Centrifugal superchargers vs. Roots Superchargers
by: Dave Coleman of SCC magazine
Supercharging Is actually a relatively simple concept. The amount of power an engine can make is proportional to the amount of air and fuel it can ingest and burn. Supercharging is the simple act of putting an air pump in front of the engine to cram in more air and fuel than it would normally be able to ingest on its own.
The most basic requirement of a supercharger is that it pump air faster than the engine it is attached to, but there is more to supercharger design than that. A supercharger also has to operate over the same wide range of rpm as the engine it is attached to, and it has to pump air as efficiently as possible without excessively heating it or absorbing too much power.
Technically, turbocharging is just another form of supercharging--turbochargers are just centrifugal superchargers that happen to be driven by exhaust gasses, rather than the crank--but in the popular car-guy vernacular, the term supercharged is reserved for those installations where the air pump is mechanically driven off the crankshaft. The seemingly minor detail of how the supercharger is driven has a profound effect on how the car drives. Since the turbocharger relies on exhaust gasses for its power, there is a slight delay or "turbo lag" when the throttle is opened, air is ingested, burned and finally expelled into the turbine to drive the turbo. Depending on the design and sizing of the turbo, this lag can be anywhere from a few agonizing seconds, to a barely noticeable fraction of a second. In contrast, a mechanically driven supercharger should be trying to pump air even before the throttle is opened, resulting in nearly instantaneous boost.
As the rotors mesh at the bottom, the air chamber on one rotor is filled with the lobe of the other rotor. Left with nowhere else to go, air is forced out the discharge port. The small slots on either side of the discharge port allow a little bit of pressurized manifold air into the chamber before the main port is open. This smoothes the discharge flow significantly, reducing noise, especially at high rpm.
Another fundamental difference between turbochargers and superchargers is how each regulates boost. Turbos use a wastegate to bleed off the exhaust gasses driving the turbo when the desired boost level is reached. What this means is that turbos run a fixed boost level, at least in theory. In a perfect world, a turbo system with the wastegate set at 6 psi would make 6 psi of boost under all wide-open-throttle conditions. A supercharger, by contrast, runs a fixed speed relative to the engine. If a supercharger is geared to pump 1.5 times more air than the car breathes naturally aspirated, the boost will be different at different rpm. In the parts of the rev range where the engine doesn't breathe well, the supercharger's insistence on pumping air will cause air to pile up in the manifold, increasing more pressure. In areas where the engine does breathe well, there will be less of a back-up, and less boost. The end result is that turbochargers tend to simply magnify the stock powerband of the engine they are attached to, while superchargers are more likely to totally change the powerband to reflect the characteristics of the supercharger.
One thing turbos and superchargers do have in common is the need for intercooling. It is a common myth that turbos need intercooling and superchargers don't. There is no truth to this statement. Compressing air makes it hot; this is an inescapable fact of nature. The more efficient the compressor, the less it heats the air, but the heating still occurs and it is still significant. Cooling that air before it goes into the engine can have a profound effect on performance. In general, turbo compressors are more efficient than supercharger compressors, but this obviously depends on the particular turbo and supercharger you are comparing. The end result is the same: Superchargers stand to benefit from intercooling as much as turbos do.
A centrifugal supercharger is very simple in function, though the design of the compressor blade is quite involved. Air trapped between vanes is simply flung off the end of the compressor wheel at high rpm.
Roots Blowers
The Roots concept is quite simple. In its plainest form, a Roots blower has two rotors with either two or three lobes each. Like giant gear teeth, the lobes on these rotors intermesh inside a tight-fitting, peanut-shaped housing. If air is allowed to enter the waist of the peanut, or the narrow portion right where the lobes are unmeshing, the air will be drawn into the gap between lobes on each rotor. As the rotors turn, chambers are formed between two adjacent teeth of each rotor and the wall of the rotor housing. Air is simply carried around both sides of the supercharger in these chambers until the lobes try to mesh again at the other end. Here, an open discharge port allows the air somewhere to go as the meshing lobes take up all the space in the chamber.
A Roots blower, then, has a discharge port that pulsates as pockets of air are expelled one by one. Eaton put a twist on the Roots design by twisting the rotors and moving the intake port from the top to the rear. Air is still carried around the sides of the blower, but the twisted rotors help the intake and discharge ports move air more smoothly and thereby the whole unit operates more quietly.
The Roots blower is the most prominent in a family of positive-displacement superchargers. A positive-displacement supercharger, by its nature, pumps an essentially fixed amount of air for every revolution. Because of their very linear pumping qualities, engines using positive-displacement superchargers tend to have very linear powerbands and excellent, low-rpm response.
You'll notice the Roots design is referred to as a blower, while a centrifugal supercharger or the pumping side of a turbo is referred to as a compressor. The reason is simple. A Roots blower doesn't actually compress air, it simply picks it up and moves it. This is one of the main reasons most Roots blowers are less efficient than most centrifugal compressors. All the compression of the discharge air happens when the air is first exposed to the backup of air waiting to get into the engine. The high-pressure air in the manifold will rush into the back of the blower until the air in the blower is at the same pressure as the air in the manifold; then, as the lobes turn, they will push the air back out of the blower again. This turbulent airflow hurts efficiency, though some creative discharge port design on the Eaton blowers helps minimize this turbulence.
Centrifugal SuperchargersA centrifugal supercharger is basically the same type of pump as the compressor side of a turbocharger. That explanation, of course, is of no help at all if you don't know how the compressor side of a turbocharger works. Let's try again: It's just a radial fan. No? OK, look at the shape of the compressor wheel and visualize the air that is trapped between those blades. As the compressor spins, the air trapped near the edge of the wheel gets flung off and the air closer to the center slides in to take its place. Remember centrifugal force? Here it is in action.
The air that gets flung off the end of the wheel is collected in the snail-shaped compressor housing and delivered to the engine. Because the air trapped between each set of blades travels down a progressively smaller path as it nears the edge of the wheel, the air is actually compressed as it is pumped. This, and the inherently smooth nature of the air's journey through the compressor, lead to relatively high efficiency. Where the Eaton blower runs at about 65-percent adiabatic efficiency (meaning the air is heated 35 percent more than the laws of physics demand) the Vortech V5 centrifugal supercharger used on the Civic Si runs at up to 73-percent efficiency.
To pump effectively, the compressor wheel has to be spinning extremely fast. Where an Eaton supercharger starts running out of steam around 12,000 to 14,000 rpm, the Vortech V5 centrifugal supercharger used on the Civic Si is barely starting to work at 30,000 rpm, and can run all the way to 60,000 rpm. The smaller centrifugal compressors on turbochagers can run over 100,000 rpm. The high speed of the centrifugal supercharger means some sort of secondary gearbox is needed to spin the supercharger up to 10 times faster than the engine.
While the higher efficiency of the centrifugal supercharger is appealing, the design does have its downside. At low speeds, a centrifugal compressor can hardly pump at all, meaning there is very little boost at low rpm. This can be solved by spinning the supercharger faster, relative to the engine, but overdo it and you end up with too much boost at high rpm. This is where a turbocharger has an advantage. A turbo can be spooled at low rpm, and the wastegate used to keep it from overboosting at high rpm.
The end result of the centrifugal supercharger's high-rpm nature is a very peaky powerband. The Vortech Civic Si made an astounding 272 hp at the wheels, but the power curve is nearly exponential in shape, meaning that most of the time the car makes far less than peak power. As always, it pays to look at the whole powerband.
So there you have it, a crash course in forced induction. Now read on and relish the visceral rewards of air pumps chasing air pumps, and never, ever try to supercharge a supplement.
