On our local car club forum a new member asked a question about adding
a turbocharger to his exhaust. The way he describes it is instead of
forcing air into the engine you will in a sence suck it out of the
exhaust by routing the exhaust outlet on the turbo to the air inlet on
the turbo, then connecting the air outlet to the exhaust. Note that
this is not a typical turbo setup where the air outlet is fed back
into the intake manifold. Anyhow the topic has basically turned into a
flame fest, and he claims to know several people that have done this
to gain horsepower from their engine without having to deal with
electronic and fuel issues typically associated with adding a turbo to
an engine that originally came without one. My question is basically
can you do this, and if you can is their anything to be gained or lost
from it? If you could provide some links to sucessful implimentations
of this it would be very helpful. Thank you.

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Request for Question Clarification by smudgy-ga on 19 Nov 2002 10:55 PST

Hi,

Let me make sure I am understanding this correctly. I am not a car
expert but I think I know how a turbo charger works:

Exhaust from engine enters turbo charger (Call this "exhaust-in") and
powers a turbine which accelerates air from the air intake (call this
"air-in").
Exhaust leaving the turbocharger ("exhaust-out") goes to exhaust pipe.
Accelerated air leaving turbocharger ("air-out") goes to engine.

So are you saying that your friend has attached "exhaust-out" to
"air-in" in the hopes that the exhaust will power the turbine, thus
accelerating the exhaust and increasing total flow through the engine?

In other words, is this his proposed setup? Exhaust from the engine
goes into "exhaust in", powering the turbine. Exhaust from
"exhaust-out" goes to "air-in," which gets accelerated by the turbine.
Accelerated exhaust leaves the car through "air-out".

Do I have this right?

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Clarification of Question by intendedacceleration-ga on 19 Nov 2002 11:02 PST

Yes you do indeed have this correct. This is what he claims to have
seen several times. Nobody else has ever heard of it.

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Request for Question Clarification by smudgy-ga on 19 Nov 2002 11:05 PST

How much detail would you want in an analysis of this situation? I can
give you an explanation about why this will/won't work from a physics
standpoint--do you want a simple physical argument? Do you want an
answer that goes into detail about open and closed energy systems? Do
you want an explanation in purely automobile engineering terms?

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Clarification of Question by intendedacceleration-ga on 19 Nov 2002 11:15 PST

I would like as detailed of an answer as you are willing to give. If
this will not work I would like to give him a reason why in
indisputable terms.

Hi,

I hope the following explanation will help convince you and your
friend that his device (as you have described to me) will not actually
improve the total flowthrough of his engine. If for some reason this
is unclear, please request a clarification before rating and I will do
my best to eliminate any confusion.

A turbocharger normally works by taking some of the waste energy in
the exhaust stream and using it to compress the air in the intake
stream. It's a positive feedback system: by compressing the air, this
allows the engine to run at a higher pressure, creating higher
pressure exhaust, which causes the turbines to compress the air
further, until everything is at a stable pressure (i.e., the exhaust
runs the turbine at a pressure which is just enough to maintain the
pressure at which it is operating). This operating pressure is
generally about 50% higher than atmospheric pressure. (1)

At operating speed, the added power from the engine provides added
pressure in the exhaust. This pressure is what provides added pressure
for the intake stream, which allows the engine to run at the higher
power level required. So in a sense, the turbo system is coaxing the
engine into powering the turbo. Not all of the added power in the
engine goes to the exhaust stream--a lot of it goes to the
transmission, which is where the power boost from the turbo goes.

In your friend's scenario, he has the exhaust running the turbine,
which is in turn propelling the exhaust stream out of the car. This
will not provide any added benefit to his total throughput--in fact,
in all likelihood it's decreasing his throughput. Here's why:

The turbo in your friend's car turns by taking energy from the exhaust
stream going into the turbo. So at this point the turbo has some
energy stored in it, in the form of the spinning turbine. It got this
energy from the exhaust stream. The exhaust coming out of
"exhaust-out" has less energy than the exhaust that went into
"exhaust-in". The difference in energy is what it took to turn the
turbine.

Now the exhaust stream comes into the "air-in" pipe. This is the same
low-energy exhaust from the "exhaust-out" stream. The turbine
accelerates the exhaust and spits it out the "air-out" stream where
the exhaust leaves the car. The energy has been transferred from the
turbine back into the exhaust stream. But the turbo can only transfer
as much energy to the exhaust as it has stored in it--which is exactly
how much it borrowed from the exhaust in the first place.

Look at this scenario very closely. Let's put some numbers so it's
easier to follow. Let's say the "exhaust-in" exhaust has 100 units of
energy. It transfers 10 units to spin the turbine. Then the turbine
has 10 units of energy and the "exhaust-out" exhaust has 90 units.
When that same exhaust makes it into the "air-in" stream, it still has
90 units. The turbine transfers its 10 units of energy to the exhaust,
so now the turbine has no energy and the exhaust has 100 units. No net
change has occurred.

This is a highly simplified scenario, ignoring lots of details about
fluid dynamics and such, but the basic premise is accurate. Your
friend can't get more energy out of his turbo than he puts in. If
there isn't a convenient "energy pump" such as an engine to help dump
energy into the system, then he can't ever get his turbo to accelerate
his exhaust. At best he will have no net effect on his exhaust
pressure, and at worst, he will lose some exhaust pressure due to
mechanical energy losses such as friction in the turbine.

The difference between his scenario and the normal turbo scenario is
that in the normal scenario the engine is smack in the middle of the
air throughput diagram, helping to accelerate the air. Where is the
extra energy coming from? The fuel. The turbo tricks the engine into
running at a higher fuel consumption rate due to the increased amount
of air in the engine. More fuel+air equals more combustion energy.
This is totally lacking in your friend's exhaust loop.

This is a straightforward application of the laws of conservation of
energy. What your friend is doing is akin to trying to get an electric
fan to drive a generator windmill that's powering the fan. If the
system is spinning at the start and there are absolutely no extraneous
energy losses, then the fan might be able to power the generator,
which can power the fan with just enough energy to keep it running.
However, things like friction and heat generation cause the system to
lose energy and eventually the fan will not have enough power to drive
the generator.

It's conceivable that you could set up something on the exhaust side
of the air system that would accelerate the exhaust and increase
flowthrough, but it would have to be powered externally, for instance,
by the engine. So for example a supercharger connected to the exhaust
output (powered by the engine) might help your total airflow, but
notice that the supercharger is powered by an outside source relative
to the exhaust stream.

I hope this makes sense to you. Once again, request a clarification if
you find anything unclear.

Good luck,
smudgy.

(1) How Stuff Works: How turbochargers work.
http://www.howstuffworks.com/turbo.htm

Search strategy: Google <how turbo chargers work>