What is the function of the manual primer in the fuel system of a Cessna 172?

The manual primer in the fuel system of a Cessna 172 is designed to inject fuel directly into the cylinder intake ports. This procedure is particularly useful during engine start-up, especially when the engine is cold. By manually priming the engine, you ensure that an adequate amount of fuel is available for ignition, which helps to facilitate a smoother and more reliable engine start.

This functionality is essential because, without sufficient fuel in the intake, the engine may struggle to start or may not start at all. The manual primer operates via a pump mechanism that enables the pilot to draw fuel from the fuel lines and inject it into the cylinders, providing a temporary boost to get the engine running efficiently. This distinct role sets it apart from other components of the fuel system focused on measurement or regulation of fuel flow.