On 8/23/2017 7:46 PM, Kas Kastner via Fot wrote:
> This is a very interesting subject.? In ?the thousands of hours of
> dyno 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 TDC 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 :-)
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.
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.
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.
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.
Or maybe it's something else.? :)
Cheers.
--
Michael Porter
Roswell, NM
Never let anyone drive you crazy when you know it's within walking distance....
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