A bullet moving through a vacuum has no pressure until it impacts
something. Yet it still has kinetic energy. M x V^2 An internal combustion

engine

is similar to a gun.  An exhaust port opening into an exhaust pipe, which
initially is at atmospheric pressure, imparts kinetic energy to the

exhaust

gas molecules and all the molecules in the exhaust pipe. This results in
a sensible increase in pressure in that that part of the exhaust pipe due
to the static inertia of the molecules. At the other end of the exhaust

pipe, which

is open to the atmosphere, the pressure is the same as the atmosphere. Yet
the exhaust gas molecules now have kinetic energy minus the heat lost

through

conduction to the exhaust pipe itself. They are now capable of imparting
that energy to a turbine by impacting the blades. The question remains
how big of a turbine?

Paul Lamar

pressure moves the gas through the pipe period. Is this not your argument
for larger runners and less back pressure, and better breathing? from whence
does this energy come? Why does an IC engine benefit from a decrease in back
pressure? greater expansion. IF you insist on using the engine as a pump to
drive the turbine it is simply the dog chasing its tail.

What causes the kinetic energy of this mythical bullet? combustion of a
propellant, and subsequent expansion (like a piston, or rotor) EXPANSION!
Back pressure is a loss! not something to be desired. Negative work is
required to move the exhaust gas to the end of the tail pipe. Chop it off
and you can expand to a lower state and make more power.

I am going to modify your bullet in a vacuum by placing it in the
atmosphere, because that is where we are operating, not in a vacuum. Using
your analogy of the bullet and the gun what you are suggesting is to take
and place a washer over the end of the barrel with a small hole in it.
connect a large piston to the bullet and duct air from in front of the
bullet to the back of said piston. The idea is you will take energy from the
compressed air in front of the bullet an put into the bullet as it
accelerates down the barrel. The fallacy is the energy to compress the air
in front of the bullet comes from the expansion of the combustion and you
cannot do anything with the excess energy from the explosion. Now if you
could somehow connect the front of the piston to a vacuum source it would
work. But we are stuck discharging to the atmosphere. In a vacuum there is
no lower state to even make this work. The rejection state relative to the
state of the exhaust gas in the manifold is key.

I have no argument against a turbocompound aircraft engine.

Monty

Residual pressure in the exhaust chamber accelerates the gas molecules 
down the exhaust pipe when the port opens. The pressures is totally converted 
to kinetic energy at the end of the pipe. If the absolute pressure in the 
exhaust pipe is higher there will be less acceleration. 
I already mentioned the air molecules in the pipe reducing the acceleration
of the exhaust gas molecules. We are in total agreement on this. 

A radial inflow turbine results in higher back pressure I think. The 
residual pressure  in the exhaust chamber results in negative work on the e-shaft.
If the back pressure is high this delays the mitigation of the residual
pressure increasing the negative work on the e-shaft. We are in total
agreement on this too. 
 
I don't have a lot of data on the radial inflow turbine yet but when I 
get a chance to go to UCLA I have a few good references to look up.

There are two kinds of axial flow turbines. Impulse
type and the pressure type. According to NACA Report 786 no significant back 
pressure was measured upstream  of the axial flow turbine. I suspect NACA was 
using the impulse type.

I agree we need numbers. All of this is speculation without the numbers.

Here is what we know so far: 

We know there is 50 HP worth of BTU's in the exhaust of a 240 HP rotary engine 
generating 40 HP at part throttle. 

We know a turbo charger will generate some boost pressure at that HP level.
We know it takes HP to generate boost pressure. Obviously there is some 
kinetic or pressure energy in there or the turbo charger would not spin.
We also can hear the exhaust and noise takes HP to generate.

Here are some of the things we don't know yet:

We don't know how much is kinetic energy, pressure energy or is just plain 
heated gas at atmospheric pressure. 

Paul Lamar
 
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