Monosoupape engines some of the most reliable of the era.

Earlier Gnome (as opposed to Le Rhône) designs used a unique arrangement of valves in order to avoid needing pushrods and other complex devices to operate the inlet of the engine cycle. 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 intake valve instead was operated by a counterweight and placed right in the middle of the piston head, where it opened to allow the charge to enter through a hollow crank from the center of the engine. Although clever, the system had several drawbacks. One was that maintaining the intake valve, which could easily become jammed, required the cylinder heads to be removed. Another was that in order to get the timing and pressures right for the rod-less operation, the inlet valves opened at times that were not efficient; the Gnomes had even poorer fuel economy than other rotaries, which were bad enough. So the valve was made redundant by a piston-controlled port inlet, similar to a 2-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 fuel "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.

The piston completed its exhaust stroke until top dead center (TDC) was reached, but the valve did not close. By being open to the slipstream, total scavenging occurred as the air moving past the cylinder created a partial vacuum inside. 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 caused a partial vacuum to form in the cylinder. When the piston uncovered the transfer ports it sucked the balance of the charge as a result of the partial vacuum in the cylinder and the atmospheric pressure in the crankcase.

The charge was an overly rich mixture of fuel and 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 to the firewall, whose high-voltage output terminal passed in close proximity by the spark plug terminals. This arrangement eliminated the need for points, distributor, high-voltage wiring and capacitors. This ring gear also drove the oil pump, which supplied oil to all bearings, and through hollow push rods to the rockers and valves. This ring gear also drove the air pump that pressurized the fuel tank as an early form of fuel injection. There was no carburetor, saving more weight.

With no carburetor or throttle, and constant fuel pressure, there were only two power settings: full throttle or none; the engine did not even have the ability to idle. To adjust power it was possible to lean the engine by reducing fuel pressure. Like most rotaries, the Monosoupape's were equipped with a "blip switch" that could cut the ignition. This had to be used sparingly, as the engine would continue to pull fuel into the crank and cylinders, so turning the ignition back on after too long a period could cause the engine to explode.

Because the entire engine rotated, it had to be precisely balanced. So castings and forgings could not be used, instead, precision machining of all parts was made necessary. As a result, Monosoupapes were extremely expensive to build, the 100 hp models costing $4,000 in 1916, about $65,000 in year 2000 dollars.