The differences between air-to-air intercoolers and air-to-water intercoolers
Have you noticed this distinction when reading the data sheets of some cars? Have you ever wondered what these terms mean? Let's get a little bit of clarity to better understand what this is all about.
The intercooler is a heat exchanger whose function is to lower the temperature of the air compressed by the turbocharger before it is sent to the intake manifolds. The compressor compresses the air coming from the air filter using centrifugal force. This results in very high air temperatures of up to 150° C. This is an entirely normal and unavoidable phenomenon because the heat energy in the ambient air is concentrated in a smaller volume, which is why the temperature at the turbocharger outlet-when supercharging-must necessarily be hotter.
However, having very hot air sucked into the engine has at least two contraindications. The first is related to volumetric efficiency issues. The higher the compressed air temperature, the lower the oxygen load it carries inside the cylinders (thus, the less fuel it is possible to inject in both Diesel engines and Otto cycle engines). The other downside for gasoline engines is the risk that the high temperature of the intake air will lead to self-ignition of the mixture (a phenomenon that occurs on older turbocharged gasoline engines with carburetors or those with indirect gasoline injection). That is why it is necessary to lower the temperature of the air entering the engine.
Air-to-air Intercoolers
Air-to-air intercoolers are called this way because they use air to cool the compressed air before it is sent to the intake manifolds. This heat exchanger is quite similar to the engine water cooling radiator. It works very well when the vehicle is in motion at medium to high speeds because of the strong flow of air over the surface of the vehicle. It normally requires lengthy and large pipework back and forth to the front of the vehicle. This results in a small additional delay in supercharging because the turbocharger has to pressurize the entire volume of the intercooler + that of the pipework.
Air-to-water Intercooler
These intercoolers use water to cool the compressed air before it is sent to the intake manifolds. The main advantage of this solution is that the volume of the intercooler (at the same efficiency) can be far less, plus this heat exchanger is mounted directly on the engine with very short piping. For example, AUDI produces engines like the 1600TDI and 2000TDI that have the intercooler integrated into the intake manifold.
This solution provides some advantages. First of all, it allows for greater efficiency and stability even at low speeds (it is less sensitive to the amount of air hitting the front of the vehicle). The reduced volume of the exchanger and the reduced volume of the piping reduces the time with which the turbocharger can generate supercharging, with obvious advantages in terms of response and readiness. The water used for the intercooler can be taken from the cold water outlet of the engine radiator, or it can have an independent circuit with an additional radiator to cool only the liquid used for the air-water intercooler. Aftermarket kits normally employ this second solution, while standard car systems normally employ coolant from the same engine cooling system.
Some examples of cars that employ an air-water intercooler are, in addition to the aforementioned Audi, some Mercedes models like certain 2-liter turbo gasolines or 2200 common-rail diesels. One of the first to employ this solution was Lotus on the Esprit with the 2200 turbo. Jaguar also employed this refined solution on the V8 with a supercharger.
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