From: parfrk@rs6405.ecs.rpi.edu (Karl Andrew Parfrey)
Subject: Re: Its a Carbon world out there [was: carbon brakes] 
Reply-To: parfrk@rpi.edu
Organization: Rensselaer Polytechnic Institute, Troy, NY.
Date: Mon, 12 Oct 1992 19:53:07 GMT

sorry about the late followup.

I believe that advanced composites are used in F1 engines.  this is 
pure speculation (this stuff is very top secret) but there are a lot
of indications that it is likely.

Ceramic engine parts have been around for years.  The Porsche 944 turbo
engine uses ceramic liners for the exhaust ports (I am recalling an
article I read on the original 2.5t engine.  I think the new 3.0 atmo
engines use these as well)  In addition to higher temperature tolerance
ceramics would have a lower heat transfer.  Less heat into the 
engine block means less cooling requirements - smaller radiators and 
less aerodynamic drag.  At some point piston and cylinder temperatures
become so high that detonation becomes the limiting factor.

Ceramics also have lower coefficients of friction than metals, thus 
less internal friction losses.

Honda has a new road engine that uses metal-matrix ceramics (I believe
it was silicon carbide fibers in aluminum).  They cast a SiC/Al sleeve
cylinder liner into the block.  As the engine wears in the aluminum
matrix at the liner face wears away, leaving only SiC in contact with the
piston rings.  The stronger liners permit thinner cylinder walls and the
reduced heat transfer means smaller coolant channels are needed.
Thus the cylinders can be spaced much closer together.  I can't remember
which engine this is but I think it is in the new Accords... It seems
unlikely that their road car engines would be technologically ahead
of the F1 engines, so assume that the F1 engines use this too... I'll
try to remember where I read this so I can give you more details.

There have been published experiments with carbon fiber/epoxy con rods
and aluminum pistons with carbon fiber/carbon matrix crowns.  The former
are lighter and the latter permit higher combustion temperatures.

Metal matrix composites are typically ceramic fibers/whiskers/platelets in
aluminum or magnesium; carbon fiber is never used as it reacts chemically with 
both metals.  Such materials would be ideal for con-rods and pistons.  As
they are already making inroads into aircraft engines it seems likely
that they are being used in racecar engines.  Such parts would be 
lighter and result in higher RPM and hence power.

Cosworth has really outstanding casting facilities at their factory.
They use high pressure pumps capable of handling liquid aluminum 
originally developed for liquid sodium in French nuclear plants.  
When they cast an aluminum engine they squirt highly pressurized 
aluminum into a vacuum in the engine mold.  The resulting casting has
virtually no voids or porsosities.  I read that the Cosworth turbo V6 
of years back was made with this method and was consequently as stiff
and strong as an iron block.  Recall that Honda and BMW at this time
had iron blocks.... Given this sophistication it seems likely that
Cosworth is also using metal matrix ceramics.  This would explain the
revving ability of the HB engines.

Again this is pure speculation but I believe that a all-ceramic piston might
actually be a bad idea in F1.  If I recall correctly the turbo engines
used oil-gallery pistons in which a stream of oil squirts into the bottom
of the piston, cooling it.  A cooler pistion allows higher compression
ratio before detonation occurs.  A ceramic piston would tend to run
hotter and thus would be worse in terms of detonation control.  Perhaps
tailoring the fuel brews would overcome this... 

Many of the examples I have seen have been diesel engines.

Carbonfiber wheels are specifically outlawed in the new 93 F1 regulations -
this would tend to indicate that they were already being developed!
Such wheels would be lighter but probably not the thing you'd want to
clip chicanes with; but then again, magnesium is a very brittle metal as
well.  I believe the rule stemmed from cost-cutting concerns.  I've
heard rumors that BMW is developing carbon fiber wheels for the new
M3.

Carbon-carbon composites are made as follows:  you make a layup of 
carbon fibers in a polymer or pitch (tar) material and allow it to 
cure.  Put the finaly piece in a very high temperature intert-atmosphere
oven and the hydrocarbons in the matrix carbonize.  The process is 
similar to burning a piece of toast; you end up with pure carbon but
it is very pourous.  You re-impregnate the pourous matrix with the
polymer or pitch and re-pyrolize it.  Keep repeating this and hopefully
you end up with something non-pourous.  Alternatively you can put it
in a methane atmosphere at high temperature and the carbon atoms from
the methane "condense" on the part solidifying the pourosities.
The proplem is that all of the little pourous channels tend to plug 
up at their entrances before the nooks and crannies have filled; thus
you end up with a pourous and weak component.  The details of the
densification process are not only an industrial but a "national
security" secret as well (its relevant to ICBM reentry vehicles, 
hypersonic aircraft, advanced jet engines, etc).  There are only a
handful of companies worldwide who can do it well and I believe you need a 
security clearance to know the details (I don't do this stuff but there
is a "vapor-phase composites lab" here at RPI).

Carbon-carbon brakes are very much lighter than iron brakes.  In addition,
this is _unsprung_ weight, meaning that you make the wheels lighter.
Lighter wheels follow the road better and hence the tires grip better (less
wheel hop, etc).  This was the rationale behind "inboard brakes" on 
F1 cars of the 70s.  The front "driveshafts" needed to operate such a
system resulted in a higher total mass, but most of the mass wasn't
sprung mass and thus theoretically the lighter front wheels followed the road
better.  In reality F1 cars have so little suspension travel that
the improvement was probably theoretical.

John Barnard has suggested that carbon-fiber suspension links are in
the works.  My guess is that the advantage is not so much lightness as
stiffness.  A modern F1 car has extremely small suspension movements.
The pushrods and suspension links would tend to bow and stretch and
thus their deformations would become a significant part of the total
wheel movement.  Carbon/epoxy or metal matrix composite suspension links would
produce more accurate, predictable wheel control.  This would be 
phenomenally expensive.  Most F1 teams throw out the suspension components
after each race.  Kind of makes you want to rummage through a dumpster
in Woking or Didcot, doesn't it.... In addition, feeding highly 
concentrated point loads into a composite and incorporating threaded
inserts is always a tricky business and thus such suspension components
would be difficult to develop.

Most metals decrease their coefficient of friction with heat - thus 
the "brake fade effect".  As the brakes get hotter the disks get
slipperier.  Carbon-carbon composites have the opposite effect: 
as the material heats up the coefficient of friction increases.
Thus the harder you use carbon-carbon brakes the better they work.
In addition, you can work carbon-carbon composites to temperatures
hot enough to melt iron - provided that there is a coating to
prevent the carbon from oxidising.  I don't think racecar brakes
have this so they gradually turn into carbon dioxide.

The temperature effect caught a lot of drivers off guard....
The starting friction coefficient of carbon-carbon is lower than 
metal but the high-temperature friction coefficient is higher 
and thus in balance they tend to work better.  There are two suppliers
of carbon-carbon to F1: Carbone Industrie and Hitco.  One of
the materials (I forget which is which) had a higher temperature
effect and a higher friction coefficient; the other had a lower
friction coefficient which changed less.  It became a matter of
personal preferance between the drivers whether they preferred
predictable brakes or more effective but nonlinear brakes.  

Carbon-carbon vs metal depends strongly upon what circuit you are at; 
carbon-carbon brakes are better on street circuits with lots of
heavy braking; metal is better on high-speed circuits like ovals
(the exception is events like LeMans - I believe carbon-carbon
brakes last longer and thus don't need to be replaced during the
race).  Ironically, the problem is frequently keeping carbon-carbon
brakes _hot_ rather than cool.  Sometimes at high-speed circuits
with minimal braking needs you see duct tape covering part of
the brake cooling ducts.  This helps keep the brake disks hot
and thus at their higher friction coefficient.

		-Kap


-- 
Kap Parfrey   (parfrk@rpi.edu)			The opinions expressed
Center for Composite Materials  		herein are not my own
 and Structures 				but rather those of my
Dept of Mechanical Engineering 			employer. I am merely a
Rensselaer Polytechnic Institute		puppet of the regime.
Troy Ny USA 						- Pinochio