07/01/2003

Turbocharging and Ceramics

Reference - Material Science 301.  Student must have remedial math and reading
completed, and excellence in Thinking 101 and Research 201.

Ceramic coatings of the chamber, piston and valves evolved under the adiabatic
engine projects carried out by US Army TankCom, Caterpillar and Cummins were
prime subcontractor. South Western Research was the Brain Tank.  Diesel stuff
- Tuner Boy can now auto-fellate as it was not for a Huuuuuundaaaaaa Pharte
Bomber and therefore of no value.

The project basically involved developing an engine that met the definition of
adiabatic compression - heat neither added or lost during compression.  The
piston, chamber and valves were relatively easy - leaving a more difficult
part of the cylinder walls.

Studies were done with both partial ( semi ) adiabatic and full adiabatic
engines actually built and field testing in a number of working army vehicles
under real usage conditions driven by real soldiers in the normal performance
of their duties.  Silly bastardo's could have learned a lot more by the
mutilation and destruction of computer models, but they chose to build things,
try them and actually see if they worked.  Someday the army will learn that
only computer models will truly tell them how something works.

The semi-adiabatic engines greatly reduced the coolant needs.  Caterpillar
found ( and patented ) a methodology where the water based cooling was
replaced with motor oil.  The cylinder was cooled by a thin film oil
circulating around the cylinder.

The full adiabatic engines ran without a coolant system for the block and
heads - with a moderate, controllable increase in the lubrication temperature.

As part of this project, South West Research made a ringless piston - which in
tests ran to 20,000 rpm.  3 inch piston, 2" stoke, crosshead design, no rings
- gas lubricated.  Probably not good enough for a Boutique  Baka Bomber.

Dave Vizard was and is a leading advocate of ceramic coating.  Several of his
works popularized intelligent use in engines.  Most of his works are still on
sale today.  Although not specific for Phart-O-Matics, they have a large
resource of material applicable to most engine.  Research 201 needed here.

In combustion, there are three major regions to understand.  The first region
is the induction phase ( blue flame ) at around 500c, the Enflamement Phase (
the flame front and peak temperature ) and the sustained burn - where most of
the energy of the fuel is fully released.  The second two phases are generally
know as Effective Combustion - which Ricardo defined as the period of time
when the pressure differed from the nPVT curve of the gas.  IE when the
combustion actually did good.

There are two common materials used for heads - aluminum and iron.  An iron
head is always recommended for life and economy.  It has low heat conductivity
and can retain significantly more heat than an aluminum head.  An aluminum
head, however, generally allows us to run at least one compression ratio
higher and is much easier to machine to needs.

Now in combustion, flame and afterburn temperatures are high enough to consume
the metal in the walls and will do so under certain conditions.  Fortunately,
a thin layer of unburned gas forms an insulating barrier as the temperature
rises during combustion.  This barrier cannot tolerate the scrubbing action
induced by the acoustically resonant wave of knocking and gets scrubbed off.
This exposes the metal directly to the heat of combustion and may be enough to
cause the metal to enter into combustion. If you have ever looked at AA Fuel
dragster heads and pistons and noticed missing metal - it was lost to
combustion.  Plus the resonant wave forces at high frequency large quantities
of charge to be driven into the flame front - so the unprotected walls are now
impacted with much higher than normal temperatures.  

Consider also piston diameter.  The larger the piston - the hotter the surface
becomes.  The hotter the surface - the more prone to surface combustion it
becomes.  The heat transfer mechanism for the piston is to transfer the bulk
of the heat thru the rings.  The further away the metal is from the rings, the
hotter it gets because of heat transfer rate.  Coat the top - bingo - the
metal receives one thousandth the heat - the underlaying piston is cooled
enough to take it out of charge heating equations.  For the big bore phreaks -
this means the compression can go up on the big block because we lost a major
heat sink.

Back to normal combustion.

With an iron head, the metal is much slower at conducting heat away than
aluminum.  This is critical during the induction period as there is
insufficient heat to form a good barrier.  Aluminum, conducting more heat,
causes the induction period to be longer ( more spark ) or may require more
compression ( heat addition ) to optimally work.  As the flame forms, the
insulation is fully formed and there is no further difference between the two
metals during combustion.  

Because Iron retains heat much better than aluminum, after exhaust, the Iron
head rejects significantly more heat into the intake charge than the aluminum
head - allowing the aluminum head to run a higher compression without knock.

Then came ceramics.  Ceramics are an insulator.  A very good insulator.
Slathering the chamber, piston and valves - makes a nice improvement in
combustion because of its insulation - but only during the induction period.
Yes, more heat is retained within the charge because of its physical
insulation and the induction period is seriously shortened - but by the time
of flame - the charge has built its own insulation barrier.  Now we have one
barrier on top of another barrier - guess what - No changes to combustion
during flame and afterburner.

The saving grace is that because it is an insulator and behaves like
firebrick, when the cylinder is opened to induction, there is only an
extremely small heat transfer to the charge.

Properly done, we get better efficiency of combustion than an Iron head and we
get a greater reduction in charge temperature than aluminum.  Material becomes
irrelevant.

Smokey Yunick revealed that circle trackers were cooking their aluminum heads
with a layer of sodium silicate.  This coated the head on the cooling side
with an insulator.  Less heat was lost to the coolant, so the head acted more
like iron during combustion, but the ability to dissipate large amounts of
heat left it still cooler to the incoming charge.  Please do this Tuner Boy -
cause if you add a tad to much sodium silicate, the head stops transferring
heat and destroys itself shortly afterwards.  Smokey never admitted how many
heads were sacrificed to sodium silicate - but it was a bunch.

Normally, an engine loses between 25 to 40% of its energy to the cooling
system.  An adiabatic engine - with no cooling system - should show a major
increase in power because of the lowered loss.  But several full adiabatic
engines showed at best a 5 to 7% increase in power.  What's up Doc?

During the effective combustion - the head is effectively insulated from the
charge either way - so the heat loss change here does not matter.  What
happens is that heat that would be lost to coolant is transferred to the
residual energy.  The better the insulation, the greater the retained heat. On
a semi-adiabatic engine - ceramic head - cooled block they noted that their
was statistically insignificant power differences ( which is what Vizards Dyno
testing revealed ) but significant increases in residual energy.

Note to readers - please note that dyno said no power differences - yet the
engines were far more capable of handling knock, etc and - if you believe in
the turbo fairey - a whole lot more residual energy to drive a turbo.  Your
choice - Dyno or Army.

Since the total energy of the exhaust is increased by ceramics, think through
the possibilities.  More gas faster to drive a turbo harder.  OK.  

But most importantly - go back to heat loss data.  The smaller the charge, the
greater the heat loss to the walls.  With ceramics, there is little heat loss.
Regardless of the compression ratio, the part throttle milage will improve
because less charge is needed to make the same bmep.  Vizard noted as well as
others, that the BSFC fell with ceramics - even if the power did not increase.

Effective octane of the fuel is affected by the charge heating.  As an
example, on a Chubby V-8 intake valve coated with ceramic, the charge fell
about 60 degrees.  30 degrees charge temperature = about 1 octane change.
Just the intake valve can make the difference between pump and race gas - then
consider the rest of the issue.

Another thought to consider.  The lower the compression of the engine, the
greater the area for heat loss.  Certain desirable chamber designs were
abandoned because other designs reduced the heat loss enough to partially
compensate for less desirable conditions.  By reducing the heat loss to
practically zero, heat loss is no longer a relevant issue and less compression
is needed to bring the induction charge to enflamement.  Partial compensation
for the "loss" of economy and response a lowered compression is supposed to
give.

In summary, on a turbo engine, a proper ceramics job can significantly
increase the energy in the exhaust gas at the same load and thus a larger
compressor can be driven earlier and harder than a smaller one with a normal
chamber.  More boost faster = more fun.  It also will significantly increase
the part throttle response and milage.

Not applicable to Baka Bombers - the pharte pipe doesn't sound right
afterwards.

For advanced credit - consider variable coolant temperature upon the engine.
Since the chamber's combustion reaction is essentially divorced from the
cooling system, we need only to consider what the coolant does at various
times.

It is a known and proven relationship that the higher the coolant temperature,
the greater the BFSC of an engine is.  An air cooled cylinder is significantly
hotter than a water cooled at all time.  A prime reason that when Dr Porsche
designed the VW, he made it air cooled.  Maintance was another issue and no
water for hot climates was a bonus.  For economy then, we want the cylinders
as hot as possible.

There is even a term and process for this in thermodynamics.  Its called
regeneration.  Some of the heat from the previous cycles is captured and used
by the fresh charge for the current cycle.  

It takes a certain amount of heat to make a specific BMEP.  Regenerating some
heat reduces the fuel needed to make that BMEP and thus BSFC improves.

At the other end of the spectrum, a cooler cylinder transferred less heat to
the charge, and the charge is less likely to detonate under servre load.  

It is possible to think of the cylinder barrel as an itercooler in a high
boost engine.  As long as its temperature is less than charge temperature, it
tends to cool the charge.  As long as its temperature is greater, it tends to
increase the temperature of the charge.

Non EPA Enviro-Nazi cars routinely see power improvements down to 160 degrees
coolant or less.

Your assignment, should you chose to accept it, is to contemplate a variable
temperature cooling system that's temperature could be optimized to what you
need for the regime your engine is operating in.  Hint - cruise control can
help you a lot.