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Gnome Monosoupape

Type of aircraft rotary engine From Wikipedia, the free encyclopedia

Gnome Monosoupape
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The Monosoupape (French for single-valve), is a rotary engine design first introduced in 1913 by Gnome Engine Company (renamed Gnome et Rhône in 1915). It uses an arrangement of internal transfer ports to replace the automatic centre-piston inlet valve found on earlier Gnome engines. The elimination of the inlet valve removed a common source of engine failure but also resulted in increased fuel consumption.[1]

Quick facts Monosoupape, Type ...

French aircraft manufacturers ordered relatively few Monosoupape engines preferring other types including Gnome et Rhône's own Le Rhône series.[1] 2,188 Monosoupape engines were manufactured in Britain,[2] with additional licensed production taking place in Italy and the United States.[1]

British aircraft designer Thomas Sopwith described the Monosoupape as "one of the greatest single advances in aviation".[3] However the type was also criticised for its poor reliability with Royal Flying Corps commander Hugh Trenchard describing it as a "beast of an engine".[4]

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Background

Unlike other rotaries, the early Gnome engines like the Gnome Omega, Lambda and Delta used a unique arrangement of valves in order to eliminate pushrods 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 to perform maintenance of the intake valves, and to adjust the timing correctly. Fuel economy suffered in comparison to other rotaries because the inlet valves could not be opened and closed at the ideal times.

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Design

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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.[1] Beginning with the power stroke, the four-stroke engine operated normally until the piston had traveled half way to 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 no exhaust manifold could be practically fitted to the spinning crankcase and cylinders.[5]: 7–12 

During the exhaust stroke, 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 remained open. The piston began to move down on its intake stroke with the valve still open, pulling new air into the cylinder. It remained open until it was three quarters 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.[5]: 7–12 

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.[5]: 7–12 

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 distributor and high-voltage wiring found in conventional mechanically timed ignition systems.[5]: 17–22  This ring gear also drove the oil pump, which supplied oil to all bearings, and through hollow pushrods to the rockers and valves. The ring gear could also be used to drive a petrol pump or and air compressor to pressurize the fuel tank.[5]: 27–33  The later 160 hp (120 kW) 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.

Control

Monosoupapes had a single petrol regulating control used for a limited degree of speed regulation.[5]: 44–49  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 fouling the spark plugs, since it was only safe to be used when the fuel supply was also cut. The later 160 hp (120 kW) output 9N subtype also featured an unusual method of functioning with its integral dual-ignition setup, that allowed output values of one-half, one-quarter and one-eighth power levels to be achieved through use of the coupe-switch and a special five-position rotary switch that selected which of the trio of alternate power levels would be selected when the coupe-switch was depressed, allowing it to cut out all spark voltage to all nine cylinders, at evenly spaced intervals to achieve the multiple levels of power reduction.[6][a] The airworthy reproduction Fokker D.VIII parasol monoplane fighter at Old Rhinebeck Aerodrome, uniquely powered with a Gnome 9N, often demonstrates the use of its Gnome 9N's four-level output capability in both ground runs[7] and in flight.

Lubrication

Thumb
The Sopwith Tabloid reproduction shows the sheet-metal cowling used to redirect the oil sprayed by the rotating engine.

The lubrication system, as with all rotary engines, was a total-loss type in which castor oil was pumped into the fuel–air mix. Castor oil was used because it did not readily dissolve into the fuel, and because it offered lubrication qualities superior to other available oils. 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 cowls, with the lowermost quarter omitted to direct the spray of castor oil away from the pilot.[8] Unburnt castor oil from the engine had a laxative effect on the pilot if ingested.[9] However, Monosoupape engines used less lubricating oil, and weighed slightly less, than earlier Gnome series engines with intake valves in the piston crown.[10]

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Variants

Gnome Monosoupape 7 Type A
Seven-cylinder rotary engine, 80 hp (60 kW). Bore and stroke: 110 mm × 150 mm (4.3 in × 5.9 in) .
Gnome Monosoupape 9 Type B-2
Nine-cylinder rotary engine, 100 hp (75 kW). Bore and stroke: 110 mm × 150 mm (4.3 in × 5.9 in).
Gnome Monosoupape 11 Type C
An 11 cylinder version, 200 hp (150 kW).
Gnome Monosoupape 18 Type C
An 18 cylinder version, 240 hp (180 kW).
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), increasing capacity to 15.8 L (960 cu in). Bore and stroke: 115 mm × 170 mm (4.5 in × 6.7 in).
Gnome Monosoupape 9 Type R
180 hp nine-cylinder rotary engine, development of 9N with same 170 mm (6.7 in) stroke.

Applications

Monosoupape 7 Type A

Monosoupape 9 Type B

Monosoupape 9 Type N

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Engines on display

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Specifications (Monosoupape 9 Type B-2)

Data from Lumsden[2]

General characteristics

Components

  • Valvetrain: Single overhead valve with cylinder 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

Performance

  • Power output: 86 kW (115 hp) at 1,300 rpm (Maximum power)
  • Compression ratio: 4.85:1
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See also

Comparable engines

Related lists

Notes

  1. [a]
    In order to keep the engine running smoothly on reduced power settings, it was necessary for the selector switch to cut out all cylinders at evenly spaced intervals. It was also beneficial to have all cylinders firing periodically to keep them warm and to prevent the spark plugs from fouling with oil. The selector switch has five positions, zero (0) for off and four running positions, one through four (1-4) (see Photo 5). The Gnôme 9N had two magnetos (and two spark plugs per cylinder) and the selector switch was wired to the right magneto only, so it was necessary for the pilot to turn off the left magneto if he wanted to change the speed of the engine.[6]
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References

External links

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