User:MinorProphet/Draft subpages/Gnome aero engines



There is a vast amount of misinformation and misidentification concerning Gnome rotary aero engines, and their copies made under licence by Oberursel, Thulins, Airco/Daimler et al. Thus this article.

More intro paras blah ...


 * NB This is a very rough draft and full of inaccuracies.

Nomenclature
From 1908 until c 1918 Gnome made two consecutive series of unusual rotary aero engines of between 7 and 18 cylinders and from 50 to 200 hp. In both types, the fuel inlet mechanism is hidden inside the engine and cannot be seen from the outside. Neither series were named sequentially by the manufacturers.

Greek-letter variants
For the first series (designed from 1908 to 1912 and built up to c1919), Gnome used the names of Greek letters: Omega (Ω), Sigma (Σ), Gamma (Γ), Lambda (Λ) and Delta (Δ) (in the Roman alphabet: O, S, G, L, and D). Gnome used the single Greek letters (such as 'Σ' or 'Δ') rather the full name on the brass engine identification plates: e.g. "Moteur Gnome: Type Ω No. 693"  See pic on right.

Rotary engines consist of a number of cylinders (often 7 or 9) arranged around a central crankcase. The power of an engine can be nearly doubled if a second row of cylinders is added behind the first. Gnome's own double-row models were named, for example, Omega-Omega (Ω-Ω) or Lambda-Lambda (Λ-Λ). Sometimes they are informally referred to as e.g. a 'double Sigma'. The Delta, for example developed a claimed 100 hp, and the Delta-Delta around 200 hp. Manufacturers' figures around the time often tended to be a little optimistic.

All Gnome engines of both Greek-letter and later Monosoupape types feature a large overhead exhaust valve in the top of each cylinder head, actuated by a single external push rod (see below). In addition, the Greek-letter models used an unusual inlet valve integral with the piston crown (not visible without dismantling the cylinder head). Thus, all these early Omega variant engines have two physical (if somewhat unconventional) valves per cylinder.

In contemporary trade press reports, all models of this type were were often simply referred to as "an 80 hp Gnome" or "the new 140 hp Gnome". The reports often did not refer to a specific model type such as the Gnome Sigma: even detailed articles tended not to mention the engine names.

Monosoupapes
Instead of the unusual inlet valve-in-piston of the Omega variant series above, the 'Monosoupape' (lit. 'Single valve') engines have transfer ports in the cylinder walls like a two-stroke motor, although they work on a four-stroke cycle. The Monosoupape is not thus specifically named for the single visible exhaust valve (which is exactly the same arrangement as on the Omegas): but they are so named because they really do have only one valve per cylinder (although you can't the see other valve on the Omega variants anyway). Because of this, a certain amount of confusion may have arisen through a tendency to call any Gnome engine a 'Monosoupape' because all of them appear to have only the one exhaust valve.

These Monosoupape types (built from 1912 to c1919) are identified by a letter preceded by the number of cylinders: the first of these new engines was the 7-cylinder, 80 hp Monosoupape 7A. Like the Omega variants, the letters were not used in strictly sequential order, namely: A, B, C, N, and M. Other examples include the 14A, the 18C and the 9N. See tables below for full list of engine models.

In the contemporary trade press, the newer engines were usually distingushed from the previous Omega variants as "the 80 hp Gnome Monosoupape" or "a 100 hp Gnome of the Monouspape type", while leaving the description of the older machines unchanged.

However, Gnome made three engines of completely different cylinder dimensions which all produced the same power:
 * 100 hp Double-Omega (c1910), 14 cyls., bore x stroke 110 x 120 mm, 15.96 litres
 * 100 hp Delta (c1912), 9 cyls., bore x stroke 124 x 150 mm, 16.30 litres
 * 100 hp Monosoupape 9B-2 (c1913), 9 cyls., bore x stroke 110 x 150, 12.82 litres

In this case and similar others, it can sometimes be very difficult to identify which particular engine is being referred to. In addition, a large number of images on the internet, and even some museum exhibits, are wrongly identified or captioned. Read on.

1908-1914

 * Original single-cylinder Gnom engine made by Oberursel workshop/factory in Germany
 * The Seguin brothers in Paris bought a licence (probably) to manufacture it, founded the Société des Moteurs Gnome (The Gnome Engine Company).
 * Various efforts, perh. a five-cyl motor?
 * 1908: Omega (7-cylinder 50 hp). Further engines with Greek-letter model names, but Gnome didn't produce them in any sort of alphabetical order: Omega, Sigma, Gamma, Lambda, Delta (O, S, G, L, D)
 * In UK, Bristol Aircraft were sole selling agents. Manufacturing license bought by Holt Thomas of Airco, formed the Gnome Engine Company Ltd. Manufactured by Peter Hooker Ltd, printers' engineers of Walthamstow (also owned by Airco). Airco fitted Gnome engines to French-designed Farman biplanes, for which Holt Thomas had also previously bought the manufacturing licence. Bristol made the Boxkite in retaliation, copying the plans for the Farman which had been published in a magazine—but with enough differences to win a patent case brought against them by Farmans (& Holt Thomas?).
 * 1912: first Monosoupape appeared, the 80 hp 7A. This was followed in December 1913 by the 100 hp, 9-cylinder type 9B, which made its first appearance at the Paris Air Show that year (or maybe it was the 9B-2.) This was the last? model to go into production before World War 1 in August 1914.
 * March 1914: Holt Thomas founded Airships, Limited to manufacture French-designed Astra "Torres" dirigibles (powered by Gnome engines?) Sold the first one to the Navy's R.N.A.S. in late autumn? 1914.

Also connections between Holt Thomas, Astra, Surcouf, Airault, Daimler-Renard Road Train, and the Gnome. Daimler also made the Airco D. H. 10 and the 105 hp Daimler engine for Mks. I-IV tank.

World War I

 * August 1914: War. Holt Thomas negotiated a licence to build Mono-soup apes in August 1914, and sub-contracted the manufacturing to Daimler. The blueprints were unavailable, so Daimler reverse-engineered a 100 hp Monosoupape 9B-2 in just eight weeks.
 * 1915: Gnome merged with le Rhône at the French government's instigation. In the UK, Holt Thomas's Gnome Engine Company was renamed as the "British Gnôme and Le Rhône Engine Company Ltd.".
 * 1914-18: Gnome in France continued to develop the Monosoupapes throughout the war with the 9C and the 18C, the 9N in 1916, ending up with the 9M of 200 hp. Other UK manufacturers made Gnomes (possibly the 100 hp Deltas, not the Monos), and le Rhosnes (for which HT had the licence as a result of the French merger. [NB But were le Rhones being made in the UK before the war? Who had the sales agency?]

Post-war
After the war, a number of aircraft manufacturers went under with the glut of unwanted planes flooding the market.
 * Holt Thomas sold Airco to BSA/Daimler in January 1920, and (as the major shareholder) with Geoffrey? de Havilland went on to found the de Havilland Aircraft company with the profits. but Airco was already utterly insolvent, and crashed the next year/a few years later?
 * Peter Hooker Ltd in liquidation in 1922
 * Gnome & Rhone continued to make engines (radial ?), became part of SNECMA in 19xx

Design
Unlike a radial engine in which the propeller is attached to the rotating crankshaft, in a rotary engine the crankshaft is stationary, and the cylinders with the pistons inside them - with the propeller attached to them - rotate around it: it is the entire engine which visibibly rotates.

Crankcase
In all Gnome rotary engines, just as in two-stroke engines, fuel enters the combustion chamber via the the crankcase. Both in the Gnome Omega variants and in the Monos, a mixture of neat fuel and lubricating castor oil (NB is this true?) is pumped by air pressure down the hollow stationary crankshaft. The mix is vaporized by a nozzle on the end of the crankshaft and enters the crankcase.

Fuel transfer
The means by which the fuel reaches the combustion chamber from the crankcase differs considerably between the two main Gnome engine types.
 * Omegas: fuel inlet valve in piston crown (opens some time after air inlet valve closes, valve closes around BDC for the compression stroke.
 * Monos: transfer ports like a 2-stroke (open 20 degrees before and after BDC)

'Exhaust' valve
All Gnomes of both types feature a single large valve visible in the top of each cylinder head. Although it is often called the exhaust valve, it actually functions as both an exhaust and as an air inlet valve, and its mechanism works in exactly the same way on all Gnome rotary engines.

The valve is actuated by a cam inside the engine connected to an external push rod and rocker arm. On every Gnome model the design of these externally visible components is slightly different, which can aid in identifying a particular engine type.

As the exhaust valve, it opens during the downward power stroke (Monos: 85 degrees after TDC. Greeks: When??? - probably rather later, maybe 50 degrees before BDC?) Inside the cam cover, the cam (which is turned by planetary gears) lifts a cam follower wheel connected to the the valve plunger. The plunger is pushed upwards through a brass guide and outside the engine. This lifts a push rod connected to the rocker arm, and the central valve (which constitutes most of the top of the cylinder head) opens.
 * Operation

The valve functions as an exhaust valve as the rising piston expels the burnt gases directly into the atmosphere. On Monos, the exhaust cycle lasts 3/4 revolution, past BDC until TDC.

Inlet stroke
In a conventional 4-stroke engine, the carburettor delivers the correct fuel-air mix through the inlet valve into the combustion chamber. In a two-stroke engine, the carburettor is connected to the crankcase. In all Gnome rotaries the fuel and the fresh air arrive from opposite directions, and the inlet cycle takes place in three stages (air inlet - suction - fuel inlet).

1. The valve remains open at the end of main exhaust stroke, and functions as the air inlet valve, drawing fresh air into the cylinder. The valve then closes (Monos: 65 degrees before BDC. Greeks: when???). 2. Since both valves are closed, the descending piston creates a partial vacuum for a time inside the cylinder. 3. The two Gnome engine types have very dissimilar mechanisms by which the fuel reaches the combustion chamber.
 * 3a. Omegas: The partial vacuum in the cylinder causes the fuel inlet valve (located in the piston crown) to open, and the over-rich air/fuel mix is drawn into the cylinder. As the piston slows towards BDC, the fuel inlet valve starts to close: the compression stroke begins at around BDC (or whenever the valve is closed).
 * 3b. Monos: Like in a 2-stroke engine, the descending piston uncovers the transfer ports drilled in the cylinder wall at 20 degrees BDC, and the over-rich air/fuel mix is drawn into the cylinder. The piston rises from BDC and covers the transfer ports again 20 degrees after BDC: this begins the compression stroke.

Overview of Omega series operation
An automatic inlet valve is located within the piston crown. The valve is forced shut by pressure during the compression, power, and exhaust strokes, and opens through centrifugal force? and vacuum? only during the downward inlet stroke when there is no/negative pressure above the piston. The inlet valve opens to admit vaporized fuel from the crankcase direct into the combustion chamber, where it is ignited and forces the piston down. The exhaust valve opens as described above, and the burnt gases are expelled directly into the atmosphere.

Overview of Monosoupape series operation
Instead of an inlet valve, the Monosoupape series have transfer ports in the cylinder walls exactly like a two-stroke engine. These ports, connecting the crankcase to the combustion chamber, are uncovered by the piston during the downward inlet stroke and admit fuel above the piston. As the piston rises again during the compression stroke, the ports are closed: the spark plug ignites the compressed fuel/air mixture, which forces the piston down again. The exhaust valve opens as described above, and the burnt gases are expelled directly into the atmosphere.

A. Inlet Stroke
Piston is at TDC (top dead centre) at the beginning of the inlet stroke. Inertial momentum ? pulls the piston downwards, creating a slight vacuum. The exhaust valve is still open, allowing fresh air to be drawn into the cylinder.
 * All models

Because of the vacuum and the negative pressure? on the top of the piston, the automatic inlet valve in the piston crown opens. Fuel vapour in the crankcase starts to fill the cylinder and mixes with the fresh air drawn in through the open exhaust valve. The exhaust valve closes and the inlet valve stays open until the piston is at BDC.
 * Omega variants (designed 1908-1912)

The valve closes at 25 degrees after TDC. The descending piston creates a partial vacuum in the cylinder. At 20 degrees before BDC the transfer ports drilled in the cylinder wall are uncovered, and the very rich fuel mix is drawn into the cylinder from the crankcase. The inlet stroke ends at BDC.
 * Monosoupape models (designed 1912-c1917)

Compression stroke
At BDC (D) the piston starts to rise. The exhaust valve is already closed: the inlet valve automatically closes as the fuel/air mix is increasingly compressed. At letter A in the diagram, the spark-plug ignites the compressed fuel/air mixture at around 15-20 degrees (average 18 degrees) before TDC. The compression stroke ends at TDC (B).
 * Greek-letter models (Omega variants)

Transfer ports close at 20 degrees after BDC. Mixture is compressed until ignition occurs at around 18 degrees before TDC, as in Greek-letter models above.
 * Monos

Power/exhaust stroke
The resulting pressure drives the piston down, forcing the inlet valve shut. Because the crankshaft is stationary and fixed immovably to the rest of the airframe, the entire engine rotates around it: the propeller is attached to the end plate at the front of the engine, and thus turns with it. Essentially the forces on the fixed crankshaft turn the engine, rather than the more conventional.
 * Greek-letter models (Omega variants)

After only 85 degrees of rotation after TDC, the exhaust valve opens (C): inside the cam cover, the exhaust cam (turned by planetary gears) lifts a cam follower wheel connected to the the valve plunger. The plunger is pushed upwards through a brass guide and outside the engine. This lifts a push rod connected to the rocker arm, and the central exhaust valve (which constitutes most of the top of the cylinder head) opens.

Ignition occurred at around 18 degrees before TDC (letter A), and the combustion pressure forces the piston downwards. The exhaust valve opens at only 85 degrees after TDC, and the burnt gases are expelled to the atmosphere. There is no exhaust pipe. The exhaust valve remains open all the way round, past BDC, through the conventional 4-stroke exhaust stroke, past TDC. and only closes for more than one entire rotation. all the way
 * Monos

D. Exhaust only stroke
Inside the cam cover, the exhaust cam (turned by planetary gears) lifts a cam follower wheel connected to the the valve plunger. The plunger is pushed upwards through a brass guide and outside the engine. This lifts a push rod connected to the rocker arm, and the central exhaust valve (which constitutes most of the top of the cylinder head) opens. The rising piston expels the burnt gases directly into the atmosphere.

Identification

 * Q. How do you identify a Gnome or Oberursel engine at fifty paces?
 * A. You can't. All Gnome pre-1918 engines (and copies made under licence) are externally very similar, including: the original Greek-letter engines (Omega variants); Oberursel U.0, U.I and U. III; Russian-made copies {name?]; and the later Monosoupapes.

Only two early Gnome engines are readily and uniquely identifiable:
 * 60 hp Sigma ('double' rocker arm on the exhaust valve)
 * 80 hp Monosoupape 7A (twin bolts in the webs between the cylinders)

Correct identification of all other Gnome engines (and Oberursel copies) up to 1918, involves various combinations of two or more items from the following list. There are at least eight designs of rocker arm (nine with the Oberursel), at least twelve pushrod couplings (several shared between two models), five arrangements of the spark plugs: all of these are beautifully, strangely, and soul-destroyingly different.

Topmost cylinder, from top to bottom:
 * 1) Rocker arm on the exhaust valve
 * 2) Spark plug - placement and number
 * 3) Cylinder base attachment to the crankcase
 * 4) Crankcase cover studs & nuts - In pairs behind the pushrod ends, except on the Mono 7A & 9M, where they are evenly spaced all the way round.
 * 5) Pushrod coupling to the exhaust valve plunger through the circular cam casing - each Gnome model is slightly but significantly different (see pix)
 * Another obvious factor is the number of cylinder fins (see tables), but various combinations of the above five items are enough to identify any particular model.

Rocker arm
NB In these diagrams, the rocker arm is shown in the valve open position - when closed, it is angled upwards. The pushrod is on the left, with the v


 * rowspan="6"

Spark plugs
Stuff

Cylinder base
Stuff

Crankcase cover studs
In pairs behind the pushrod ends, except on the Mono 7A & 9M, where they are evenly spaced all the way round.

Pushrod coupling
Coupling to the exhaust valve plunger through the circular cam casing - each Gnome model is slightly but significantly different (see pix)

Another wikitable of ASCII art?

Other
Another obvious factor is the number of cylinder fins (see tables), but various combinations of the above five items are enough to identify any particular model.