[Vwdiesel] INCREASING AND DECREASING FUELING

[James Hansen]

Remember what I had stated earlier.
The fuel takes time to burn, and smoke is a product of insufficient burn
time, as fire leaves the cylinder when the exhaust valves open.
Increase the compression (oxygen) and the reaction takes place quicker,
allowing you to extract more power from a given amount of fuel in a shorter
period of time while it is still in the cylinder..
This is considered bad from the nitrogen emissions standpoint, which is the
main obstacle in diesels in N america. Poor fuel quality poisons the
catalysts that can deal with nitrogen emissions.
Compression is always a good thing in a diesel, the compression is lower to
allow boost pressures (and less head pressure at boost), and easier starts,
then the turbo compensates through increaseing VE.
It's kind of walking a narrow ledge between too much and too little.
That's where a VNT comes in.  It compensates very well for inadequacies in a
fixed wastegated design which is really all about compromise in the first
place. No real boost off  idle with wategate designs.
VNT are beautiful turbos... add ball bearings to the tubine shaft and you
really have something special.

Compression is kind of an elastic band thing too.
The more work expended to compress it, you get most of that back as the
piston goes back down off the compression. Negligible lossses to friction
and leakage.  That's why total seal rings make some sense- the losses to
leakage are almost eliminated, at the cost of a tad more friction.

So... I guess you add backpressure, to do work, which you get back minus
friction, for an increase in efficiency of the fuel used would about sum it
up. The increase in efficiency is more that the losses, within reason.
Grossly overboosting will cause trouble in the long run with other thing
mechanical, efficiency aside.  If the system is built to withstand it,
you're away.  If not, you break stuff.  Watch NOPI drags on speed sometimes,
those are good examples of gross overpressure accomplishing a lot of work in
a relatively short time and lifespan.
-James

-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com]On
Behalf Of Libbybapa at wmconnect.com
Sent: Monday, December 12, 2005 9:07 AM
To: vwdiesel at vwfans.com
Subject: Re: [Vwdiesel] INCREASING AND DECREASING FUELING


Okay, I got very clearly that an engine just pumping air will not produce
boost and so dropped the idea of underfueling when accelerating with boost
in a
TD.  But the idea just wouldn't play dead in my head.  The fact is we're not
just pumping air.  Obviously the volume of gasses leaving the engine
compared to
that entering the engine is far greater.  That volume of gasses is what
spins
the turbine.  Even at idle with an extremely minimalal fueling for the
amount
of air, the turbine will spin.  The VNT turbo with vanes closed will
actually
produce boost at idle on a 1.6.  Although there is no throttle plate, there
still would be pumping losses when pushing that air in and pushing the
gasses
out especially considering the losses of some of the gasses passing the
rings.
I understand that fuel makes boost, but still feel that a condition could
exist where the amount of boost air entering the engine is above that where
any
benefit in work is achieved and therefore a detriment to fuel economy.

I understand Svend's claims to high economy in "ECO mode".  I also have
talked to others that tried "ECO mode" with a significant drop in fuel
economy
accompanying the abysmal drop in performance.  I have also heard from
several
folks that the 1.9TD ECO (not sold in North America) had resultant lower
fuel
economy.  That is second hand and unconfirmed by me.  Also, if I remember
correctly, the most recent Fuel Economy numbers produced by Svend resulted
in mid to
high 40's (US gallons), figures that my '86 N/A Jetta could easily beat with
300,000 miles on the engine, horrendous compression (400, 320, 320, 230) and
200,000 miles on the injectors.

Andrew
_______________________________________________
Vwdiesel mailing list
Vwdiesel at vwfans.com
http://www.audifans.com/mailman/listinfo/vwdiesel
--
No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.1.371 / Virus Database: 267.13.13/197 - Release Date: 12/9/2005

--
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.371 / Virus Database: 267.13.13/198 - Release Date: 12/12/2005