nyoung
Pre-takeoff checklist
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?
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?