Air/Liquid Intercooler Tech
Article coutesy of
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
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.
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 intercooler, 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
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.
Copyright © Tom Coleman 1999