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 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!
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.
The Complete Guide to Intercooling - Part 1