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

[James Hansen]

> An engine can't make full boost anytime you press on the pedal.

Well... mine can.  Come drive it some day.  It's all in the fueling setup.
Diesels are not like gas cars.  When you ask for power by depressing the
pedal, you are telling the governor inside the fuel pump that the engine
speed is too low.  The pump responds by increasing the fuel injected
quantity to bring the rpm's to what your foot is asking for.  How you have
your max fueling set up will dictate how quickly max boost will be attained,
as the amount of fuel injected dictates directly how much exhaust gas volume
is produced, and how much energy is available to drive the turbocharger.
More energy available equates to a quicker ramp up in boost pressure.  Once
your engine rpm's get to the point of engine speed you are asking for with
your foot, then the fuel slacks off, and everything is back to a steady
state.  Remember, this is the most significant difference between diesel and
gas... the diesel uses only the amount of fuel that is required to maintain
the rpm you ask of it.  Gas engines use fuel based on the stoichiometric mix
of fuel air that you admit to the combustion chamber.


> Generally when one talks about "performance" with cars they are talking
> about acceleration (or possibly handling, braking, etc.). I wouldn't
> expect to open a catalogue that says "Performance Parts" and see
> products intended to improve fuel economy. The definition of a word is
> whatever common meaning the sender and receiver have in mind. Many words
> are expected to have different meaning than their dictionary definitions
> when talking about cars on a internet e-mail list ;)

Sure.  Dieselers are smarter and better looking too.  Most are interested in
performance WITH economy, something rarely attainable with a gas engine.
But, done properly, any engine is a blend of the two.  If it wasn't the
country would all be driving 300hp 12 mpg  suv's.  Oh yeah....

Jake, now that you are back, what's your quarter mile time these days??



> You don't think companies put turbos on almost all modern diesel engines
> at least partially because it makes them respond more like gasoline
> motors? Automotive manufacturers know that how the vehicle drives sells
> it a lot more than some power or fuel efficiency statistics on a sheet
> of paper. If that's all the turbo were put there for, then engines would
> have much bigger and less responsive turbochargers that are more
> effective at improving power and efficiency.

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

> Those variables are not sufficient to account for the huge difference in
> feel between a non-turbo Volvo D24, and a TDI. In fact, I am certain of
> it because the turbocharged D24 which has basically the same "Cam
> design, timing, injection timing, advance, etc." feels more responsive
> than a TDI.

What you are feeling is a combination of other things as well... gearing,
tire diameter, tranny gear ratio spacing to name a few.  I could take
identical engines as I am sure you are well aware, gear them differently,
and put the passenger out the rear window accelerating to thirty miles per
hour.  Then there is torque.  your volvo makes more torque at low rpm than
the tdi I am guessing.  Torque is a function of displacement, and more
displacement will give a better seat of the pants push to a point.

There are so many things that make driveability pleasant...  It's like
racing stock cars.  Only half of a winning car is the engine.  The other
half is suspension design, gearing, and setup.  (The third half is the
driver;)   It's the package, not just the parts.

Here's a design challenge for you.  Build me a 350 chevy stock car engine,
use only stock unmodified parts, stock intakes and exhaust manifolds, no
porting.  Go look for a camshaft.  You will learn how important the cam
parameters are. It's an interesting and informative task, trust me.



> such as Volvo might use to test

They have ALL the toys. See Mark's response again.

>
> With gasoline engines of the same size, the turbocharged one will almost
> always get worse fuel economy, but with diesel engines the turbocharged
> one will almost always get better fuel economy despite increased power
> output. With a turbocharged diesel, the additional air allows for a much
> leaner condition, which promotes more complete burning of the fuel, and
> therefore more power and fuel efficiency. The turbocharger itself
> requires very little additional power from the engine, (from
> backpressure) because most of its energy is extracted from the exhaust
> heat, which would have otherwise been lost


"They ain't nuthin fer free."
There is a power cost to run any device, don't fall into this trap.  If you
raise backpressure, you lose out on cylinder emptying, lowering effective
VE, requiring more fuel to attain the same level of power output.


(this increases the
> 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).

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 etc,but
are really negligible in the real world.

 Many
> studies have been done, some of which by Gale Banks Engineering proving
> that a turbocharger can increase both power and fuel efficiency on a
> diesel engine.

And marketing has absolutely NO influence on results or interpretation of
those results, right?  uh-huh.  If you ask for the power, you spend on the
fuel. fficiency gains are easily eaten away.  Only time this scenario works
is if you are overloaded making black smoke all the time at your specified
load.  Getting more air in or improving VE will increase efficiency, but to
make blanket statements that this is the case is not accurate.  Don't get me
wrong, we agree actually, but for different reasons than you state.


My Volvo turbo diesel gets 6.5mpg better than the
> non-turbo diesel, and the turbocharged suburban gets 2mpg better than
> the non turbocharged one, and even more when pulling a trailer. I'd be
> willing to bet that most other list members experience the same thing.
> The ECOdiesel has a turbocharger just for increasing fuel efficiency,
> and only adds a few horsepower because it has no fuel enrichment. If it
> were more efficient to run a diesel without a turbocharger, than why do
> virtually all semi-trucks have them, when any savings in fuel that they
> could make would go strait to improving profit margins?

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, operating
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 advantage.
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 efficient.
HOWEVER, this never happens.  People will use the available power, as they
get wood over such things as performance.  As soon as you demand more power
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, this
is for real, with imperial gallons. I used to work on his car for him.
Driving style...


 I also suggest
> that most of the improvements that one sees in a TDI over, say a 1.6 NA
> motor comes from the very advanced turbocharger that it has.

It's the whole package, not one definable thing. trust me in this.


I gotta go, another semi showed up to load with grain before the ground
thaws...
it's been nice to sit for a bit. Loaded two already this morning.
-James
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