Rotary Engine - Rotary Engine Control

Rotary Engine Control

It is often asserted that rotary engines had no carburetor and hence power could only be reduced by intermittently cutting the ignition using a "blip" switch. This was literally true only of the "Monosoupape" (single valve) type in which the air supply was taken in through the exhaust valve, and so could not be controlled via the crankcase intake. The "throttle" (fuel valve) of a monosoupape therefore provided only a very limited degree of speed regulation, as opening it made the mixture too rich, while closing it made it too lean (in either case quickly stalling the engine). Early models featured a pioneering form of variable valve timing in an attempt to give greater control, but this caused the valves to burn and therefore it was abandoned.

Most rotaries however, had normal inlet valves, so that the fuel (and lubricating oil) were taken into the cylinders already mixed with air - as in a normal four-stroke engine. Although a conventional carburetor, with the ability to keep the fuel/air ratio constant over a range of throttle openings was precluded by the spinning cylinder block, it was possible to adjust the air supply through a separate flap valve or "bloctube". The pilot needed to set the throttle to the desired setting (usually full open) and then adjust the fuel/air mixture to suit using a separate "fine adjustment" lever that controlled the air supply valve. Due to the rotary engine's large rotational inertia, it was possible to adjust the appropriate fuel/air mixture by trial and error without stalling it, although this varied between different types of engine, and in any case it required a good deal of practice to acquire the necessary "knack". After starting the engine with a known setting that allowed it to idle, the air valve was opened until maximum engine speed was obtained.

Throttling a running engine back to reduce revs was necessary to allow pilots to fly in formation - they would close off the fuel valve to the required position then re-adjust the fuel/air mixture to suit. This process was more difficult, so that "throttling back", especially when landing, was often accomplished by temporarily cutting the ignition using the blip switch.

Some engines were equipped with a switch that cut out only some rather than all of the cylinders to ensure that the engine kept running and did not oil up. A 9-cylinder rotary with this capability, typically allowed 1, 3, or 5 cylinders to be kept running. Some 9-cylinder Monosoupapes had a selector switch which allowed the pilot to cut out either three or six cylinders so that each cylinder fired only once per three engine revolutions, but the engine remained in perfect balance. Contemporary photographs of the cockpit of Fokker Eindecker fighters show a rotary selector switch to cut out a selected number of cylinders showing that this was also true of German rotaries.

Cutting of cylinders using ignition switches had the drawback of allowing fuel to continue to pass through the engine, causing the spark plugs to oil up and prevent the engine from restarting. A raw fuel/oil mix would also collect in the cowling. As this could cause a serious fire when the switch was released it became common practice for part or all of the bottom of the basically circular cowling fitted to most rotary engines to be cut away, or fitted with drainage slots.

By 1918 a Clerget handbook advised that all necessary control was to be effected using the fuel and air controls, and the engine was to be stopped and started by turning the fuel on and off. The landing procedure recommended involved shutting off the fuel using the fuel lever, while leaving the blip switch on. The windmilling propeller allowed the engine to continue to spin without delivering any power as the aircraft descended. It was important to leave the ignition on to allow the spark plugs to continue to spark and keep them from oiling up, while the engine could easily be restarted simply by re-opening the fuel valve. Pilots were advised to avoid the use of ignition cut out switches as it would eventually damage the engine.

Pilots of surviving or reproduction aircraft fitted with rotary engines still find, however, that the blip switch is useful while landing rotary-engined aircraft, as it allows pilots a more reliable, quicker source of power in case it should be needed, rather than risking a sudden engine stall, or the failure of a windmilling engine to re-start as expected, at the worst possible moment.