Turbo

Definition

A turbocharger is a turbine (as it name states it) that pushes air into the engine, in order to increase the engine´s power. The turbocharger is driven by the car´s exhaust gases, making it more efficient than a supercharger, and lighter as well.
The idea of forcing air into the engine is to increase the quantity that enters the cylinders, thus making the cycle more efficient per stroke. Also, by compressing the air into the cylinders, it is also possible to increase the amount of fuel fed into the engine, thus increasing power and torque. The power gain is proportional to the engine and turbo size, as well as the pressure. One must select a turbo according to the engine, since it must be sufficiently powerful to make it rotate, otherwise, it´s useless. Some turbo systems use intercoolers to cool down the air before it enters the turbine, and all of them have a system known as the wastegate, which lets some air escape to control the turbo´s rotational speed and pressure from getting too high and damaging the engine. This is what gives turbocharged automobiles their distinct whistling sound. 

Advantages and Disadvantages

One well-known disadvantage that turbochargers have is turbo-lag. Naturally aspirated and supercharged cars don´t have this issue since the power gain and delivery is instantaneous, relying mainly on the engine´s revs. However, in a turbocharger, there are occasions when the driver reduces the speed and stops demanding power from the engine. This means that the air that was already pumped and ready to go into the cylinders is not immediately needed and can´t be released, which means that it has to go back through the turbine in reverse flow. This effect obviously causes the turbo to spool slower than ussual. When the accelerator is opened again, it takes some time for the turbo to regain it´s lost momentum and speed, and the car will suffer from a momentary power loss. This lack of response is known as turbo-lag. Modern turbos have coped with the problem, by being variable in geometry. This means that, according to the engine´s condition and revving, as well as the amount of air, they can change the angle of the turbine´s flaps in order to avoid the turbo from losing too much momentum when reversed flow of air occurs. This is mainly controlled by a computer, and it makes the turbo incredibly efficient, erasing lag almost completely.
Turbo´s are a simple way of increasing a car´s power, they are very light (although not exactly cheap). An important advantage they have over superchargers is that they don´t "steal" power from the engine, because they work using a waste product from combustion.