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Gnome Monosoupape
The Monosoupape (French for single-valve), was an engine design used by Gnome et Rhône's later rotary engines, and first introduced in 1913. It used a clever arrangement of internal transfer ports and a single 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".[1]
Contents
Background
Earlier Gnôme (as opposed to Le Rhône) designs 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. The inlet valve, which was operated by a counterweight, 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 for both maintenance of the intake valve, which could easily become jammed, and in order to get the timing and pressures right for the rod-less operation; and the Gnômes exhibited even poorer fuel economy than other rotaries because the inlet valves opened at times that were not efficient.
Description
In 1913, Louis Seguin and his brother Laurent (engineers and founders of the Société Des Moteurs Gnôme [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 created a partial vacuum 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 caused a partial vacuum to form in the cylinder. When the piston uncovered the transfer ports again, the partial vacuum sucked 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.
Control
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.[2] 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 six cylinders so that each cylinder fired only once per three engine revolutions but the engine remained in perfect balance.[3]
Lubrication
The lubrication system 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 three-quarters cowl ring, open at the bottom. The cowl directed the spray of castor oil, along with sparks from the exhaust, away from the flammable aircraft structure.[4]
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 Template:Convert models costing $4,000 in 1916 (approx. $65,000 in 2000 dollars). However, they weighed slightly less than the earlier two-valve engines and also used less lubricating oil.[2]
Motorcycle use
The German Megola motorcycle which was produced from 1921 to 1924 featured a small Template:Convert 5-cylinder Monosoupape rotary engine mounted within its front wheel.
Aircraft uses
- Airco DH2 – 100 hp version
- Morane-Saulnier A1 – 160 hp version
- Nieuport 28 – 160 hp version
- R.A.F F.E.8 – 100 hp version
- Vickers FB.5
See also
References
Notes
- ↑ Nahum, Andrew (1999). The Rotary Aero Engine. NMSI Trading Ltd. ISBN 190074712X.
- ↑ 2.0 2.1 Vivian, E. Charles (2004). A History of Aeronautics. Kessinger Publishing, 255. ISBN 1419101560.
- ↑ Donovan, Frank; Frank Robert Donovan (1962). The Early Eagles. Dodd, Mead, 154.
- ↑ Gnome Monosoupape Type N Rotary Retrieved on 18 February 2009.
- McCutcheon, Kimble D.. Gnome Monosoupape Type N Rotary (PDF). Aircraft Engine Historical Society. Retrieved on 2008-05-01.
External links
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de:Monosoupape es:Gnôme Monosoupape
| This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gnome Monosoupape". |
