Hi folks ! As a newbie in 4c engines, could comebody explain why does backfire occur, how do we prevent it from happening, and what are the precautions we need to take in case a backfire occurs ? Much obliged....................

The glow model airplane engine whether it be 2 cycle or 4 cycle runs in a semi diesel manner. The ignition timing is created by the heat in the glowplug plus the rate of burning of the fuel. More heat - sooner combustion. Less heat - later combustion.

A rich fuel mixture will cool the combustion process thereby retarding the timing.

A lean mixture creats more heat and therefore advances the timing.

Keep in mind that the combustion process is somewhat more efficient in a 4 cycle engine. Therefore there is less fuel difference in mixture required to effect a greater change in heat produced.

Therefore your backfires are created by a lean mixture or the glowplug is too hot for the mixture you are running.

Backfires most often occur when the engine is trying to accelerate but can happen in level flight too if too much heat builds up in the engine.

Enjoy,

Jim

Thanks w8ye. What is the effect of the backfire on the motion of a propeller ? Does it decelerate suddenly and then continue to gain speed back to normal ? Or does it reverses its direction momentarily, and then continue to gain speed back to normal ? Thanks again.............

Hi w8, just to clarify my question above, when I say propeller speed, I am referring the the rpm. Thanks............

In the beginning with a mild backfire you just hear what sounds like a very mild popping noise, usually around the intake. If you are standing beside the motor. Nothing happens to the prop or anything.

In a sevier backfire, it is almost like a big pop. The engine usually stops running.

Sometimes in the case of a sevier backfire the prop will just loosen.

Sometimes the prop and prop nuts come completely off.

Sometimes the prop is displaced with it's relationship with the spinner. Sometimes it strips off the seriations on the backplate. Sometimes it blows the spinner cone completely off.

This is usually caused by the motor instantly stopping and the prop and spinner are still trying to rotate.

A sevier backfire is caused by a lean condition and the backfire itself blows what fuel there is left out of the engine. The engine may make a revolution or two in the reverse direction but that's all.

Most highly tuned 4 strokes are highly subject to this phenomina. Likr fot example the Y-S and some of the Saito's. But I have seen OS and Magnums do it too. But the old OS 52 just usually quits and nothin happens.

Jim

Woww........the stress on the crankshaft must be tremendous in severe backfire !! I wonder if anyone have experienced bent or damaged crankshaft after a backfire ?

I've often wondered too. The only one I've had to backfire while running and that is the Y-S 91. All the other engines have backfired while starting them. Kinda think the starter helps undo the nut and spinner?

Havn't had a broken crank yet.

Jim

Hi w8, I can think of a few possible solutions:

1) Adjust the engine slightly richer and not too lean

2) Use a fuel with "anti-knocking" agent. Not sure if this is available in rc world ?

3) Design prop nuts with anti-clockwise threads to prevent loosening at starting. I know that this will not happen against current standards

What do you think ? Thanks..........

Well, on the Y-S, it helps a lot if your fuel is 15% or greater and to keep it on the rich side.

The other things you mentioned sound nice but meanwhile we are going to live with what we got.

Enjoy

Jim

With normal running the engine tries to tighten the prop nut everytime it fires so a left hand thread nut would be counter productive. At all times other than on the power stroke the prop is actually driving the engine, which means of course that the prop and engine are slowing down slightly. A backfire when starting is the most likely to throw the prop because the starter is applying a torque to the prop nut trying to unscrew it plus with the back fire the engine is trying to go backwards.

A backfire can only happen if the mixture ignites too soon or the pressure rise after ignition is too rapid. Too much power from the glow driver makes the plug glow too hot and advances the ignition on start up. The ideal is to have the plug glow just enough for easy starts and no hotter. A lean mixture burns faster than a rich mixture so even if both begin to burn at the same point then the lean mixture will build up pressure much faster which can then drive the piston back down before it's had a chance to go over TDC. Another culprit is too much nitro which has the affect of advancing the ignition point.