Originally written by Jim Middlebrook of Vortech
Superchargers January 4, 2000 Reprinted with permission September 4, 2000 The Terminology An aftercooler is a heat exchanger placed
between the compressor and the engine’s inlet. Vortech
uses the term "aftercooler" as we feel it is more accurate;
it is "after" the compressor. "Intercooler"
means a heat exchanger placed between two compressors in a multi-stage
system, but has been used as a synonym for an aftercooler ever since
it was incorrectly stuck on the backend of a Volvo in about 1981.
Aftercoolers and intercoolers are both also called charge
coolers.
The History of Aftercoolers> The earliest aftercoolers appeared on supercharged
racecars shortly after the turn of the century. They took
the form of fins placed on the exterior of the discharge ducts which
extended into the air stream. Both aftercoolers and intercoolers
were used with great success on WWII aircraft. This was the
golden era of supercharging. More advances were made on superchargers
and on charge cooling systems at this time than at any other time
since. During this time, the two stage centrifugal supercharged,
air-to-water aftercooled Rolls Royce Merlin engines were paired
with the P51 Mustang to produce the most impressive piston engine
fighter aircraft in history. Vortech’s engineering staff
has years of experience with charge cooler designs for many types
for supercharged and turbocharged applications such as automobiles,
boats and aircraft. In 1992, Vortech engineers evaluated using
an air-to-air aftercooler with a Vortech V-1 supercharged 5.0 Ford
Mustang. Multiple configurations were tried, all with similar
results: the expected performance improvements from cooling the
charge air were just not realized due to the significant frictional
losses in the charge pressure. We felt that improving supercharger
efficiency was more important, and with improved efficiency, an
aftercooler was unnecessary with the lower boost levels popular
at that time.
The Current Design For heat exchanger design purposes, the gases
in air are classified as a low-density fluid. Air going through
a supercharger is called "charge air". A supercharger
compresses the charge air before it enters the engine. The
act of compression both increases the energy and density of the
charge air, but this act also generates a proportionate amount of
heat. Heating is undesirable, as it tends to decrease the
density of the charge air. One way to deal with heating is
to cool the charge air after it leaves the supercharger and before
it enters the engine. However, in designing the charge cooling
system, which is, as we know, an aftercooler and not an intercooler,
you should try to cool the air without losing the increased density
through "frictional losses". Frictional losses means
the pressure drop which is naturally caused by running the compressed
air through ducting and twists and turns and lengths of tubes. In
fact, frictional losses caused by using a cooling mechanism can
be so great that there can actually be a net loss in air density.
That means if the cooling mechanism is not designed right,
you start at 10 lbs, run through twists and turns and lengths losing
4 lbs, and end up at 6. So what’s the point? It’s
almost not possible to compensate for that sort of misdesign. Therefore,
the designer must consider both the heat transfer rate and the frictional
losses for any system under consideration. This is called
the friction-power expenditure. The friction-power
expenditure increases with flow velocity as much as the cube of
the velocity and never less than the square. Math aside, the
impact of this equation is more clearly stated in the 1998 revision
of the seminal heat exchanger design book, Compact
Heat Exchangers:
However, for low-density
fluids such as gases, it is very easy to expend as much mechanical
energy in overcoming friction power as is transferred as heat.
And it should be remembered that in most thermal power systems
mechanical energy is worth 4 to 10 times as much as its equivalent
in heat.
Vortech’s primary design goal is to
limit frictional losses and pressure drop through the aftercooler
to an absolute minimum. For this, Vortech chose the intrinsic
benefits of an air-to-water system. At temperatures consistent
with aftercoolers used in automobiles, water is about nine times
more conductive than air. (Think about a hot frying pan. If
you want it to cool quickly, do you dunk it in water or do you wave
it around in the air? Which method cools faster?…) The
more conductive nature of water over air allows both a smaller design
of the cooling mechanism and a more effective design. The
flow path length through the aftercooler core can be a fraction
of that of an air-to-air core with the same cooling capabilities.
Therefore extensive additional ducting to run the charge air
out to an external air source and back to the engine are not needed
and therefore associated frictional losses are almost eliminated.
We bring the "cool" to the charge air, not the charge
air to the "cool". Remember that in these cases,
just cooling the charge won’t matter much if you are losing
a lot of the charge through frictional losses and pressure drop.
What's Better? Air-to-Water or Air-to-Air?
In an air-to-air aftercooler system,
the charge air at the supercharger discharge is ducted to a heat
exchanger assembly, cooled, and then continues on to the engine
inlet. The heat exchanger assembly must be placed where the
outside cooling air passes through it. The cooling possible
is dictated by the heat exchanger design and size, and the flow
and temperature of the air passing through it. You also need
to take into consideration the boost loss, packaging within the
vehicle, interaction with other cooling systems, and convection
heating of the system. Air-to-air systems have little significant
thermal transition. This means that the relatively low mass
of the heat exchanger system heats and cools almost instantly and
the system relies on its effectiveness for any benefits.In an air-to-water
aftercooler system, the charge air at the supercharger discharge
is ducted to a heat exchanger assembly, cooled, and then continues
to the engine inlet very much like the air-to-air. However, the
air-to-water heat exchanger assembly does not need direct exposure
to outside cooling air, and it can be much smaller. Because
of this, the heat exchanger can be placed right in the existing
path between the supercharger discharge and the engine inlet. With
no additional ducting or tubes with bends, and a more effective
and compact air-to-water heat exchanger, there are much lower frictional
losses. Air-to-water aftercooler system employs a second heat
exchanger or "radiator" to remove the heat from the system.
The second heat exchanger sits in the cooling air, right ahead
of the engine’s radiator. The heat exchange actually
occurs air-to-water-to-air, but this is still called an air-to-water
system. In this system, water is routed from a storage reservoir
to the "radiator", where it is cooled, up through the
heat exchanger assembly, then back to the reservoir. This
cooling loop is completely separate from the engine cooling system
and continuously cools the water.
Consider the Thermal Transitions In a typical supercharged automobile, boost
is made at wide open throttle ("WOT"), and boost makes
heat. Most cars use boost intermittently and the engine is
running in vacuum most of the time. (How often and how long
can you drive your car at WOT?) Therefore most of the time,
a supercharger is generating little heat. In this typical
mode of operation, with an air-to-water system there is continuously
cooled water stored in the reservoir since there is virtually no
heating from the charge air. When the engine is used under
boost conditions, the heated charge air will begin to exchange heat
into the water. That heat in the water is then exchanged from
the water to the air in the "radiator". Depending
upon the system and reservoir capacity, boost level and duration
of the acceleration, there may be very little impact of boost on
the total system temperature. Continuous use at WOT over an
extended time would have to take place for the system to "stabilize",
which means that the reservoir and radiator water are as hot as
the charge air. Then, the system would be dependent solely
on the effectiveness of the heat exchanger radiator removing the
heat. These conditions rarely occur. In-house testing shows
the Vortech Max Flow® air-to-water system operating at a continuous
8 p.s.i.g. boost pressure would require approximately 18 to 20 minutes
to stabilize. Even stabilized, the air-to-water aftercooler
system can deliver superior overall performance due to the minimizing
of boost loss intrinsic with the design approach. This system
is intended to employ thermal transitions as part of the
design criteria and delivers significant cooling with minimal
frictional losses. For sustained wide open throttle use,
more cooling may be desired. Vortech recommends the use of
a second or larger "radiator" heat exchanger for road
racing or top speed competitions. Some salt flat streamliner
speed record cars use no "radiator" heat exchanger at
all. Instead, they successfully depend totally on thermal
transition by installing a very large coolant reservoir.
The Benefits
Better charge cooling– up to 80% effective
(water only)
Reduced frictional losses (boost loss)
The supercharger does not have to work as hard
to overcome boost loss. This means that the supercharger will
be making less heat in the first place and consume less power
doing it.
Maintain engine cooling–does not block
cooling air to radiator causing engine overheating
Maintain ground clearance–no low hanging
ad-on parts
Better street performance
Even Better strip performance
Allows the use of ice for added charge cooling
for drag racing.
