I have been wondering,what if the coil in the plug fails during the flight ,will the engine quit? since our glow engines are half diesel. lets think about it a bit.

Sometimes they do and sometimes they continue to run until you shut the engine off. Sometimes, they quit when you go to idle. It depends on where the element breaks and how much of it shorts against the body of the plug.

Enjoy,

Jim

Once the engine is running, it doesnt matter what the coil is touching (it cant be shorted out because there isnt any electrical current flowing anyhow).

The answer is yes, and no - it depends upon how much of the coil is left. I have personally had engines keep running like nothing is wrong, but the coil became broken near the weld. It wouldnt start again (open circuit) but ran fine untill the engine stopped.

I have also had coils more than 50% disintegrate, and then ususally the engine dies since there isnt enough coil left to continue the catalytic reaction.

One funny thing, I do often in a jam without a spare plug, is if the plug does go open circuit, I take a small screwdriver and push the coil up against the plug body to make contact. Then, you can restart the engine and run it for another flight. But not recommended unless you are hard up for one last flight and havent a spare plug handy..

AJC

I remember when I first started flying the only plug available was the brass knobbed KLG's. Every so often the coil would break near the centre and you'd never know until the next time you hooked up the battery. We got quite good at sticking a pin through the coil and stretching it a little until the broken ends made contact then off for another fly

Shakib,

A glow engine IS NOT a half Diesel.

The ignition results from a catalytic reaction of the platinum in the glow element, on the methanol in the fuel.
The methanol breaks down and heat is released, keeping the glow element very hot.

The heat of the burn will not suffice in keeping this glow element hot. If you use gasoline/petrol in your glow engine, it will run ONLY when the battery is connected to the glow plug. The engine will shut down as soon as you remove the plug driver, despite the higher burn temperature of the petroleum fuel.

Well that's half right but let's not confuse Diesel with compression ignition which is what a glow engine is partly.

For the catalytic reaction to occur two things are necessary (and for the pedantic, I'm ignoring surface area of the coil). First, a high enough temperature of the coil. The temperature needed is the inverse of the pressure of the mixture around it. Which is a fancy way of saying that the higher the pressure the lower the temp needed in the coil. But even an electrically boosted temperature won't necessarily fire the mixture...ask anyone who's got a worn out engine

A glow engine is a combination of a compression ignition engine and an ignition hot spot (usually called a glow plug ). The catalytic reaction isn't a requirement, it's just a fortunate circumstance that methanol happens to be a good fuel and allows us to throw away the battery once the engine is running.

For those who still don't think it's a partial compression ignition engine, if you lower the compression ratio then you retard the ignition point and vice versa.

Not too long ago I ran my Saito .80 on Davis Diesel fuel, I was totally surprised when it started right up with the glow battery connected and even more surprised when it would run without the glow battery connected but the biggest surprise of all was when it started and ran without the glow battery at all. The compression on the Saito was too high to run it higher than half throttle.

Dar, check this out [link]http://www.rcuniverse.com/forum/gas_fuel_with_glo_plug_no_ignition/m_1267448/tm.htm[/link]
gas can be run without a battery connected to the plug...i guess idle is still a prob but i'm sure it could be worked out. do a google on "smart plug" and you'll find some interesting reading: a glo plug type thingy designed to work on full scale aviation engines running gas, battery power needed only for starting!




dave

Ususlly you don't notice it till the next time you start the engine.

If the coil just breaks, the engine will just keep on going (most of the times)... But the next start will be a bit of a problem... (it won't work)

If a part of the coil gets released (and passes through the exhaust port without doing further damage) the engine will keep on running at full throttle, but it will sag a little... If you go to part throttle or idle when this happens, it will die most of the times...

If the plug coil completely fails (releases) the engine will quit... and unless you are a very lucky person you will be shopping for a new liner and piston...

So use the right plug... and it won't fail... think about it...