Charge Air Cooling

This will primarily be a discussion for forced induction applications; there is not much that can be done for naturally aspirated cars other than placing the inlet into the intake close to fresh air and away from the radiator/radiant heat.

Why does the air heat up?

Compressing air via a turbocharger or a supercharger will cause the intake air to heat up.  This is due to the ideal gas law equation PV=nRT.  If volume (V), amount of moles (n), and the ideal gas constant (R) all stay the same (which in our case, they do), as pressure (P) is increased, the temperature (T) has to increase as well to keep the equation equal.  There is no way around increasing pressure without increasing temperature, this is physics.

What does this mean?

• Air at higher temperatures is less dense then lower temperature air.  A denser (colder) charge increases the number of air molecules that make it into the combustion chamber on the intake stroke, more air + more fuel = more power.  So the opposite is true with hotter air and thus reduced power and performance are realized.

• Disregarding that higher temps can cause pre-ignition/detonation and reduced timing as a result, for every 10 degrees F that the intake air temp (NOT ambient air temp) increases or decreases, a resulting ~1% in horsepower is gained or lost.

• However, if we do consider that high temps can cause pre-ignition/detonation, then power can suffer significantly and engine damage could occur.

How can we keep the air cool?

There are multiple systems that can cool this charge air.  We will split these into conventional and non-conventional systems.  Conventional refers to the use of charge air coolers (CAC), commonly referred to as intercoolers; while non-convention refers to adding fluids to the intake charge to reduce the temps.  Some fuels require less or no cooling as well due to the fuels ability to cool the intake charge.  OEMs generally rely on conventional tactics as they are safe, maintenance-free, and highly effective.

Conventional Systems:

Air to Air (A2A) charge air coolers

These would be the large front mounts that we all love.  Other packages have these located in side-pods, in the rear, and up top in air ducts.  The charge air flows through passageways ways through the core, and exchanges charge air heat, with the ambient air, similar to how a radiator works.  These systems need ambient airflow flowing through the unit to work properly.  There are two main variants of A2A cores, bar plate, and tube and fin.  There are multiple styles of each of these two as well, with different inserts, fins, and tubes used.  Each of these offers its own set of benefits and drawbacks.

OEM Style Tube and Fin

Extruded Tube and Fin

Bar and Plate

Positives:

• Extremely simple design and operating principle

• Not much can go wrong with this system.  Leaks would reduce performance but not cause anything catastrophic to occur.

• Costs are generally reasonable with the lower level construction cores.

• With higher level cores (think 4 figure pricing) and adequate airflow, near ambient temperatures, can be reached.

Negatives:

• Impossible to cool temps below ambient

• Blocks airflow to the radiator and condenser.

• Some variants, typically the cheaper ones, are extremely heavy.

• The cheaper cores are generally not efficient, but still, get the job done.

• Performance is highly dependent on airflow.

Air to Water (A2W) charge air coolers

These are the intercoolers found inside the intake manifolds of some OE cars, Edelbrock’s s/c, Cosworth’s s/c, and Innovate’s s/c.  With the use of its own cooling circuit, these can be placed in confined spaces and are generally used to cool the charge air temperatures from a supercharger but can be used with turbo systems as well.  This cooling circuit generally involves a low temp heat exchanger upfront, the intercooler in the plenum, a water pump, a reservoir for filling, and the lines.  Water flows from the low-temperature heat exchanger, removing heat from the water, and then flows to the intercooler in the plenum, which exchanges the heat from the charge air and puts it back into the water.

Low-Temperate Heat Exchanger

Positives:

• Ability to be packaged in small spacing and placed within the intake manifold itself.

• Great for drag cars when used with ice, which can reduce air temps below ambient.

• These can reduce pumping losses from reduced tubing and bends.

• The intercooler portion itself can be much smaller than an A2A and still reduce air temps drastically.

• Less impact on the radiator cooling as the heat exchanger is generally very thin.

Negatives:

• These systems are more expensive as they involve more components.

• When used without ice, air temps will be higher than a properly sized A2A.

• While unlikely and uncommon, they can leak water into the engine.

• Less dependent on airflow as the heat exchanger can be placed remotely and a fan used to pull air through them.

Less Common, Non-conventional Systems

Water Injection
By injecting a small amount of water into the intake tract, the water (preferably vapor) can turn to steam and with it, take a significant amount of energy out of the air charge.  Water’s high latent heat of evaporation makes it a great fluid to absorb the energy and thus heat from the inlet air.  This change in phase (water vapor to steam) is the key to its success.  This system can be used in cases when an intercooler is not enough to cool the intake charge.

Water Injection Kit

Positives:

• Increased knock resistance

• Reduced inlet charge temps

• Can produce slightly more horsepower

Negatives:

• Maintenance

• Cost of the system

• Have to remember to fill the reservoir

• Possible failure point (wouldn’t be significantly destructive if it fails though)

Methanol/Alcohol Injection
Methanol exhibits two main differences from water when being injected into the intake stream.  One, methanol is a fuel with the octane rating of ~109 RON, which will greatly increase the knock resistance.  The other difference being methanol evaporates at a much lower temperature than water.  Place a small amount of rubbing alcohol on your hand and you can feel it become cool to the touch, more so than water.  Water will remove more heat per unit through its latent heat of evaporation due to its physical properties; however, we are restricted by the amount of water we can inject before it will bog the engine.  On the other hand, with methanol being a fuel, we are not limited by the amount of liquid we can inject.  We can inject a large amount, which in turn can actually lower the intake charge significantly, all while increasing the knock threshold significantly through the increase in octane.

M1 Methanol Fuel

Positive:

• Increased octane, reduced knock threshold

• Ability to run more timing

• Ability to run more boost pressure

• Reduced air temps

Negatives:

• Added costs (system and the fuel)

• Remembering to fill the reservoir

• Potential failure point that will ruin the engine

• Calibration challenges

Non-Intercooled Applications:

Some vehicles, generally drag cars, do not need intercoolers due to the primary fuel that is used. These vehicles run on either alcohol, methanol, or nitro-methane.  For reasons previously discussed, these fuels can cool the intake air beyond that of an intercooler, rendering the use of one obsolete.  Ethanol (E85) reduces the need for intercooling but does not offer enough of a cooling effect to abandon the use of one in most cases.

Conclusion:

We recommend the use of the conventional systems listed above as we share a similar mindset to the OEMs.  These systems work well and if designed properly, you won’t need the use of chemical (fuel) intercooling.  When chemical intercooling is used and it is not the primary fuel (methanol injection), this can lead to a very dangerous condition and could lead to the demise of an engine.  If used with the right precautions though, it is a viable alternative for those willing to take the risk for the reward.

As always, please let us know if you have any questions and we will do our best to answer.