08/31/2003

Knock and Heat Transfer

First - the time has zero to do with the time the engine is run.

The time factor is the time that combustion takes.

After a few seconds of firing, an cylinder inner surface stabilizes to within
a few degree.  The surface area will be this region.

The surface of the chamber/piston will warm to 5000 degrees Rankine plus - if
its let to.  A small area of gas insulates these walls from the actual heat.
As the intake event happens, the insulation is scrubbed off and cool charge
lowers somewhat the surface temperature. Then the cycle is repeated.

Since each combustion event starts without an insulating charge, early energy
"heat" needed to establish combustion is lost to the surfaces.  Typical
chamber heat for head and piston is around 600 f at the crown - and combustion
reactions don't really start till around 900-1100 f - so by the time
combustion starts - heat is being lost to the surfaces.

The cold gases forming an insulation layer each cycle is all that allows the
components to exist.  Knock, by generating a regenative acoustical wave both
scrubs the insulating gases off the surfaces and forces the introduction of
fresh fuel at an ehanced rate raising the combustion temperature
significantly.

The advent of ceramic coatings places an insulating layer on the surfaces.
This insulating layer so greatly reduces heat movement to the chamber and
piston that there is only one advantage to an aluminum head and that is weight
and strength is the only remaining advantage to a ferrous head.  When, because
if it won't knock, its not making enough power, knock occurs, it still scrubs
the insulating gases off - but - there is a high insulation value undercoating
of ceramic that will resist the temperatures for minutes.  Ceramics save the
metal as well as the heat needed for combustion.

--------------------------------

With a too small piston wall clearance, the land trapped gases simply lower
the potential power by the amount of gases trapped there compared to the total
charge.  Consider that the gases are trapped at the peak of combustion
pressure and are slowly released late in combustion.  Its the volume times the
pressure at peak pressure is the potential for power that can be lost.

The normal response has been to reduce the trapped gas volume and move the
ring nearer the top.  So sayeth the lord.

But, what would happen if you INCREASED the volume?  There was a paper a
couple of years ago concerning that exactly.  Some researchers thought out of
the box.  

They increased the clearance to about .100 or thereabouts ( strange number ),
and a 4 mm chamfers on the edge.  Best results were full circular around the
piston, with 4 prefers over 2.  By other adjustments compression ratio was
held constant for all size pistons.

At this value, the emissions dropped startlingly and in opposition for what
they expected to happen.  And both power and bfsc were improved.  WTF.

It seems that by expanding the size appropriately, a sliding constant volume
combustion chamber was created in the ring land gap and given pressure over
time, the trapped gases burnt almost completely in CV combustion - by around
20 degrees atdc.  

Cross this back to the Waukeeshaw patent about dual plugs for detonation
control from the thirties.  WM started a second combustion front that got
squished out at TDC but, by pre-burning the end gases, left an in cylinder
generated EGR to quench the main combustion as it reached the end gas region.

This generated a "backfire ( forestry term - nothing to do with human methane
) - very similar to what the ring gap combustion would do, and since its
unquenched would start the second flame front - all the way around the piston
edges moving towards the flame kernel.  This largely enhanced flame front
would consume the end gases before knock could occur.

The pressure traces indicate that the 50 to 95% of combustion time was
markedly reduced - tending to back this conjecture.  Faster burn will less
emissions by doing the exact "wrong" thing.

It was in this time frame that certain pistons started appearing with deepens
ring gap and "rings" that was supposed to reduce detonation.  You don't think
someone other that me may have seen the well published SAE paper on this
subject - soon to be part of fangle base if not already there.

By diddling with the head gasket thickness and gap - similar activity was
noted.

OBTW, the larger the bore diameter, the more end gas could be trapped and burn
in this manner - even more reducing the big bore tendency to detonate.

I guess the moral is never read air pollution papers if you are looking for
power right?  



On Sun, 31 Aug 2003 05:16:12 -0400, you wrote:

>> >> Ring placement been's argued from before Harris was a kid .
>> >> Trapping gases, vs strenght.
>> >> Not to mention in relationship to the wrist pin and rocking forces.
> >
> >Anyone want to clue the still somewhat new guy in on what the consensus
> >is on this?  As far as I'm concerned (unless someone gives me a really
> >good argument otherwise) I don't believe that trapped gasses... are an
> >issue for most of our applications (they are an issue from an emissions
> >perspective, but I'm not much of an environazi), but durability is, and
> >my current belief on the whole deal is that placement should be as low
> >as necessary and top ring gaps as large as necessary so that you're not
> >at all worried about breaking things, and any slight losses due to
> >higher leakdown/lower ring placement can be made up for in other places
> >(boost?).  On that note, I want gapless rings to work, but I don't know
> >that I've ever seen a set that haven't caused other problems (never used
> >them myself).