Improving flame front

[Will L.]

Warning bumped my head again and got to thinking...all insults welcome:hihi:

So I watched a rerun of mythbusters about blowing manhole covers 50 feet in the air. The jest of it is with clean passages the flame front could barely pop off the manhole covers, but with a bunch of obstructions in the way it had a massive increase in flamefront and of coarse increase in power. I didn't find a video of the small test that shows it, but here is a caption:

For small-scale testing, Adam and Jamie built a miniature sewer pipe fitted with a full-length viewing window and three manholes. They pumped in enough methane to reach a 9% concentration in air (the center of its flammability range) and used a spark to set off the mixture. A test with both ends of the pipe open gave only a small flash of flame; when the ends were closed, all three covers flew off. The addition of metal debris to the pipe, simulating junk that might collect in a real sewer, launched the covers even higher due to a faster, larger flame front moving around all the obstructions.

I cant help but wonder about some type of slinky / stainless steel brillo pad , or some perforated steel in the top of the chamber magically glued to the head, of coarse avoiding the valves and cylinder walls.
At Timet they make titanium, and there is this stuff called sponge. Its a very porous state of the metal being formed - something like that if anyone is familiar with it in a foundry.

I did some play with a simple pipe and gas fumes tests with sos pads and wow. I am years away from being able to put this to real experiments with scrap engines to break. Just thought I'd throw this out there for any one in a playful mood.

 

[schiker]

But where was the spark in relation to the debris and which way was the flame front traveling. I did not see the show but can imagine lots of different things.

Flame front and combustion speed depends on several things. In a pipe a little restriction may slow combustion at first but increase temperature rise during combustion behind the obstruction ..... but increased temperature increases flame front speed. Has some snowball affect? I have lit fires with diesel and when its cold it burns pretty slow but as the fire warms it burns faster. Might be the fire creates some natural convection to increase air mix???? So debris in a pipe might also create some better mixing from turbulence as roughly as a nozzle would ? In large smoke stacks they baffle and change geometry to mix the air so is that what the obstruction does?

There are other ways to make porous metal. Sintered metal if done properly can be very porous but maybe not flexible. A sintered precup ? But would soot clog it up?

 

[JayTheCPA]

Am presuming the question is focusing on the Ricardo Comet and not a potato cannon

Perhaps I am missing something . . . When the flame front comes out of the pre-chamber, it continues to expand and push the piston. Is the goal to get the flame front out of the pre-chamber at a higher velocity? If so, it seems that a faster moving flame front will call for a change to the pre-chamber opening and possibly the piston head as well.

A few months ago Ferm started a discussion on a better pre-cup design and coincidentally I had just finished a tour through the Technical Library. Per the misc tech articles, IIRC, to get better power (efficiency) out of the IDI, the better focus was to further limit heat leaching out of the pre-cup chamber to the coolant. For the OBD systems, it also looks like the system can benefit from split injection, but GM did not enable this feature in the ECM's code.

 

[WarWagon]

Mythbusters possibly could have simulated detonation by lighting things in two places at once to get the rest of the air and fuel hot enough to light by itself. Heat and pressure cause the spark ignited fuel to start burning elsewhere and the ping is when the remaining fuel explodes instead of burning. Combustion chamber design is a real art on gas engines with a goal of avoiding this. IMO Mythbusters was in over their heads in the ability and know how to do this on a large scale. They appear to have chickened out by not trying more than one large scale 'test'.

Vietnam veterans say you can casually burn C4 shavings in a fire, but, it takes something else to make it explode.

Pre-ignition is a gas engine is different. This is the fuel lighting off before the spark plug fires. It is very hard to light the fuel and air being compressed and gets harder as it is compressed more. :skep: Thus pre-ignition is rare. You wouldn't hear it until it quickly melts a hole in the center of the piston.

Diesel engines relatively quickly light the fuel and never mix all the air before ignition occurs. Timing being advanced can waste fuel driving the engine in reverse before the power stroke and cause the same damage as pre-ignition above.

Running an engine on explosions vs. smooth burning of the fuel isn't desired. Damage from detonation is broken: top rings, top ring lands, pistons, shattered cylinder walls, popped head gaskets, heads departing the engine from broken head bolts.

If the emissions are reduced than there is a market for a detonation proof engine.

 

[Will L.]

http://www.bing.com/videos/watch/video/amazing-small-scale-sewer-test/pfxqmn0


You have to watch the first 2 videos and 2 commercials. 1st shows without stuff in the pipe about 20 seconds into 2nd video shows with it, but you have to watch the flame front, how much faster it goes and how much higher the Minnie manhole covers go. Sorry I just cant find a video where they did the slow-mo comparison like they did in the show. I suck at finding things online.