[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [New Search]
Sorry, I really hate doing this, but I just couldn't let it pass.
My apologies. Read on....
On 8 Jun 99, at 22:45, Squareback wrote:
> Ok, quick lesson in Heat Transfer. There are 3 forms of heat transfer.
> Conduction, Convection, and Radiation.
This is the conventional wisdom, but, of these, only radiation
applies directly to the case where heat is transferred from one
material to another. There are obviously others ways for our engines,
or any other hot body, to get rid of heat, but they are neither
conduction nor convection, although those play an obvious part.
> Conduction and Convection are the main forms of heat transfer in the VW
> motor. Conduction is straight forward. It is dependent on surface area,
> thickness, and the conduction coefficient, which is different for all
> materials. Convection has more factors. Convection is dependent on the
> fluid temperature, fluid velocity, surface area, surface temperature, and
> the convection coefficient for the fluid. Fins and rough surfaces are one
> way to increase surface area, which will increase the heat transfer due to
> convection.
This is heat transfer across a boundary. It really is neither
conduction nor convection, and, yes, it is very complicated. It is
affected by everything!
> Color does play a large role in radiation heat transfer. A black
> case/engine opG s a black body radiator. A black body radiator is a
> perfect emitter. This means that it does not store any of the energy that
> it receives in the form of radiation from the surrounding, i.e., hot engine
> sheet metal, exhaust, etc., and it also radiates the maximum amount of
> energy that is generated from inside the engine. A white case/engine will
> radiate all the energy that it receives from the surroundings, but will not
> radiate any from within the engine. This explains why a black asphalt road
> is very hot and the white stripe dividing the road is much cooler.
This is where I really have to disagree. A "black body" is a
theoretical perfect radiator, and a perfect radiator is also a
perfect absorber. It's radiation depends only on its temperature and
it has no way of knowing if it is hot because it received energy from
within or without.
Nothing comes very close to a perfect "black body," certainly not a
VW case in any condition or color.
The real "secret" is that the coeffecient of absorption and the
coefficient of emissivity are the same, so if you want to make
something that radiates well when it's hot, all you have to do is
make something that absorbs well when it's cool. Painting it black is
one very easy way to do this. Even this, however, is not a simple
thing because adding the paint layer adds just a bit of thermal
insulation, so it is not clear that this will give you an advantage.
This is a VERY complicated problem. It would all depend on how
important radiative cooling is relative to "conductive" transfer to
the air (because black paint helps radiation, but hurts heat flow out
of the case). It would also depend on engine temp and air temp.
In general radiative losses go up with the T^4 where T is in Kelvin.
Jim
-
*******************************
Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
*******************************
-------------------------------------------------------------------
Search old messages on the Web! Visit http://www.vwtype3.org From ???@??? Thu Jun 10 07:23:53 1999
X-POP3-Rcpt: gregmpop@maria.yourwebhost.com
Received: (from gregm@localhost)
by maria.yourwebhost.com (8.9.3/8.8.4)
id VAA18472; Wed, 9 Jun 1999 21:28:22 -0400
Resent-Date: Wed, 9 Jun 1999 21:28:22 -0400
X-Authentication-Warning: maria.yourwebhost.com: gregm set sender to type3-request@vwtype3.org using -f
From: "Keith Park" <TopNotchResto@worldnet.att.net>
To: <type3@vwtype3.org>
Subject: Re: [T3] Rebuild or T4
Date: Thu, 10 Jun 1999 09:22:00 -0700
X-MSMail-Priority: Normal
X-Priority: 3
X-Mailer: Microsoft Internet Mail 4.70.1155
MIME-Version: 1.0
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
Message-Id: <19990610012738.DYTL2001@keith-park>
Resent-Message-ID: <"i3Xbp2.0.YW4.pInNt"@maria.yourwebhost.com>
Resent-From: type3@vwtype3.org
X-Mailing-List: <type3@vwtype3.org> archive/latest/13040
X-Loop: type3@vwtype3.org
Precedence: list
Resent-Sender: type3-request@vwtype3.org
Get the gene Berg catalogs! even if you dont order from them the catalogs
are full of good info!
Keith
----------
> From: Steve Glavas <steveglavas@email.com>
> To: type3@vwtype3.org
> Subject: [T3] Rebuild or T4
> Date: Wednesday, June 09, 1999 5:51 PM
>
> As you know, my head is falling off...
>
> So, here I am with this wonderful decision to make: Rebuild a
1600(stock,
> FI) or install a T4 (stock, FI). What I am looking for is some good
advice
> about rebuilding a very solid, reliable, cost not really an object stock
> 1600. Let's put it this way, I want it to last 150,000 US miles, 24000
> Swedish miles, or 240,000 km before the next rebuild. Longer would be
> preferable.
>
> If you have answered this stuff before, just point me in the right
> direction. I will be buying the "Hot rodding" and "Rebuilding Air
Cooled"
> books very soon. Any and all resources for this stuff would be very
> helpful.
>
> I know the exhaust valves need to be of the best quality (which would
> be...?).
> A balanced crank shaft would be very nice, too (will that affect the FI?
I
s? think so, but I am not sure--stock dimensions, though).
> Case savers, as I have learned, are of ultimate importance. (are those
> needed in a T4?) Anything specific I should know about them (16 plus one
> deep one, for example)
> And, of course, a full flow oil filter (any suggestions on how or where?)
> How would I handle T4 dropped seats, rather, prevent them?
>
> Now, I just need to find out about good quality P/Cs, probably a head or
> two, maybe some rods, and anything else you think would be ideal for
making
> the dream stock engine. Brand names, I need brand names! Maybe sources,
> too.
>
> What is your stance on hydraulic lifters? Yes/No? Drawbacks? Advantages?
>
> What are some GOOD companies for rebuilds (maybe we should make a list of
> this) and some BAD companies?
>
> What price range should I be looking at? (for excellent quality)
>
> I would love to do this myself, but working and going to school prevent
> this. My square is also my only source of groceries (the stores are
> driving, not biking distance).
>
> I appreciate your help on this matter, guys.
>
> Thanks,
>
> Steve and the Headless Square ('71)
>
>
>
>
> -------------------------------------------------------------------
> List info at http://www.vwtype3.org/list or mailto:help@vwtype3.org