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

[Shirley, Mark R]

> -----Original Message-----
> From: Tyler "Casioqv" Backman [mailto:casioqv at usermail.com]

I also disagree that one
> shouldn't compare gasoline and diesel engines. I think a lot can be
> learned about both by comparing their differences, and that it is
> required to determine if you want to use a gasoline or diesel engine.

You might like to pick up a copy of "Internal Combustion Engine
Fundamentals"
by John Heywood.  It's the definitive text in this area.

> An engine can't make full boost anytime you press on the pedal. I am
> talking about low rpm operation (1500-2300rpm or so). At
> 1500rpm a VW TD
> motor should produce enough power to drive the car, but not enough
> exhaust gas to spin the Garrett T3 enough for full boost, especially
> without full throttle (which would likely just lug the engine at these
> speeds).
>

Uh, yes it can.  It depends on how you setup the system.  Boost is caused
by turbine rpm on the turbocharger.  If you set the exhaust housing size
small
enough, you can reach full boost at lower rpms, even with lower loads.  For
example, If I took my trucks' exhaust housing, which has a 0.96 A/R, and
swapped
it for a 0.65 A/R housing, I would incur more boost at a lower level.  The
A/R
value is a ratio of inlet area to radial distance to outside wall of the
housing.
Note however, it's a tradoff between making the housing small enough for
good
boost response, and so small, it restricts flow at upper rpm ranges.

James' original assertion that Cam timing, lobe height, runner length and
volume,
etc defines torque peak and VE for an engine is correct.  A turbocharged
engine
and a NA engine with the same basic hardware will produce torque peaks at
the same
rpm, the only difference is that the peak will be higher on the Turbo.  The
VE will
however be different throughout the rpm range.

> 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.

Companies put turbochargers on diesels because the diesel combustion process
requires
more oxygen to properly combust all the injected fuel than a gasoline engine
does.
Remember, the typical gasoline engine has started vaporizing the fuel before
the intake
valve has even opened!  The diesel process, since it's injecting the fuel
right into the
cylinder, has to spend time vaporizing the fuel before it will burn
properly.  Increasing
the available air atoms, increasing the pressure, and increasing the
temperature all serve
to speed the vaporization process.  The fact that turbo'd diesels respond
more like
gasoline engines is a secondary characteristic.


> I was aware of all this, but I was just suggesting that a small
> dynometer that a car manufacturer such as Volvo might use to test it's
> gasoline engine cars, might not put sufficient load on a diesel engine
> to measure the full torque that the engine is capable of. A dynometer
> usually works by having the cars wheels (or engine directly)
> accelerate
> a large heavy drum, and derives torque from its radial acceleration,
> because the mass of the drum is known. If the drum were of
> sufficiently
> small mass, the diesel engines slow rev characteristics would
> prevent a
> proper reading.

OEM dyno work is not done on a chassis dyno.  Typical OEM development
dynos do not use acceleration of an engine to measure anything.  Engine
dynos measure torque directly off the retarding device, regardless of the
type, such as water brake, electro, friction brake, whatever.  Torque is
all it measures.  HP is a fuction of rpms.  Nothing more.  The engine dyno
is capable of loading any engine to the point that it stalls.  There's no
problem with loading a small diesel engine.


Take for example, no load on either engine. If both
> engines (gasoline and diesel) with the same torque curve are
> given full
> throttle at the same, time, clearly the gasoline engine will reach
> maximum rpm quicker;

There's a lot more at work here than torque curve.  The gasoline engine
reaches max rpms quicker because of a number of factors.  Diesel engines
are typically built heavier, diesel fuel burns slower, etc.

>
> My understanding is that Gasoline engines tend to have much
> higher, and
> more sustained high exhaust temperatures, but it is less of a problem.

Really, you run at WOT all the time in a gas car?  They changed the melting
point of aluminium for gasoline engines?  :)

Gasoline engines have higher peak EGT's than diesel engines true, but they
don't spend a longer time at that high temp than diesels.  Aluminium still
melts at 1350F last I checked.  The only way to get 1600-1700F EGT's out of
a gasoline engine is to run WOT.  And the only way to do that is have your
foot buried in the accelerator pedal all the time.  Running around town, you
have the same lower EGT's that a diesel has, plus the heat content removed
from by the turbo, unless your gas car is turbo'd.

> In fact, almost all modern turbochargers on gasoline cars are
> water-cooled, while diesel turbochargers are not, but still
> last longer
> because they run so much cooler. I know that if I open the hood on my
> Gasoline Turbocharged Volvo after a quick full throttle run the
> turbocharger is glowing bright red, but my turbodiesel doesn't even
> after towing a trailer at 10psi of boost all day. Why do turbochargers
> last longer on diesels, despite the fact that they run in
> constant boost
> (compared to only occasional boost on gasoline engines)?

Lower peak temps, lower average temps.  That's all it is.  The oil is
sufficient
to cool the diesel turbo because it's removing less overall heat.