When my electrical system failed so spectacularly recently, there was a tremendous explosion in the exhaust system. I was running at highway speed--and with no ignition--the cylinders stopped firing, resulting in a bunch of unburned fuel dumping into the exhaust system. The explosion ripped the seams of the muffler wide open, and there is a curious dent in my transmission cover not unlike a bullet would make that didn't quite have enough energy to penetrate all the way through. My truck is running fine now, so hopefully there was no internal engine damage.

Jim

 

Perhaps someone can enlighten this ignorant old sot. It takes three things to create combustion 1) Fuel, 2) Oxygen (air), 3) Ignition source. We all know that triangle, and all three must be present. Sooo, how can there be an exhaust pop when there is no fuel present? How would adding fuel prevent an exhaust pop? This has me baffled. Maybe someone can accommodate me here.

 

Go back and read post #14 it explains it.

 

I am very curious now. Can you cite any technical papers on this? Again, what kind, brand engine are those valves you show from? Thanks for any and all replies. Just an old boy trying to learn something new.

 

 

I agree with Bigwin.

From post #14:
"What happens exactly is that the mixture leans out enough when the engine is over run with the throttle closed to where the mixture it fails to ignite consistently."
Seems to me that if there is still spark at the appropriate time in the combustion chamber there wouldn't be any unburned fuel passing through unless there was a spark timing issue, and even if so it would be extremely lean.

"This, in turn allows some un-burnt fuel to get into the exhaust. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust, causing the backfire/popping. Some carbs have an enricher circuit which cuts in on deceleration to help alleviate this problem."
Unless there is a valve timing issue, the flame front will not pass the exhaust valve. The hot exhaust manifold/pipe/muffler provides the source for ignition. The pressures created by the rapid expansion of the explosions will take the path of least resistance.

"Enriching the idle air will alleviate it or help mitigate it as it will add more fuel to help ensure more consistent ignition of the fuel air mix when the engine is over run with the throttle is closed.. "
Enriching the mixture to increase the likelihood of unburned fuel passing through the combustion process would only seem to exacerbate the situation.

Edit: I perused the SAE white papers but couldn't find anything relating to this. Try your luck here:
https://www.sae.org/publications/technical-papers

 

So, I will further speculate that the valves pictured were first wire-drawn by debris lodged between the seat and valve face, which began the erosion process and caused the eminent failure.

 

The posted ones first was a carbed CB650 the other 2 were a carbed SBC.

The Fuel air ratio for combustion is pretty narrow remove enough fuel or air and you won't have combustion. Spark or not or even with an open flame it won't support combustion as the ratio is not correct to support combustion.
In terms of the carb, under deceleration you are pulling high manifold vacuum therefore pulling more air past the closed /at rest throttle blades. The idle air circuit fuel delivery is fixed it does not adjust with air flow significantly like the main circuit does. So you lean out the mix past the point of supporting combustion. Add more fuel by opening the throttle to expose the transition circuit or start flow on the main cuircuit and the afterfire goes away as the mix has enough fuel to support consistent combustion in the cylinder.

When you have an after fire situation it is due to a lack of fuel to support consistent combustion in the cylinders. So you end up dumping raw fuel in to the exhaust along with oxygen, eventually the mix will reach a ratio that will support combustion and the whole works will ignite in the exhaust when a cylinder does decide to fire.

It is the impact of the shock waves from the afterfire that fractures valves, they are taking what are essentially repeated hammer blows on the back side of the valve from the afterfire.
Stock type valves are not designed to take this sort of impact on the back side.
In applications were afterfire is going to be encountered consistently such some racing applications the valves are designed to withstand it. The stem to seat transition will be much beefier such as these below. Valves such as these are designed to take the repeated impacts of afterfire shock waves.
An afterfire is considered a type of backfire. All afterfires are backfires but not all backfires are afterfires.

 

Are those two piece valves?
Then if I am to understand you correctly, you assert that the air/fuel mixture on deceleration is too lean to burn in the combustion cycle, and passes through the combustion chamber unburned (or partially burned??) and collects somewhere downstream in the exhaust system until enough has collected to form a combustible mixture, and then ignites? And if the air/fuel mixture is made richer, the chances of complete combustion is greater and more complete, and no unburned fuel passes through the combustion cycle and thus does not collect in the exhaust system?

 

Yes 2 piece with hardened valve tips.

And that is exactly what happens. This is the same phenomenon that happens with a lean condition exhaust backfire, I'm sure most of us have experienced that due to fuel starvation.
In the cylinder the mixture burns at a pretty constant rate and the pressure rise is pretty linear. In an afterfire in the exhaust it does not burn but detonates. If multiple pockets of fuel air in the exhaust ignite simutnatioulsy or close to each other the propagation of the shock waves can combine and drastically increase it's power, additionally the contour of the exhaust ports can also help to concentrate these shock waves. The principle of shock wave concentration is used in shaped charges for demolition or for armour piercing ammunition.