05/28/03

It takes a certain amount of work (heat) to transport and compress air from
the atmosphere to the cylinder at TDC.  This is known as the work of
induction.  It can be divided into the work of transport and into the work of
compression.

The power of an engine is directly proportional to the mass density of air.
To make a certain power takes a certain quantity of air in the combustion
chamber at TDC ignition.  Matters little on how the air got there.

To induct that amount of air takes a certain amount of work.  Again, the
methodology of performing that work is relatively unimportant to the total
power released at combustion.

With the same or similar amounts of air in the chamber, the BMEP will depend
on how efficient the work of induction is performed - and this work will be
deducted from the total power output.  All forms of supercharging ( creating a
super atmospheric fresh charge ) and NA require work from the engine to
perform this work of induction.  Tanstaafl applies - no matter what Corksheet
Bellemouth says.

All supercharged engines have two expansion cycles - one driven by the
supercharger as the cylinder fills, and the normal combustion expansion cycle.
Thus all such engines gas expansion is larger than there geometric expansion
ratio.

This intake expansion is directly related to intake manifold pressure and
increased as the manifold pressure increases.  This effectively counters some
of the expansion loss of reducing expansion ratio on supercharged engines.
Not completely usually - but it is an effective increase in the expansion
ratio.

Residual energy driven supercharger further expand the exhausting gasses to do
a significant portion of the work of induction.  However, the total energy
that can be recovered from the residual energy is limited to some portion of
the work of induction.  Other methodology must be used to increase the
"expansion" attributed to the residual energy utilization.

To increase the effective expansion and it's work recovery, our primary option
is to INCREASE the work of induction.  The fastest way to increase the work of
induction is to RAISE the manifold pressure.  The higher the manifold
pressure, the greater the work of induction and consequently, the greater the
total expansion ratio of the engine - both from the increased work recovered
during the intake stroke and the effective expansion after the opening of the
exhaust valve.

Consider now the effective combustion expansion.  Assume an air engine - which
is similar to an otto or diesel - save the working fluid is air and all the
heat is magically released at TDC making for the most effective expansion of
the air.

When the piston has moved downward one clearance volume, the pressure will be
halved.  Thus a two to one ratio delivers 50% of all possible expansion
because the pressure is halved.

Move the piston downward to three clearance volumes, the pressure will be one
third as at TDC and 66% of all possible expansion will have occurred.

Move the piston downward to four clearance volumes, the pressure will be one
fourth as at TDC, and 75% of all possible expansion will have occurred.

Move the piston downward to five clearance volumes, the pressure will be one
fifth as at TDC, and 80% of all possible expansion will have occurred.

All combustion expansion effectively stops as the exhaust valve opens and 
blowdown commences.  Consider that at this point we open the exhaust valve.

We have a 5 to 1 effective expansion ratio engine with a 6:1 geometric ratio
engine and we have recovered 80% of the possible expansion.

Draw this out.  Then next to it, draw another cylinder - except divide the
clearance volumes in half and the clearance in half.  This is a 12 to 1
expansion ratio engine.

If we open the exhaust valve at the same point, we will have expanded the
charge 10 times or a one tenth pressure and 90% of all possible expansion has
taken place.  Thus we have doubled the expansion of the charge but we have
only gained 10% more expansion than if we were at 5:1 effective expansion.
Clearly - there is the matter of greatly diminishing returns.

Then consider the time the piston is at a given expansion.  The piston on the
low compression engine takes twice the distance to move down the cylinder of
the high compression engine to get a certain expansion.  At the same rpm, this
means the low compression gets twice as much time to apply its energy to the
reciprocation assembly than the high compression engine.

And it has twice the distance and effective lever arm to convert its
reciprocating energy into rotating energy.

Real combustion - being slower and longer - enhances this advantage - because
the later burning real charge will be at a lower effective volume for a longer
time and will not have lost as much volumetric expansion as the high
compression engine.

Because it is a supercharged engine, manifold pressure can be used to raise
the peak compression pressure to the same point as a higher compression /
expansion NA engine so the starting pressures are the same.

Even casual observation shows that a low compression high boost engine gives
little away to an NA engine at the same power.

Factor in the effective expansion of driving the piston downward by positive
manifold pressure and the ratio's get closer.  Add the residual energy
recovery by the turbine, and there will be little difference.  How little -
all the residual energy compressor has to do is, in combination with the
enhanced intake expansion, recover 10% of the expansion to bring it equal to
the 12 to 1 NA engine.

No computer models were endangered in this exercise.