Flame propagation speed

[NickB]

I'm trying to get a rough idea of the combustion speed inside a combustion chamber.

Is there any number i can use as a rough estimate? I know it will be dependant on MANY things, including air/fuel ratio, type of fuel (pump gas in this case) octane rating, and chamber shapes. Does engine speed also affect combustion speed? How does combustion camber pressure before ignition affect it?

Based on what i've seen, a higher pressure mixture will burn faster than a lower pressure mixture. I'm basing this off the fact tat ignition timing is usually advanced under vacuum, and retarded under boost, and the advance is roughly proportional to the absolute pressure. How far off am I?

What about chamber shape? How would a spherical chamber compare to a hemi chamber, or a pent-roof DOHC chamber? The two cases i'm specifically interested in are a pent-roof chamber, and a chamber that is similar to a wedge design but with the valves shrouded along one side (chrysler 2.2/2.5)

How much of an affect does spark energy cause?

I'm trying to generate some ignition maps for some race engines, and it would be nice to have a good idea where to start. Thanks again guys, you're a great help!!!

 

I've always heards that 3 milliseconds (3/1000 second) is a good rough estimate. Obviously, as you note, there are many factors which could change things. I'm curious to see what the combustion speed is with BMW's multi-spark system...

Jack Rosebro (rosebro@mindspring.com)

 

[NickB]

I was told once by someone who builds hondas superbikes it was 10ms, then i did a little mental math and realized it took 10 ms for a complete engine rotation at 6000 rpm... obviously something wasn't right. Even at 3ms, that's 108 degrees of crank rotation....

The more i learn, the more i see i need to learn.

 

[buzz41]

Some numbers from the Gasoline FAQ by Bruce Hamilton, as I recall:

Under STP, a stoichiometric mixture has a flame speed of ~0.5 meters/sec. A figure for flame speed in an 8:1 CR engine was ~10 meters/sec. ( not very fast at all. ) So, the more revs, the more turbulence needed for optimum combustion and the more compact the charge volume shape should be, I would guess.

The flame slows down greatly with leaner mixtures. That is what most of the cruise power advance is for, I believe, to compensate for a leaner mix. In fact, if the mix is very, very lean, it will slow down and still be burning as the intake valve opens again for the next cycle. This then lights off the incoming charge and causes a backfire. This seem to be the case, be it a lawn mower or Wright R-4360; they all backfire if they begin running out of fuel and go extra lean.

The FAQ gives flame speeds of most gasolines within a small range. Hydrogen is something else though! 20 to 50 or more times faster. For hydrogen, ignition would be at TDC or nearly so from idle to redline.

Interesting subject, internal combustion.

 

[NickB]

Ah yes, i forgot about how turbulence in the chamber will affect the flame travel across the chamber. Since the flame is travelling through a moving medium, the flame front can cover the entire chamber faster than if the mixture was still. Hrmmm.....