11/20/2003

Brayton Cycle

If you are capable of viewing a turbocharged engine with at least an
Otto cycle engine with a superimposed Brayton Cycle composed of the
compressor, the Otto Combustion Unit, an afterburner if needed and an
exhaust turbine that runs the compressor, the following is an absolute
truth.

A.  With a single stage compressor, 30 psi is available continuously
from idle to max power.

B.  Maximum air flow is possible from idle rpm to max crankshaft rpm.

C.  The sole purpose of a wastegate is to limit the maximum pressure in
the manifold.

D.  The limiting factor to available air flow is the exhaust turbine
temperature and nothing to do with the Otto intake tract or displacement
of the Otto expander.  

E.  Virtually any turbocharger can be used for any displacement engine
within reason.  The only need for an aftermarket "matching" a turbine to
an engine is to enhance the experts wallet.  In fact, the closer matched
a turbine is to the engine - the more efficient the Brayton Cycle
becomes but has zippity do dah nada influence on the air flow into the
engine or power.

F.  The biggest difficulty making the above come true is the inability
of the tuner to understand the Brayton Cycle and how a Brayton Cycle
engine works.

Someday I'll post it to the web site - but today, I leave it to the
student as an exercise to figure out what a Brayton Cycle is, what
practical engines its on and how it relates to a turbocharger.

Ignore the advantages and capablities.  We all know that rammie whammie
runners make far more difference to an engines power than a Brayton
Cycle Optimization.

--------------------------------

Having realized that I am partially responsible for Robs understanding
and use of a turbo on a suburaro next season, it would not be fair to
leave him hanging out there.

A Brayton Cycle described not a cyclic device - it describes a process
flow device.  It consists of a continuous flow compressor serially
feeding into at least one combustion chamber and the gas serially going
thru at least one expansion device to drive the compressor and
optionally a second expansion device to extract rotating mechanical
power from.  In other words - a Gas Turbine engine.

In the late 40's and early fifties, many efforts were made to improve
the efficiencies of a gas turbine engine.  The major limit to thermal
efficiencies has always been the metal.  An intermittent device such as
a combustion chamber on an otto, is exposed only partially to the flame
and is then subsequently cooled well below melting before the process is
repeated whilst the gas turbines combustion chamber is limited to the
heat that the metal can continuously absorb.

In Fangle Base - you will find reference to the Napier Deltic 2 cycle 12
cylinder aircraft diesel engine.  This is a classic example of an
attempt to combine the thermal advantages of densely combustion with the
process advantages of a Brayton Cycle.  The non Brayton Cycle components
were used to fire the fuel at very high temperature and energy release
and to provide a partial Non Brayton expansion mechanism to work in
conjunction with several Brayton Cycle expanders.

Sultzer and Nordberg, in a similar vein, proceeded to develop and
produce a number of Free Piston Brayton Cycle Engines.  The pistons were
freed from a crankshaft and allowed to float.  The sole purpose was to
slam from head to head to compress fresh charge to ridiculous
compressions, inject fuel and fire, then repeat.  They became piston gas
generators to replace the continuous flow combustion chamber of a gas
turbine - and left the work to the remaining gas turbine elements.

On any engine with a turbo-charger, if you "replace" the continuous flow
combustion chamber with an intermittent flow Otto or Diesel cycle, it
becomes blindingly obvious that you have described a Brayton Cycle
engine.

Even casual inspection will reveal that the Otto/Deisel cycle downstream
and in series with the compressor has, but best, moderate effect on the
air flow thru the process.  The rpm of the compressor determines the air
flow and performs the compression.  The Otto/Diesel combustor is but an
air flow resistance to the compressed air.  Consider rammie whammie
tuning like a fat guy, having drunk too much stout, pharteing against a
stiff wind and claiming it made a difference and lo - even changed the
direction of the wind.  At least he had the enjoyment of the stout
before he made a fool of himself.  Unlike those who unthinkably cling to
desperate old thoughts.

Further inspection shows that only the quantity of heat and mass of the
combusted gas affects the rotation of the expansion turbine.  There is
no connection to the Otto/Diesel save the output gases.  The expansion
turbine neither knows nor cares about the Otto/Diesel Cycles
displacement, rpm or compression ratio.  Only the available energy in
the gas stream.

The Brayton Compressor and the Brayton Expander are directly connected
to each other.  The rpm of the compressor is directly tied to the rpm of
the expander and thus, the air flow thru the entire engine is not
dependent upon any characteristic of the Otto/Diesel Cycle, but soley on
the available energy to the expansion turbine.  Not rpm, not
displacement, not combustion ratio, not swoopy whoopy rammie whammie
tracts of any kind - nothing nada.

Quick and partial summary - going to work to solve no money no honey
potential problem.  Somebody has to pay for Coronas and Condoms.

All turbo-charged engines are a Brayton Cycle engine, with the
continuous combustion chamber replaced with an imbedded and dependant
Otto/Diesel  Cycle.

Expansion for power may take place in the Otto/Diesel cycle (i.e. the
Fangle Base Napier Deltic) or in the Brayton Cycle (free piston) or both
as in the numerous Turbo Compound engines with an expansion device
geared to the output shaft.

The availability of gaseous energy and pressure from any combustor
determines the rotating speed of the turbine expander.

The rpm of the turbine expander driven compressor determines the air
flow thru the system.

There is no direct relationship between rpm, displacement, compression
ratio, the number and location of the cams, the presence or absence of
Rammie Whammie or any other factor in the performance of the Brayton
Cycle.  The Otto/Diesel is a passive combustion chamber and optionally,
another serial expander.

More when I get to work unless someone comes in and waves money at me to
fix their silly little computer.  Oh wait - that's a customer.  Back
later.



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If you understand the Brayton Cycle - which pre-dates the Otto Cycle by
several years and was the most popular cycle engine up to about 1900,
then smokeys engine is simple and straightforward to understand.

A brayton cycle has independent compression and expansion ratios and a
form of heat addition to the gas between the two phrases.  Brayton uses
internal combustion - and can be continuous or intermittent ( eg free
piston ).

In fact, the difference between a brayton - joules cycle and a stirling
cycle is that the heat is added from an external source.