[Vwdiesel] Turbo vs. Non-turbo [was My TURBO test (more scientific)]

Tyler "Casioqv" Backman casioqv at usermail.com
Tue Apr 8 13:59:45 EDT 2003

 > Mark answered this most eloquently. Driving characteristics are
secondary to
 > the design requirements of the diesel engine.

What you are trying to say is that for some reason _all_ engine
manufacturers put a turbocharger on for just one reason, and do not
consider many of the other advantages and disadvantages when they make
their decision? Even ones that would affect sales more than the
efficiency or peak power output? If all diesel engine manufacturers were
complete morons, than why do many of the engines sell so well, and work
so good? Sheer luck? I doubt it. While I am not a diesel engineer, I am
sure that every design decision they make is made based on many factors,
none of them are inadvertent side effects that were never considered.
What my main point was, is that there are very few reasons to not
turbocharge a diesel engine, other than cost.

 > To a very limited point.  accelerating a mass costs what it costs,
you have
 > to pay the piper no matter how you do it.  Conservation of energy and all
 > that.  Doing it quicker requires more power than doing it slower.
Think of
 > it this way.  If you could accelerate the shuttle at twice the rate, you
 > would need less fuel to reach escape velocity.  Is that right?  No
way man,
 > but that's essentially what you just said.  The only savings are in less
 > time spending money on parasitic losses such as water pump, friction
 > are really negligible in the real world.

That is not what I said. What I said is that increased thermal
efficiency can lead to gains in both horsepower and torque, or just one
or the other depending on the setup. A Turbocharger does not violate
conservation of energy, but does increase efficiency because most of the
power that drives it comes from the exhaust heat energy, which is
otherwise wasted. Some energy comes by increasing backpressure to the
engine, and therefore decreasing the power output of the engine, but not
most of it, and certainly not all of it. Do not underestimate how much
of the energy in a diesel engine is "wasted" as heat, it is a very large
percentage, and regaining just a small percentage of that can do a lot
to improve the overall thermal efficiency of the engine. If my shuttle
was powered by a diesel, adding a turbocharger would allow me to reach
the escape velocity with less fuel, and still accelerate it a bit faster
(but not twice as fast, unless I wanted to run tons of boost and fuel,
canceling out all of my efficiency gains) ;)

 > I never said that it was more efficient to run a semi without one.
You are
 > comparing apples and grapes.  In a little 1.6 volkswagen rabbit,
 > at highway speeds, the 1.6 na will outdo 1.6td in my experience from the
 > mileage perspective.  From the power perspective, the td has the
 > IF you drive the td in EXACTLY the same fashion as the na, same rate of
 > acceleration, same speed, then yes, the turbo will indeed be more
 > HOWEVER, this never happens.  People will use the available power, as
 > get wood over such things as performance.  As soon as you demand more
 > output, you quickly negate the efficiency advantage of the turbo.
But in a
 > diesel volks, who really cares.  You can drive in a very spirited manner,
 > and this will cost you maybe at worst 5mpg over average driving.
Drive with
 > an egg under your foot, you maybe get 5mpg better than average. Heck,
I had
 > an old buddy that got 96mpg with his na 82 jetta diesel.  No kidding,
 > is for real, with imperial gallons. I used to work on his car for him.
 > Driving style...

Certainly, if you drive them both in the same fashion, or do not have
much boost fuel enrichment, than the turbocharged engine will provide
better fuel efficiency. If you want, you can move all of that additional
efficiency over for making power, by driving the vehicle hard and making
the fuel mixture as rich as a NA engine, and you will have more power
but worse fuel efficiency, because it is just as if you had a larger
displacement NA engine instead of gasoline. It is all in how the driver
drives the vehicle, and how the boost and enrichment curves are set up.
On my Volvo Turbo Diesel the engine produces about 7psi of boost on
level ground at 3000rpm during normal 75mph cruising for a long trip
(what I mostly use the vehicle for). This, coupled with a very lean
mixture (no visible exhaust smoke) is putting out the same power output
as a non-turbo diesel, but with less fuel, and likely accounts for the
greatly increased fuel economy that I see. When I climb a steep grade,
the non-turbo diesel would have to slow down, but I can continue at
75mph under a rich mixture, and I am likely getting worse fuel economy,
but I don't mind because it's so much more fun ;)


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