05/28/2003

High Boost and Reliability

As shown earlier, a supercharged engine has lower peak cylinder pressures than
a non supercharged engine at the same power.  Peak pressure = peak stress in
the cylinder.

Consider that the highest stress on the reciprocating assembly is as the
piston approaches TDC on the exhaust stroke and that it is a tension on the
rods.  Raising the inlet pressure reduces the tension and turns part of the
load into compression.  The higher the pressure, the lighter the load on the
reciprocating assembly - for the same total output at the same rpm.

There is a relationship that shows that the former intake stroke becomes an
expansion stroke under positive pressure and its effect can be estimated to be
about 1 mep per psi of boost.  Thus, instead of a single expansion stroke, all
supercharged engines have two expansion strokes per cycle - one driven by the
incoming gas from the supercharger, and the second from combustion.  The
higher the manifold pressure the more effective of an expansion cycle is
created on induction, and the lower ( less load ) the combustion expansion has
to be for the same load.

There ain't no such thing as a free lunch.

All super-atmosheric devices require power from the previous cycles to
function - whether by increasing internal pumping work for various air tuning,
mechanical effort for engine driven compressors, or increased negative power
from average exhaust back pressure ( at the rate of 1.5 to 2.0 pounds of Mep
for each average pound of back pressure ) for an exhaust turbine driven
compressor.

Depending on the efficiency of these devices to pump air compared to a larger
IC engine, they can be more efficient at moving the air and thus reduce the
load on the reciprocating assembly - compared to a reciprocating assembly
pumping the same amount of air for combustion.  To the extend they are more
efficient, is the extant they can reduce the mechanical load.

It should be apparent that two engines producing the same power at the same
rpm, that a supercharged engine will be more reliable than the NA engine as
its peak pressure, and its load distrubion is more favorable than a similar
power NA.