ALT-1 What does a Turbocharger do
A supercharger is a compressor that increases (thus the "super," part of the name, meaning "over or above" atmospheric pressure) the pressure of the air/fuel "charge" (thus that part of the name) in the intake manifold of an internal combustion engine.
More pressure means more of that charge is in the cylinder when the spark (or compression in a diesel) ignites the mixture. More fuel/air means a bigger "bang" and more pressure in the cylinder pushing down on the piston, which means more power in each power stroke.
If the compressor is driven by a mechanical link, such as a belt or gears, it's called simply a "supercharger." These units draw power from the engine and that power subtracts from what's available from the engine to move the vehicle.
A turbocharger is a variation on the supercharger that is powered by an impeller or turbine (thus the prefix "turbo" - the full correct name is "turbo-supercharger") spun by the pressure of escaping gasses in the exhaust. Those gasses are normally wasted, so although there is some efficiency lost to exhaust back-pressure, a turbocharger is somewhat more efficient than a mechanically driven supercharger.
Each has its advantages.
The drain on power mentioned is a disadvantage of mechanical superchargers. Another is a loss of efficiency as rpms build.
A mechanically driven supercharger is usually a "positive displacement" compressor, meaning that it is more like a pump than the turbocharger's "fan." So manifold pressure rises immediately with crankshaft speed, and there is little lag in building up power.
Turbos take time to "spool up" from low rpms when there is little exhaust flow to spin them, so there may be "turbo lag" between when you stomp your foot on the accelerator and when you feel the boost kick in.
Some engines have "compound" or "sequential" turbos. One has a light impeller that spools up almost immediately, and a larger capacity turbo supplies the volume needed to feed the engine at high rpms. The BMW 3-Liter inline six in the 335i and others has such a system. Its maximum torque is reached at a mere 1,400 rpms, and never drops off until redline.
Top fuel dragsters, which need power and responsiveness above all else, with no consideration whatever to fuel efficiency, use gigantic belt-driven superchargers sometimes called "blowers," originally designed for use on huge GM diesel engines and now manufactured specifically for the purpose.
WWII Military aircraft, which needed to carry enough fuel to complete a mission, and also needed to be durable enough to last through many, used turbochargers to compensate for the loss in atmospheric pressure at altitude.
The Detroit Diesels used in some military hardware, such as tanks, have a sort of hybrid compound pressurization, with both a turbocharger and a mechanically driven supercharger. These take advantage of the low rpm qualities of the "blower" and the higher rpm capacity of a large turbo driven off the high volume of exhaust gasses.
