How is it that the incoming charge does not get ignited by the remaining exhaust gas in between the exhaust and intake strokes? The exhaust stroke is pushing out 1600-2000 degree gas and the intake valve opens BEFORE the exhaust valve closes. There must be some remaining superheated gas that comes in contact with the fresh charge.... Is it because of the vacuum and that it is not compressed? Even so, it's fuel. Why doesn't it light, even in it's negative pressure state?

 

It will light if there are hot spots, and then you get detonation/dieseling.

I could be wrong, but blowing 2000*F inert (exhaust) gasses at a fuel/air mixture will not lead to combustion.

However injecting fuel into 2000*F combustible gasses will ignite, that's how the diesel works

 

correct me if i am wrong-
but without compression, not much is going to detonate.


secondly

i dont think you should be dumping fuel in there when it is in the exhaust cycle?

 

yeah trey's right no compression no bang bang.

as for the not dumping fuel when its in the exhuast cycle i think he's refering to overlap trey. the superheated gasses from the compression and cumbustion would most likely leave the cylinder during the first part of the exhuast valve being open seeing as they would be hottest (duh) they would be at the top of the chamber (this is purely a guess) have the most energy and exit the exhuast port first(?) never really sat down and thought about it.

 

Correct me if I'm wrong but gasoline does not need compression to ignite.

Now in a closed system gasoline will not ignite, but the cylinder is not a true closed system since the gas has already been mixed with air.

Unfortunately I don't have my IFSTA with me and I've been out of the academy too long to remember the ignition point and flammable (explosive) range of gas.

edit...due to the wonders of the internet:

The ignition point of gasoline is 500-860* F.

And the flammable range is between 1.4-7.6% which is where the mixture will be upon entering the cylinder.

Hmmmm...that's a good question. But I would assume that the intake charge isn't exposed to the exhaust long enough to heat it to its ignition temperature.

 

The problem with your thinking is that the incoming A/F mix doesn't see anything like 2000 degree temperatures. This only occurs after ignition when the compression and temperature are greatest.
During the power stroke the expanding gas cools rapidly. At (X) degrees before BDC when the exhaust valve opens the cylinder pressure is reduced to ambient and the exhaust temperature is dropped even further.
By the time the intake opens the remaining exhaust temperature is well below the ignition temperature of the incoming A/F mix.

 

On top of all of this the highest pressure is on exhaust valve opening, so a large majority of the gasses are gone. There is always a little remnants left over which is residual exhaust. You'll see more at lower RPM. One of the reasons why modern cams have such big LSA numbers.

 

Even if it is no longer 2000 degrees, it can still get hot enough to make your headers glow (well, perhaps not my car, but some cars do this)... that should be in the 500-860F ignition point of gasoline range.

I believe now that it is the combination of no compression and the, shockingly, stability of gasoline. In talking to other people i've heard stories of putting a cigarette out in a small container of gasoline without lighting it.

I've also heard that the firing temperature of a sparkplug is in the thousands of degress, perhaps even up to 3000 degress F. Can anyone verify this?