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

James Hansen jhsg at sk.sympatico.ca
Wed Apr 9 01:44:52 EDT 2003

> 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?

Of course not.  If I try to say something it usually comes out pretty close
to what I say. I said "Driving characteristics are secondary to
the design requirements of the diesel engine."  design requirements are to
meet emissions, and develop power sufficient for the task which is to meet
the criteria set out.
Guy building a car wants "X" hp, because market research tells him that with
"X" hp, so many units will be sold, and will maximize his bottom line
balanced with powertrain costs.  With X+1 hp, his bottom line goes down, as
the engine costs more to produce and he does not increase sales enough to
offset the increased costs.  Make a smaller engine, he loses the performance
aspect, loses sales, and makes less bottom line dollars, even tho the engine
is cheaper. Shareholders get grumpy, and boot his ass. Oversimplified, but
you get my drift.  Point is, if they could do it without the turbo, it would
happen, as it's a cheaper engine to build.  But, emissions have to be met...
so turbo it is. Sure, they look at everything down to the last nit as it
effects the bottom line.

> 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.

Actually, my space shuttle reply was responding to your statement:

"> efficiency of the engine). One also saves fuel by accelerating quicker
> on energy that would have otherwise been lost, and therefore spending
> less time accelerating to speed (despite increased fuel delivery)."

Accelerating quicker does not save energy. Period.

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.

Very true.

> Certainly, if you drive them both in the same fashion,

But Tyler, if you don't drive them in a similar manner, how do you compare
engines/vehicles in real world conditions? You have standards, man. That's
how published epa ratings on window stickers of new cars are arrived at, a
standardized test to determine in average conditions how much fuel you will
use at legal speeds and conditions the vehicle was designed to be operated
in at the time of design. not so?

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.

Plus a few other parameters, but yes, that is exactly what I said.  we
agree. except for the gasoline part, you are confusing me on, yeah, the
driver will tend to use the available power. Give the driver more available
power, you will use more fuel. He will drive at 75 instead of 65.

> 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).

What is max boost for this car, still ten?  Seven is higher than a Jetta
needs, but it is lighter than a volvo I expect.

 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.

And at 75mph, I would expect to see some smoke behind your na version too,
no?  I expect that your na is built to cruise in the 55-60mph range, no? ...
just guessing.   Any engine operating near or at peak power will use more
fuel than at "cruising speed" (a factor of power and gearing, resistance
etc).  In percentage of available power, your na would be nearer the top,
and not at it's most efficient level, where the turbo is farther from it's
max power, and more in the cruising range. Just where you are operating in
the available power range of an engine will effect the economy quite a bit.

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 ;)

But of course!

Just one thing.  One email that had a space in the subject line, got stuck
elsewhere,(thank you Microsoft Outlook) you said----
"At the factory, VW decided to put a
rather large turbo on their motors (for the displacement) and therefore
they cannot produce full boost instantly at low rpms, but must first
reach higher rpms (with more throttle as James Hansen said).  This is not
reaching full boost at any rpm, because your rpms are increasing to a
level capable of flowing enough air for the turbocharger to provide full

 If you go back far enough in diesel technology, yes, this would be the
case.  HUGE turbos, that were sized to keep the rpm down, because metal and
support technology had not progressed to the point where they could spin a
compressor at 100000rpm without exploding into a collection of shiny bits.
The sequence of events in a diesel is totally different than your gas cars.
Stop comparing them and it will be easier, the combustion process is
different.  This was what we started off disagreeing about... The scenario
is, you give it part throttle under load with the intent of going faster.
Your fueling instantly increases, as the governor speed limit has been moved
via your moving your right foot. You have more fuel injected as the governor
is calling for it, up to the fueling limit as set by aneroid and limit
screw.  You now have instantly more exhaust gas volume and heat, the turbo
spins up almost instantly (almost instantly in my volks, takes about two
seconds in the semi with 3406 cat, about one second in electronically
controlled cummins in my one tractor).  Your engine rpm's have not increased
in any significant amount at all yet maybe twenty rpm, but the turbo is
going like the lawn mower from hell. Engine rpms are less strongly linked to
turbo rpms in engines  that run non-stoichiometric ratios because the energy
in the exhaust products is much more variable.  That was what I was trying
to get across in my original reply. In a gas job, the energy in your exhaust
is a relative constant, just more or less of it by volume, because the fuel
air ratio is supposed to be a constant factor.   You do not need to have
engine rpms increase to have a turbo wind out and produce full boost in a
diesel, you just need to put more fuel in. This can happen at any operating
rpm.  Well other than idle...



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