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Indirect charge air cooling

In our “Introducing a product” series, we give you the essential information from the experts about modern vehicle technologies and equipment in a compact, uncomplicated form. Today, it’s all about indirect charge air cooling. We’ve also got tips and tricks for cooler installation and repair, summarized in a concise manner.

There’s even more to it

In our last article we looked at direct charge air cooling and described how it works. Today, we’re focusing on another, more efficient, and—most of all—more structurally advantageous alternative, the indirect variant.

Even more efficient

There’s a second even more efficient way of resolving the conflict between installation space and power requirements: the concept of indirect charge air cooling.

In the passenger car, this cooling system consists of a complete coolant circuit that is independent of the engine cooling circuit. A low-temperature radiator and a charge air/coolant cooler are incorporated in this circuit. The charge air’s waste heat is initially transferred to the coolant and then dissipated to the ambient air in the low-temperature radiator. This cooler is integrated in the vehicle’s front end, where the charge air/air cooler is located in conventional air-cooled or direct charge air cooling. Since the low-temperature radiator needs significantly less space than a comparable charge air/air cooler, space is freed up in the front end. This also means that the bulky charge air lines from the vehicle front end to the engine are no longer needed.

The advantage of this system is that the temperature of the compressed intake air can be regulated according to the situation via controllable charge air cooling. This helps to optimize the engine operating temperature. In diesel engines, the particulate filters also regenerate more quickly. The significantly simpler design in the front end improves cooling air flow through the engine compartment accordingly.

Differences between the indirect and direct charge air cooling circuit

  • The charge air is not cooled by air but by a liquid coolant (water–glycol mixture).
  • In the case of coolant-cooled charge air coolers, the charge air cooler can be installed in almost any position.
  • As the overall installed size is small, the intake section and charge air cooler form a single unit.
  • The low-temperature radiator is housed in the vehicle’s front end and requires significantly less space than conventional charge air coolers. The bulky charge air lines are no longer needed.

Advantages of indirect over direct charge air cooling:

  • Significantly reduced charge air pressure loss
  • Improved engine dynamics thanks to the lower volume of charge air
  • Higher dynamic cooling capacity
  • Improved engine efficiency due to the increase in charge air density
Coolant circuit in indirect charge air cooling

Identifying defects and checking components

Causes and effects of a defective charge air cooler

The causes and effects of a defective charge air cooler described here apply equally to the direct and indirect charge air cooling circuits.

Symptoms of a defect:

  • Inadequate engine output
  • Air escaping from the charge air cooler
  • Increased emissions
  • Greater fuel consumption
  • Coolant loss (only with water-cooled charge air coolers)

Classic causes of failure:

  • Damaged/blocked hose/coolant connections
  • Coolant loss or air infiltration due to leakage
  • External damage (stone chips, accident)
  • Reduced air flow rate (surface dirty)
  • Internal contaminants (oil, foreign objects, corrosion, sealant)

Cause of failure after mechanical turbo damage:

  • If there is a mechanical defect in the turbocharger, metal swarfs can accumulate in the charge air cooler.
  • Contamination or blockages can impair engine performance.
  • If chips leave the charge air cooler and end up in the combustion chamber, this can cause engine damage.
  • Metal particles can also come loose in the charge air cooler, even a long time after turbocharger replacement, and enter the combustion chamber—a delayed effect.

IMPORTANT: To prevent the damage scenarios and delayed effects described above, you should check the charge air cooler and attachments thoroughly every time the turbocharger is replaced. If the turbocharger suffers a mechanical defect, the charge air cooler should always be replaced.

  • Shavings in charge air cooler
  • Shavings in charge air cooler
  • Mechanical turbocharger defect

Instructions for installing a new charge air cooler

When installing a new charge air cooler, the peripheral parts also need to be taken into account. Always consider the following points when carrying out the replacement:

  • Check the air guide between the turbocharger and charge air cooler for contamination, foreign objects, blockages, or any constriction of cross sections.
  • Check the air guides between the turbocharger and intake manifold for contamination, foreign objects, blockages, or any constriction of cross sections. Damaged, blocked, or clogged air guidance parts or attachments should be cleaned and replaced if necessary.
  • Replace any gaskets on the air guides and coolant connections (in the case of water-cooled charge air coolers).
  • Make sure all connection elements are fastened tightly and there are no leaks, so that excess air is not sucked in.
  • Check the charge air pressure.

In the third part of our “Introducing a product” series, we look at the exhaust gas recirculation cooler, or EGR cooler for short. So, stay tuned!


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


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