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, Wed, 15 May 2002 15:03:29
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Piston pumping and exhaust gas power.
The argument you hold is certainly valid
to a point, should one not look at the
problem any further it even could seem
that your representation gives the full picture.
But in life things are a little different then
they seem at first. If you take ie. a BMW 330
6 Cyl engine, and you replace the exhaust with
an ultra low backpressure design and a better intake
plus updated software, you will not be able to gain
more the 20% at identical RPM. For truly more power
on this non Turbo engine, you would have to increase
the REV band by using many special internals that
will resist higher RPM. Look at any ATMO engine,
the big power gains come with lots more RPM and
different internals.
Why is that SO? In fact up to 80% of the exhaust gas will
leave the cylinder without any pumping action given a
low back pressure exhaust.
For our Turbo engines, we thus start with up to 80% of
free energy, 20% being pumped out and thus reducing
HP. As we fit the Turbo it is certain that we increase
backpressure to a less ideal amount, thus more losses
will be due to backpressure, but unlike an ATMO engine
we still recuperate lots of power from the reduced FREE available
amount to drive the turbine, which will drive the compressor.
As we now feed more air/fuel into our cylinders, there
comes a point where the amount is such that our relative
FREE energy will drop because of exhaust gas valve sizes
and also because it simply takes longer to evac more hot
gases out of the same volume, through the same small valve
apertures.
As you said it correctly the Turbo sizes we use are small
to have low RPM response, thus these add more backpressure
then models that are less restrictive, but that only work at higher
RPM.
Now what many do is to add a larger Turbine downpipe, which
reduces backpressure from the turbine outlet to the exhaust,
this will increase the pressure/temp. drop between the turbine and the
exhaust and thus mainly increase the shaft HP of the turbine
allowing technically to drive a bigger compressor.
But over all, the backpressure at the manifold prior to the
turbo, got less benefit then one would think, this
for one simple reason, the size of our turbine and its casing
are unchanged and thus still offer identical backpressure
characteristics. But the additional turbine shaft HP do allow
for quicker turbo response and also more boost if the power
consumption of the compressor was such that the turbo was
not able to reach maximum safe levels, at some point too high
rpm, and compressor efficiency losses dictate the use of
a larger lower RPM compressor.
Considering that in many cases the compressor intake is
obstructed in such a way that maximum compressor efficiency
with available turbine shaft HP can't be reached, larger
compressor intake pipes along with less restrictive air filters
and improved cold air intakes do allow for substantial gains
without even touching the turbine/exhaust side.
Also by using more effective chargecoolers we can operate the
compressor at levels where some efficiency is lost because of the
raise in pressureized air temperature it being compensated by
the more effective chargecooler.
Thus, the ideal for every day applications is to optimize the
available turbine shaft HP and to fit a compressor and chargecooler
that give us maximum usable not to hot compressed air.
It is clear that in our setups, the HP loss due to the small
turbine and casing will be higher then on a pure high rpm
race car setup, but nevertheless the amount of free energy we
harvest across the RPM range is substantial, and clearly less
at max power RPM, but in mid range we're mostly doing very well.
As such, yes at high boost pressures the pumping losses will
make out a higher % of the FREE energy, but at low to midrange
boost levels we can operate very efficiently.
Considering everything, mid range efficiency and high maximum
HP figures when need be, turbo engines offer mainly advantages
over ATMO engines, even if our sfc figures are bad under full
load, they can be very good at low to midrange loads, offering more
midrange torque then any same size ATMO engine can offer.
Now that you have read this, I am sure that you can make up
your mind on what to do next to your engine to make it fit
your needs, and clearly this shows that pumping losses at max
output while substantial can be more then compensated with the
actual obtained output, even if this is achieved at the expense
of less ideal sfc figures at full load.
Lots of boost at max power = bad sfc at max power = still lots of
power = still lots of midrange torque.
Regards,
Coolknight
posted by 212.77.37...
, Wed, 15 May 2002 15:03:29, Thu, 16 May 2002 10:36:39, Thu, 16 May 2002 17:56:04, Fri, 17 May 2002 10:56:00
, Wed, 15 May 2002 17:40:35
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