If you'd like your car to have some more horsepower without making any drastic changes to the cylinders or engine block, a turbocharger could be just what you're looking for. Turbochargers use an exhaust-driven turbine compressor system to force more air into the cylinder, resulting in more air for more complete burning of the existing fuel. This, in turn generates substantially more horsepower.
Naturally Aspirated Engines
Most internal combustion engines use the intake stroke of the piston to create a partial vacuum over the piston, which causes ambient air pressure to push air and fuel into the cylinder. The air/fuel mix burns to generate the energy that pushes the piston down on the combustion stroke. Because atmospheric pressure can only push just so much air into the cylinder, there is an upper limit to its efficiency and therefore the horsepower that the engine can generate using natural aspiration.
A turbocharged engine uses a compressor linked to an exhaust-driven turbine. The hot exhaust gases flowing over the turbine cause it to spin, which in turn drives the compressor through their common axle. The compressor then works to squeeze more air into the cylinder's intake valve. The increase in air available to burn fuel increases the energy generated to push the piston down on the downward (combustion) stroke, thereby boosting the horsepower. As efficient as this process is, there is a danger of over-stressing the cylinder head by forcing too much air into the cylinder. To protect against this, the intake pressure is controlled by a valve called a "waste gate," which bleeds off exhaust gases from the turbocharger to stabilize its boost and protect the engine from overpressure.
Turbochargers vs. Superchargers
Standard superchargers work the same way in that they improve the engine's output efficiency by using a compressor to force more pressurized air into the cylinder, improving on the upper limit imposed by natural aspiration. This method of boosting horsepower is called "forced induction." Superchargers, however, use the engine's crankshaft to generate the centrifugal force used to drive the compressor instead of the exhaust, which causes a reduction in the energy gain due to friction loss. Turbochargers, on the other hand, use heat energy from the exhaust gas that would otherwise go to waste to produce additional horsepower without giving any back to friction loss.
The Myth of "Free Energy"
Many people think that using this otherwise-wasted energy means that turbochargers get their extra horsepower for free. Unfortunately, this isn't true, because when the exhaust is used to rotate the turbine wheel, it encounters resistance from back pressure, which the engine must work to overcome. This, in turn, gives back some of the gained efficiency. Turbochargers are still more efficient than superchargers, though, because, as mentioned above, superchargers give back even more energy generated by the engine due to friction loss, which can only come from part of the engine's overall output.