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How Intercoolers Work
An intercooler, or charge air cooler, is a device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing the amount of charge in the engine and lowering charge air temperature thereby increasing power and reliability. It is also known as a charge air cooler, especially on larger engines that may easily self-destruct with high intake-air temperatures. The inter in the name refers to its location compared to the compressors; the coolers were typically installed between multiple stages of supercharging in aircraft engines. Modern automobile designs are technically aftercoolers because they appear most often at the very end of the chain, but this term is no longer used. Also the thicker and large the Intercooler, the more Turbo Lag it produces.
Turbocharging
Turbochargers and superchargers compress incoming air, causing it to become heated (see the ideal gas law). Since hot air is less dense than cooler air at the same pressure, the total charge delivered to the cylinders is higher than non-compressed air but still less than it could be. By cooling the charge after compression, the stream experiences further compression which is naturally tied with cooling of matter—upon cooling matter shrinks occupying less volume (usually, see Coefficient of Thermal Expansion). With this further compression even more charge can be delivered, increasing power. Additionally, intercoolers help to increase the total amount of boost possible without causing engine knocking. One of the most efficient intercoolers is water injection—it cools the intake charge and cools down the combustion temperature.
Charge cooling
An alternate design, often referred to as a charge cooler, (heat exchanger) uses water or a water/antifreeze mix to cool the charge, then cools the water in a separate radiator. While heavier and more complex, charge coolers can often make arranging the rest of the engine much simpler. This approach is also known as Water To Air (WTA or A/W). A variation on this type of charge cooler substitutes a reservoir of coolant for the radiator, allowing the use of an icewater mixture or liquid nitrogen that can bring outlet temperatures well below ambient air temperature even under very high boost pressure. Because of the limitations on the volume of coolant that can be stored and circulated, this approach to charge cooling is only practical for short durations, making it most common in drag racing and land speed record attempts.
In at least one land speed record attempt, Gale Banks used nitrous oxide, not internally as a power-adder, but as the medium into which the heat was transferred from the charge air. The nitrous oxide was held in bottles and released through the intercoolers' cooling fins and exhausted directly to the atmosphere. Extra cooling by nitrous oxide spraying on the front of the intercooler is now a related commercially available upgrade.
Extra cooling of the charge air can be achieved also by externally spraying water on the front of the intercooler. This can be activated automatically or manually, and is far cheaper to refill than nitrous oxide.
Air to air intercoolers need to be mounted so as to maximize air flow and promote efficient cooling. Most cars such as the Toyota Supra, Nissan Skyline, Saab (except the Subaru WRX-based 9-2X Aero), Dodge SRT-4, Mitsubishi Lancer Evolution use front mounted intercooler(s) (FMIC) mounted vertically near the front bumper, in line with the car's radiator. While FMICs' offer the best form of charge cooling, the car suffers a slightly larger amount of "turbo lag", as it takes more time for the turbo to push the intake air through the additional piping. Many older turbo-charged cars, such as the Saab 900, Volkswagen Audi, and Turbo Mitsubishi Eclipse use side-mounted intercoolers (SMIC), which are mounted in the front corner of a bumper, in front of one of the wheels. Side-mounted intercoolers are generally smaller and less efficient than front-mounted intercoolers. Cars such as the Subaru Impreza WRX, MINI Cooper S and the MAZDASPEED 6 use top mounted intercoolers (TMIC) which are mounted horizontally on top of the engine (due to a low hood line) and use a hood scoop to force air over the intercooler. While TMICs' offer a fix to the "turbo lag" caused by the FMIC, they experience an effect known as "heat soak", where the heat from the engine warms the intercooler, lowering it's efficiency to cool the intake charge. Due to limited room in the engine bay, TMICs' tend to be small, and can only efficiently cool small amounts of air. At high horsepower levels, or when using large turbochargers, FMIC's are much more efficient than TMICs. Some World Rally Championship cars use a reverse-induction setup, where air from ducts in the front bumper is forced up over a horizontally-mounted intercooler and then vented through ducts in the top of the hood to further maximize aerodynamic benefits.