[Vwdiesel] More on diesel efficiency

[mikitka]

You also get hot starts in a dual spool axial flow turbo fan jet engine like
in the Pratt & Whitney F100-220E.  It is controlled by a DEEC  Digital
Electronic Engine Control. And at times it will hot start, especially if the
main starting system cuts out to early for some other reason..
Nick

-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com] On
Behalf Of hwy9fergs at comcast.net
Sent: Friday, December 09, 2005 1:29 PM
To: vwdiesel at vwfans.com
Subject: [Vwdiesel] More on diesel efficiency

Svend said-
Ferguson ?   shouted down ?   --count on Svend not shouting.

Thanks for bearing with me.  I was hoping to make some sense out of this
mess.
I wish I could take more time to devote to this stuff, but the reality lies
elsewhere. I myself even get a little bored with this technical
pontificating. Anyway, a quick note on hot starts in turbine engines.  The
compressor section of the turbine engine is on the same shaft as the turbine
section.  The hot exhaust flowing past the turbine is what drives the
turbine- which drives the compressor, which supplies the air to the
combustion chambers, which supply the driving force for the turbines
which---------keeps the cycle going.  Once the engine has spooled up to
normal operating speeds, the compressor puts out an abundance of air for
combustion, as well as the air that is critical to be routed as a boundary
layer to protect the combustion liners from the (extremely hot) flame.
Without the air cooling the turbine engines could not survive. .  But during
the starts, a starting motor of some sort is used to get the turbine turning
to a certain point, but through most of the start, it's a fine line that
gets danced around, adding enough fuel to keep increasing RPM, without
putting too much in for the amount of air being supplied. When too much fuel
is supplied in relation to the amount of air supplied by the compressor
during a start, then you  have the "hot start" problems. Older engines with
worn, less efficient compressor and turbine blades are more prone to this.
Huge flames coming out the tailpipe, melted turbine blades, etc.  Anyway,
enough on that.
The other thing I wanted to mention on the diesel efficiency is interesting,
in the way it differs from gas engines.  Specific fuel consumption on a gas
engine is much more linear with respect to load.  If you look at a chart
with the load and fuel use, the line would be more or less a straight line
up at an angle.  For instance, a gas Jetta at 50 mph might get 35 mpg, and
at 80 mph drop to 30 mpg.  With the diesel engines, the chart for specific
fuel consumption has a line that stays relatively low down on the fuel
consumption throughout the majority of the load range, until you get up near
the full capacity of the engine, at which point the line starts to go just
about straight up.  Looking at the way our mileage differs in the real world
bears this out.  A 1.6 diesel (IDI) Jetta traveling at 50 MPH will get in
the neighborhood of 45 mpg or better, but you take that same car up nearer
the limit of it's capablity, say 80 mph, and you might more likely be
looking at 35 or 36 mpg.  Just a little food for thought on your next long
road trip.  Peace, Doug Ferguson
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