Both compress air entering an internal-combustion engine, with the goal of increasing the amount of oxygen available in the fuel-air mixture that enters the cylinders, and thus increasing the amount of energy available to each power-stroke of each piston.
A supercharger is direct-driven from the engine itself, typically by a toothed belt from the driveshaft, but occasionally directly from the driveshaft itself.
The most common form is an external-compression, or Roots supercharger, which uses counterrotating lobed “rotors” or gears to trap air (or fluid, in other uses) against the inside of an enclosure, and move it from one side to the other. Explaining this is the exact reason animated gifs were invented:
Those gifs are two-dimensional (well, three, with the third being time rather than Z, but..) the most common implementation uses three-lobe rotors which have a helical twist, in order to minimize pulsation or “hammering” of the output. It’s worthy of note that an “external-compression” supercharger does not actually compress air, it just pumps it from a low (atmospheric) pressure side to a high (intake manifold) side, where it’s compressed.
The rate at which air that can be moved by a supercharger (or “blower” in the vernacular) is dependent upon the cross-sectional area of the void between the lobes and the enclosure, multiplied by the length of the rotors, multiplied by the rate of rotation. The more back-pressure you’re pumping against and the higher the rate, the more torque is required to turn the blower. On a top-fuel dragster, the supercharger typically consumes 1,000 horsepower in the process of producing four atmospheres of pressure, which quadruples the amount of fuel (and fuel-air mixture) that can be combusted inside the cylinders, yielding a significant net gain in output horsepower.
These three photos of Roots superchargers being built (courtesy of Al’s Blower Drives of Auckland) give a good sense of what the real thing looks like.
Superchargers were instrumental in making high-altitude fighter-plane engines work in WWII, and were first mass-produced by General Motors for use on diesel trucks. Their two models, 4–71 and 6–71 became de-facto size standards (which allowed the production of interchangeable aftermarket parts), and Phil Weiand then began producing larger variants (8–71, 10–71, 12–71, 14–71) in the 1950s for racing use.
Although in theory, you can use any size of blower on any size of engine, by adjusting the drive ratio between the crankshaft and the rotors, in fact there’s an efficiency “sweet spot” that dictates larger blowers on larger engines and smaller blowers on smaller engines: the faster the rotors are spinning, the more heat is generated and has to be dissipated. That heats the incoming air, which reduces its mass-per-volume, and causes the pump to work inefficiently or counterproductively. At the other end of the spectrum, the slower a Roots blower is turning, the more leakage there is between the tips of the rotors and the walls of the enclosure.
There are two other types of superchargers: screw-type and centrifugal. The screw-type ones are very similar to Roots blowers, but use a progressively-tightening spiral, rather than a uniform helical spiral, in order to compress air (note that unlike Roots blowers, these only work on gaseous media, not incompressible fluid) within the blower; so they’re also called “internal compression” superchargers.
They’re a better, more efficient design, but require much closer tolerances, which makes them more finicky, and much more difficult to maintain; not the best qualities in a race car, where the more common choice is to treat 14–71 blowers like popcorn, and just try to contain the popping under a kevlar blanket.
Then there are centrifugal blowers, which spin a centrifuge to compress air. These require much higher rotational speeds to work, and aren’t nearly as efficient.
Finally, we get to turbochargers, which are a pair of centrifuges running back-to-back on a common free-spinning shaft.
The inlet of the turbine section takes exhaust gas which has been compressed by the exhaust stroke of the engine, and uses it to spin a turbine, which spins the shaft. The shaft in turn spins a compressor centrifuge, which pulls in atmospheric air, and compresses it into the intake manifold. The problems with this are many, most notably the “turbo lag” which is the relatively high latency between when the throttle is pressed (dumping in more fuel), higher exhaust pressure is achieved (spinning the exhaust turbine faster) and finally more air is pumped in (to provide more fuel/air mixture). But they’re cheap and easy to integrate (because all they need is ducting, no belts or shafts), so cheap Japanese cars and their derivatives tend to use them.
I’m sure this will call the fury of ricer trolls down upon me, but facts are facts. If turbos were useful in producing more horsepower, grownups who actually need more horsepower would use them. Instead, in racing and aerospace, you find pretty much exclusively superchargers. Mostly Roots-type.
supercharger
turbocharger
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