Date: Wed, 22 Oct 1997 02:39:15 -0400
Reply-To: Scot Douglas <sdoug@CONCENTRIC.NET>
Sender: Vanagon mailing list <Vanagon@Gerry.SDSC.EDU>
From: Scot Douglas <sdoug@CONCENTRIC.NET>
Organization: Mitsubishi Motors R&D of America
Subject: Re: heads - anti-corrosion
Content-Type: text/plain; charset=us-ascii
Todd Francis wrote:
> They say some of the heat that would be absorbed into the head
> will be blown out the exhaust if you also coat the combustion chamber. I
> would think that is a good thing also.
> --
I'm not sure if this the best idea, although it sounds great in theory.
I have discouraged aircooled owners wanting to try this modification on
several occasions (914 owners will try anything to get more power).
Ceramics are excellent insulators; see also space shuttle exterior.
Ceramics are available (I may have the url of such a company - I'll have
to check after I get back to the office).
A ceramic coating on in your combustion chamber and valve faces will
lower the heat rejection to your cylinder heads, allowing lower head
temps. Benefits are: More combustion energy will go into expansion i.e.
pushing the piston down, and more heat would be transfered out of the
exhaust.
Drawbacks are: While that hotter gas is expanding, your cylinder walls
are heated to higher temps; mean in-cylinder temps rise above normal
operating temps - will the cooling system support this load? Probably,
since it would normally be exposed to the non-ceramic head. *But* now
the incoming charge of fresh air/fuel is pre-heated and expands,
lowering the volumetric efficiency (volume of air drawn in during intake
stroke divided by cylinder volume). Next, when this pre-heated mixture
is compressed and temps rise in the cylinder (due to the compression and
insulation of the ceramic coated head) there is a good chance of
pre-ignition or knock. Next, when the hotter than normal exhaust gas is
pushed out through the exhaust port, there is more rejection to the
port, since it isn't ceramic coated.
My opinion is to say no to the ceramic bug when it bites. There are
also durability issues of ceramic coatings. Personally, I believe if
you want to go with ceramics, the exhaust port is the best place to
apply it; it doesn't change the characteristics of your engine's
combustion, but it will keep some of the heat from the exhaust gas from
being rejected to the head. Porsche did this in a few of their engines,
but they cast the aluminum head around a ceramic port for durability.
If anyone has any further questions, I can try to answer them...
--
Scot Douglas
Powertrain Performance Engineer,
Mitsubishi Motors R&D of America
Ann Arbor, Michigan
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