Usually, the alternate air comes from inside the engine cowling, where it bypasses a clogged air filter. Some alternate air sources function automatically, while others operate manually.
Two types of induction systems are commonly used in small aircraft engines:
- The carburetor system, which mixes the fuel and air in the carburetor before this mixture enters the intake manifold
- The fuel injection system, which mixes the fuel and air immediately before entry into each cylinder or injects fuel directly into each cylinder