While it is true that a well designed and properly placed intercooler can improve engine power on a forced induction engine, it is also true that effective cooling of the intake air is a rather exact science, and therefore, even slight mistakes or miscalculations can have hugely negative effects on the efficiency of the air intake system as a whole. In this article, we will discuss the various types of intercoolers that are available today, as well as the advantages and disadvantages of each, starting with this question-
In simple terms, an intercooler is a simple heat exchanger whose purpose it is to remove the heat from intake air that was imparted to it both by being compressed into a smaller volume, and the heat the intake air had absorbed from the hot turbo/supercharger casing.
Removing heat from the intake air increases its density, which translates into the fact that more air and therefore more fuel can be forced into the cylinders, which not only increases engine performance, but actually reduces emissions because combustion is improved as well. As a rule of thumb, but assuming that an intercooler is properly designed and matched to the application, power increases of between 10% and 20% are possible for the range of boost pressures that are common on standard, unmodified vehicles, which brings us to this question-
There is no clear, simple, or definitive answer to this question, except to say that everything depends on the application, and its intended use. However, air-to-air, air-to-water, and refrigerated intercoolers all have distinct advantages and disadvantages, so let us look at each type of intercooler in turn-
As the term suggests, an air-to-air intercooler uses the cooling effect of ambient air flowing through the core of the unit to carry away heat, in exactly the same way that radiators use air flowing through the radiator core to shed heat from hot engine coolant.
This is the arrangement that is most commonly used on commercial scales in the OEM automotive market. However, as with anything else, air-to-air intercoolers have advantages as well as serious disadvantages, so let us look at some of the-
Advantages of air-to-air intercoolers
Ease of installation
For the most part, air-to-air intercoolers can be installed almost anywhere on a vehicle, provided the airflow at the installation site is strong enough to flow through the entire depth of the intercooler core. Nonetheless, since air-to-air intercoolers are “spliced” into the inlet tract, with only some additional ducting, these units are usually very easy to install, and will usually produce measurable power increases even though the airflow through the core may not always be optimal.
Great reliability
If the intercooler is properly constructed and insulated against excessive vibration, these units are largely trouble-free and should outlast the vehicle. However, the cores of air-to-air intercoolers do need to be debugged from time to time to maintain an effective airflow through the core.
Reasonably cost effective
There are many suppliers of off-the-shelf air-to-air intercoolers that carry a range of intercoolers that are specifically designed for street use. Since the volume and flow characteristics of the unit and the additional ducting plays a critical role in the efficiency of these intercoolers, purchasing a ready-made intercooler for a specific application takes all of the guesswork out of the equation.
Disadvantages of air-to-air intercoolers
The single greatest disadvantage of these intercoolers is that their efficiency depends on both the ambient temperature and the road speed of the vehicle if the airflow through the intercooler’s core is not fan-assisted.
In cold climates, these issues generally do not present much of a problem, but in the Australian context where ambient temperatures are usually high, the combination of high ambient temperatures and low road speeds makes air-to-air intercoolers particularly susceptible to the effects of both heat soak, and the fact that intake air temperatures rise sharply at high engine speeds.
As a practical matter, factory-fitted air-to-air intercoolers are very often nothing more than cosmetic selling points, and under conditions of high ambient temperatures, excessive heat soak, and low road speeds, some of these units can become largely ineffective, and may in fact, actually hamper the flow of air through the air intake system.
The image above shows an example of a heat exchanger that cools down the coolant used in an air-to-water intercooler. In these designs, a liquid such as an antifreeze/water mixture is circulated through one set of passages in the actual intercooler, while the intake air is passed through another set of passages. Heat is therefore exchanged at the metal interface between the coolant and the intake air in the intercooler.
Theory dictates that since the heat-absorbing capacity of water is about four times that of air, air-to-water intercoolers should therefore be our times as effective in removing heat from hot intake air, but this is seldom, if ever the case. Nonetheless, while air-to-water intercoolers do have some advantages, they also have serious disadvantages, so let us look at the-
Advantages of air-to-water intercoolers
While it is true that the efficiency of air-to-water intercoolers does not depend on vehicle speed, and hence, airflow quite as much as air-to air intercoolers do, this is true only up to a point. If the application is used for say, drag racing, the race is over before the liquid coolant could have absorbed meaningful amounts of heat from the intake air, but during continuous use over extended periods, heat soak can become a real issue on some installations, which brings us to the two biggest-
Disadvantages of air-to-water intercoolers
Complexity
Intercoolers of this type depends on the efficient circulation of a liquid coolant, which in turn, requires a supply of liquid coolant, a circulating pump, hoses, a reservoir to store the coolant, and most importantly, a heat exchanger to remove heat from the stored coolant.
The problem with this however, is that all the elements in the system have to be matched to each other if the system is to work even reasonably effectively. For instance, the total inner surface area of the intercooler that the coolant is in contact with has to be big enough to allow for effective heat transfer, while the flow rate of the coolant has to be high enough to carry away the absorbed heat effectively.
Moreover, the heat exchanger has to be big enough to shed heat from the hot coolant effectively. If it is not, heat will continue to accumulate and be stored in the coolant, and given the fact that heat has more barriers to cross in an air-to-water intercooler than in an air-to air intercooler, the overall efficiency of an air-to-water intercooler will decline progressively, and in direct proportion to the increase in the coolant’s temperature.
Leaks can cause engine failure
While coolant leaks to the outside of an air-to-water intercooler can be repaired relatively easily, small coolant leaks into intake air passages can cause misfires and other combustion issues. However, a sudden major internal rupture can allow enough liquid coolant to enter the engine to cause the sudden hydro locking of one or more cylinders, which as we know, usually ends in catastrophic engine failure if it happens while the engine is running.
As the term implies, these are intercoolers that are cooled or refrigerated in some away, and while most commonly used methods produce large decreases in the intake air temperature, these methods are with one exception only effective for very short periods, such as the time it takes to run a drag race.
Generally, though, short-term cooling methods include adding ice to the coolant in air-to-water intercoolers, or spraying water, CO2, or pure antifreeze over the cores of air-to-air intercoolers. How well (or otherwise) these methods work depends on the surface area of the core, the location of the core, and of course, the volume and flow rate of the substance being sprayed over the core.
Nonetheless, one method of refrigerating an intercooler over extended periods makes use of the air conditioning system. In practice, these systems divert about 50% of the A/C systems’ cooling capacity to a sealed container that encloses the intercooler. Depending on how the system is configured, the A/C system can provide cooled air to the intercooler from first start-up, or it can supply cooled air only on demand. In both set-ups though, these systems are designed to disengage the A/C compressor’s clutch above a predefined engine speed, and/or under WOT conditions to prevent damage to the compressor.
As a practical matter, this type of refrigerated intercooler can produce, and maintain intake air temperatures that are well below the ambient air temperature over extended periods, but their biggest disadvantages are their high cost and complexity, since they amount to virtually a second A/C system.
If, for the moment, we disregard the different types of intercoolers, the short answer is that heat is shed from the intake air through both radiation and convection, but how effectively (or otherwise) heat is shed depends on a number of variables. In particular though, the design and volume of the intercooler, the boost pressure and flow rate of the intake air on any given application; as well as the overall length, average diameter, and overall configuration of the intake ducting on both sides of the intercooler all play major roles in the efficiency of an intercooler.
However, the long answer is that the efficiency (or lack thereof) of any given intercooler depends on the following factors, all of which must be taken into account when the efficiency of an intercooler is calculated-
Here is an example of how some of the above factors may be used to calculate the efficiency of a non-refrigerated intercooler, regardless of whether it is an air-to-air, air-to-water unit, or refrigerated unit:
Say the turbo or supercharger heats the intake air to 650C, and the intercooler is shown to remove 400C from the intake air. In this example, we would take the removed heat (400C) as a percentage of the temperature of the air that entered the intercooler (650C), which yields a value of 61.53%. Thus, the efficiency of the intercooler in our example is 61.53%. However, it should be noted that the efficiency value in this example represents only one data point, and to obtain a more complete assessment of any intercooler’s efficiency, accurate temperature and pressure readings must be taken at a minimum of six engine speeds, boost pressure settings, ambient temperatures, and road speeds.
Consider the example below, in which several readings of the intake air temperature in an aftermarket air-to-air intercooler on a stationary 2.2 L Ford diesel engine was taken at different engine speeds, and at 5-minute intervals. Note though that while the readings in this example are accurate, this example is purely illustrative, and will therefore not apply to all intercoolers under all possible conditions.
From the above example, it should be obvious that the efficiency of intercoolers declines sharply at higher engine speeds. It is worth pointing out that in the above example, the drop in the intake air temperature is only one degree above the ambient temperature at 4800 RPM, and it is more than likely that in this case, more energy is spent in creating that one percent drop than the amount of energy that could be extracted from it.
The same is largely true for A/C assisted intercoolers if one considers that at steady highway cruising speeds the energy expended in creating super cool intake air is likely to exceed the energy spent in creating super cool intake air. Moreover, since steady highway cruising rarely produces major accelerations, the energy expended by the A/C system will likely exceed the energy that can be, or is being extracted from the cold intake air.
Similarly, in city driving conditions, the added power cannot be used due to the stop-start nature of city traffic, which means that the only viable application of A/C assisted intercooling is in competitive environments, where performance usually takes precedence over cost and reliability.
In the final analysis, there is no single set of rules or circumstances that will produce consistently repeatable results for any intercooler on any application under all possible operating conditions. This is of course not the same as saying that all, or most, intercoolers are ineffective: far from it, but it should be borne in mind that any variable factor (or a combination of variable factors) can either make or break an installation.
Therefore, all possible variables and their effects on a particular intercooler installation should be taken into account when diagnosing general performance or boost pressure related issues on vehicles that are fitted with intercoolers, and particularly so on vehicles that are fitted with aftermarket intercoolers.
