chenery 9 cylinder gnome

[charlie9cylParticipant @charlie9cyl]

Having built a 9 cylinder rotary Gnome to Les Chenery drawings I am now having problems starting the engine. I am using a MiniMag ignition system with commutator ring and conventional points. I seem to be getting a good spark with firing set at about 18 degrees before TDC. I am assuming the timing is not super critical to get it firing, hopefully being able to adjust a few degrees either way once started. I have tried various fuel setting, even turning the engine with fuel cut off for some time to try and dry out the plugs. Likewise I have primed each cylinder head with a small amount of petrol (syringe) via the open exhaust valve to no avail.

Compression seems good on each cylinder and valve timing appears correct. Not sure where to start. I feel the ignition is the problem but not sure.

Has anyone got experience of a 9 cylinder Les Chenery Gnome and can point me in the right direction?

Thanks

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Absolutely no experience of 9-cylinder engines here … but instinctively, 18° seems like too much advance.

Hopefully someone wiser will advise.

MichaelG.

[Dave HalfordParticipant @davehalford22513]

Try 10 deg advance.

it’s possible the volts are too low to fire the plug under compression or conversely compression is in fact too low.

This is where an old skool neon timing light might help.

[JasonBModerator @jasonb]

The 1/3rd scale Gnome is timmed at 18deg BTC so you should be in the right ball park.

How are you activating your points to get the correct firing. Also most of the electronic ignitions fire the opposite to a set of points when the contacts close

[Andy StopfordParticipant @andystopford50521]

Firing order wrong/reversed maybe? Is there any sign of ignition at all?

[ChrisLHParticipant @chrislh]

Charlie, you have a PM

[noel shelleyParticipant @noelshelley55608]

Jason should know but to me 18* is far to early. The original gnome had a strange induction where over rich mixture was drawn in and brought to a combustible mixture by the ” exhaust ” valve staying open to allow more air in ( or something like that), followed by compression and ignition. In the manual the mixture is described as incombustible – initially ! The fuel was dribbled into the hollow mounting tube that fed the crankcase. Are you using some form of carb ? Noel.

[Michael GilliganParticipant @michaelgilligan61133]

This page is presumably well-known to the cognoscenti … but fellow idlers might find it of interest:

https://modelenginenews.org/ed.2007.04.html#t1

MichaelG.

.

Edit: Lovely video here of an Avro 504K being started:

https://youtu.be/4fMrzVzHr5Q?feature=shared

.

[JasonBModerator @jasonb]

Noel, it does sound a lot but as I posted above the larger 1/3rd scale one gives 18degrees TDC so would seem to be in the right ball park for the 1/5th scale version. Like a lot of the IC engines I build the 1/3rd scale one has adjustable advance and retard so easy to try a few different angles.

[noel shelleyParticipant @noelshelley55608]

Yes Jason, and as I sit here thinking what the engine is, the mass of the rotating cylinders would allow for early ignition and less risk of firing backwards. Thank you. Noel.

[Howard LewisParticipant @howardlewis46836]

I thought that the Gnome Monosoupape was so called because it had only the one (Exhaust) valve for each cylinder, with mixture being fed in via the hollow crankshaft, with the inlet valve being within the piston.

This arrangement, couple with a huge exhaust valve clearance, and inhaling some air via the exhaust valve, night account for an unusual ignition timing, compared to conventional four strokes.

Rotary engines seemed to have no throttle, engine speed being reduced, when required by the blip switch on the control column, to short out the magneto.

When we lived at Shoreham by Sea, the son of the chairman of BEA or BOAC used to fly his replica Vickers Gun Bus, on a Sunday morning, blipping the engine as he came into land.

Apparently aircraft such as the Gun Bus, DH2 and the like would roll more quickly one way than the other because of the gyroscopic, and torque, effect of the rotary engine.

Howard

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