What is a turbocharger?
A turbocharger consists of two separate devices linked by a common shaft: A centrifugal supercharger and a gas turbine. The supercharger's compressor fan--with greater pumping capacity than the engine it is boosting--forces air into the intake manifold at higher than atmospheric pressure--providing extra oxygen so the engine can burn much more fuel and thus produce dramatic increases in power and torque. Meanwhile, the turbine harnesses exhaust gas pressure and heat against jet-like fan blades, providing the shaft horsepower and torque needed to drive the compressor beyond the 40,000 or so RPMs typically required to make measurable boost.
The smallest Garrett turbochargers safely rotate up to as much as 250,000+ RPMs, with many larger automotive units toping out at a little over 100,000 RPMs. Larger-sized turbos--like larger jet engines--tend to be a bit more efficient.
As the compressor "spools up" and begins to create boost, the engine starts to burn more fuel and create more exhaust--which spins the turbine faster, which spins the compressor faster, which creates more boost, which creates more exhaust, and so on--a self-feeding cycle in which--to a degree--the more power you make, the more power you can make. The time it takes to accelerate the turbine and compressor to operating speed and make boost when you put the pedal to the metal, is known as turbo lag. Careful design can effectively eliminate turbo lag as a practical consideration, although there may then be a tradeoff in performance at peak power.
A turbo compressor is precisely analogous to a centrifugal supercharger like the Vortech, which is scaled up in size and fitted with radial (rather than reverse-curved) blades for increased pressure ratio at lower compressor speeds (at the cost of efficiency and a narrower operating envelop) and powered off the crankshaft. The turbo's centrifugal compressor and turbine each consist of a superficially-similar centrifugal fan freewheeling within a scrolled housing. The inlet side ("inducer") of the compressor fan is smaller than the outlet ("exducer") side that flings air into the compressor housing and discharge, and vice versa for the turbine.
The "cold-side" compressor housing is machined from cast aluminum, while the turbine's "hot housing"--which must survive temperatures that can approach 2000 degree F--is made of thick, machined cast iron. The compressor and turbine wheels are attached to a common shaft, with a housing in the center locating the shaft on bushings or ball bearings that are lubricated with high-pressure engine oil and may also be cooled with radiator fluid pumped through the center section close to the bearings. Oil drains back into the oil sump via gravity, while the engine's water pump circulates coolant from the turbo back to the radiator for cooling.
It takes about 30 horsepower to generate 100 horsepower worth of boost. When a centrifugal compressor is driven by a belt or gears from the crankshaft, the 30 HP is wasted in the form of parasitic drag. Not so on a turbocharger: Eighty percent of the energy that drives a turbocharger's turbine comes from exhaust gas heat that would otherwise be wasted out the tailpipe; the remaining 20 percent comes from exhaust pressure which does have a cost in the form of decreased pumping efficiencies resulting from higher backpressure in the exhaust manifold. Therefore, the "cost" of a turbo conversion is only 5-10 horsepower out of 100 gained rather than 30. Bottom line, turbochargers are simply more efficient power-adders than blowers or nitrous, which is why turbo engines dominate all forms of racing where they are allowed to compete without crippling sanctions.
Turbochargers, like jet engines or dogs, come in a bewildering variety of sizes, from miniature units that easily fit in the palm of a human hand (suitable for boosting a tiny motorcycle engine) to gigantic fixtures as big as a spare truck tire used to increase the power output of a 6,000 HP locomotive or even a 25,000 HP container ship. The size and operating characteristics of a compressor and turbine can be mixed and matched on a turbocharger to optimize performance for a specific application.