The Gnome engines were built by the Société Des Moteurs Gnome - a French Engineering company founded in 1913 by the engineers Louis Seguin and his brother Laurent. They introduced the single valve type (Monosoupape) engines which eliminated the need for an inlet valve. In 1915, the company was renamed to Gnome et Rhóne.

The principal licensee manufacturer of the Gnome and Le Rhóne engines in Britain was Peter Hooker Ltd, Walthamstow (Essex).

Gnome 9 B2 - Operation 1

In the Gnome 9 B2 (Monosoupape) engine, similar to the transfer ports in two stroke engines, the mixture is admitted from the crankcase into the cylinder through a row of ports (36) in the base of the cylinder. This can only happen when these holes are uncovered by the piston at their very bottom of the inlet stroke. However, the Gnome Mono operates on the normal 4 stroke cycle, although the timing of the single large exhaust valve in the cylinder head is somewhat unusual.

When the piston descends down the cylinder during its working stroke, the exhaust valve is opened at 85 degrees after TDC, earlier than usual. This relieves the cylinder pressure and little exhaust gas will flow through the inlet ports when they are opened by the piston in its lower position. This limited the contamination of the crankcase mixture.

Gnome 9 B2 - Operation 2

During the upstroke, the exhaust valve stays open and the remaining exhaust gas is discharged. The piston then descends on the induction stroke, the exhaust valve is still held open to allow fresh air to be drawn into the cylinder.

The exhaust valve closes at 60 degrees before BDC and as the piston continues its downward stroke, a partial vacuum is created within the cylinder.
When the piston next uncovers the inlet transfer ports, the petrol-rich mixture from the crank case is drawn into the cylinder and mixes with the fresh air to form a combustible mixture.

Crank Shaft

The crankshaft serves the double purpose of supporting all rotary parts and provides entrance for oil and petrol supply pipes.
To meet the requirements for erection and stripping, the crankshaft consists of two pieces, the main hollow main Crankshaft and the Maneton, the smaller part. The large (front) end of the Crankshaft is open to admit air and contains the petrol jet.

The petrol jet has eight holes which project the petrol downwards in the direction of the mixture flow towards the inlet ports in the cylinders.

Thrust Cover flange (Thrust box)

The Thrust box contains the Main ball-bearings (thrust bearings) and is screwed onto the crankcase rear half.

Unlike more conventional engines, rotary engines such as the Gnome, have a stationary crankshaft around which the crankcase and cylinders as a whole unit rotate. The propeller, attached to the propeller-hub and nose piece, are bolted to the crankcase.

Master & Auxiliary Rods and Maneton

The Master rod connects the Auxiliary Rods to the crankshaft. It enables the thrust from several pistons to act successively through the same point.

The big end (banjo) of the master rod contains two ball-bearings and eight wrist pins through which the auxiliary rods are connected.

Controlling the Engine

The Gnome 9 B2 engines had no proper throttle. The engines ran at full throttle. The only way of controlling the engine was by cutting the ignition to a number of cylinders, or "blipped" to reduce power when necessary.

Later WW1 engines like the Clerget 9B, Bentley BR1/BR2 and Le Rhóne, where fitted with the Tampier Bloc-tube carburetor and throttle-mixture control levers, but reducing power when landing involved simultaneously adjusting the throttle and mixture controls was not that straightforward. It became a common practice during landing to "blip" the engine as well.

Gyroscopic Effect

The gyroscopic action of such a large metal mass spinning at the front of a fairly light wood and fabric airframe must have been extremely powerful, especially in turns. However, due to the compact construction, the engine weight is concentrated near the center of gravity. The propeller produces a large part of the total gyroscopic effect

Another interesting fact was that these engines had no exhaust system (the burnt gasses were simply released from the tops of the cylinders). They used a “total loss” oiling system, where the oil was exhausted with the burnt fuel, coating the aircraft with a heavy sheen of castor oil. The fabled white scarf worn by these pilots had less to do with fashion, and more to do with wiping the oil off their faces and goggles.

Valve Gear Case

The Valve Gear Case (or Cam Box) contains the cam followers (tappets), tappet guides and houses the timing gears and exhaust cams.
There is one cam per cylinder and the cam stack (on the cam shaft) rotates at half engine speed through the use of reduction (planet) gears.

Push Rods

The Gnome Monosoupape (French for single valve) used a clever concept of internal mixture admition ports around the base of the cylinders and a single push rod operated exhaust valve.

By reducing the number of moving parts found on more conventional engines, the Gnome "Mono" engine was considered as one of the most reliable engines of the era.

Valve Gear Case Cover (Cam box)

The Valve Gear Case (Cam box) cover contains the two sets of planet gears, each 30/20 teeth, which gear into the 30 teeth tooth gear attached to the Maneton/Crankshaft.

Valve Gear Case Cover Con't.

The Maneton gear (30 teeth) is key-locked attached to the Maneton, the small front part of the (stationary) crankshaft.

The Cam box cover is bolted onto the Cam box and therefore rotates with the engine. The Maneton with its gear is key-lock "fixed" to the stationary Crank shaft.
The Camshaft with its tooth gear of 40 teeth, contains the stack of 9 Cams. Driven by the revolving Cam box cover with the planet gears, a 20/40 = half of the engine speed is obtained.

Nose Piece/Propeller Shaft to Cam box

Various lengths and forms of propellers shafts from time to time have been used to fit the various types of aircraft.
The shaft portion is hollow and turned to a long taper to tightly fit the internally tapered propeller hub.

Thrust Cover flange (Thrust box) and Distributor

The Distributor contains a series of 9 contact segments, arranged concentrically, is screwed onto the Thrust box by the Main gear.
The carbon brush within the brush holder mounted on the central support, will make contact with the appropriate segment. The brush is connected to the Magneto through the high tension cable. The terminals on the circumference of the Distributor, connected with the segments, have uncovered brass wires connected through which the current is led to the Spark plugs.

The magneto is set to give a spark in the cylinder at 15 to 20 degrees before the top dead center (TDC), the end of the compression stroke.

Central Support and Rear Supports

The function of the Central Support and Rear support is to carry the Crankshaft. They are the sole means of attachment of the engine to the aircraft structure. Together with the crankshaft they are the only stationary parts of the engine.

The Central support carries the majority of the engine's weight. In addition, the Central Support has mounted on its rear face the auxiliary fittings e.g. pressure-air pump, the magneto and the oil pump.

The engine driven air-pressure pump, driven by the Main gear, would provide air-pressure to the petrol tanks.

A single A.D.S. type magneto was used for the ignition. The magneto is also driven by the main driving wheel and is electrically connected via the carbon brush in de brush-holder to the ring of contact segments on the distributor.

The high tension magneto is fitted with a pinion (32 teeth) with a crown wheel allowing for fine adjustment. The pinion is driven by the engine's main driving wheel (72 teeth).

Oil Pump and Reduction Gear Box

The Oil pump, also driven by the Main gear, with the Reduction Gearbox mounted.

Ignition - Electrical Wiring

Each cylinder has one spark plug and is powered by a magneto instead of a battery. This design improves reliability and this is an important consideration in aviation.

Petrol Air-pressure pump.

This engine driven pump is driven by its pinion, and through worm gear.
This pump has no suction valve, the air being admitted through the 4 holes in the barrel when the piston is near the inward end of its stroke.

(*) This engine driven Petrol Air-pressure pump was used for aircraft that didn't have and external air-pressure pump, e.g. the Rotherham air driven pump, mounted to the rear right cabane wing strut. To avoid stress and damage to the cabane strut (due to vibration), the pump was later fitted to the under carriage. This however was not liked by the pilots because they could not see the pump working.

Petrol Air-pressure pump.

The delivery valve is of the plate type and of ample size. The pump, in addition to supplying the air required to displace the petrol used, will deliver an ample margin to allow for reasonable leakage from the delivery pipe or petrol tank, and to deal with this excess air and adjustable relief valve is provided.

Petrol Air-pressure pump Cut-away view

The two valve plates are visible in front of the valve bodies on the bottom left. Each valve body contains a spring that presses the valve plate into its closed position (closing the holes in the cylinder head) during the suction stroke.
The air is being admitted through the holes in the cylinder barrel when the piston is near the bottom stroke of its stroke. During the compression stroke, the valve plates are pushed against their springs, thus opening the holes in the cylinder head, allowing the air to flow out through the cut-away channels in both valve bodies. The relieve valve plate movement is adjustable as to leak the excess air.

Rotherham - Petrol Air-pressure pump

The Rotherham & Sons Ltd (Coventry) Patent Mechanical Air Pump was designed for putting Air Pressure in petrol tanks of Aeroplanes. The pump is adjustable and gives a range between one and ten pounds (psi). The pump is rotary with a stroke of 5/8 inch (15.875mm.) and is driven by a spindle and propeller.

The bottom nut, with a spring loaded brass plunger under the connecting rod, is an oiler. Oil collects in it and every time the connecting rod pushes it down, a jet of oil shoots up the inside of the connecting rod.

Various Petrol Air-pressure pumps.

This image shows the various air-pressure pumps that were used by Aero plane engine manufacturers.

The left version is the standard (Clerget) engine driven air-pressure pump.
The pump in the middle has been seen on the Gnome and other engines (e.g. Clerget, Le Rhone).
The pump on the right is the wind driven Rotherham air pump which was commonly installed on the Sopwith Camel, mounted on the wing strut or under carriage.

“AVIA” Magneto A.D.S. type).

Here the end cap is taken off to show the rotating contact-breaker assembly.
The contact-breaker assembly is attached to the rotating armature. The contact-breaker rocker is activated through the two "half moon" shaped shoes, which are screwed onto on the inside of the rear housing.

“AVIA” Magneto (A.D.S. type)

Also shown is the provision for an earthing switch, used to prevent the magneto from producing sparks when not required by means of short-circuiting the primary coil. Therefore any inductive effect in the secondary coil is prohibited. This shut-down switch will be connected to the screw terminal, protruding through the rear magneto end.

   See Magnetos simply explained for additional details.

Cross Section of the “AVIA” Magneto.
This image shows the cross section of the magneto, its various parts and the coil with the Primary (inner) and Secondary winding (outer). To the right, inside the brass end of the armature, the condenser / condenser “plates stack” is visible.


   See Magnetos simply explained for additional details.

Contact Breaker

Close up of the contact breaker assembly which is attached to the rotating armature.
The “L” shaped rocker is connected to the frame (earth) through the blade spring. One end of the primary winding of the armature coil is connected to the insulated central member (by the long central bolt), the other end is also connected to the frame (earth).
The fiber contact-breaker rocker heel will be pressed inwards when touched by either one of the two cam shoes inside the cam-ring. This causes the opening of the contact points, breaking the circuit (twice per revolution) and causes the induction of the H.T. (hight voltage) in the secondary winding.

   See Magnetos simply explained for additional details.

Oil Pump Operation

The oil pump, filled with oil, is driven by its pinion through the worm gear. Two cams, formed on the worm shaft, acting against the internal springs, force the regulating plunger (R) and the valve plunger (L) to descend.



   Click the “Animate” button to see the operation of the oil pump.

Oil Pump Operation Con'd

The oil flows into the inlet chamber, through the gauze strainer, and into the annular space around the reduced portion of the valve plunger; the valve plunger descends, and the regulator plunger ascends, drawing the oil after it through the opening between the two chambers; the valve plunger then ascends, and the regulating plunger descends, forcing the oil back through the opening into the space under the valve plunger, and from there into the oil supply pipe.

   Click the “Animate” button to see the operation of the oil pump.

Oil Pump / Reduction Gear Box.

The speedometer drive/reduction gear box is mounted on the oil pump and drives the cockpit RPM indicator, which is connected by means of a flexible cable.

The function of this reduction gear box is to reduce the driving flexible cable speed to prevent undue wear.

Speedometer Drive / Reduction Gear Box

The oil pump pinion (32 teeth) is driven by engine's main gear (72 teeth), making the oil pump rotating at 2 1/4 of the engine speed.
The reduction gear box reduces the RPM to limit undue wear of the flexible cable.

Reduction Gear Box

Cut-away view to show the internal gears in detail.

The nut on the left is screwed onto the oil pump main shaft and drives the inner reduction gears through the spring coupling.
A flexible cable that connects to the RPM indicator in the cockpit is screwed onto the brass end of the gear box.