The Complete Guide to Intercooling
Water/Air Intercooling - Everyone always talks about air/air
intercoolers. So what are the pros and cons of water/air designs?
By Julian Edgar
Water/air intercooling is used less frequently than the
air/air approach. However, it has several benefits, especially
in cramped engine bays. A water/air intercooler uses a compact
heat exchanger located under the bonnet and normally placed
in-line with the compressor-to-throttle body path. The heat is
transferred to water which is then pumped through a dedicated
front-mounted radiator cooled by the airflow generated by the
car's movement. A water/air intercooler system consists of
these major parts: the heat exchanger, radiator, pump, control system, and plumbing.
Technically, a water/air intercooler has some distinct
cooling advantages on road cars. Water has a much higher
specific heat value than air. The 'specific heat value' figure
shows how much energy a substance can absorb for each degree
temp it rises by. A substance good at absorbing energy has a
high specific heat value, while one that gets hot quickly has
a low specific heat. Something with a high specific heat value can
obviously absorb (and then later get rid of) lots of
energy - good for cooling down the air.
Air has a specific heat value of 1.01 (at a constant
pressure), while the figure for water is 4.18. In other words,
for each increase in temp by one degree, the same mass of
water can absorb some four times more energy than air. Or,
there can be vastly less flow of water than air to get the
same job done. Incidentally, note that pure water is best - its specific heat value is
actually degraded by 6 per cent when 23 per cent anti-freeze is added! Other
commonly available fluids don't even come close to water's specific heat value.
The high specific heat value of water has a real advantage
in its heat sinking affect. An air/water heat exchanger
designed so that it has a reasonable volume of water within it
can absorb a great deal of heat during a boost spike. Even
before the water pump has a chance to transfer in cool water,
the heat exchanger has absorbed considerable heat from the
intake airstream. It's this characteristic that makes a
water/air intercooling system as efficient in normal urban
driving with the pump stopped as it is with it running!
To explain, the water in the heat exchanger absorbs the heat
from the boosted air, feeding it back into the airstream once
the car is off boost and the intake air is cooler. I am not
suggesting that you don't worry about fitting a water pump,
but it is a reminder that in normal driving the intercooler
works in a quite different way to how it needs to perform
during sustained full throttle. However, the downside of this
is once the water in the system has got hot (for example,
after you've been driving and then parked for a while), it
takes some time for the water to cool down once you again drive off.
The Heat Exchanger:
Off the shelf water/air heat exchangers are much rarer than
air/air types. Water/air intercooling has been used in cars
produced by Lotus, Subaru and Toyota. A few aftermarket
manufacturers also produce them. If you want to make your own,
the easiest way to go about it is to jacket an air/air core.
Pick an air/air intercooler that uses a fairly compact core
that still flows well. If it uses cast alloy end tanks (as
opposed to pressed sheet aluminium) then so much the better.
(Plastic end tank types need not apply!) The core is then
enclosed in 3mm aluminium sheet, TIG welded into place. Water
attachment points can be made by welding alloy blocks to the
sheet metal, with these blocks then drilled and tapped to take
barbed hose fittings. Pressure-test the water jacket to make
sure that it actually does seal, and make sure that the water
flow from one hose fitting to the other can't bypass the core.
Small baffles can be used to ensure that the water does fully
circulate before exiting.
Another type of water/air heat exchanger can be made using
a copper tube stack. These small heat exchangers are normally
used to cool boat engine oil, exchanging the heat with engine
coolant or river or seawater. While the complete unit uses a
cast iron enclosure and so is too heavy and large for car
applications, the core piece itself can be enclosed to make a
very efficient heat exchanger. Comprising a whole series of
small-bore copper tubes joining two endplates, the core is
cylindrical in shape and relatively easy to package. The
induction air flows through the tubes while a water-tight
sheet metal jacket can be soldered around the cylinder. The
resulting heat exchanger is a little like a steam engine
boiler, with induction air instead of fire passing down the
boiler tubes! The one here is shown installed on a car
undergoing fuel pump testing.
As with air/air designs, the more efficient that you can
make the heat exchanger, the better is the potential system
performance. If you plan to use an off-the-shelf heat
exchanger that has specifications available for it, you will
be interested to know that the 150kW turbo Subaru Liberty
(Legacy) RS uses a factory-fitted water/air exchanger that has
a 4kW capacity. This heat exchanger also works quite
effectively when power is increased to about 210kW. Remember
in your design considerations that you want a reasonable store
of water in the actual heat exchanger (2 or 3 litres at least)
to help absorb the temperature spikes.
Radiator and Pump
The front-mounted radiator for the water/air intercooler
should be completely separate to the engine cooling radiator.
Some turbo trucks use the engine coolant to cool the water/air
ntercooler, but their efficiency is much reduced by taking
this approach. Suitable radiators that can be used include
large oil coolers, car air conditioning condenser cores, and
scrap domestic air conditioning condensers. If you use a car
airconditioning condenser there is likely to be available a
small dedicated electric fan that attaches to the core easily.
This fan can be triggered to aid cooling when the vehicle is
stationary. The radiator should at least match (and
preferably) exceed the cooling capacity of the heat exchanger,
but again finding proper specifications is often difficult.
The Subaru Liberty (Legacy) RS with the 4kW heat exchanger
uses quite a small radiator, only 45 x 35 x 3cm.
An electric pump is the simplest way of circulating the
water, with the type of pump chosen influenced by how the pump
is to be operated. Some factory systems have the pump running
at low speed continuously, switching to high speed at certain
combinations of throttle position and engine airflow. If you
follow a similar approach, the pump that is chosen must be
capable of continuous operation. Another approach is to
trigger the pump only when on boost, or to trigger a timing
circuit that keeps the pump running for another (say) 30
seconds after the engine is off-boost. The latter type of
operation will mean that the pump operating time is drastically reduced over continuous running.
Twelve volt water pumps fall into two basic types -
impeller and diaphragm. An impeller pump is of the low
pressure, high flow type. In operation it is quiet with low
vibration levels. A diaphragm pump can develop much higher
pressures but generally with lower flows. A diaphragm pump is
noisy and must be rubber-mounted in a car.
Suitable impeller type pumps are used in boats as bilge
pumps and for deck washing. They are relatively cheap and have
very high flows - 30 litres a minute is common. However, they
are not designed for continuous operation and generally
don't have service kits available for the repair of any worn
out parts. Diaphragm pumps are used to spray agricultural
chemicals and to supply the pressurised water for use in boat
and caravan showers and sinks. They are available in very
durable designs suitable for continuous running and have
repair kits available. Flows of up to 20 litres a minute are
common and they develop enough pressure (45 psi) to push the
water through the front mounted radiator and heat exchanger
without any problems.
The factory water/air intercooler system in the Subaru
Liberty RS uses an impeller-type pump rated at 15 litres a
minute (all flow figures are open-flow). It is automatically
switched from low to high speed as required. This is an ideal
pump because it was designed by Subaru to circulate the water
in a water/air intercooling system! However, it is a very
expensive to buy new, but if one can be sourced secondhand it is ideal.
A cheap and simple impeller pump is the Whale GP99 electric
pump. It is so small that the in-line pump can be supported by
the hoses that connect to it. It flows 11 litres a minute and
has 12mm hose fittings. It is 136 x 36mm in size and is
suitable for discontinuous operation. This pump is available
from marine and caravan suppliers.
The Flojet 4100-143 4000 is a diaphragm pump suitable for
water/air intercooler use. The US-manufactured pump uses a
permanent magnet brush-type fan-cooled motor with
ball-bearings and is fully rebuildable. The pumping head uses
four diaphragms which are flexed by a wobble plate attached to
the motor's shaft. The 19 litre/minute pump uses ¾ inch
fittings and is 230mm long and 86mm in diameter. It is
available from companies supplying agricultural spray equipment.
The Flojet pump needs to be mounted either vertically with
the pump head at the bottom, or horizontally with the vent
slots in the head facing downwards. This is to stop any fluid
draining into the motor if there are any sealing problems in
the pump head. At its peak pressure of 280 kPa (40 psi), the
pump can draw up to 14 amps; however, in intercooler operation
the pressure is vastly less and so the pump draws only about
5.5 amps at 12 volts. The pump is noisy (as all diaphragm
pumps are) but mounting it on a rubber gearbox crossmember
mount effectively quietens it. Note that these pumps are much
louder when mounted to the car's bodywork than they are when
sitting on the bench!
Control Systems
As already indicated, there are a number of ways of
controlling the pump operation. The simplest is to switch the
pump on and off with a boost pressure switch. This means that
whenever there is positive manifold pressure, the pump
circulates the water from the heat exchanger through the
radiator and back to the heat exchanger. If boost is used
frequently and for only short periods, this approach works
well. However, it is better if a timer circuit is used so that
the pump continues to operate for a short period after boost is finished.
A suitable pressure switch is an adjustable Hobbs unit
(pictured), available from auto instrument suppliers. However,
this switch is relatively expensive and a cheaper unit is
easily found. Spa bath suppliers stock a pressure-operated
switch that is ideal for forced aspirated car use. The
pressure switch is designed to work as part of the
air-actuated switching system which is used in a spa bath so
that bathers don't have to directly operate high voltage
switches. The switch triggers at around 1 psi and costs about
half that of a traditional automotive pressure switch. If a
switching pressure above 1 psi is required, simply tee a
variable bleed into the pressure line leading to the switch.
Adjusting the amount of bleed will change the switch-on point.
Another approach to triggering pump operation is to use a
throttle switch. A micro switch (available cheaply from
electronics stores) can be used to turn on the pump whenever a
throttle position over (say) half is reached. A cam can be cut
from aluminium sheet and attached to the end of the throttle
shaft. If shaped with care, it will turn on the switch gently
and then keep it switched on at throttle positions greater
than the switch-on opening throttle angle.
If a two-speed pump operation is required, the pump can be
fed current through a dropping resistor to provide the slow
speed. When full speed is required, the dropping resistor can
be bypassed. Suitable dropping resistors are the ballast
resistors used in older ignition systems or the resistor pack
used in series with some injectors. The value of the resistor
that is used will depend on the pump current and its other
operating characteristics. In all cases, the resistor will
need to dissipate quite a lot of power and so will need to be
of the high wattage, ceramic type. The resistor will get very
hot and can be placed on a transistor-type heat sink mounted
within the airstream, perhaps behind the grille. When
experimenting with resistors and a pump, you should know that
placing the multiple resistors in parallel will increase pump
speed while wiring the resistors in series will slow the pump.
Another approach is to use a temperature switch, so that
the pump doesn't run when the intake air is not actually hot.
This situation can occur on boost if the intake air
temperature is very low because the day is cold. Overly cold
intake air can cause atomisation problems, although this is
not normally a problem in a high performance car being driven hard!
However, running the pump when the intake air
is perhaps only 5 is pointless and it can be avoided by
placing a normally-open temperature switch in series with the
boost pressure or throttle position switches. If the switch
closes at temperatures above (say) 30 degrees, the pump will
operate only when it actually needs to. A range of low cost
temperature switches is available from RS Components (stores
world-wide). Note that in all pump control systems a relay
should be used to operate the pump
The Water Plumbing:
The most obvious place for the pump to be within the system
is immediately after the radiator, so that it is then
subjected only to relatively cool water temperatures. However,
this can't always be done because some designs of pump are
reluctant to suck through the restriction posed by the
radiator. Depending on the design of the radiator, its flow
restriction may be substantial. During the assembly of the
system it is therefore wise to set it all up on the bench.
Check water flows with the pump running (at different speeds,
if this is the approach to be taken) and with the pump in
different positions within the system. The pump position that
yields the greatest water flow should be the one adopted -
even if that places the pump immediately after the heat
exchanger. In practice, the temperature of the water exiting
the heat exchanger will not be extremely high if the water
volume circulating through the system is adequate.
A header tank should be positioned at the highest point of
the system. This should incorporate a filler cap and can
actually be part of the heat exchanger if required. Note that
a water/air system can be pressurised if required by the use
of a radiator-type sealing cap. Be careful that the system
design allows air to be bled from any spots where it will
become trapped. Air in the system degrades performance and can
cause pump problems. A filter placed in front of the pump is a
good idea and very cheap water filters can be found in the
garden irrigation section of hardware stores. These filters
use a fine plastic mesh design and can be easily placed in-line.
Selecting an Intercooling System:
Both air/air and water/air systems have their own benefits
and disadvantages. Air/air systems are generally lighter than
water/air, especially when the mass of the water (1kg a
litre!) is taken into account. An air/air system is less
complex and if something does go wrong (the intercooler
develops a leak for example), the engine behaviour will
normally change noticeably. This is not the case with
water/air, where if a water hose springs a leak or the pump
ceases to work it will not be immediately obvious. However, an
air/air intercooler uses much longer ducting and it can be
very difficult to package a bulky air/air core at the front of
the car - and get the ducts to it! Finally, an air/air
intercooler is normally cheaper than a water/air system.
A water/air intercooler is very suitable where the engine
bay is tight. Getting a couple of flexible water hoses to a
front radiator is easy and the heat exchanger core can be made
quite compact. A water/air system is very suitable for a road
car, with the thermal mass of the water meaning that
temperature spikes are absorbed with ease. However, note that
if driven hard and then parked, the water within the system
will normally become quite warm through underbonnet heat soak.
This results in high intake air temperatures after the car is
re-started as the hot water takes some time to cool down.
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Type of
Intercooling
Advantages
Disadvantages
Air/Air
eg Nissan 200SX
Water/Air
eg Subaru Liberty RS