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<div class="moz-cite-prefix">On 8/23/2017 7:46 PM, Kas Kastner via
Fot wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAGiQU1DSV-BrCs4wdvxJN6e3OqA6B5=ZNNiA==aMaxmw2cGwXg@mail.gmail.com">
<div dir="ltr">This is a very interesting subject. In the
thousands of hours of <span class="" id=":ji.1" tabindex="-1"
style="">dyno</span> testing I did in the 60's, the squish
area clearance was never addressed with exception of making
certain the piston did not hit the head. Off hand, when the
clearance is too close it rather sounds like there is
insufficient volume in the squish at <span class="" id=":ji.3"
tabindex="-1" style="">TDC</span> to do its job of properly
exciting the balance of fuel mixture when it squirts back into
the combustion chamber thus making a better mixed and thus more
powerful combustion moment. Or maybe something else :-)</div>
</blockquote>
<br>
There's probably some arcane and obscure hydrodynamics formula that
explains this phenomenon, of which I'm completely unaware, but,
thinking this through, I doubt that the effects on turbulence at
precisely TDC are all that important in this context. The jet effect
produced by the squish area is the result of the rising piston
approaching TDC.<br>
<br>
I suspect that this has to do with stagnant mixture acting as a
buffer to and barrier against expanding gas intrusion into the
squish area at TDC, and there may be some thermodynamic effects, as
well, because of the proximity of the squish area to the piston (gas
in that region may be cooler and denser than in the flame front, and
the nearness of the two surfaces may extract more heat from the
volume of mixture trapped there). Peak BMEP ideally arrives a
smidgen after TDC. We know that gas pressure exerts equal force in
all directions, but that's at equilibrium. Bernoulli's equations
show that, dynamically, there are different pressures at different
points.<br>
<br>
As the piston approaches TDC, the mixture is already burning,
pressure is going up, and the velocity of the mixture escaping the
squish area is rising rapidly. Research into Reynolds effects show
that as the flow increases, even if that flow is turbulent, the
boundary layer of stagnant fluid gets thinner and denser, and its
kinetic viscosity goes up. The squish area and the piston, when in
very close proximity, form a classic flow through two plates, which
is defined by the distance between the plates. I suspect that when
the distance between the two surfaces decreases to the point that
the two boundary layers meet, flow stops, and the fluid begins to
behave more like a solid. Dynamically, gas pressure in the cylinder
and combustion chamber sees a little thin lateral ridge of dense and
viscous gas and pushes against it, rather than against the squish
area and the piston. The effect is that of applying BMEP across a
smaller area of the piston for a very brief period of time, thus
reducing the amount of total force applied to the piston, hence,
less power. <br>
<br>
This near-instantaneous effect might be small, except that at this
point in crank rotation, the piston isn't moving much because for a
significant number of degrees of rotation before and after TDC, the
connecting rod big end is mostly moving sideways, so that effective
gap between squish area and piston crown is changing very little.
It's not until that gap starts to increase that the expanding gas
can get between the boundary layers and exert force on the entire
piston crown.<br>
<br>
Or maybe it's something else. :)<br>
<br>
<br>
Cheers. <br>
<br>
<br>
<pre class="moz-signature" cols="72">--
Michael Porter
Roswell, NM
Never let anyone drive you crazy when you know it's within walking distance....</pre>
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