Since aircraft operate at altitudes where the air pressure is lower, it is useful to provide a system for compressing the fuel/air mixture. Some systems are used to normalize the air pressure entering the engine. These systems are used to regain the air pressure lost by the increase in altitude. This type of system is not a ground boost system and it is not used to ever boost the manifold pressure above 30 inches of mercury. A true surpercharged engine, called ground boosted engines, can boost the manifold pressure above 30 inches of mercury. In other words, a true supercharger boosts the manifold pressure above ambient pressure.
Since many engines installed in light aircraft do not use any type of compressor or supercharging device, induction systems for reciprocating engines can be broadly classified as supercharged or nonsupercharged. [Figure 3-9] Supercharging systems used in reciprocating engine induction systems are normally classified as either internally driven or externally driven (turbosupercharged). Internally driven superchargers compress the fuel/air mixture after it leaves the carburetor, while externally driven superchargers (turbochargers) compress the air before it is mixed with the metered fuel from the carburetor.