Under Pressure

The Basics of Forced Induction

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A blower motor is the ultimate for most, but there are those that ascribe to the more-is-better school of thinking.

A naturally aspirated (read that as carbureted) engine relies on the combustion cycle's intake stroke and the scavenging effect of the exhaust as a means to draw air and atomized fuel through the intake manifold and into its cylinders. Forced induction (known as supercharging) on the other hand, uses a mechanical means to increase the pressure of the intake air charge, commonly referred to as boost.

Boosting the intake charge thrusts a larger quantity of air into the cylinder. More air, when mixed with more fuel, makes for a more powerful combustion burn. The stronger burn exerts more force on the piston, which in turn, pushes harder on the crankshaft, obviously creating more power. To burn a given amount of fuel, an exact amount of oxygen is required if the mixture is to be consumed without leaving excess fuel or oxygen behind. This chemically correct mixture is called stoichiometric (Stoichiometric or theoretical combustion is the ideal combustion process during which all fuel is burned completely). Most engines have to operate at or near this chemically correct mixture.

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The Roots-type blower is the most identifiable of the three types of supercharger. Based on the early GMC units, this style has a long history of use in the race and street performance niche.

Many people assume that running a supercharger, and hence added intake boost, puts added strain on an engine's parts. This is not necessarily true, because engine damage is almost always caused by rpm. Because a supercharger helps the engine produce more power at lower rpm, supercharged engines will make the same amount of horsepower as their naturally aspirated counterparts at substantially lower engine rpm, where today's street engines are designed to run (around 6,000 rpm). Another concern some people have toward using a supercharger is that they think it will increase the engine's compression to the point that it will cause detonation inside the combustion chamber. Detonation exists when the combustion pressure is raised so high that the inlet charge ignites itself before the spark plug fires. When this happens, combustion takes place while the piston is still traveling up in the cylinder bore, which puts tremendous loads on the piston, rod, and crank. While it is true that a supercharged engine creates boost and increases the engine's compression, most supercharger kits include a boost-timing retard chip that delays the engine's ignition timing under certain conditions to prevent detonation.

Superchargers have enjoyed a rise in popularity during recent years, mainly because of their cost efficiency, reliability, performance, and of course, because of the "WOW" factor elicited by a peek under the hood.

Supercharging an engine often results in huge power increases, making them great for racing, hauling heavy loads, or just burnin' the tread off you tires. Although super-chargers carry a fairly big-ticket price when compared to other single performance upgrades ($1,500 - $4,000), nothing provides more horsepower for your dollar. In fact, aside from laughing gas, nothing even comes close. Because of the way superchargers work, they provide power only when the engine is under full throttle or under load--not under normal cruising conditions. This means that the supercharger will not affect the engine's reliability, longevity, or fuel economy under normal driving conditions.

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These diagrams of both two- and three-blade units illustrate the principle of the Roots blower...

SUPERCHARGING METHODSThe two most common methods of forced induction in the performance car world are supercharging and turbo charging (which we'll cover in a separate story in this issue).

A supercharger is an air pump that is driven by a mechanical link with the engine, often a belt that's connected to the crankshaft.

Centrifugal superchargers and screw-type superchargers are called "internal compression" blowers because the air compression takes place inside the supercharger. Roots superchargers are "external compression" blowers because the air compression takes place outside of the supercharger.

The supercharger's mechanical link to the engine means it is an "instant-on" device. As soon as the engine is turning over, the supercharger is working. In cars, the device is used to increase the effective displacement and volumetric efficiency of an engine, and is often referred to as a blower. Boost is created at the point when the supercharger's internal impeller pushes enough air to overcome the vacuum force naturally created by the engine's air intake. Therefore, air is being forced, rather than pulled, into the air intake. Boost is measured in pounds per square inch, or psi. Again, more boost equates to a more dense air charge to the engine's combustion chamber, which allows the engine to burn more air and fuel, and to create more horsepower. By boosting or pushing the air into the cylinders, it's as if the engine had larger valves and cylinders, resulting in a "larger" engine.

SUPERCHARGER TYPESThe feature that differentiates superchargers is the type of compressors being used. There are three different types of supercharger compressors: centrifugal, twin-screw, and Roots-type. Roots-type blowers are also called external compression pumps because no compression of air actually takes place inside the blower case. This is the key point of difference between the Roots, centrifugal, and screw-type blowers. Centrifugal and screw-type blowers are considered internal compression superchargers, since the impellers in each compress air inside the unit itself, and then send the compressed air to the intake manifold. It would be helpful to familiarize yourself with these three types of superchargers before deciding which one is best for your application.

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...Air is trapped between the rotating lobes and the casing in which they ride and is forced around from inlet to outlet.

THE CENTRIFUGAL SUPERCHARGERThe centrifugal supercharger compressor creates its boost via a rapidly rotating impeller that draws air into the center of the supercharger compressor. Many centrifugal blowers have round, snail-like cases that resemble turbochargers.

The impeller design inside a supercharger compressor is very similar to a turbocharger's compressor impeller. After drawing the air molecules into the center of the supercharger compressor, it throws them outward toward the into-the-supercharger scroll. The supercharger scroll acts as a chamber to collect the air molecules and channel them toward the supercharger discharge tube so they can be forced into the engine's air intake. The diameter of the scroll increases as it moves farther away from the center of the supercharger, which slows the flow of the air while increasing the pressure of the moving air.

The centrifugal supercharger compresses the air primarily at the point that the air leaves the impeller and is forced into the scroll; and from there through a venturi-shaped bore. The compression peaks at the apex (narrowest point) of the venturi before being released into the scroll for discharge. This compression method allows the centrifugal supercharger to produce a fairly high degree of thermal efficiency. However, in order to generate substantial amounts of boost, the impeller must spin at very high rpm. In fact, the amount of boost produced by a centrifugal supercharger is proportional to the square of its impeller speed, enabling the centrifugal supercharger to make a substantial amount of boost in the upper half of the engine's powerband.

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The twin-screw supercharger, at first glance, appears 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 toward each other. The rotors mesh together and draw air from the back 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 rpm is unparalleled.

THE ROOTS-TYPE SUPERCHARGERThe Roots-type supercharger is a positive displacement-type device that consists of two counter-rotating, meshed lobed rotors. The two rotors trap air in the gaps between rotors and push it against the compressor housing as they spin toward the outlet/discharge port. During each rotation, a specifically fixed amount of air is trapped and moved to the outlet port where it is compressed, which is why the Roots-type supercharger falls under the broader class of fixed-displacement superchargers (like the twin-screw supercharger).

Though most superchargers are typically driven directly from the engines crankshaft via a belt, in order for the Roots-type supercharger to deliver air at greater pressure than atmospheric, its gotta be geared so that it spins faster than the engine. Out of the three basic supercharger types, the Roots is considered the least efficient. However, it is also widely used and thus is invariably the most cost-efficient. Over the years, much has been done to improve the efficiency of the Roots-type supercharger, but because it doesn't have internal compression it will never have the same potential as the centrifugal or twin-screw-type supercharger. The external-compression nature of the Roots pump creates more heat in the inlet air charge than the internal- compression centrifugal blower does, thereby making the air from a Roots blower less dense. So, the argument goes, even if the boost levels are the same, you'll get more power from 5 pounds of boost from a centrifugal blower than you will from 5 pounds of Roots blower boost.

Still, the Roots-type supercharger is known for its ability to produce a decent amount of boost while spinning at relatively low speeds. This trait has played to its success in drag racing and has made it ideal for use on smaller engines that traditionally struggle in the lower half of the rpm range. Another benefit of the Roots-type super-charger is the simplicity of its design. It has very few moving parts and spins at relatively low rpm, making it one of the more reliable and durable supercharger designs.

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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 TWIN-SCREW SUPERCHARGERMr. Alf Lysholm originally invented the twin-screw supercharger in the 1930s. The twin-screw design was developed to fill the tremendous voids that centrifugal and Roots superchargers have. This concept was intended to meet the requirements of a high average efficiency under most varied conditions of pressure and speed, a high maximum efficiency--preferably above 85 percent, with small bulk, low weight, and also suitable for direct drive. This concept lead to the development of the twin-screw supercharger which was compact and light, had a very high efficiency over varied conditions of pressure and speed, and an incredibly high maximum efficiency. This concept became a reality in the late 1930s and early 1940s when the Lysholm twin-screw supercharger was produced. Because of high manufacturing costs at the time, the screw compressor did not find its way to OEM use on automobile engines, but rather on industrial applications for air, refrigeration, and air-conditioning compressors.

The twin-screw supercharger is a positive displacement air mover, which means it moves a fixed amount of air per revolution, like the Roots-type blower. Unlike the Roots however, which is only an air delivery system, the twin-screw supercharger is also a compressor. The counter-rotating lobes and chambers of the twin-screw are designed for a screw-like tapering effect running intake air into a smaller space for output, thus compressing it. The rotors have very close tolerances yet never touch. Compressed air is delivered into the compression environment of the intake manifold with very little leakage or energy loss.

The twin-screw supercharger creates boost the instant the throttle is touched, and generally reaches full boost by 2,000-2,400 rpm. Full boost is then available all the way to redline. A positive displacement compressor is ideal for street performance. Because of the increased mechanical efficiencies of the superior design, the output air temperatures of the twin-screw positive displacement supercharger are radically improved from the Roots type--often producing an adiabatic efficiency in the 70- to 80-percent range across the whole powerband.