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A finely made one fifth scale model of a seven cylinder Monosoupape rotary engine; developed in the early part of the nineteenth century the seven and later nine cylinder engines went on to power many successful aircraft in the First World War.

Earlier rotary engines had featured two valves per cylinder, with a atmospheric inlet and cam driven exhaust. The inlet valves proved troublesome in service, leading to unreliability.

In 1913 Laurent and Louis Seguin, founders of the Société Des Moteurs Gnome - later Gnome et Rhone - designed an engine overcoming the problem by doing away with the inlet valve completely, with the "Monosoupape" - literally "single valve" - engine.

In this engine, the exhaust valve opened early in the combustion stroke, letting exhaust gas to vent directly to atmosphere through the valve cage on top of each cylinder, before the piston carried on down to uncover a row of transfer ports lower down the bore - the crankcase contained a rich fuel mixture, fed through the hollow crankshaft from a carburettor mounted at the rear.

The piston continued up on the exhaust stroke, then started down on the induction stroke. The exhaust valve remained open until part way down the stroke, allowing air to be drawn in through the exhaust valve, before it closed. The piston continuing to move down created a partial vacuum - when the row of transfer ports opened on this stroke. a quantity of the rich mixture contained in the crankcase was sucked up into the cylinder. The piston started up on the compression stroke, closing the transfer ports and entrapping a mixture of fresh air and rich mixture, ready for the next combustion stroke.

Unlikely as it sounds, the system worked well - Thomas Sopwith called the engine "one of the single greatest advances in aviation".

Mounted on a test stand, the engine turns over freely with good compression. It uses a cam ring driven by epicyclic gears, roller followers actuating the pushrods giving a 1-3-5-7-2-4-6 firing order.

Complete with a wooden propeller and carrier.