Aircraft Engine Backfires - why does it happen?

[nyoung]

Question about backfires... why does it happen? I am specifically refering to backfires where the mag switch is accidentally switched off and then back to the L,R, or both setting?

I was trying to think this through in my head... and at first thought that the lack of spark (the mag=off setting) allows excess/unburned fuel in the cylinder that is ignited when the spark comes back (ie mag=on).

But, I think this is not accurate for 2 reasons:
1. Even if the mags=off... if the engine is still turning (windmilling) the suck/squeeze/bang/blow cycle occurs. Excess air/fuel mixture should be getting pushed out the exhaust valves in the blow cycle.

2. Mags are timed to the cylinder suck/squeeze/bang/blow cycle. So, in theory, turning the mag back on should only apply spark during the bang cycle. which is when it should occur anyway. I don't see how that would cause a backfire.

So then I read on the internet (gotta love wikipedia) that a backfire typically occurs in the exhaust/tailpipe. At first, this too made sense to me... excess fuel gets pumped into the hot exhaust header, and voila it ignites. But then, I realize that the backfire typically only occurs once the mag is turned back on...

So now, I'm on to this theory...
1. Mag off=excess fuel in cylinder
2. Blow cycle pushes the unburned fuel into the exhaust
3. For some reason the exhaust does not burn the fuel immediately, nor does the unburned fuel completely leave the exhaust header.
4. Switching the mags=on restarts combustion, thereby exhausting 'new' hot exhaust into the header which then ignites the excess fuel, causing the backfire.

As you can see, I've been through a few iterations with this and am pretty confused. So, A&P/engine experts... what's the scoop?

 

[Capt. Geoffrey Thorpe]

So now, I'm on to this theory...
1. Mag off=excess fuel in cylinder
2. Blow cycle pushes the unburned fuel into the exhaust
3. For some reason the exhaust does not burn the fuel immediately, nor does the unburned fuel completely leave the exhaust header.
4. Switching the mags=on restarts combustion, thereby exhausting 'new' hot exhaust into the header which then ignites the excess fuel, causing the backfire.

You got the basic idea. With mags off, you are pushing the combustable air/fuel mixture into the exhaust pipes. Mags back on, the hot gas from the blow down will ignite the stuff in the pipes.

Bang.

With older cars (that still had carburetors) this is what you did in a tunnel or under an overpass to get the echo effect. Well, at least you did it until you blew out the exhaust sytem and decided it wasn't worth the price of repairs

 

[gismo]

Capt. Geoffrey's answer is correct except that AFaIK technically what you're describing is called "afterfire" and a true "backfire" occurs when the fuel mixture upstream of the intake valves (e.g. in the intake manifold) ignites. FWIW, the most common cause of that is a leaking intake valve. You can also get an afterfire when the mixture is so rich that it doesn't ignite in one cylinder letting the compression and exhaust strokes dump the unburned fuel into the exhaust system where it finds enough oxygen to light off. This is the "pop,pop" sound you sometimes hear when you close the throttle quickly. Of course, for some reason, almost everyone calls that a "backfire", probably because the word itself sounds better.

 

[Dan Thomas]

This is the "pop,pop" sound you sometimes hear when you close the throttle quickly. Of course, for some reason, almost everyone calls that a "backfire", probably because the word itself sounds better.

"Backfire," they think, because it comes out the back. Like you say, it's an afterfire. If you turn the mags to OFF in the runup, say, the engine will fill the exhaust system with raw air/fuel mixture that will ignite when the mags are turned back on. Since at anything above a high idle there's some flame coming out of the cylinder past the exhaust valve, that flame does the igniting. The resulting bang is known to crack exhaust components, which are not cheap.

We tell our students that they must let the engine die if they accidentally go to OFF in the runup. It will restart without the bang and subsequent damage. The accidental OFF is usually because they're holding the key by its edges instead of its flats, and it can snap further than we want it to if we just squeeze a bit too much. But just try to get them to hold the key by the flat sides instead of the edges...nope. And so we sometrimes find cracked mufflers.

A true backfire can be due to a too-lean mixture on startup, or sometimes if the engine is leaned in flight too aggressively. The lean mixture is slow to burn and there may be a bit still burning at the top of the exhaust stroke where there's a bit of overlap between the closing of the exhaust and opening of the intake, and the flame sets off the charge in the induction system. Some guys have managed to clear serious induction ice by blasting it like this. I've never been able to get a backfire doing it. An overprimed engine can backfire, too, and set overflowed fuel afire in the airbox or cowling.

As kids we used to make big afterfire bangs with our cars by turning the ignition off for a few seconds while coasting in gear, and then back on. Sure shortened the life of the exhaust system. My first car, a Beetle, had a Zoom Tube, a four-inch diameter by two-foot side-facing chrome pipe that fed off the muffler outlets, and it would let loose a three-foot flame with a loud crack. Handy for making aggressive hitchhikers jump back.

Dan

 

[Cap'n Jack]

Dan- thanks much- I read about clearing the carb of ice as you described on other threads and you explained how it works very well.