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Gnome Monosoupape / 7 Type / 9 Type


1917 Gnome 9N 160 hp Monosoupape


The Monosoupape (French for single-valve), was a rotary engine design first introduced in 1913 by Gnome Engine Company (since 1915 called Gnome et Rhône). It used an arrangement of internal transfer ports and a single pushrod-operated exhaust valve to replace a large number of moving parts found on more conventional rotary engines, and made the Monosoupape engines some of the most reliable of the era. British aircraft designer Thomas Sopwith described the Monosoupape as "one of the greatest single advances in aviation".

Produced under license in Britain the engine was built in large numbers in both seven and nine-cylinder versions, the latter being produced with two different displacements.

Contrary to the Le Rhône designs, earlier Gnome engines like the Gnome Omega, Lambda and Delta 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. 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 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.

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. 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.

The Monosoupape had no carburetor or throttle, and since most of its air supply was taken in through the exhaust valve, it could not be controlled by adjusting the air supply to the crankcase like other rotaries. Monosoupapes therefore had a single petrol regulating control used for a limited degree of speed regulation. In early examples, engine speed could be controlled by varying the opening time and extent of the exhaust valves using levers acting on the valve tappet rollers, but this was later abandoned due to causing burning of the valves. Instead, a blip switch was used, which cut out the ignition when pressed. This was used sparingly to avoid damaging the engine, since it was only safe to be used when the fuel supply was also cut. Some later Monosoupapes were fitted with a selector switch which allowed the pilot to cut out three or six cylinders instead of all nine when hitting the blip switch, so that each cylinder fired only once per three engine revolutions but the engine remained in perfect balance.

The lubrication system, as with all rotary engines, was a total-loss type in which castor oil was injected into the fuel–air mix with a small pump. Castor oil was used because it could not be easily dissolved into the fuel, and because it possessed lubrication qualities superior to mineral oils of the day. Over two gallons of castor oil were sprayed into the air during each hour of engine operation. This explains why most rotaries were fitted with a roughly 270º perimeter "horseshoe"-shape ring cowl, with the lowermost quarter of the cowl omitted to be open at the bottom. The cowl directed the spray of castor oil, along with sparks from the exhaust, away from the flammable aircraft structure. Unburnt castor oil from the engine tended to be hurled into the pilot's face, often having a laxative effect on the pilot if ingested.

Because the entire engine rotated, it had to be precisely balanced, requiring precision machining of all parts. As a result, Monosoupapes were extremely expensive to build, the 100 horsepower (75 kW) models costing $4,000 in 1916. However, they weighed slightly less than the earlier two-valve engines and also used less lubricating oil.



Gnome Monosoupape 7 Type A
(1916) Seven-cylinder rotary engine, 80 hp (60 kW). Bore and stroke: 110 x 150 mm (4.3 x 5.9 in).
Gnome Monosoupape 9 Type B-2
(1916) Nine-cylinder rotary engine, 100 hp (75 kW). Bore and stroke: 110 x 150 mm (4.3 x 5.9 in). 2,188 units produced under license in Britain.
Gnome Monosoupape 9 Type N
(1917) Nine-cylinder rotary engine, larger diameter crankcase than the B-2, 150 or 160 hp (112 or 119 kW). Bore and stroke: 115 x 170 mm (4.5 x 6.7 in).



Monosoupape 7
Avro 504
Avro 511
Bristol-Coanda G.B.75
Sopwith Pup


Monosoupape 9 Type B-2
Avro 504
Airco DH.2
Airco DH.5
Alcock Scout
BAT Bantam
Blackburn Scout
Blackburn T.B
Blackburn Triplane
Bristol-Coanda T.B.8
Bristol Scout
Coventry Ordnance Works Biplane No 2
F.B.A. C Flying boat
Nieuport 12
Nieuport 28
Royal Aircraft Factory B.E.8
Royal Aircraft Factory F.E.8
Short S.70
Short S.80
Short Type C
Sopwith Circuit Seaplane
Sopwith Gunbus
Sopwith Tabloid
Sopwith Sociable
Sopwith (Admiralty Type ) 807 Folder Seaplane
Sopwith Two-Seat Scout
Sopwith Schneider
Sopwith Pup
Sopwith F.1 Camel
Vickers Gunbus (FB.2, 3, 5, 6 and 7)
Vickers E.S.1 Bullet
Vickers F.B.12
Vickers F.B 19 Bullet


Monosoupape 9 Type N
Nieuport 28C
Sopwith F.1 Camel
Morane-Saulnier AI



Monosoupape 9 Type B-2
Type: 9-cylinder, single-row, rotary engine
Bore: 110 mm (4.3 in)
Stroke: 150 mm (5.9 in)
Displacement: 12.8 L (781.63 cu in)
Length: 107.4 cm (42.3 in)
Diameter: 95 cm (37.4 in)
Dry weight: 137.4 kg (303 lb)
Valvetrain: Single overhead valve with piston ports
Fuel type: 40-50 Octane petrol
Oil system: Total loss, castor oil
Cooling system: Air-cooled
Reduction gear: Direct drive, right-hand tractor, left-hand pusher
Power output: 86 kW (115 hp) at 1,300 rpm (Maximum power)
Compression ratio: 4.85:1



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