11/19/2003

Otto, Brayton, Ericsson Cycles

Historically, the Otto cycle has been used to define the thermodynamic
performance of a supercharged/turbo-charged engine.  

Just as the Carnot Cycle has been abused by dweebs for a 100 years and
is thoroughly misunderstood, the Otto cycle differs widely from the
super atmospheric engine actual results.

There has been some non list communication about how to properly deal
with the supercharged cycle engines and a number of partial attempts to
try to rationalize thermodynamics with actual results.

Basically, the turbocharged engine actually consists of at least 3
different overlapping cycles with at least 3 expansion stages.  In
thermodynamics with multiple cycle systems, cycles that operate at
greater heat than the major cycle are called topping cycles, and those
that operate at lower heat are called bottoming cycles.

The turbo engine has a major ( Otto ) cycle preceded by what appears to
be a bottoming Ericson Cycle and followed by another bottoming Brayton
(Joules) cycle.

For those that do not consider Dr. Ferdinan Porsche a complete
blithering idiot, you could consider adding a third bottoming cycle
after the Brayton Cycle - a vapor rankine cycle.  Dr Porsche ( you know
the silly Prussian who invented the VW and some dweebish air cooled
sporty car that nobody has heard of ) has a number of patents on using
the exhaust heat to generate steam and run a steam turbine in a compound
manner.

This btw is the Dr. Porsche who did his apprenticeship with Tatra and
basically repeated his training with the VW and the Porsche.  Still
learning more - but the Roots Supercharged Mercedes Engine in Humpries
looks exactly like Dr Porches patents in the twenties on shaft driving a
roots, clutching a roots, carboration for a draw thru roots supercharged
engine etc.  

Ignore any thoughts of a Rankine Bottoming Cycle.  Can't be done.  If it
could, Corkshiit Bellony would be marketing it and writing more
commercial propaganda about it.

The Ericsson Cycle adds heat in the expansion cycle at constant
temperature and pressure, then expands the gas at constant pressure,
then rejects the heat at constant pressure and temperature.  The gas is
then compressed and Cycle repeats.

The compressor can replace the compression portion of the Ericsson Cycle
perfectly and the output of the compressor at constant pressure and
constant temperature ( result of adiabatic compression with less than
perfect efficiency ) can then be expanded by the piston from TDC to BDC
on the former intake stroke.  Instead of rejecting the heat to a cooling
system, the heat is rejected to an Otto Cycle at BDC beginning the
Compression Stroke.

We know what happens.  This gives us a closer to reality predication and
contemplation model and allows a better judgement of what to deal with
and how.

The Brayton cycle is generally associated with a continuous combustion
gas turbine, but in the case of a turbocharged engine, it is an ideal
description of the thermodynamics of the turbine.  The compressor and
combuster are replaced by the exhaust of the Otto cycle.  Up until the
waste gate begins to open, all of the exhaust of the Otto cycle is used
to drive a turbine.  This turbine drives the compressor for the Ericsson
Cycle - completing the loop.

The Brayton Turbine is limited to the power needed to drive the
compressor and thus the Friction MEP is lowered accordingly - minus the
additional loss's in the Otto cycle due to exhaust back pressure.

The addition of an afterburner in the exhaust ahead of the turbine such
as popularized by the French Hyperbar system, allows one with
functioning neurons to change the shape of the air curve of the turbine
to one closely approaching a positive displacement compressor such as a
lyscholm or a roots.  This after burner will add heat in the manner of a
Rankine Reheat cycle to the exhaust stream.  Depending on preferences,
either air or fuel or both can be added to the exhaust and thusly
increase the gas flow thru the turbine independently of the gas flow
thru the engine.  

Since this flow depends only on the afterburner, you could conceivably
have 30 psi just off idle.  Perkins on a turbo-compound engine used a
roots or lyscholm as a parasitic load on a differential drive and wound
up with only 2 gears to properly propel an 80,000 pound capacity diesel
truck.  The slower the output - the higher the boost pressure of the
roots.  This same pressure curve can be achieved with an afterburner.
The only limit to the pressure curve is your imagination.

The normal convention is to determine the expansion by the expansion
ratio of the Otto cycle - which is a good and close approximation of the
actual efficiency of a Naturally Aspirated engine.  But with a
supercharged engines, you have at least two expansion cycles - the Otto
expansion and the bottoming Ericsson Cycle expansion. If turbo'd - you
add another bottoming Brayton Cycle expansion.

If you bump your intake to 30 psi, that's a 3 to 1 ( 3 bar ) Ericsson
Cycle expansion to be considered with whatever you Otto expansion is.
An 8 to 1 expansion Otto at 30 psi is running around an 11 to 1
expansion.  Then toss in some from the Brayton Cycle.

The point here is to consider the efficiency of the engine as a whole.
The baseline Otto cycle is doing good to do 30% efficiency - but a
Rankine Cycle Steam plant breezes past 50% and some very efficient and
large late model super-critical plants can push 70%.  The key is that
nothing is the Rankine cycle by itself can approach the temperature of
an Otto, so each step is much less efficient.  But if you put together
enough small steps, the end result is significantly better than any step
in the process.

This is an ongoing mental process.  The Sterling Cycle is similar to the
Erection - both are external combustion, but the mechanism appears to be
closest to the Ericsson Cycle.  

Some input not related to standard freshman thinking will be accepted
and once the ducks are confirmed to be in a row, it will go up with
crayon drawing onto one of dave's web pages.

As I said, simple thermodynamic brain food.  Not perfect - but
approaching good enough.