Intercooler
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- For the Australian rock group, see Intercooler (band).
An intercooler, or charge air cooler, is a device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing the amount of charge in the engine and lowering charge air temperature, thereby increasing power and reliability. It is also known as a charge air cooler, especially on larger engines that may otherwise easily self-destruct with high intake-air temperatures. The inter in the name refers to its location compared to the compressors. In aircraft engines, coolers were typically installed between multiple stages of supercharging. Modern automobile designs are technically actually aftercoolers because they appear most often at the very end of the chain; this term, however, is no longer used. Also, the thicker and larger the intercooler, the more turbo lag it produces.
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[edit] Turbocharging
Turbochargers and superchargers compress incoming air, causing it to become heated (see the ideal gas law). Since hot air is less dense than cooler air at the same pressure, the total charge delivered to the cylinders is higher than non-compressed air but still less than it could be at lower temperature. By cooling the charge after compression, the stream experiences further densification (see Coefficient of Thermal Expansion). With this further densification even more charge can be delivered, increasing power. Additionally, intercoolers help to reduce tendency for engine knock. One of the most efficient intercoolers is water injection—it cools the intake charge as well as the combustion temperature.
An intercooler is similar to a radiator, but tuned for high air flow rates and to increase density of the charge air by lowering its temperature. Most designs use ambient air for cooling, flowing through the radiator core, and often co-located with other radiators for oil or cooling fluid. When air is directly used with no intermediate fluid (such as water), the approach is known as Air To Air (ATA) intercooling.
In at least one land speed record attempt, Gale Banks used nitrous oxide, not internally as a power-adder, but as the medium into which the heat was transferred from the charge air. The nitrous oxide was held in bottles and released through the intercoolers' cooling fins and exhausted directly to the atmosphere. Extra cooling by nitrous oxide spraying on the front of the intercooler is now a related commercially available upgrade.
Extra cooling of the charge air can be achieved also by externally spraying water on the front of the intercooler (evaporative cooling). This can be activated automatically or manually, and is far cheaper to refill than nitrous oxide.
Air to air intercoolers need to be mounted so as to maximize air flow and promote efficient cooling. Most cars such as the Toyota Supra, Nissan Skyline, Saab (except the Subaru WRX-based 9-2X Aero), Dodge SRT-4, Mitsubishi Lancer Evolution use front mounted intercooler(s) (FMIC) mounted vertically near the front bumper, in line with the car's radiator. While FMICs' offer the best form of charge cooling, the car suffers a slightly larger amount of "turbo lag", as it takes more time for the turbo to push the intake air through the additional piping. Many older turbo-charged cars, such as the Saab 900, Volkswagen Audi, and Turbo Mitsubishi Eclipse use side-mounted intercoolers (SMIC), which are mounted in the front corner of a bumper, in front of one of the wheels. Side-mounted intercoolers are generally smaller and less efficient than front-mounted intercoolers. Cars such as the Subaru Impreza WRX, MINI Cooper S and the MAZDASPEED 6 use top mounted intercoolers (TMIC) which are mounted horizontally on top of the engine (due to a low hood line) and use a hood scoop to force air over the intercooler. While TMICs' offer a fix to the "turbo lag" caused by the FMIC, they experience an effect known as "heat soak", where the heat from the engine warms the intercooler, lowering its efficiency to cool the intake charge. Due to limited room in the engine bay, TMICs' tend to be small, and can only efficiently cool small amounts of air. At high horsepower levels, or when using large turbochargers, FMIC's are much more efficient than TMICs. Some World Rally Championship cars use a reverse-induction setup, where air from ducts in the front bumper is forced up over a horizontally-mounted intercooler and then vented through ducts in the top of the hood to further maximize aerodynamic benefits.
[edit] FMIC
FMICs generally require open bumpers, and front spoilers, which will force air into the bumper and provide down force as well, are also beneficial. In general, because of the location, a front mount intercooler tends to cool air more efficiently than a similarly sized TMIC (top mount intercooler) or a SMIC (side mount intercooler). FMICs have some disadvantages, however. One obvious drawback is the vulnerable position of the intercooler in front of the car - any moderately serious frontal impact will significantly damage the FMIC. FMICs, by virtue of their sitting in front of the radiator, block airflow to the radiator, as the air that passes through the intercooler is several degrees hotter than the air on the other side. While on most piston engines, this is not too major a concern, on hot-running engines, and rotary engines in particular, this can lead to problems. FMICs also require the most plumbing of any intercooler setup, which means that there is much more volume that the turbocharger or supercharger must pressurise before it can deliver positive boost. Because of this, many manufacturers opt to use SMICs or TMICs to avoid excessive turbo lag. Several manufacturers including Ford (with the 2003/04 Mustang Cobra and 2007 GT 500), Mitsubishi (Lancer Evolution) and Dodge (2003-05 SRT-4) are shipped from the factory with FMIC's.
[edit] TMIC
A top mounted intercooler (TMIC) is an automotive intercooler mounted within the engine bay, above the engine. Because of restricted airflow to this location, a hood scoop is virtually a necessity for a TMIC. The TMIC may be placed close to the turbocharger and/or supercharger compressor and to the engine's intake. As a result, the intake tubing can be kept short. The longer the path from the intercooler to the engine, the more air must be pressurised within the hoses when a change in pressure is demanded - and the greater the lag imposed. When used in combination with quick-spooling turbochargers, such as ball bearing turbochargers, the result is a more responsive engine. Unlike front-mounted intercoolers, TMICs do not block any airflow to the radiator and/or oil cooler, allowing better engine cooling. Heat from the engine may be conducted through to the intercooler, usually while trying to escape through an open hood vent. For this reason hot, cramped or poorly ducted engine bays (commonly the case with rotary engines) negatively affect the performance of TMICs. TMICs tend to be less efficient than similarly sized front mount intercoolers, due to the smaller amount of cold air flow through the hood scoop compared to the front grille area in most car designs.
TMICs are used in many street cars, such as all current intercooled Subarus, the MINI Cooper'S and also in older cars such as the Mazda RX-7 (86-91 model).
A properly designed top mount intercooler's advantage in responsiveness is preferred over longer piping found in front mount intercoolers in situations where responsiveness is more important than total power - notably in rallying, drifting, autocross and touge. Shorter piping results in less turbo lag.
[edit] V-Mounted Intercoolers
The V-Mounted Intercooler is a hybrid system, developed to provide superior air cooling to a front mounted intercooler, yet still retain the short intake piping and radiator airflow of the TMIC. In this case, the intercooler is mounted horizontally, directly in front of the engine (although it can be at an angle). Most VMIC setups place the radiator below the intercooler, at a great angle, tilted back until it is almost touching the motor. Ducts are used in the front of the car to duct air through the intercooler, creating a ram-air effect, while the remainder of the air flows over the radiator, normally. The air is usually removed via a hood vent (a vent recessed into the car's hood near the front of the car; if it is mounted too far back, it will actually suck air into the engine bay), although in the case of a bottom-mounted intercooler, the air is allowed the exit underneath the car (although this is dangerous because is places the intercooler at extreme risk to damage from bumps and rocks). VMIC setups are typically utilized on Front Midship cars, as the location of the engine, far back in the engine bay, allows room for the system.
VMICs were pioneered on the Mazda RX-7, because rotary engines have a tendency to run hot. It was intended to be a compromise between a TMIC or a side-mounted intercooler (2nd Generation and 3rd Generation RX-7, respectively) and a FMIC. An intercooler in the stock position would not support high airflow (and thus limit top power, or create severe detonation in the engine, which damages rotary engines more easily than piston engines), while FMICs would block airflow to the radiator, leading to overheating. The RX-7 is the only car that currently has a VMIC kit available for it. VMICs on other cars are custom made, usually used on track cars and require significant investment and fabricating skills to properly set up and tune.
[edit] Charge cooling
An alternate design, often referred to as a charge cooler or Water To Air (WTA or A/W) intercooler, is a heat exchanger, using water or a water/antifreeze mix to cool the charge, then cooling the water in a separate radiator. While heavier and more complex, charge coolers can often make arranging the rest of the engine much simpler and do not add significant intake pipe length, unlike some air-to-air intercoolers. A variation on this type of charge cooler substitutes a reservoir of coolant for the radiator, allowing the use of an ice water mixture or liquid nitrogen that can bring outlet temperatures well below ambient air temperature even under very high boost pressure. Because of the limitations on the volume of coolant that can be stored and circulated, this approach to charge cooling is only practical for short durations at high boost levels, making it most common in drag racing and land speed record attempts.
[edit] External links
- Very large diesel engine air coolers
- www.chargecooler.co.uk - Information on automotive chargecooler systems
- FAQ on Waterinjections - "Chemical intercooling"
- The Effects of Water/Methanol Injection on Diesel Combustion, Cooling, and Lubricant Quality
- Zurich Egineering, Inc: information about how charge coolers work.
