thumb|right|Front-mounted air-to-air intercooler
thumb|Top-mounted air-to-liquid intercooler (the silver cuboid-shaped part) on a [[BMW S55 turbocharged engine]]
An intercooler is a heat exchanger used to cool a gas after compression.
Method of heat transfer
Air-to-air intercoolers are heat exchangers that transfer heat from the intake air directly to the atmosphere. Alternatively, air-to-liquid intercoolers transfer the heat from the intake air to intermediate liquid (usually water), which in turn transfers the heat to the atmosphere. The heat exchanger that transfers the heat from the fluid to the atmosphere operates in a similar fashion to the main radiator in a water-cooled engine's cooling system, or in some cases the engine's cooling system is also used for the intercooling system. Air-to-liquid intercoolers are usually heavier than their air-to-air counterparts, due to additional components making up the system (e.g. water circulation pump, radiator, fluid, and plumbing).
The majority of marine engines use air-to-liquid intercoolers, since the water of the lake, river or sea can easily be accessed for cooling purposes. In addition, most marine engines are located in closed compartments where obtaining a good flow of cooling air for an air-to-air unit would be difficult. Marine intercoolers take the form of a tubular heat exchanger with the air passing around a series of tubes within the cooler casing, and sea water circulating inside the tubes. The main materials used for this kind of application are meant to resist sea water corrosion: copper-nickel for the tubes and bronze for the sea water covers.
Alternatives
An alternative to using intercoolers - which is rarely used these days - was to inject excess fuel into the combustion chamber, so that the vaporization process would cool the cylinders in order to prevent knocking. However the downsides to this method were increased fuel consumption and exhaust gas emissions.
Air compressors
thumb|left|Top-mounted intercooler on a 1910 [[Ingersoll Rand two-stage air compressor]]
thumb|Schematic of a two-stage compressor with intercooler
Intercoolers are used to remove the waste heat from the first stage of two-stage air compressors. Two-stage air compressors are manufactured because of their inherent efficiency. The cooling action of the intercooler is principally responsible for this higher efficiency, bringing it closer to Carnot efficiency. Removing the heat-of-compression from the discharge of the first stage has the effect of densifying the air charge. This, in turn, allows the second stage to produce more work from its fixed compression ratio. Adding an intercooler to the setup requires additional investments.