Charge air coolers: Reviewing the basics

Charge air coolers: Reviewing the basics

Today's high tech air induction systems are designed to optimize engine efficiency and decrease emisisions across a wide range of operating conditions and loads.  The charge air cooler (CAC) is a critical component of that system. 
Increasing engine boost, through the use of increasingly efficient turbo-chargers and charge-air coolers, permit smaller and lighter engines to deliver the equivalent horsepower of older, larger, and heavier engines.  And, smaller and lighter engines means better fuel economy.


At the same time, charge-air cooler performance can significantly impact engine emissions.  Nitrogen oxide (NOx) are a byproduct of high temperature combustion in diesel engines.  By decreasing the temperature of the air to the combustion chamber, the charge air cooler also helps to substantially reduce Nox emissions. 
Engine manufacturers will continue to experience pressures to increase engine 'boost' for fuel efficiency and decrease emissions through lowered combustion temperatures.  These efforts will increasing stress existing charge air cooler designs and require increased maintenance and replacement.  Engines with advanced EGR (Exhaust Gas Recirculation) systems, which meter various amounts of exhaust gases into air intake system depending on engine speed and load, will also place additional stress on the CAC. 
According to recent report on aftermarket cooling components by Frost & Sullivan, "... increasingly stringent tailpipe emissions regulations will drive demand for charge air coolers for medium and heavy duty trucks over the next five to seven years.. Notably, charge air cooler manufacturers expect to increase unit shipments at 3.5 per cent annually during this period:

Basic Air Intake System
Figure 1 shows a basis air intake system using a turbocharger and charge air cooler.  The red circuit indicates the hot exhaust gases, and the blue circuit shows the compressed ambient air.
Hot exhaust gas moves from the cylinder to the turbocharger, where it powers the turbine wheel, before it exits to the exhaust manifold.  The spinning turbine powers the compressor wheel which compresses the incoming ambient air.
Due mainly to compression (which heats the air according to the Ideal Gas Law) the air leaving the compressor can be heated to as much as 400 degrees F.  Without cooling, an excessively hot intake charge would be delivered to the combustion chamber and would significantly reduce the performance gains of turbocharging.  In addition, increased charge temperatures can cause premature detonation, excessive wear, and potential heat damage to the engine block - operating temperatures could exceed the engine's design limits and cause the engine block to warp or crack.
As a result, the compressed airflow must be cooled by the charge air cooler.  As the ambient air passes through the cooler, it extracts heat from the cooler tubes and fins and cools the compresses airflow in the cooler tubes.  The cooled air is still dense and promotes more complete combustion and power output.
The vast majority of the charge air coolers on the road today are made from aluminum alloys.  Aluminium offers light weight, strength, corrosion resistance and excellent heat rejection.  A number of OEM's worldwide are beginning to specify copper-brass charge air coolers which can afford greater reliability at higher temperatures, but this article will focus on the aluminium charge air coolers which continue to account for the majority of the CAC business for most HD shop owners.
CAC Failure Modes
Charge air coolers operate in an extremely harsh environment.  Engine designers are pushing the limits of CAC design as engine horsepower, operating temperatures, as well as turbo-charge "boost" pressures are increasing.  Thermal cycling is more frequent and occurs over greater temperature ranges and vibration is constant.  All of this means that charge air coolers will continue to fail.  The number of service and replacement opportunities will continue to increase as the number of CAC equipped trucks on the road increases each year.
Charge air coolers are most often damaged by debris and oil contamination due to a failure of the turbo-charge.  In these cases, the cooler should be removed, thoroughly cleaned of all debris, flushed with a detergent solution and back flushed with hot water.  The unit should be tested to ensure that it meets specifications and if not, replaced.
Other than turbocharger failures, cracks in tubes and fractures in the tube-to-header joints cause small leaks which, if not repaired, will only get worse.  Eventually these leaks will significantly degrade CAC performance causing decreased engine performance and fuel economy.  In these cases, CAC failure mechanisms include: separation of the top plate from the header, core corrosion and tube distortion and failure.
Separation of the top plate from the header sometimes occurs as a result of the long term effect of vibration.  The top plate is usually more rigid that the somewhat more flexible core.  Due to the difference in rigidity between the top plate and the core, vibration (as well as temperature and pressure cycling) plus stress on the weld between the top plate and the core.  Separation can also occur as a result of the pressure spikes caused by the closing of the waste gate on the turbocharger.
Corrosion of the core often occurs as a result of road salt used in northern states, and sea spray in coastal areas, acting on the external surface of the aluminium.
Aluminium in normal applications is pretty corrosion resistant.  When exposed to air, the exterior surface of the aluminium forms a thin barrier of aluminium oxide which resists further corrosion.  However, road salt and debris strikes can degrade this barrier and eventually cause "pitting".  Over time, such pitting can penetrate the aluminium and cause tiny holes.
Tube distortion and failure can be caused by temperature and pressure cycling over time working to deform the brazing of the internal fins to the tubes.
Servicing the CAC
According to Recommended Practice (RP333) of the American trucking Associations Technology and Maintenance Council (TMC), "Basic maintenance consists of keeping airways clean and free of obstructions, minimizing corrosion, minimizing air loss, inspecting for proper mounting and inspecting for structural integrity".
All external surfaces of the cooler should be visually inspected and periodically cleaned removing any leave,s debris or dirt that might obstruct airflow over and around the tubes and fins.  A steam cleaner or high pressure washer with mild detergent can be used to perform the cleaning as part of a regular maintenance routine.  
Clean the core in the reverse direction of the normal airflow and rinse it with clean water.  In areas where the CAC is exposed to road alt or sea spray, regular warm water rinsing will reduce the potential for corrosion.  In situations where the CAC is mounted immediately behind the radiator, place a flexible shield or screen between the CAC and the radiator to avoid blowing debris into the radiator.  Caustic chemicals or acid used in the cleaning process can facilitate corrosion and must be rinsed completely.
When possible, the cooler should be removed from the vehicle and boil-out vat or an ultrasonic cleaner should be used to clean the interior of the tanks and tubes.  To avoid any debris from entering the CAC intake, place a damp shop rag over all openings.
Mounting brackets, rubber mounts and grommets should be checked and replaced if necessary.  Missing or broken mounting elements can lead to excessive weld stress and failure due to vibration.  Also, examine other nearby components which may be loose or vibrating against the cooler.  Any component that contacts the cooler can abrade the surface, bend fins, and lead to corrosion or decreased airflow.
Inspect the cooler for cracks or leaks in the tank or core, especially where the tanks are welded to the header plates and joined to mounting brackets.
And, finally perform a pressure test.  Air loss within allowable limits specified by the truck manufacturer is permissible.  Normal leakage is usually approximately 5 psi within 15 seconds, as 25 psi boost.
The TMC lists the average life expectancy of CAC's under optimal conditions and with regular preventative maintenance as five years or 700,000 miles.  However, it is not unusual for CAC's to fail at 300,000 to 500,000 miles.

The Cooling Journal - October 2009