05/29/2003

Considering a turbo'd engine as a combined rankine and otto cycle engine
simplifies many of the thermodynamic quandaries associated with a super
atmospheric engine.

The rankine inlet cycle is not very efficient - but it generates more power
than a similar larger NA induction.  And it gives a solid thermodynamic basis
for dealing with issues.

For example, as I have been saying - the total power that a turbo charger can
add to an engine as compared with a non-residual energy device - is the work
of induction of the engine minus the increased FMEP that the raised exhaust
back pressure generates.  This then is the expansion offered by an exhaust
turbine driven supercharger.

We can increase the expansion of the exhaust only by increasing the work of
induction - generally higher pressure.  

As Gary has pointed out, increasing the expansion increases the efficiency.

A super-atmospheric engine always has at least two stages of compression - an
external compressor of some sort and the piston on the otto cycle compression
stroke. ( Ram Air, Acrostic Tuning, Hemholz Resonator - all pressure wave
tuning that raises the air pressure within the manifold are external
compression devices - and thus applicable in a super atmospheric engine ).
Normally, one would work to reduce the work of induction to increase the
efficiency of an engine.

But here, since our actual expansion of the charge depends on the total work
of induction, reducing the work of induction to maintain the same power out,
would reduce our effective expansion and increase the amount of fuel needed to
maintain that power out.

Taylor talks about an increased expansion engine - where the intake is cut off
early, the piston expands downward to bdc and does not start compressing the
charge until well after bdc and then expands the charge by the full geometric
expansion ratio.  Taylor "borrowed" this concept from Lysholm ( the screw dude
- among various other devices including hydraulic transmissions etc ).

Thus we can cut off the intake earlier - exactly the same as a semi-expansion
rankine engine, and expand the charge in the intake and gain more power
recovery.  

It takes more work of induction to create a higher pressure, then cut it off
and let it expand in the cylinder than a no cut off induction cycle,
there-fore more work is extracted from the residual energy and we have
increased the effective expansion ratio.

We had a theoretical basis for the improvement - not by twisting a cycle or
magic knowledge - but by understanding that the induction cycle is an
in-efficeint rankine cycle - and that by increasing the efficiency of the
rankine cycle - we could perform more work ( work of induction ) and that
these work, being recovered from residual energy, would effectively increase
the expansion and the total engine efficiency.

Further, by using an intake cutoff and raising the pressure to accommodate the
same mass density of air, we can use the expansion as an intercooler.  Gas
expanding cools.  Gas - having heat ( power ) removed cools.  The power
extracted during the in-efficient rankine induction cycle cools the charge
exactly as a intercooler - except that instead of dissipating the heat as
friction, the heat is consumed as additional rotating power.

Further, because the compressed gas will be at a higher temperature with
higher pressure, more of the heat can be absorbed by the cooling system on
induction.

And intercooling the charge by power extraction is they most effective and
efficient way of cooling the charge.

And BTW, Obert in his "Internal Combustion Engines and Air Pollution" (I
think), shows nordberg and others deliberately increasing the work of
induction by cut off valve and other devices to increase the BSFE of their
large diesel engines - but without giving a thermodynamic basis for what they
were doing.