- This article or section should be merged with Rotary piston engine.
The Monosoupape engine is a style of rotary-radial engine once seen in high performance WWI fighter aircraft.
An example of an aircraft powered by this engine was the 9-cylinder French LeRhone Gnome Monosoupape.
Engine Configuration
The conventional orientation of a radial aircraft engine is seen when the crankshaft protrudes from the front of the engine and the propeller is mounted to it. The Monosoupape engine was a reverse orientation in that the crankshaft protruded from the rear of the engine and was firmly mounted to the airframe at the firewall. Whereas a conventional crankshaft rotates the propeller alone, on the Monosoupape it was stationary and hollow to allow passage of components. The propeller was instead mounted to a rigid extension protruding from the front of the engine crankcase and, in order for the propeller to turn, the entire engine including the cylinders rotated.
The crankshaft was hollow, but not in order to save weight, rather it was the name of the style of air intake for the fuel injection system. The name Monosoupape is derived from the French mono meaning one, and sauppe meaning valve. Thus, this unique design had only one valve per cylinder in contrast to modern multi-valved engines, a strange concept considering that it would seem logical to have one valve for each of fuel intake and exhaust output.
Engine Operation Beginning with the exhaust cycle, the four-stroke engine operated with the piston at the bottom of its stroke (bottom dead center or BDC) with the valve already open when the piston was just before reaching BDC. This relieved the cylinder of exhaust gas pressure before the piston uncovered 36 small bypass (transfer) ports, spaced at 10 degrees apart around the base of the cylinder. No transfer took place then since there was no pressure differential. The overhead valve exhausted directly into the slipstream since there was no exhaust manifold in order to save weight.
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 was passed in close proximity by the spark plug terminals. This arrangement eliminated the need for points, distributor, high-voltage leads (spark plug wires), and capacitor. This ring gear also drove the oil pump which supplied oil to all bearings, as well as through oil journals to the main, rod and wristpin bearings, and 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.
The piston completed its exhaust stroke and when TDC (top dead center) was reached, the valve did not close. By being open to the slipstream, total scavenging occurred and the piston began to move down on its intake stroke. The valve was still open and obtained fresh air from the slipstream until the piston was 2/3 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 obtained the balance of the fuel-air mixture as a result of the partial vacuum in the cylinder and the atmospheric pressure in the crankcase.
The balance of 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 next cylinder rotated into position next to get its own charge of fuel and air. The compression stroke was conventional, and the Monosoupape power stroke differed from the conventional power stroke in that the poppet exhaust valve opened just before the piston uncovered the transfer ports, relieving exhaust pressure and preventing exhaust gases from entering the crankcase.
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. And because the entire engine rotated, it had to be precisely balanced which eliminated the use of castings and forgings, and required precision machining of all parts.
See Also
External Links - 1916 Gnome Monosoupape Type N 160 HP (PDF) (http://www.enginehistory.org/Gnome%20Monosoupape.pdf)
- Illustration of Gnome Monosoupape 50HP (http://www.glwason.com/gw_pages/Gnome1.htm)
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