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Thermal Efficiency
Thermal efficiency is the ability of an intercooler
to remove heat. This is measured in relation to the
ambient air temperature. For example, if the ambient
air temp is 70 °F and you are running at 5500 RPM
at 14.7 psi of boost pressure, the air entering the
turbocharger or supercharger will be in the range of
75-85 °F (accounting for heat absorbed from engine
compartment). During compression in the turbocharger
or supercharger the air is heated (adiabatic process)
to the range of 230-260 °F. This hot intake air
then passes through the intercooler where heat, if the
intercooler is properly designed, is removed. The closer
the post-intercooler intake air temperature gets to
the ambient air temperature the more thermally efficient
the intercooler.
Thermal
Inertia (Thermal Momentum)
Thermal inertia, also known as thermal momentum, is
the heat sink capacity of the intercooler or the amount
of heat the intercooler can absorb based on the mass
of the intercooler. An intercooler with a high thermal
inertia is able to absorb more intake air heat before
intercooler outlet temperatures rise than an intercooler
with low thermal inertia.
To see it another way, consider two different masses
of aluminum, a torch and your hand. If we place a 6”
x 6” piece of aluminum foil in the palm of your
hand and then heat the exposed side with a torch, you
will notice the heat on your palm almost immediately.
Now try it again with a 6” x 6” plate of
one inch thick aluminum. The timeframe before you feel
any heat getting through will be much longer. The aluminum
foil has very little mass with which to absorb heat,
the thick plate, however, has considerable mass. In
much the same way a larger intercooler will be able
to absorb more intake air heat than a smaller intercooler
with the same base temperature.
Thermal inertia is the combination of the mass of the
intercooler (a larger mass being able to absorb more
heat than a smaller mass), the base temperature of the
intercooler (usually a steady temperature of the intercooler
at idle or cruise) and the time required for the temperature
at the intercooler outlet to increase with the introduction
of hotter intake air for the turbocharger or supercharger.
It is important to note that even with high thermal
inertia the intercooler outlet temps will begin to rise
slightly immediately after introduction of higher intake
air temps. These outlet temps will continue to rise
slowly over the duration of the increased intake air
temperature event until the thermal inertia of the intercooler
is overcome, at which point the outlet temps will rise
sharply to the a stabilized temperature. This new stabilized
temperature reflects the thermal efficiency of the intercooler.
In short, thermal inertia is the amount of time required
between an increase in the intercooler inlet temperature
(from increased boost pressure and/or engine rpm) and
the subsequent increase in intercooler outlet temperature.
If the increased intake air temperature is removed prior
to overcoming the thermal inertia of the intercooler
the outlet temperatures will never reach the thermal
efficiency temperature.
Pressure Drop (Pressure Efficiency)
Pressure Efficiency, more commonly known as Pressure
Drop, is the measure of the internal resistance of the
intercooler or how much energy or boost pressure is
lost as the intake air passes through the intercooler
system. While this pressure drop is most often seen
at the intercooler core, there is also opportunity for
boost robbing resistance in the tubes leading to and
from the intercooler, as well as the in the intercooler
tanks. Every inch of tubing provides some drag resistance
to the intake air, as do bends and turns in the tube
system. Also smaller diameter tubes increase the drag
resistance compared to larger tubes. The optimal intercooler
system uses the most direct tube routing available with
tube diameters properly sized for the air flow associated
with the target power level.
Resistance at the intercooler core is directly related
to two elements: internal flow area and internal resistance.
Internal flow area is the amount of space available
for the air to move through the core. The larger the
internal flow area, the lower the resistance and the
lower the pressure drop. The smaller the area, the higher
the resistance and higher the pressure drop. Internal
resistance, as you might reason, is the resistance encountered
within the intercooler core. This is primarily the result
of the internal cooling fin design and density with
a secondary source being potential restrictions at the
entry point to the core.
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