THE "ROTARY" ENGINE DIDN'T WORK QUITE LIKE A MODERN ENGINE,ALTHOUGH IT WAS THE SAME BASIC "4-STROKE" CYCLE.
For example: In the Gnome the induction was through the crankcase. There
was a single large valve in the top of the cylinder that provided both
intake and exhaust services. The spark plug was on the side, just above
the piston.
The spark was provided by a magneto. There was an insulated ring on the
back of the crankcase with a hightension carbon brush rubbing on it that
came from the hot side of the magneto. The magneto did NOT have a
distributor in it, but merely generated sparks. The nine cylinder Gnome
has a magneto that generated nine sparks every two revolutions of the
crankcase. The magneto was mounted stationary and driven by gears on
the back of the crankcase. As each cylinder approached top dead center
a wire from the spark plug down to a copper button on the insulated ring
would contact the carbon brush. The high voltage would be connected to
the plug and it would fire the cylinder.
Near the bottom of the fireing stroke the cam in the nose of the engine,
there was one cam for each cylinder on the Gnome, would open the valve
in the head through a large rocker arm. There was a light spring to
assist centrifugal force in keeping the valve closed normally. The
weight of the pushrod tended to try to open the valve. This is one of
the factors that contributed to the relatively low maximum RPM of the
rotary engines. Most peaked out at around 1000 RPM. Also, above that
speed it took more power to rotate the engine faster than the increase
in power gained by the additional RPM.
The valve was held open until the piston made it all the way to top
dead center on the exhaust stroke and then left open a little longer.
When the piston started back down, with the valve still open, air was
drawn in through the same valve we just used for exhaust. About half
way down this "intake" stroke the valve was closed.
Of course the piston continued down to the bottom of its stroke, making
a vacuum inside the cylinder. At the bottom of the stroke an excessively
rich air/fuel mixture was admitted into that partial vacuum either past
the piston through a bypass port in the cylinder wall in the monosoupape
version, or through an inertially operated valve in the center of the
piston in the earlier Gnome engines. This reduced the partial vacuum
and provided the fuel. This rich mixture was leaned down by the air
charge that had been drawn into the cylinder on the intake to make a
burnable mixture.
This mixture was then compressed on the upstroke of the piston and as
the spark plug wire went by the magneto brush on the firewall, it was
fired near to top dead center, and the cycle began again.
Notice that the air charge drawn in on intake was not throttled or
throttleable. That meant that any attempt to throttle the carburetor
merely adjusted the final mixture, rather than the fuel quantity. As
a result, restricted the fuel merely leaned out the engine and caused
it to overheat. These engines only ran at one speed. Run or NOT run.
To throttle the engine for landing the pilot has a "coupe" or KILL
button on top of the stick. This button shorted out the magneto and
kept the engine from fireing. That reduced the power for landing.
To get a shot of power, you merely released the button and the engine
would start again. ( So much for shutting off the mags to simulate an
engine failure. How would you like to do that for THROTTLE control? )
That is the source of the unique sound of the WWI fighter. The movie
maker who made the first big WWI movie, back in the twenties, liked
the sound of the Tommy Morse fighters he taped with their rotary engines
so much, that he dubbed that sound in for the V-8 powered Kansas Fokkers
( actually elephant ear Travelairs with Hispano Suiza V-8 engines like
the one that Dennis Trone flies ) giving us the unique situation of a
easily throttable V-8 engine that sounded like a "coupe" killed rotary!
The rotary feature of these early engines also provided another problem.
Oil was pumped into the bearings to keep the engine lubricated. With
the entire engine rotating and the valve mechanism exposed to the air,
there was no way to recover the oil after it passed through the engine.
There was also a problem with the rich fuel/air mixture in the crankcase.
If they used mineral oil, the gasoline in the crankcase washed the oil
away and shortened the life of the engine. Since the engine was designed
to a 50 hour TBO, significant shortening of that life made any kind of
serious flight difficult. To overcome that problem, they lubricated the
engines with a vegetable oil from castor beans, castor oil, that would
not wash away in the gasoline mixture in the crankcase.
Unfortunately, the excess castor oil pumped into the engine eventually
made its way up the valve pushrods and past the piston into the valve
area, where the excess was evenly distributed into the airstream. The
world war one pilots sat quite closely behind this rotating mass of
iron and breathed that fine oil mist whenever they were flying. WWI
fighter pilots were noted for incontinence on the ground, which was
clearly due to a steady diet of slightly used castor oil while flying.
This is far form a complete article, but hopefully will answer any
questions out there about how these interesting engines worked.
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