[Vwdiesel] Diesel octane or why can you run a diesel engine lean?
val at swamps.roc.ny.us
Sun Mar 20 00:35:00 EST 2005
> how could compression affect point of ignition? seems to me
> it's going to be the hottest at TDC, since that's when the
> air is compressed the most. when it starts to expand it's
> going to cool down again. that stays the same whether or
> not it's good compression. it's just a matter of how hot it
> gets, not when in the cycle it's the hottest.
There is point of ignition, and there is rate of burn.
Flash point varies depending upon temp, pressure and
fuel mix. Fuel mix is a primary factor. Turbulence,
atomization, fuel chemistry variations are some of the
fuel mix parameters. The object is to get the fuel
and oxidizer molecules close together.
Temp is pretty obvious. A certain degree of compression
will affect temperature. So will a cold engine vs a
engine up to temperature. High temps get the molecules
closer together, resulting in flash.
Pressure is another direct factor on ignition. Higher
pressures force fuel molecules closer to oxidizer (atmospheric
oxygen), and affect flash.
If injection were to be advanced, so that there was injection
prior to TDC, then there may be insufficient pressure and
temperature to ignite the fuel. Similarily, if injection
were delayed (well, practically speaking, sustained) far
down the power stroke, the pressure would become less, and
less efficient combustion (soot) would result. Practically
speaking, a real life engine would not run well with these
Ultimately, it's not really an issue of when it's hottest,
merely that it's hot enough, with enough pressure. The timing
of the fuel burn helps optimize the extraction of energy
from the burn, without high mechanical stresses.
> i would expect droplet size would affect how long it took
> the flame to burn the entire fuel charge, not point at
> which it starts to burn. if the drops were too big then it
> might not get burned completely. but i still don't think it
> affects when it starts to burn.
Well, mixing is a big factor. If you have large droplets,
you will have less efficient combustion (soot) because
some of the droplet will not work it's way to oxygen
in time. You are right in that a larger droplet may
start burning at the boundry at the same temp/pressure,
however the finer atomization will better support the
burn. Practically speaking, you need to burn almost
all the fuel.
In a Otto cycle gasser, the fuel and oxidizer are presumably
completely mixed, so that the whole charge of fuel will be
nearly completely burned. Pressure and temperature will
rise rapidly, after ignition, and the flash will propagate
very quickly. By the way flame front propagation is also
a function of pressure and temperature. Under higher
temps and pressure, the flame propagates faster.
> --- Val Christian <val at swamps.roc.ny.us> wrote:
> > Erik,
> > You have the picture.
> > Point of ignition could be delayed if compression isn't
> > quite up
> > (old engine, cold engine, etc.). Droplet size influences
> > things
> > as well. But those differences are microseconds, not
> > milliseconds.
> > Val
> > >
> > >
> > > > PS : Injection point and point of ignition are
> > NOT
> > > > the same. And Flashdance ?
> > > > I saw the Royal Ballet of Copenhagen ---and I doubt
> > that
> > > > any of those dames could spin on
> > > > shoulderblades like that. ---And NO stealership can
> > TUNE
> > > > an old Rabbit to my satisfaction
> > > > NO how NO way.
> > >
> > > Are you maybe using these words in a different way than
> > the
> > > rest of people? I'm honestly asking and not trying to
> > infer
> > > anything.
> > >
> > > Because everything I've read on the diesel process says
> > > that the temperatures in the cylinder are so hot that
> > the
> > > fuel bursts into flame the instant that it is
> > introduced to
> > > the cylinder. So the timing of when that happens is
> > > critical to proper running of the engine. This seems to
> > me
> > > to completely contradict what you say about ignition
> > point
> > > and injection point NOT being the same thing. From my
> > > understanding they are necessarily the same, unless the
> > > pump is grossly out of time. (I once managed to time
> > mine
> > > 180 degrees off! Embarassing, but true. It was the
> > first
> > > pump I had ever timed and couldn't find the mark so
> > marked
> > > it myself and forgot that it too ran at 1/2 crank
> > speed.)
> > > So then ignition didn't start until the cylinder
> > actually
> > > had compression. The darn thing still ran, but had all
> > > kinds of white smoke and no power. If the pump is timed
> > > right it seems to me it starts injection just a hair
> > before
> > > TDC and the fuel flashes to flame immediately.
> > >
> > > Thanks,
> > > Erik
> > >
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