What does the air-injection system do in a vehicle?

The air-injection system in a vehicle is designed to reduce emissions by injecting air into the exhaust stream, specifically targeting unburned or partially burned fuel that is still present after the combustion process. By introducing additional air, the system helps facilitate a secondary combustion reaction, which effectively burns off the remaining hydrocarbons and carbon monoxide that would otherwise enter the atmosphere as pollutants. This process contributes to meeting environmental regulations and enhances the overall efficiency of the engine by ensuring more complete combustion of the fuel.

The other options do not accurately depict the function of the air-injection system. While cooling the engine and filtering pollutants are important for vehicle performance and emissions control, those tasks are handled by different components such as the cooling system and air filtration systems, respectively. Improving fuel quality is not a role of the air-injection system, as fuel quality is primarily determined by the fuel itself and how it is processed before reaching the engine.