Gnome Monosoupape

Gnome Engine Company
Industry	Aircraft engine manufacturing
Founded	1905
Founder	Louis Seguin, Laurent Seguin
Fate	Became Gnome et Rhône in 1915
Products	Rotary aircraft engines (including Monosoupape series)

Template:Infobox Aircraft Engine

The Monosoupape—meaning single valve in French—was a rotary engine concept launched in 1913 by the Gnome Engine Company (which became Gnome et Rhône in 1915). Its design incorporated a distinctive layout of internal transfer ports and utilized a single exhaust valve operated by a pushrod, replacing many of the moving components typical in standard rotary engines. This innovation contributed to its reputation as one of the most dependable powerplants of its time. British aviation pioneer Thomas Sopwith regarded the Monosoupape as "one of the greatest single advances in aviation."

Manufactured under license across Britain, Russia, Italy, and the United States, the engine was produced primarily in two nine-cylinder variants: the 9B-2 and the larger displacement 9N. The 9N model featured an almost cylindrical crankcase and incorporated a dual ignition system to improve flight reliability.

In Britain alone, 2,188 units were built under license. Meanwhile, Russia and later the Soviet Union developed an enhanced version, with two such engines powering the Soviet TsAGI-1EA single lift-rotor helicopter during 1931–32.

Background

Earlier Gnome models like the Gnome Omega, Lambda, and Delta employed a unique valve configuration that eliminated the need for inlet pushrods found in conventional engines. Each cylinder head contained a single exhaust valve actuated by a pushrod at the conclusion of the power stroke. The inlet valve was pressure-actuated and balanced by a counterweight to counteract centrifugal forces; it was positioned centrally on the piston crown, allowing fuel–air mixture to flow from the crankcase.

Despite its innovative approach, this setup had limitations: servicing intake valves or adjusting their timing required removal of cylinder heads. Additionally, fuel efficiency lagged behind other rotary engines due to inability to optimize inlet valve timing.

Design

In 1913, Louis and Laurent Seguin—founders of Société Des Moteurs Gnome (established 1905)—introduced the Monosoupape series which replaced traditional inlet valves with piston-controlled transfer ports akin to those found in two-stroke engine designs. During operation, the four-stroke cycle proceeded normally through most of the power stroke until near bottom dead center (BDC), when the exhaust valve opened early to expel hot combustion gases while the piston was still descending. This reduced exhaust pressure and prevented gases from entering the crankcase. As the piston continued downward, it uncovered thirty-six small ports around the cylinder base that connected to the crankcase containing additional fuel–air mixture; however, initial transfer was prevented due to pressure equalization.

The overhead exhaust valve vented directly into the slipstream, thereby eliminating an exhaust manifold. This reduction in components decreased overall weight and minimized gyroscopic effects during flight.

During the exhaust stroke, airflow past the open exhaust port aided scavenging by lowering internal pressure. The piston reached top dead center (TDC) with the exhaust valve still open and began its intake stroke while this valve remained ajar, drawing fresh air into the cylinder. The valve closed approximately two-thirds down during intake, creating a low-pressure environment for the remainder of that stroke. When transfer ports reopened near BDC, this low pressure facilitated drawing in the remaining charge.

Fuel-rich air–fuel mixture was delivered through a hollow crankshaft via a stationary nozzle aimed at the cylinder base near transfer ports; this nozzle remained fixed as cylinders rotated into position. Compression then followed conventional four-stroke timing.

The spark plug was mounted horizontally at each cylinder’s rear without requiring high-voltage wiring. An internal-tooth ring gear on the engine's rotating assembly drove a fixed-position magneto mounted on the firewall; its high-voltage terminal passed close to spark plug terminals as they rotated past, eliminating distributors and wiring common in traditional ignition systems. This ring gear also powered an oil pump supplying all bearings, hollow pushrods operating rockers and valves, as well as an air pump pressurizing the fuel tank. Later versions such as the Gnome 9N introduced dual ignition systems featuring twin spark plugs per cylinder wired electrically to central magnetos driven by crankcase rotation for improved operational safety.

Control

The Monosoupape engines employed a single throttle mechanism for modest speed regulation. Initial variants adjusted engine velocity by altering the exhaust valve timing through levers attached to the valve tappet rollers; however, this approach was discontinued due to damage sustained by the valves. Subsequently, a blip switch was implemented to momentarily interrupt ignition, though its use was limited to prevent fouling of the spark plugs. The advanced 160 CV 9N model introduced a novel dual-ignition arrangement, allowing power output to be modulated at half, quarter, and one-eighth levels via a coupe-switch paired with a five-position rotary selector. This system permitted ignition to be interrupted at evenly spaced intervals among cylinders, ensuring smooth engine function while minimizing spark plug contamination. The operational Fokker D.VIII replica at Old Rhinebeck Aerodrome showcases this four-tier power modulation during both ground testing and flight demonstrations.

Lubrication

The engine utilized a total-loss lubrication system, relying on castor oil blended into the fuel-air mixture. Castor oil was selected because of its exceptional lubricating properties and its resistance to dissolving in the fuel. During operation, more than two gallons of oil were expelled each hour, which required engine cowlings to feature an open lower section designed to divert oil spray away from the pilot’s cockpit. Ingesting unburnt castor oil had a laxative effect on pilots.

Given that the entire engine rotated as a unit, meticulous balancing and precision machining were critical, contributing to the high production costs of Monosoupape engines—the 100 hp versions were priced at $4,000 in 1916 (equivalent to approximately $89,000 in 2017). Despite this, they used less lubricating oil and weighed slightly less than earlier two-valve rotary engines.

Variants

Gnome Monosoupape 7 Type A: Seven-cylinder rotary engine with bore and stroke of .
Gnome Monosoupape 9 Type B-2: Nine-cylinder rotary engine featuring bore and stroke of .
Gnome Monosoupape 11 Type C: Eleven-cylinder variant with specifications of .
Gnome Monosoupape 18 Type C: Eighteen-cylinder configuration with details of .
Gnome Monosoupape 9 Type N: (1917) Nine-cylinder rotary engine equipped with an enlarged crankcase compared to the B-2 model, delivering 150 or 160 hp (112 or 119 kW) and increased displacement of . Bore and stroke measure .
Gnome Monosoupape 9 Type R: Development of the 9N producing 180 hp in a nine-cylinder rotary layout, maintaining the same stroke length.

Monosoupape 7 Type A

Monosoupape 9 Type B

Monosoupape 9 Type N

Engines exhibited

Nine-cylinder Monosoupape engines can be found on display at the Royal Air Force Museum London, the Aerospace Museum of California, and the Musée aéronautique et spatial Safran.
A seven-cylinder Monosoupape engine is showcased at Solent Sky.
The Museo Nacional de Aeronáutica de Argentina in Morón features a Monosoupape built by Peter Hooker, utilized to demonstrate rotary engine operation.
A restored B-2 variant of the Monosoupape is presented at the New England Air Museum in Windsor Locks, Connecticut, USA.
In Australia, TAVAS preserves two Monosoupape engines for use in flying aircraft exhibitions.
The Reynolds-Alberta Museum houses a restored Monosoupape 9 Type N on display.
At the South Australian Aviation Museum, an original, operational nine-cylinder Monosoupape engine is publicly exhibited, with videos showing it in action accessible online.

Citations

Bibliography

Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. .

External links