An intercooler, or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged (forced induction) internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through isochoric cooling. A decrease in air intake temperature provides a denser intake charge to the engine and allows more air and fuel to be combusted per engine cycle, increasing the output of the engine.
The inter prefix in the device name originates from historic compressor designs. In the past, aircraft engines were built with charge air coolers that were installed between multiple stages of supercharging, thus the designation of inter. Modern automobile designs are technically designated aftercoolers because of their placement at the end of supercharging chain. This term is now considered archaic in modern automobile terminology since most forced induction vehicles have single-stage superchargers or turbochargers. In a vehicle fitted with two-stage turbocharging, it is possible to have both an intercooler (between the two turbocharger units) and an aftercooler (between the second-stage turbo and the engine). The JCB Dieselmax land speed record-holding car is an example of such a system. In general, an intercooler or aftercooler is said to be a charge air cooler.
Intercoolers can vary dramatically in size, shape, and design, depending on the performance and space requirements of the entire supercharger system. Common spatial designs are front mounted intercoolers (FMIC), top mounted intercoolers (TMIC), hybrid mount intercoolers (HMIC). Each type can be cooled with an air-to-air system, air-to-liquid system, or a combination of both.
Turbochargers and superchargers are engineered to force more air mass into an engine's intake manifold and combustion chamber. Intercooling is a method used to compensate for heating caused by supercharging, a natural byproduct of the semi-adiabatic compression process. Increased boost pressure can result in an excessively hot intake charge, significantly reducing the performance gains of supercharging due to decreased density. Increased intake charge temperature can also increase the cylinder combustion temperature, causing detonation, excessive wear, or heat damage to an engine block.
Passing a compressed and heated intake charge through an intercooler reduces its temperature (due to heat rejection) and pressure (due to flow restriction of fins). If properly engineered, the net result is an increase in density. This increases system performance by recovering some losses of the inefficient compression process by rejecting heat to the atmosphere. Additional cooling can be provided by externally spraying a fine mist onto the intercooler surface, or even into the intake air itself, to further reduce intake charge temperature through evaporative cooling.
Intercoolers that exchange their heat directly with the atmosphere are designed to be mounted in areas of an automobile with maximum air flow. These types are mainly mounted in front mounted systems (FMIC). Cars such as the Nissan Skyline, Saab, Dodge SRT-4, 1st gen Mazda MX-6 and Mitsubishi Lancer Evolution and all use front mounted intercooler(s) mounted near the front bumper, in line with the car's radiator.
Many older turbo-charged cars, such as the Toyota Supra (JZA80 only), Nissan 300ZX Twin Turbo, Mitsubishi 3000gt, Saab 900, Volkswagen, Audi TT, and Turbo Mitsubishi Eclipse use side-mounted air-to-air intercoolers (SMIC), which are mounted in the front corner of the bumper or in front of one of the wheels. Side-mounted intercoolers are generally smaller, mainly due to space constraints, and sometimes two are used to gain the performance of a larger, single intercooler. Cars such as the Subaru Impreza WRX, MINI Cooper S, Toyota Celica GT-Four, Nissan Pulsar GTI-R, MAZDASPEED3, MAZDASPEED6 and the PSA Peugeot Citroën turbo diesels, use air-to-air top mounted intercoolers (TMIC) located on top of the engine. Air is directed through the intercooler through the use of a hood scoop. In the case of the PSA cars the air intake is the grille above the front bumper, then flows through under-hood ducting. Top mounted intercoolers sometimes suffer from heat diffusion due to proximity with the engine, warming them and reducing their overall efficiency. Some World Rally Championship cars use a reverse-induction system design whereby air is forced through ducts in the front bumper to a horizontally-mounted intercooler.
Because FMIC systems require open bumper design for optimal performance, the entire system is vulnerable to debris. Some engineers choose other mount locations due to this reliability concern. FMICs can be located in front of or behind the radiator, depending on the heat dissipation needs of the engine.
As well as allowing a greater volume of air to be admitted to an engine, intercoolers have a key role in controlling the internal temperatures in a turbocharged engine. When fitted with a turbo (as with any form of supercharging), the engine's specific power is increased, leading to higher combustion and exhaust temperatures. The exhaust gases passing through the turbine section of the turbocharger are usually around 450 °C (840 °F), but can be as high as 650 °C (1200 °F) under extreme conditions. This heat passes through the turbocharger unit and contributes to the heating of the air being compressed in the compressor section of the turbo. If left uncooled this hot air enters the engine, further increasing internal temperatures. This leads to a build up of heat that will eventually stablise, but this may be at temperatures in excess of the engine's design limits- 'hot spots' at the piston crown or exhaust valve can cause warping or cracking of these components. This effect is especially found in modified or tuned engines running at very high specific power outputs. An efficient intercooler removes heat from the air in the induction system, preventing the cyclic heat build-up via the turbocharger, allowing higher power outputs to be achieved without damage.
Compression by the turbocharger causes the intake air to heat up, rather than the air being heated by contact with the hot turbocharger itself. The extra power obtained from forced induction is due to the extra air available to burn more fuel in each cylinder. A significant benefit of turbocharging petrol engines is that a lower compression ratio may be used, allowing a wider mapping of ignition timing advance before detonation occurs (for a given fuel's octane rating).
Exhaust gas temperatures (EGTs) can be considerably higher than those quoted above, even for diesels, 850 °C is often quoted as a steady state maximum, with up to 1000 °C for short periods.
Air-to-liquid intercoolers (aka Charge-Air-Coolers) are heat exchangers that reject intake charge heat to an intermediate fluid, usually water, which finally rejects heat to the air. These systems use radiators in other locations, usually due to space constraints, to reject unwanted heat, similar to an automotive radiator cooling system. Air-to-liquid intercoolers are usually heavier than their air-to-air counterparts due to additional components making up the system (water circulation pump, radiator, fluid, and plumbing). The Toyota Celica GT-Four had this system in the 1988-89 version and also in the Carlos Sainz RC Version.
A big advantage of the air-to-liquid setup is the lower overall pipe and intercooler length, which offers faster response (lowers turbo lag) , giving peak boost faster than most front-mount intercooler setups. Some setups can use reservoirs that can have ice put into it for intake temperatures lower than ambient air, giving a big advantage (but of course, ice would need constant replacement).
Ford had adopted the technology when they decided to use forced induction (via Supercharger) on their Mustang Cobra and Ford Lightning truck platforms. It uses a water-cooled intercooler inside the intake manifold, just under the supercharger, and has a long heat exchanger front mounted, all powered by a Bosch pump made for Ford. Ford still uses this technology today with their Shelby GT500.