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Chrysler has done it again. They introduced a car with insane power to the public that’s literally called “Demon.” However, if you’re relatively new to forced induction systems, you’re probably wondering just what on earth a “supercharger” really is and how it works. Even if you are well versed in the forced air world, you’re probably wondering just what on earth the “SRT Power Chiller” is and how exactly it works. We’ll explain that and more in this supercharger technical article.
First, let’s start with what exactly a supercharger is. By definition, it is a means of adding power by artificially increasing the amount of air an engine is intaking. In other words, it’s forcing more air into the intake than what the natural draw of the piston moving down during the intake stroke can bring in. You’re probably looking at that and saying, “isn’t that what a turbocharger does?” Yes it is, and, technically, it is a supercharger. However, there is a distinction between a turbocharger and a supercharger because despite having the same goal, they achieve that goal by different means. If you want to know more about turbochargers, you’ll want to follow this link here: http://www.racingjunk.com/news/2017/04/21/should-i-go-with-turbos-for-off-road/. Even though it is off-road centric, the tech and explanation of the turbocharger isn’t.
A supercharger is a belt driven forced induction system that uses the engine’s crankshaft to drive its compressor lobes or the gearbox attached to the compressor wheel on centrifugal style. Inside the snout of the supercharger is a pair of gears which transfers the spin of one lobe to the other. It’s also where the supercharger gets its famous whine noise.
A roots style supercharger uses twin, triple or even four straight compressor lobes, and the rotary screw (or Lysholm) uses a pair of twisted compressor lobes. The advantage of the rotary screw is that the twin-screw exhibits internal compression, which is the ability of the device to compress air within the housing as it is moved through the device instead of relying upon resistance to flow downstream of the discharge to establish an increase of pressure in the case of the roots supercharger.
A centrifugal supercharger looks like a turbocharger compressor with a box in the back where the turbine housing should be. A centrifugal supercharger uses a compressor wheel and compressor housing much like a turbocharger does, but instead of being driven by exhaust gasses, it’s driven by a gearbox and belt from the driveshaft (or sometimes a belt from the driveshaft to a transfer shaft pulley to another belt on that to the compressor pulley, but that’s complicated). This is also what makes it not a turbocharger at all but instead a supercharger, as it is belt driven.
Now that we’ve got that out of the way, let’s talk about what “boost” is. You must understand what an engine is able to pull in naturally without a supercharger. Well, the atmosphere is the pressure you feel around you right now, which is 14.7 PSI at sea level and decreases as you increase your altitude and increases as you decrease. That’s also the reason why a boost gauge never reads in absolute pressureö it just reads boost pressure above your current atmospheric pressure. A turbo may increase the engine’s atmospheric pressure by seven PSI, for example, but seven PSI for me may be twenty-one PSI for you depending on what your altitude is.
The advantage of superchargers is near instant boost. Instead of waiting for the turbine to spool up the compressor wheel to create boost, a supercharger will typically start making boost in as few as 2,000 RPM, sometimes less depending on how it’s designed, the pulley size or how it’s geared on a centrifugal supercharger. The centrifugal is in between when it comes to boost lag (the word for that spool up time). However, it is more efficient at making boost and can potentially make more power than a roots or a twin-screw because it can also make more boost.
The advantage for the aftermarket when it comes to superchargers is that it is easier to get a supercharger 50-state legal. This is because the turbo can act like a giant heat sink that will not allow the catalytic converter to heat up to proper temperature to convert exhaust gasses into safer exhaust. However, in the OE world, we’re starting to see this become less common as we understand the metallurgy of the turbine side, use metals that can take higher heat and take advantage of direct-injection. On a non-OEM turbocharged engine, you don’t have to change out your exhaust manifolds and the exhaust side emissions remain the same because the compressor is driven by a belt rather than the exhaust.
When it comes to boost pressure management for a supercharger, there are four options: The pop-off valve, the burst panel (or plate), the blow-off valve and the recirculation valve. The pop-off valve and bust panel are both usually found on the twin-screw and roots superchargers, though they can be found on centrifugal systems as well. These are designed so that, if excess boost can’t be taken by the engine or the engine is beginning to blow up and shoving pressure back into the intake system, these devices will open to allow excess pressure out and save the supercharger.
The pop-off valve is sprung, so that if the condition goes away it closes back up and normal operation continues. The bust panel, however, is one-time use, as the name implies. The panel is mounted on the intake manifold on a roots supercharger, but a twin-screw can have it attached directly to the supercharger body. Keep in mind, unlike a wastegate on the turbocharger system, the panel and valve are not designed or intended to control boost. They are only there to protect your supercharger in the event of a failure.
On a centrifugal supercharger system, you might see a device called a blow-off valve. A blow-off valve opens when you close the throttle body to relieve the pressure from the boost side of the compressor. If you didn’t have this valve, you would get compressor surge and could damage the compressor wheel, bearings, shaft or all of the above. Again, just as the case of the pop-off valve and bust panel, it’s only there for the protection of the supercharger when the throttle body butterfly closes. That’s it – it’s not for boost control.
The reason this is true is because of how the compressor is being turned. Instead of using a turbine attached to the compressor to drive it, the belt drives it instead. The faster you drive the compressor, the more air is being shoved in and the more boost you will create. This is true for all supercharger systems (including the turbocharger). The catch for the supercharger is that the gearing in the gearbox and the size of the supercharger pulley determines the speed of the compressor’s lobes or wheel and, in turn, boost pressure. So, while you have the advantage of instantaneous boost from a supercharger, you do lose the ability to change your boost pressure on the fly like you get from a turbocharger and manipulating its wastegate.
Now, there are ways to manipulate the supercharger’s compressor speed at the pulley side that can be done to increase or decrease boost pressure or even not drive it at all. Toyota did it as an OE design on the supercharged AW11 MR2 (1988-1989 chassis with the 4A-GZE 1.6-liter engine with a roots supercharger) as well as Mercedes Benz on certain “KOMPRESSOR” option cars, like the C32 AMG engine, SLK230 and S55 AMG. The pulley was mounted to an electromagnetic clutch, just like you see on your air conditioning compressor, and would only activate when needed for an extra boost in power. There are superchargers on the aftermarket side that do this as well. Eaton has continued to pioneer this technology, while Japanese aftermarket manufacturer Blitz has kits for various Toyota powered vehicles.
There have been other attempts at making a variable boost supercharger, but the most current development (well, as of 2012) has been with ProCharger and their i-1 Centrifugal Supercharger. Instead of using an electromagnetic clutch, the i-1 uses a CVT-type transmission between the pulley that the engine’s serpentine belt drives and the gearbox drive of the supercharger. Just like a typical automatic scooter or the way that some hybrid car transmissions work, the pulleys change the diameter of the belt to act as a change in gearing to drive the supercharger faster or slower and follows the demand of the engine from low rpm to high rpm as well as boost needs.
The idea of the intercooler is that it will reduce the temperature of the boosted air charge before it reaches your intake manifold. There are two ways to get this done: either air-to-air intercoolers or air-to-water intercoolers. First, we’ll start with the simplest design: the air-to-air intercooler.
An air-to-air refers to how the intercooler cools the charged air by using the core as a heat sink. Charged air passes through the tubes of the intercooler while ambient air flows over the tubes to remove the heat the charged air transferred to the intercooler tubes. These are further helped by the little fins or plates between the tubes.
Bar-and-plate, on the other hand, is far stronger and more efficient at cooling charge air. You get more area because you braze or weld the core rather than relying on a large metal seal that reduces its actual core size. It’s also stronger thanks to using bars and plates over tubes to create the intercooler core. This not only reduces the chances of boost killing road damage, but can usually take far higher boost pressures than tube-and-fin. However, it is heavier.
Then there is the air-to-water intercooler, and it is a super-efficient design. Water is a far better way of transferring heat away from an object, and you can actually use a smaller intercooler than you would have needed over the air-to-air intercooler. This reduces losses in boost pressure as you now have a shorter route to the intake and you can make more power!
Not only that, but some designs allow you to use a small chest to fill with ice and allow the water to flow through, further cooling boosted air. You are also less reliant on keeping your vehicle moving since you don’t need air flowing over the intercooler to keep it cool. However, not only is this system heavier thanks to all the components involved (water pump, water tank, water, ice chest or heat exchanger for the water to cool it) it’s also harder to package over an air-to-air. Despite that, air-to-water intercoolers are the preferred method of OE manufacturers that do use intercooling on their supercharged efforts, including Dodge and the Challenger SRT Demon.
What the Demon does differently, though, is how it further cools the liquid for the Supercharger’s integrated intercooler. This is where the SRT Power Chiller comes into play. Now famously, the Power Chiller says that it uses the air conditioning system to cool the boosted charge air. It does this by passing the intercooler’s coolant into a cooler that reduces the temperature of the coolant further than is possible with a regular heat exchanger. That chilled coolant is sent to the supercharger’s intercooler and reduces the boosted air a further eighteen-degrees Fahrenheit than what the ambient temperature would be with just the standard heat exchanger. This allows the Demon to run 14.5-PSI of boost with a 9.5:1 compression ratio of the 6.2-liter Hemi and produce up to 840-horsepower on 100-octane (or 808-horspower with 91-octane).
So now you can see that the belt-driven supercharger has many advantages in the many forms it comes in. Thanks to ever-developing technology on how a supercharger is driven, the advantages the turbocharger once had over the supercharger are becoming a thing of the past. With the addition of being more emissions friendly than a turbocharger system for an OE naturally aspirated engine, the supercharger will be hard to beat for fifty-state compliance over the turbocharger. Additionally, with developments on the OE front on intercooling, like that of the SRT Power Chiller, making more boost is becoming easier. Now that you’ve seen the advantages, will you go with a belt-driven supercharger?
