Aircraft engine, rotary air-cooled nine cylinder, Le Rhone, steel / copper, Societe Des Moteurs Le Rhone, France, 1918. This Le Rhone 9Z rotary engine has nine finned cylinders arranged radially around its crankcase, to which the propeller is attached. The 'crankshaft' is hollow and designed to be fixed to be body of the aircraft, while the cylinders and propeller rotate. Cylinder bore is 112 mm, piston stroke is 170 mm, compression ratio is 5:1 and power is 90 kilowatts (120 horsepower) at 1200 rpm.
The engine operates on the four stroke cycle, and air is first drawn through the carburettor thence through the hollow crankshaft to an annular chamber behind the crankcase and finally via copper pipes to the engine cylinders. This tortuous passage severely limits engine performance, but this is offset by the low engine mass of the rotary design.
A block-tube carburettor with a simple fuel jet is attached to the rear end of the crankshaft. Since the crankcase volume in a rotary engine does not vary as the cylinders rotate, there is no pumping action to force a fresh mixture into the cylinders. Instead, the cylinder is filled due to the suction generated inside it as the piston travels down after the exhaust stroke. Because the only forces moving the fuel-air mixture through the engine are centrifugal force in the transfer passages and suction generated in the cylinders, rotary engines require a pressurised fuel system to spray petrol into the carburettor.
A single cam-actuated push-pull rod and rocker arm, pivoted near its centre, operates both the intake and exhaust valves. When pulled down it opens the intake valve and when pushed up it opens the exhaust valve. To achieve this action two cams are used, and the positive action of the cam followers is accomplished through levers. This system suffers from the disadvantage that the intake valve cannot open before the exhaust valve has closed, limiting the speed at which the engine can run efficiently. The settings for valve events are: intake opens 18 degrees after top dead centre (TDC) and closes 35 degrees after bottom dead centre (BDC); exhaust opens 55 degrees before BDC and closes 5 degrees after TDC. Ignition occurs 26 degrees before TDC.
A single magneto, driven at 2.25 times engine speed, fires the spark plugs mounted on the side of each cylinder. Ignition occurs in alternate cylinders during each revolution of the engine. The engine was started manually by turning the propeller. It normally ran at full speed, which could only be varied by intermittently cutting the ignition via a 'blip switch' on the control column.
All the connecting rods couple directly with the crankshaft. A split doughnut-shaped ring, which surrounds the crankpin, is provided with three concentric grooves. Into these grooves are fitted slippers of connecting rods 1,4 and 7 in the inner groove, 3,6 and 9 in the middle groove and 2,5 and 8 in the outer groove. This design allows all pistons to experience identical dynamics during the engine cycle.
It is impossible to fit an exhaust system to spinning cylinders, so the exhaust valve on each cylinder opens directly into free air inside the cowling. Because there is relatively little pressure in the cylinder when the exhaust valve opens, the engine is relatively quiet.
Lubrication is a total loss system. Castor oil is injected into the carburettor by an engine-driven pump, and the unburnt residue forms part of the exhaust. The fuel-oil-air mixture flows through the crankshaft and into the crankcase, where it lubricates the bearings before being transferred into the cylinders.