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|04-09-2006, 11:45 AM||#1|
Join Date: Mar 2005
understanding how an intercooler works
Any compressed fluid will increase in temperature. Because hot air is less dense (meaning less oxygen per unit of volume), however, in an automotive application, hot air is our enemy. Less oxygen means less power, and lower temperatures reduce the risk of engine-destroying detonation.
Just like a radiator, an intercooler is a simple heat exchanger, built in two primary parts: the endtanks and the core. The core is a collection of fins, each one passing a small part of the air charge by a small part of the cooling medium (outside air in an air/air unit, or water in an air/water unit), who’s low temperature and high flow around the core absorb the heat of the air charge. The endtanks simply take the air from the pipe and distributes it to the fins of the core, or vise versa.
Turbulators are curved pieces of sheet metal seen on both the inside and outside of intercooler cores between the fins. They do much to increase drag, but much more too increase heat transfer. Their job is to separate the air charge (as well as the cooling medium) into even smaller units, forcing virtually all of the two fluids to do their part in heat exchange. The more turbulators there are, the better the heat exchange, but the greater the flow restriction as well. Therefore, in an application with lots of internal flow area, more turbulators can be reasonably employed, while in an application with less internal flow area the reverse is the case.
While more volume will provide more space for heat transfer, just as overly large piping will result in more lag and less throttle response, a larger intercooler will take more time to pressurize and have the same effect. The goal of designing/selecting the proper intercooler for each application is to find the balance between maximizing the ability to remove heat from the system while minimizing flow restriction and pressure loss.
Internal flow area
In the design of intercooler cores, there is always a balancing act between too much and not enough drag. If the air charge has a hard time passing through the core and spends more time in it, it will consequently have more time to give up its heat to the cooling medium and cool down. However, if it spends too much time in the core, it will experience large pressure drop, forcing the compressor to do extra work to achieve the same boost level, and thereby reducing efficiency.
Because as the cooling medium passes through the core and absorbs the heat of the air charge it heats up, by the end of the core the efficiency of the heat transfer decreases significantly; so much that the second half of the intercooler only does one half of the work. Increasing depth also increases the air for the passing medium, which can be a considerable issue for air/air intercoolering. If the outside air can more easily pass around the intercooler then through it, then it will do just that, thereby decreasing the amount of heat exchange that can possibly occur. Fortunately, proper positioning of the intercooler and ducting to it can be used to counter this problem, as will be discussed later.
Although aluminum tends to be the material of choice for core construction with generally all automotive heat exchanged, several other materials offer distinct advantages and disadvantages. Silver has a lower coefficient of heat then aluminum, and will thus support more heat exchange over aluminum in an otherwise identical situation, and the increase in price is surprisingly small. The core manufacture Blackstone, used by Porsche, Ferrari, Saab and Johnisenglish uses all silver cores, as well as several other foreign companies. For air/water cores, copper offers even better heat properties at a fraction of the cost of both aluminum and silver. Unfortunately, due to copper’s corrosive properties, it generally isn’t appropriate for street use in an air/air situation.
Air/Air or Air/Water?
The main draw of an air/air intercooler is its low cost and simplicity. The outside air offers a virtually limitless supply of cooling medium, and offers excellent efficiency at high speeds. Unfortunately, an air/air intercooler can never lower charge temperatures below ambient air temperature, and in some applications (such as the top mount unit found in WRX’s) at lower speeds can function more as an interheating then an intercooler (although this isn’t a huge problem, as at low speeds lower charge temperatures generally aren’t as important, if important at all).
The Air/water intercooler’s biggest performance draw is its ability to lower air charge temperature below ambient air below ambient air temperature. To do this, as pre-cooled cooling medium boost be employed, such as an ice water tank. However, such as setup can only be used for a short time, as after the medium absorbs all the heat of the air charge, no further heat exchange can occur.
To construct an air/water intercooler capable of cooling the air charge in a street application, a separate heat exchanger (along with a pump to power it) must be employed to cool the water itself. The benefit of such as setup is the ease of piping in applications where such is a consideration, and the desirable heat properties of water (water can transfer heat 14 times more easily then air). The obvious drawback is the need for a much more complex, usually much more expensive setup.
Air/Air Intercooling Design Considerations:
As mentioned earlier, air will travel the path with the least resistance. More drag there is between the fins of a core, leads to less ambient air will flow between them and the less heat exchange. The simplest method of increasing air flow is to increase the internal flow area, but doing so lowers the amount of heat exchange, so alternate means of doing so should be employed first.
Core Flow Direction
Because greater internal flow area will increase the amount of heat transfer over a unit with the same volume but less internal flow area, the orientation of endtanks with respect to the core should always be designed to maximize internal the flow area. However, because of space considerations in our cars, its very difficult to employ an intercooler of this sort in the front bumper.
The two primary fin designs of intercoolers on the market today are bar and plate and round edged extruded. Because it is easier for air to flow around a smooth curve, round edged finds will always provide for better air flow over bar and plate designs.
The placement of the intercooler plays a huge role in the amount of heat exchange that can take place, but is almost always limited by available space. Front air dams, fender wells and practically anywhere that has consistent access to a large amount of ambient airflow will work, but for Integra purposes the front air dam tends to be the place of choice, both for reasons for success and necessity. Top mount units, such as those used in the WRX and FC RX-7 usually enjoy sufficient airflow at high speeds when paired with a hood scoop and proper ducting for the air to leave, but will always be subject to the high temperatures of the engine bay; it gets the job done, but by no means is it ideal.
Proper ducting of ambient air to and away from the core can increase airflow by up to 20%. However, contrary to what common sense might imply an opening larger then the internal flow area of the core is not the ideal size. Instead, the opening should be between 60 and 25% of the core area. This number is both a result of the fact that in a completely open situation, less then one forth of the area going towards the core would flow between its fins, and a smaller inlet tapering open towards the core will produce a low pressure area, sucking in more air and maximizing air flow. An inlet tapering down towards the core will produce a high-pressure area, and consequently hinder airflow towards the core. After leaving the intercooler, the outside air should be given an unrestricted path away from the core, thus allowing cooler air to come and take its place. However, in most Integra front mount applications, the radiator sits behind the intercooler, causing it to both restrict airflow and receive already heated air for cooling. Fortunately, the affect of this on the cooling system is generally acceptably small.
An ideal endtank will take the air charge from the piping inlet and evenly distribute it among each fin, or vise versa. However, many intercoolers on the market today employ endtanks that unevenly distribute the air, putting more work on one section of the intercooler then the other, and as a result decreasing efficiency. Un-ideal endtank design can be compensated for by using internal baffles to even out the airflow, or simply altering the shape of the endtank itself.
Although intercooler piping generally follows the best (if not only) path it can take, there are several factors to take into account. Larger diameter piping will allow for less restriction in a high pressure, high flow application, but will also require more time to fill and hinder throttle response. 2.25” outer-diameter piping tends to be the size of choice for low/mid boost street Honda applications, while 2.5” piping is more appropriate for high boost race applications.
Although there are limitless possible materials to construct intercooler piping out off (they’ve got carbon fiber intakes these days…), the cost and performance effective material is hands down aluminized steel. Cheap, common and easy to work with, it can quickly be fabricated to whatever shapes are necessary. The only drawback to aluminized steel is its heavy weight when compared to aluminum. Unfortunately, aluminum can easily cost over three times as much as aluminized steel, but fortunately, a full set of aluminized steel intercooler piping only weighs about 15-20 pounds.
Due to the high flow velocity of the air charge and the relatively low surface area of a simple pipe (when compared to that of an intercooler core), heat soak from the engine bay is generally unimportant. Insulating wrap, chrome plating, ceramic coating or different piping materials can all be used to reduce it further, but for virtually any street application the gain is power doesn’t even come close to validating the effort.
Rubber and Silicone are the two most common and readily available base materials for piping connectors. While silicone by far offers the best heat resistance (and therefore the greatest ability to hold the piping together under the high temperatures of the engine bay), many blends of rubber exist, some being better then others.
Piping blow off under boost can be a huge problem (not to mention embarrassment) if not prevented. Pressing or welding a bead around the edge of each pipe will provide the piping connectors with something to hold on to, and in most cases eliminate the problem altogether. However, piping connectors will always be the weak link in the system, and wherever possible (with easy removal in mind) pipes should be bent as one piece or welded together.
Intercooling is an integral part of an Turbocharged air induction system. Lower air charge temperatures produce more power per psi and a larger safety margin of detonation resistance. Because of this, boost levels can be safely increased by 3-4psi, increasing power output that much more.
Proper intercooler design should aim for the least possible pressure drop and the highest possible efficiency of heat exchange.
|04-09-2006, 12:44 PM||#2|
New title is new
Solid post/copy paste, good info.
For referance, it works exactly the same for superchargers, except with even better results with roots blowers (because they put out more heat for a given boost level then other blowers/turbos).
--Bobnova '89 2.0SI 4ws(309k miles), '76 Triumph TR7 FHC (63k miles)
24 years of professional automotive experience, specializing in Honda/Toyota/Subaru.
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|04-09-2006, 07:36 PM||#3|
Join Date: Mar 2006
rep for you
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