Model Engines - Gnome and Aeronca

Sorry, but I cannot help injecting some trivia on the Gnome engine. The Gnome is a "rotary" not a radial. The crankshaft is bolted to the firewall while the crankcase and cylinders rotate with the propeller. They had exposed flapper exhaust valves (well shown in the photos below). They were a 2 cycle engine that burned a mixture of gasoline and castor oil. On WWI fighters such as the Sopwith, you can see what looks like a cooling shroud over the upper half of the engine. I am told that this was not so much a cooling shroud as a method to catch the spray of castor oil mist blowing off the engine. I have read that breathing the mist during a mission resulted in a powerful laxative effect on the pilots. The engine had a very rudimentary carburetor which required the pilot to adjust both the air and the fuel flow. The carb was so difficult to manipulate that most pilots found a good mixture for full power and left the carb set there for the entire flight - including the landing. In order to reduce power to land (and still have full power in reserve) the pilot would switch the ignition on and off during the approach to landing. This resulted in the rrrrup ----- rrrup ------ rrrup sound that is reproduced in the old movie "The Blue Max." The gyroscopic effect of the rotating mass of the engine put tremendous loads on the air frame during maneuvers. Some of the violent WWI maneuvers were made possible by using the gyroscopic effect to advantage. I have read that the typical time between overhauls was about 20 hours.

Charlie, you are building a couple of fine looking models.

Terry S.
 
I was wondering if anyone was going to catch that. Yes the Gnome is not a Radial Engine. Radial Engines have a moving Crankshaft, Rotary Engines do not. Nice catch. ------ "Billy G"
 
Nice rotary charlie.
But I am confuserd by Terry's comment that its a 2 stroke. as i understand it the gnome engines are 4 strokes
Ok they have ports at the bottom of the cyl's but thats to admit a rich mixture fuel- air to mix with the air that has been drawn into the cylinder during the inlet stroke, thro the single valve.that serves as exhaust and inlet this helps keep the valve cool.
Early engines had a valve in the piston to admit a correct mixture but that lead to crank case explosions.

Sorry if this sounds a bit garbled but i am typing on the fly as they say.

Brian
 
The Gnome is a standard Otto cycle engine with a definite exhaust stroke. The crankcase is part of the induction system like a two stroke, so it is easy to confuse it with one, but there are no ports in the cylinder wall. Intake of the fuel/air mixture into the combustion chamber is achieved through a valve in the head of the piston itself. The single exhaust valve in the head is cam operated from the crankshaft.

Tom
 
I stand corrected.
Tom and Brian are right. The Gnome used a fuel/oil mixture induction through the crankcase (similar to a modern 2 cycle), but the combustion cycle was 4 cycle.
Terry S.
 
Hi Tom. yes that is correct for the early engines. but this system has two faults ---if the valve in the piston developed a leak or got a little sticky Hot gases got into the crank case and fired the mixture distroying the motor. also the exhaust valves had a very short life.


Later motors kept the valve open after the exhaust stroke drawing in cool air thus cooling the valve .

The valve was shut early causing a depression, then at the bottom of the stroke the ports opened, alowing a very rich mix to enter the cyl, this mix was to rich to burn normaly, and needed the air in the cyl to attain the correct ratio.

This mix was then compressed / fired / developed power .

On the power stroke the exhaust was opened early ( just as we do in modern motors ) so the gasses escaped before before the ports opened, so no transfer of fuel took place ( it was to rich to burn in any case

Normal scavenging took place on the exhaust stroke .And then it started all over again.

This system was only correct at one speed so hence the blipping.


MY head hurts Brian.
 
I forgot, the french name for this later motor was the Monosupape ( One valve ).

Brian.
 
Terry - have you got a reference you could share that shows a Gnome with a carb? I'd be most interested in seeing what was used.
So far the only thing I've seen that could remotely be called a carb on a rotary was the slide throttle on a Bentley, a later rotary. The Bentley carb had a sliding, tapered needle that metered fuel as the air intake was adjusted. Sort of like an SU without the vacuum bits. <grin>

BTW - my Gnome is modeled after a later version that did not have any valve in the piston. I guess the accelerated experience of wartime use taught the engineers that it "seemed like a good idea at the time" but was more trouble than it was worth.

I really do admire the engineers that designed and developed these engines ... just think of how little was really known compared to modern times. We've become very good at improving designs and ideas but how are we really doing with creating original "stuff".

Charlie
 
Hi Charlie and Terry

The valve in piston engine had a basic carb with a throttle and mixture controll.

But the Monosupape had no throttle at all, just a adjustable jet in the inlet to the crank case to provide a very rich mixture.
The lagest part of the mix was provided by the air drawn thro the Ex/ inlet valve so it was imposible to throttle the motor.
power was simply On or Off.


Regards Brian.
 
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