Talk:Twin-screw type supercharger
From Wikipedia, the free encyclopedia
 |
This article is within the scope of Wikipedia Project Automobiles, a collective approach to creating a comprehensive guide to the world of automobiles on Wikipedia. If you would like to participate, you are encouraged to visit the project page, where you can contribute to the discussion. | |
| Stub | This article has been rated as Stub-Class on the Project's quality scale. | |
| ??? | This article has not yet been assigned a rating on the importance scale. | |
 |
This article has been automatically assessed as Stub-Class by WikiProject Automobiles because it uses a stub template.
|
[edit] technical criticisms of this article
I think the following critique raises good questions. They should be addressed in the main article. Greg Locock (talk) 05:23, 2 May 2008 (UTC)
"Many cold-air kits, exhaust upgrades, and most often any engine upgrade can often be retained when installing an aftermarket twin screw, excepting throttle bodies outside the range of the twin screw's machining. Though Roots-style blowers like the Eaton are effective, even the newer Eaton TVS series supercharger show much less results from porting (what? this is nonsense). The Roots fails to surpass the twin screw (in what way? adiabatic efficiency perhaps but not in others) due to the more controlled nature of the flow between screws (again, nonsense. Internal airflow inside a Roots supercharger is controlled in the same way it is in a twin screw). Imagine trying to paddle a boat with a flat paddle; water goes everywhere, where if you have a twisted and close knit pair of screws you could easily push far more water with far less turbulence to lose power (this is ridicuous: both are positive displacement superchargers, nothing like an oar moving water around in an open circulatory system). However, the Roots is much easier to manufacture (no, simply less expensive), and the twin screw is somewhat more difficult to refine for use (not true). Unlike centrifugals, the same factor which allows the volume based compressor attitudes of the Roots and twin screws undermine their ability to run coolly at idle when boost is not allowed to flow freely into and out of the chambers of the engine. To compensate, blow-off and bypass valves are often installed and provide extreme benefits (not extreme). Surge is eliminated however (no. only when throttle is pre-supercharger), unlike with centrifugal (turbo-based aspiration driven by crank instead of exhaust) superchargers, the two "volume based" compressors work by forcing their volume through per revolution. Reliability is often improved by the lack of extreme heat caused by exhaust that plagues turbos (no, OE turbo applications have proven this for 30 years now), and self-contained twin-screws (carry their own separate and often self-cooled synthetic oil blends, as the motor's oil is often very much hotter than the supercharger (also untrue. where did you get this silly information about temps?)) are about as effective as their self-contained centrifugals ("about as effective"? Not scientific nor based on fact). However, it is far more beneficial to run a centrifugal blower above 25 psi of boost (25+ ambient 14.7psi @ Sea level) (untrue! this should be deleted) due to ease of inter-cooling (ridiculous: it's just as easy to intercool regardless of the configuration of the air pump aka supercharger) and large, often unwieldy, twin screws of proper output would require a lot more than the average engine would be able to supply (again, ridiculous. If you can intercool X amount of air out of one blower you can do it out of another. Please, people, don't write about things outside your realm of expertise. If you aren't professionally employed in that field please think twice before posting). Compression ratios also play key role in power output, a 8:1 compression ratio is about what most serious racers will shoot for, as much lower than 8:1 compression yields rough and hard idle (untrue. idle is very soft and tame; large camshafts affect idle, not compression!), near gutless power output (how scientific), and high compression ratios like 14:1 will be far too hot alone (untrue - look at late model mass production vehicles with 11:1+ ratios), and the supercharger will work as a function to that with boost which will cause absorbent amounts of heat that will (no. "may", not "will") ultimately cause engine knock and failure. Engine knock is also by far more likely to cause engine damage than boost itself, most modern engine components can withstand 12 psi with no modifications, while some models that are factory fitted with a turbo may handle much higher boost "
[edit] original essay
The twin-screw type supercharger is a positive displacement type device that operates by pulling air through a pair of meshing lobes not dissimilar to a set of worm gears. Air is trapped in pockets between the lobes and carried between the intake side and the exhaust. The supercharger is typically driven directly from the engine's crankshaft via a belt. Unlike the Roots type supercharger, however, the twin-screw exhibits internal compression. As the air passes through the supercharger the air pocket decreases in size towards the outlet. This increases the thermal efficiency of the supercharger beyond that of the more common Roots style supercharger. The twin-screw type supercharger is a positive displacement type device that operates by pulling air through a pair of meshing lobes not dissimilar to a set of worm gears. Air is trapped in pockets between the lobes and carried between the intake side and the exhaust. The supercharger is typically driven directly from the engine's crankshaft via a belt. Unlike the Roots type supercharger, however, the twin-screw exhibits internal compression. As the air passes through the supercharger the air pocket decreases in size towards the outlet. This increases the thermal efficiency of the supercharger beyond that of the more common Roots style supercharger. How Forced Induction Work
How turbo work
Article courtesy of howstuffworks.com
We have few 1UZFE that are force induced FI 1UZFE
Here is my Supercharged Lexus LS400
Here is an article comparing Intercooled Vs. Non-Intercooled with Root Style and Centrifugal Supercharager
When people talk about race cars or high-performance sports cars, the topic of how turbocharger work usually comes up. Turbochargers also appear on large diesel engines. A turbo can significantly boost an engine's horsepower without significantly increasing its weight, which is the huge benefit that makes turbos so popular!
Photo courtesy Garrett
Turbochargers are a type of forced induction system. They compress the air flowing into the engine (see How Car Engines Work for a description of airflow in a normal engine). The advantage of compressing the air is that it lets the engine squeeze more air into a cylinder, and more air means that more fuel can be added. Therefore, you get more power from each explosion in each cylinder. A turbocharged engine produces more power overall than the same engine without the charging. This can significantly improve the power-to-weight ratio for the engine (see How Horsepower Works for details).
In order to achieve this boost, the turbocharger uses the exhaust flow from the engine to spin a turbine, which in turn spins an air pump. The turbine in the turbocharger spins at speeds of up to 150,000 rotations per minute (rpm) -- that's about 30 times faster than most car engines can go. And since it is hooked up to the exhaust, the temperatures in the turbine are also very high.
In this edition of HowStuffWorks, we'll learn how a turbocharger increases the power output of the engine while surviving these extreme operating conditions. We'll also learn how wastegates, ceramic turbine blades and ball bearings help turbochargers do their job even better!
Basics One of the surest ways to get more power out of an engine is to increase the amount of air and fuel that it can burn. One way to do this is to add cylinders or make the current cylinders bigger. Sometimes these changes may not be feasible -- a turbo can be a simpler, more compact way to add power, especially for an aftermarket accessory. Where the turbocharger is located in the car
Turbochargers allow an engine to burn more fuel and air by packing more into the existing cylinders. The typical boost provided by a turbocharger is 6 to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at sea level, you can see that you are getting about 50 percent more air into the engine. Therefore, you would expect to get 50 percent more power. It's not perfectly efficient, so you might get a 30- to 40-percent improvement instead.
One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time.
The turbocharger also helps at high altitudes, where the air is less dense. Normal engines will experience reduced power at high altitudes because for each stroke of the piston, the engine will get a smaller mass of air. A turbocharged engine may also have reduced power, but the reduction will be less dramatic because the thinner air is easier for the turbocharger to pump.
Older cars with carburetors automatically increase the fuel rate to match the increased airflow going into the cylinders. Modern cars with fuel injection will also do this to a point. The fuel-injection system relies on oxygen sensors in the exhaust to determine if the air-to-fuel ratio is correct, so these systems will automatically increase the fuel flow if a turbo is added.
If a turbocharger with too much boost is added to a fuel-injected car, the system may not provide enough fuel -- either the software programmed into the controller will not allow it, or the pump and injectors are not capable of supplying it. In this case, other modifications will have to be made to get the maximum benefit from the turbocharger.
How It Works The turbocharger is bolted to the exhaust manifold of the engine. The exhaust from the cylinders spins the turbine, which works like a gas turbine engine. The turbine is connected by a shaft to the compressor, which is located between the air filter and the intake manifold. The compressor pressurizes the air going into the pistons. Image courtesy Garrett How a turbocharger is plumbed in a car
The exhaust from the cylinders passes through the turbine blades, causing the turbine to spin. The more exhaust that goes through the blades, the faster they spin.
Image courtesy Garrett Inside a turbocharger
On the other end of the shaft that the turbine is attached to, the compressor pumps air into the cylinders. The compressor is a type of centrifugal pump -- it draws air in at the center of its blades and flings it outward as it spins.
Photo courtesy Garrett Turbo compressor blades
In order to handle speeds of up to 150,000 rpm, the turbine shaft has to be supported very carefully. Most bearings would explode at speeds like this, so most turbochargers use a fluid bearing. This type of bearing supports the shaft on a thin layer of oil that is constantly pumped around the shaft. This serves two purposes: It cools the shaft and some of the other turbocharger parts, and it allows the shaft to spin without much friction.
There are many tradeoffs involved in designing a turbocharger for an engine. In the next section, we'll look at some of these compromises and see how they affect performance.
Design Considerations Before we talk about the design tradeoffs, we need to talk about some of the possible problems with turbochargers that the designers must take into account. Too Much Boost With air being pumped into the cylinders under pressure by the turbocharger, and then being further compressed by the piston (see How Car Engines Work for a demonstration), there is more danger of knock. Knocking happens because as you compress air, the temperature of the air increases. The temperature may increase enough to ignite the fuel before the spark plug fires. Cars with turbochargers often need to run on higher octane fuel to avoid knock. If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.
Turbo Lag One of the main problems with turbochargers is that they do not provide an immediate power boost when you step on the gas. It takes a second for the turbine to get up to speed before boost is produced. This results in a feeling of lag when you step on the gas, and then the car lunges ahead when the turbo gets moving.
One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier.
Small vs. Large Turbocharger One sure way to reduce the inertia of the turbine and compressor is to make the turbocharger smaller. A small turbocharger will provide boost more quickly and at lower engine speeds, but may not be able to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger of spinning too quickly at higher engine speeds, when lots of exhaust is passing through the turbine.
A large turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it takes to accelerate its heavier turbine and compressor.
In the next section, we'll take a look at some of the tricks used to overcome these challenges.
Optional Turbo Features The Wastegate Most automotive turbochargers have a wastegate, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds. The wastegate is a valve that allows the exhaust to bypass the turbine blades. The wastegate senses the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the wastegate bypasses some of the exhaust around the turbine blades, allowing the blades to slow down. Ball Bearings Some turbochargers use ball bearings instead of fluid bearings to support the turbine shaft. But these are not your regular ball bearings -- they are super-precise bearings made of advanced materials to handle the speeds and temperatures of the turbocharger. They allow the turbine shaft to spin with less friction than the fluid bearings used in most turbochargers. They also allow a slightly smaller, lighter shaft to be used. This helps the turbocharger accelerate more quickly, further reducing turbo lag.
Ceramic Turbine Blades Ceramic turbine blades are lighter than the steel blades used in most turbochargers. Again, this allows the turbine to spin up to speed faster, which reduces turbo lag.
Sequential Turbochargers Some engines use two turbochargers of different sizes. The smaller one spins up to speed very quickly, reducing lag, while the bigger one takes over at higher engine speeds to provide more boost.
Intercoolers When air is compressed, it heats up; and when air heats up, it expands. So some of the pressure increase from a turbocharger is the result of heating the air before it goes into the engine. In order to increase the power of the engine, the goal is to get more air molecules into the cylinder, not necessarily more air pressure.
Image courtesy Garrett How a turbocharger is plumbed (including the charge air cooler)
An intercooler or charge air cooler is an additional component that looks something like a radiator, except air passes through the inside as well as the outside of the intercooler. The intake air passes through sealed passageways inside the cooler, while cooler air from outside is blown across fins by the engine cooling fan.
The intercooler further increases the power of the engine by cooling the pressurized air coming out of the compressor before it goes into the engine. This means that if the turbocharger is operating at a boost of 7 psi, the intercooled system will put in 7 psi of cooler air, which is denser and contains more air molecules than warmer air.
For more information on turbochargers and related topics, check out the links on the next page!
How Supercharger Work
Article courtesy of Superchargersonline.com
In this series we'll take a slightly more in depth look at the fundamentals of supercharging that were introduced in our "Supercharger Basics" article. This is part 1 of a 3-part series. After reading these three articles you should have a fairly strong understanding of what the supercharger does, what the advantages of each type of supercharger are, and how superchargers make so much damn power. This article lays down the foundation of how superchargers came into being by taking a look at the fundamentals of creating more power, and looking back in history at where and how the technology originated.
Making More Power - Four Possibilities with One Common Thread
When it comes to extracting more power from an engine, the common goal, simply stated, is to burn more air and fuel per time. There are essentially four ways to achieve this end.
1.) The first way to make more power, is to make the engine more efficient by tuning the air and fuel delivery, reducing intake and exhaust restrictions, reducing rotating mass, enhancing spark energy, and tuning engine timing. This is the purpose of most aftermarket modifications, like air filters, ignition programmers, exhaust systems, etc. These modifications are very popular because they provide added power, they look good, and they sound good. Moreover, they can be done piece by piece, so your car can build with your budget. The problem with these kinds of modifications is that performance gains are small - often negligible and unnoticeable. This is because most engines today are tuned fairly well from the factory, and are not equipped with highly restrictive intake or exhaust components, which would reduce fuel economy. In other words, if you're looking for more moderate power gains, you'll need to get to the heart of the engine where power is really made. Most of these modifications essentially have one goal in mind - make the engine more efficient so it can burn more air and fuel in a given amount of time.
2.) You can also make more power by speeding up the engine, i.e. spinning it at a higher RPM. This technique is very effective in producing more horsepower while keeping the engine lightweight and small. If you look at some of the fastest race cars in the world, you will find them spinning at incredibly high RPMs. The only drawback is that to spin at such high RPMs requires very high quality (and expensive) engine parts that can withstand the torture from the rapid rotation. Furthermore, the increased RPM substantially increases wear and tear on the engine resulting in decreased reliability and shorter engine life. Most street cars and trucks have a redline RPM of around 4000 to 7000 RPM. Spinning the engine faster than the redline RPM in street vehicles is risky without extensive engine modifications to support the higher rotational speeds. The goal with this option is also to burn more air and fuel per time.
3.) Another obvious way to make more power is to simply use a larger engine. Bigger engines burn more air and fuel, and hence, make more power per revolution. Of course, if it were that simple, we'd all be driving around with V-12s. You can fairly easily increase the size of the engine's displacement by boring the cylinders and running a larger piston, or by lengthening the stroke of the crank, but you can only go so far before you've bored the entire cylinder away or your piston is slamming into the cylinder head. To go really big requires a bigger engine, probably with more cylinders. The drawbacks of a bigger engine include their increased size (duh!?), increased weight, and reduced fuel efficiency. In addition, using a larger engine normally is not practical because it would require an entire engine replacement, which would be prohibitively expensive, and would require extensive modifications to mount it to the vehicle. Again, though, the goal of this technique is to (yep you guessed it) help the engine burn more air and fuel per time.
4.) The final way to make more power is to pack more air and fuel into the combustion chamber before igniting it. The end result is the same as using a larger engine. The problem with this technique is that it's not as simple as telling your engine to suck more air and fuel - it's restricted by atmospheric pressure. At sea level, atmospheric pressure is 14.7 psi, which is a measure of how densely packed our atmosphere is with air molecules. As elevation rises, air thins which, as you probably noticed on your last skiing / snowboarding trip, robs power from the engine. Now imagine if you could trick mother nature by making atmospheric pressure 21psi. You'd be packing around 50% more air, which means you could burn 50% more fuel, meaning you'd be making approximately 50% more power. You've probably already figured out that this is exactly what a supercharger does - it compresses air to pressures above atmospheric pressure (boost), thus packing more air into the engine. And you've probably also figured out that the goal of this technique is to burn more air and fuel per time. By utilizing this technique, a small engine can act like a big engine. It is more efficient because it has less weight and rotating mass. In addition, because you can control when the compressor (supercharger) is sending compressed air (boost) to the engine, and when it is not, you can enjoy stock fuel efficiency when the supercharger is not sending boost to the engine (normally at half throttle or less).
In reality there are more than four ways to make more power, but these are the four most conventional ways. You can also use a more potent fuel source that has more potential energy. This is the idea behind Nitrous Oxide and other high-energy fuels - a topic beyond the scope of this article.
A Brief History of the Supercharger
You may be wondering, "Who first thought of compressing air before sending it to the combustion chamber?" Don't run to the library just yet. We'll tell you!
It seems that just before the turn of the century (1900 that is), a German engineer named Gottlieb Daimler (yes, of Daimler Benz, Daimler Chrysler...) obtained a patent for a pump to aid in the delivery of increased amounts of air and fuel to the cylinder, and to aid in the removal of exhaust gasses. He didn't call it a "supercharger" in his patent application, but that's what he was describing - this was the birth of the automotive supercharger. But in order to get to the true beginnings, we have to look ever further back in history.
Gottlieb's automotive supercharger design was modeled after a twin-rotor industrial "air-mover" invented and patented nearly 40 years earlier by Mr. Francis Roots (from Indiana) back in 1860. This technology is the foundation of the roots type "blowers" still used today! Soon after the roots air movers (they were not called "compressors because they did not compress air - they only moved it) were used in industrial applications, a German engineer named Krigar invented an air pump that utilized twin rotating shafts that compressed. This technology would go on years later to become the foundation of the Lysholm twin-screw compressor used in today's automotive applications.
Apparently our old friend Gottlieb didn't have much luck in the early stages with his new invention, but the idea inspired French engineer Lois Renault, who patented his own type of supercharger soon after the turn of the century. It wasn't long before superchargers started to show up on American race cars. Lee Chadwick is credited with being one of the first American racers to successfully use a centrifugal supercharger in competitive racing, starting in the Vanderbilt Cup in Long Island, New York in 1908. One of Lee Chadwick's early supercharged rides. Soon thereafter superchargers took to the air as World War I military engineers looked for new ways to make more powerful airplanes. Because airplanes fly at such high altitudes, the internal combustion engines that worked great on the ground, suffered at altitude in the thinner air. Although the technology wasn't successfully used in combat before the war ended, it was clear that superchargers were well on their way to becoming a mainstream power adding device. Meanwhile, back in Germany, Mercedes was hard at work trying to make old Gottlieb's supercharger work. By 1921 they found success and released a glimpse of the first production supercharged vehicle utilizing a roots-type supercharger. Mercedes went on to manufacture several supercharged models with great success in the following years.
In the racing scene, supercharged cars were finding more and more success. By 1924, superchargers made their way to the Indy 500. Around the world, racers in Mercedes, Fiats, Bugattis, Alfa Romeos, Buicks, and MGs began using superchargers to help them to the victory circle. Mercedes found great success with their supercharged Grand Prix cars, while Harry Miller's supercharged Indy cars dominated at the Brickyard.
In the mid 1930's Robert Paxton McCulloch started McCulloch Engineering Company and began manufacturing superchargers as the first large American commercial supercharger manufacturer. They began developing superchargers for use on American passenger cars and hydroplane boats. This was the start of the supercharger industry in America as we know it today. Robert Paxton McCulloch in the early days. Then came World War II in 1939, and the Allied forces had an ace up their sleeve in the form of the supercharged Spitfire fighter planes and B-29 SuperFortress bomber. These supercharged planes seemed almost unaffected by the altitude to the delight of Allied pilots and soldiers. Supercharged WWII Spitfire. After the war, superchargers took on a new life in the world of racing. Alfa Romeo and British Racing Motors used superchargers on their Grand Prix cars to the horror of the competition before they were eventually outlawed. At Indy, there was no such rule, and centrifugal superchargers howled their way to many victories.
By 1950, McCulloch had formed Paxton Engineering as its own entity, which took over the supercharger development and took on the task of creating an inexpensive supercharger fit for use by the general public. After $700,000 in research, and two years of testing, the VS57 supercharger was ready for the public in 1953. Initially it worked only on 1950 - 1953 Fords, but by 1954 kits were made for nearly every commercially available 6 and 8 cylinder engine.
The rest is history, as Paxton developed newer and better superchargers until they became a part of life, not only in the world of racing, but also in the street-legal aftermarket world. Today it's hard to keep track of all the supercharger brands and models, but that's the way we like it!
Paxton's first shop.
Paxton VS57 supercharger. That's it for part 1 of the series. Next time we'll take a look at the modern supercharger and the various technologies that make it work!
Welcome to part 2 of "Superchargers A-Z". If you haven't already read part 1, of this series, you may want to start there.
Some of you may have recognized in part 1 of this series that in the early days of supercharging, there are three types of superchargers - roots, twin-screw, and centrifugal. You may already be familiar with these buzz-words, but most people don't understand how each technology differs. Before buying a supercharger, you should familiarize yourself with how each type of supercharger works. Each has its own set of advantages and disadvantages that may make it ideal - or not - for your performance needs. Today we take a technical look at the technology behind each type of supercharger.
First lets begin with some basics. There are many components that go into making a complete supercharger system - mounting brackets, ignition controller, fuel pump, etc. In this article we look at only one component of a supercharger system - the supercharger itself (sometimes called a "head unit", "compressor", or "blower"). All superchargers, except turbochargers, are driven via a pulley that is connected either to the engine's accessory belt, or to its own belt that goes directly to a crank pulley. This is where the similarities between the different supercharger technologies end.
The Roots Supercharger (aka "blower") The roots supercharger was originally designed as an air moving device for industrial buildings. The roots supercharger features two counter-rotating lobes that trap air from the intake side of the supercharger (normally at the back of the supercharger), move it around the outside casing of the lobes, and out the bottom of the supercharger through an outlet / discharge port. Like the twin screw supercharger, the roots is a "positive displacement" aka "fixed displacement" supercharger, meaning that it moves a fixed volume of air per rotation. Notwithstanding minor amounts of air-leak at low rpms, the roots supercharger cannot flow backwards like a centrifugal supercharger, and is thus nearly as efficient in its ability to pump air at low rpms as it is at high rpms. What this means is that roots superchargers are very capable of making large amounts of boost even when engine rpms are very low. This makes for great low-end and midrange power, and also makes them great for trucks and towing vehicles. The roots is also self lubricated, and is the simplest of the supercharger designs, meaning it is reasonably priced and very reliable. This is why roots superchargers have been the choice of GM, Ford, Mercedes, and Toyota for OE applications.
The only real disadvantage to the roots supercharger is that it creates a lot of heat. There are two reasons for this. First, the roots supercharger does not compress air - it only moves from the intake port to the discharge port (i.e. it is the only supercharger design with no internal compression ratio). All of the compression is done in the intake manifold. Laws of thermodynamics kick in in favor of supercharger designs with an internal compression ratio (centrifugal and twin screw) because they do less work on the incoming air charge. We will leave the mathematics of this phenomenon to a later (much more boring) discussion. Another reason roots superchargers create higher amounts of heat is because they tend to carry some of the compressed air in the intake back into the supercharger because it gets trapped by the rotating lobes that are exposed to the hotter air in the intake manifold.
A roots supercharger ("blower"). Want to know why a roots supercharger creates more heat than a centrifugal or twin screw? Calculate the amount of work each does on the incoming air charge and measure the area underneath the curve on the Pressure Volume Graph.
The Twin Screw Supercharger The twin screw supercharger at first glance appears to look similar to a roots supercharger both inside and out. The two technologies are indeed similar, however there are significant differences. At the heart of the twin-screw supercharger are two rotors, or "screws" that rotate towards each other. The rotors mesh together and draw air from the back of the supercharger. The twisting rotors move the air to the front of the supercharger, while compressing the air before discharging through a port at or near the front of the supercharger.
Because the compression is done inside the supercharger, this design produces less heat than a roots supercharger - in fact, it is almost as thermally efficient as a centrifugal design. Like the roots design, the twin-screw is a fixed displacement supercharger (meaning that it pumps a fixed volume of air per revolution), and because the tolerances between the rotating screws are very tight, its ability to create boost at low rpms is unparalleled. These characteristics make it ideal for trucks and towing vehicles, where low to mid range power is primary in importance. Another important advantage of the twin screw compressor is its reliability. Unlike a roots supercharger, the rotors in a twin screw supercharger do not actually touch, so there are virtually no wearing parts. For this reason, twin screw compressors are commonly used to pressurize cabins in passenger aircraft. Like roots superchargers, twin screw superchargers are self lubricated and do not tap into the engine's oil supply.
One disadvantage of the twin screw design is that, because it has an internal compression ratio, the twin screw is compressing air even when it is not sending boost to the engine (i.e. under cruising or deceleration). An internal bypass valve releases the pressurized air, but because it takes work to pressurize the air in the first place, the twin screw supercharger draws more power from the engine than while not under boost. Like the roots, the throttle body must be placed before the compressor because it is a fixed displacement supercharger.
A cutaway view of a twin screw supercharger. Airflow through a twin screw supercharger.
The Centrifugal Supercharger Although the centrifugal supercharger is founded on a technology much newer than either the roots or the twin screw, it was the first supercharger to be successfully applied to automotive applications. Unlike the roots, the centrifugal supercharger is NOT a positive displacement / fixed displacement supercharger because it does not move a fixed volume of air per revolution. The centrifugal supercharger essentially operates like a high speed fan propeller / impeller, sucking air into the center of the supercharger and pushing it to the outside of the rapidly spinning (40,000 + rpm) impeller blades. The air naturally travels to the outside of the blades because of its centrifugal force created by its rotating inertia. At the outside of the blades, a "scroll" is waiting to catch the air molecules. Just before entering the scroll, the air molecules are forced to travel through a venturi, which creates the internal compression. As the air travels around the scroll, the diameter of the scroll increases, which slows the velocity of the air, but further increases its pressure.
The centrifugal supercharger enjoys several advantageous characteristics that make it the most popular supercharger design in the aftermarket world. First, it is simple and reliable because it has very few moving parts - just a few gears and the impeller. Second, the centrifugal supercharger produces very little heat because of its internal compression ratio. It is also small in size and very versatile because it can "free-wheel" and allow the engine to suck air through it or even flow air backwards. For this reason it can be placed anywhere in the intake tract - it can even "blow through" the throttle body, meaning it can be mounted nearly anywhere. It is also the most thermally efficient supercharger, meaning that it produces the lowest discharge temperature.
The only significant disadvantage of the centrifugal supercharger is that it must be spinning at a relatively high speed before it begins to make a significant amount of boost. For this reason, it is not helpful in creating boost (and power) at low engine rpms. Normally the supercharger only begins to create boost at around 3000 rpm, and the boost curve gradually and increasingly rises with engine RPM. Many centrifugal superchargers do not have a self-lubricating oil system, and draw oil from the engine's oil supply. The disadvantage to this is that you must tap the oil pan for the oil return line. However, in doing so, the supercharger becomes virtually maintenance free. Some manufacturers make a "self-contained" centrifugal supercharger that is self-lubricated like roots and twin screw superchargers.
A centrifugal supercharger. Airflow through a centrifugal supercharger.
The Turbocharger You may be wondering where the turbocharger fits in to this equation. Technically, a turbocharger IS a type of supercharger - one that is driven by exhaust gasses rather than from a pulley that draws power from the engine's crank. Because we have covered this topic in depth in our Turbos vs. Superchargers article, we will not re-examine the differences again here. Because the turbocharger relies on a technology substantially different from the three traditional supercharger technologies discussed above, it is beyond the scope of this article.
That's it for part 2 of the series - next time we'll pull everything together and discuss what goes into making a complete supercharger system, and how the supercharger works in conjunction with the engine.
Welcome to the 3rd and final installment of "Superchargers A-Z". If you haven't already read Part 1 and Part 2, of this series, you may want to start there.
So far in this series we've discussed what a supercharger is, where it came from, and what technologies drive the core of any supercharger system - the supercharger itself. Today we'll take a look at the supercharger system as a whole. Because of the radical performance differences between a supercharged engine and a normally aspirated engine, the supercharger must integrate with other critical engine systems like the ignition system and the fuel delivery system. Don't worry, though, because almost all of the supercharger systems sold today are complete supercharger systems and do not require the addition of 3rd party fuel and ignition components. With this in mind, let's break a supercharger system down into its main functional components - a discussion of the supercharger itself is not included in this article because it was the focus of part 2 of this series. Keep in mind that each supercharger system is designed for a specific application, and the specific contents of different supercharger systems vary greatly.
An example of a complete supercharger system.
The Air Intake System Because a supercharged engine draws substantially more air than a normally aspirated engine, it is important to minimize intake restrictions. To ensure a smooth delivery of air to the supercharger, most supercharger systems include a high-flow air filter as well as low-restriction tubing or ducting to deliver air from the atmosphere to the supercharger. It is important to maintain a clean air filter to minimize the particles that come into contact with the supercharger's impeller, rotors, or screws. Most supercharger systems will draw air from behind the fender wall, where there is an abundance of cool air that has not been heated by the engine. Because superchargers heat air as it is compressed, a cool air supply helps to keep the charge temperatures at a reasonable level. On a non-intercooled application, the cold air pickup can lower the charge temperature by up to 60 degrees!
On most vehicles the incoming air charge passes through a Mass Air Flow sensor (aka MAF) on its way to the supercharger, although on centrifugal superchargers, the Mass Air Flow sensor can be mounted after the supercharger ("blow-through" setup). The Mass Air Flow sensor measures, you guessed it, the mass of air that the engine is drawing. This reading allows your engine's ECU (Electronic Control Unit) to calibrate and deliver the appropriate amount of fuel for the incoming air charge.
Once the supercharger has worked its magic, the air must be delivered from the supercharger to the engine intake. Although many roots and twin screw superchargers bolt directly to the manifold, most centrifugal superchargers require an extra tube called a Discharge Tube to carry the air to the intake through the throttle body. This tube will normally be mandrel bent to minimize restrictions.
The Bypass Valve Compressor surge is a problem that affects most superchargers and develops when the supercharger is creating boost, but the throttle shaft is closed. Although not a problem on some low-boost (5psi or less) applications This condition can occur under deceleration or while shifting between gears, and can cause the car to sputter and chirp. Under surge, the compressor forces air into the closed throttle body until the pressure inside the throttle body is higher than the amount of pressure being created by the supercharger, and the air tries to pop backward through the supercharger. At that point, pressure is released inside the throttle body and the compressor forces air back through the supercharger and into the throttle body, which again has nowhere to go, and the process repeats. While surge normally is not highly damaging to the engine it is certainly annoying and can cause damage with time. To eliminate these problems under surge conditions, a bypass valve (sometimes called an anti-surge valve) is used to release the excess pressure. The bypass valve is actuated using intake manifold vacuum, which opens the vent valve and releases pressure in the air-intake. Air is either released into the atmosphere (blowoff valve) or recirculated back through the supercharger compressor (bypass valve).
The Intercooler / Aftercooler Some supercharger systems include an aftercooler (more commonly called an "intercooler"). The purpose of the intercooler is to remove heat from the air to create a cooler, more densely packed air charge - more on this in Let's Talk Intercoolers, and Aftercooling - Vortech Style. Although the intercooler is not necessary on most street applications, its performance becomes increasingly important on higher-output systems (with correspondingly higher charge temperatures). The intercooler can be compared to a automotive radiator, only instead of cooling water or coolant, the intercooler cools the air. Air-to-air intercoolers force the air through a large air-cooled finned and fluted core, normally mounted in front of the car's radiator. Air-to-water intercoolers force the incoming air charge through a much smaller finned and fluted heat exchanger that is cooled by water. The water is, in turn, cooled by a compact radiator that mounts next to the stock radiator.
The two main purposes of the intercooler are 1. to allow more boost on a given octane level of fuel without detonation, and 2. to help create more power by condensing the air charge. Thus, intercoolers are very common on high boost applications (10+ psi) and on roots-style superchargers, where discharge temperatures are higher than normal. Most street supercharger systems (5-8psi) do not come standard with intercoolers.
Here is an article comparing Intercooled Vs. Non-Intercooled with Root Style and Centrifugal Supercharager
The Fuel System As increased amounts of air are pumped into the engine with the supercharger, so too must increased amounts of fuel be delivered. This is where the power gains come from. Most stock fuel systems are not up to the task of delivering the increased volumes of fuel demanded by a supercharged engine. Without a proper fuel system, your engine may run lean, detonate, and obviously perform below its potential. Because every engine is different, the fuel system requirements vary greatly with different vehicles and with different supercharger systems. Sometimes larger fuel injectors and a larger fuel pump is required. On some applications, a fuel management unit (FMU) will do the job by restricting the fuel return line to build up fuel pressure. On other applications, additional fuel injectors are mounted to the intake manifold, while on some applications the stock fuel system works like a charm. Fortunately most supercharger systems include all of the fuel system components necessary to tune the engine to perfection. On some race kits, tuner kits, custom installations, and high output systems, it is up to the engine tuner to determine the engine's fuel requirements and tune the fuel system accordingly.
The Ignition System The engine's ignition system serves the important role of telling the spark plugs when to fire so the compressed air and fuel is ignited at the exact right time to produce maximum power. Ignition timing can be advanced, causing the spark to fire earlier, or retarded, causing the spark to fire later. Ignition timing is critical not only for performance reasons, but also for engine longevity as it used to eliminate detonation (aka spark knock). With the added air and fuel that is compressed in a supercharged engine, the engine is closer to its detonation threshold. To avoid detonation, many supercharger systems retard the ignition timing, thus reducing maximum cylinder pressures and temperatures, and moving away from the detonation threshold. Because retarding the ignition timing causes a slight loss in power, a higher octane fuel or an intercooler are recommended for optimal performance, both of which allow for more timing without detonation. To ensure a complete and cool burn, high quality, cool heat range irridium spark plugs are also recommended for use on supercharged engines.
The Pulley All superchargers are driven by a pulley that sits inline with the accessory belt or crank pulley. The size of the supercharger pulley is what regulates the speed at which the supercharger spins. Obviously, a smaller pulley turns the supercharger faster, and vice versa. The pulley is easy to change on all superchargers and is often used to increase (or decrease) the output of the supercharger. A simple pulley-swap can equate to huge power gains if the rest of the system is up to the task (in particular the fuel and ignition system).
The Rest Other components serve self explanatory roles. Mounting brackets obviously are used to attach the supercharger to the engine in a position such that the pulley can be spun from the accessory belt or an additional supercharger belt. The belt tensioner keeps the belt tight around the supercharger pulley, which is important
