What acts as the nozzle for dispensing fuel directly into the engine's air stream?

The fuel injector is the component that acts as the nozzle for dispensing fuel directly into the engine's air stream. Its primary function is to precisely atomize and deliver fuel into the intake manifold or directly into the combustion chamber at the correct time and in the right amount, ensuring optimal combustion and engine performance.

Fuel injectors operate based on a combination of engine signals and pressure, enabling them to inject fuel with precision. This process helps increase efficiency and reduce emissions compared to older systems, such as carburetors, which mix air and fuel before it enters the engine. The fuel injector system is crucial in modern automotive engines, as it enhances power, responsiveness, and fuel economy.

The other options do not fulfill the role of dispensing fuel directly into the air stream. The throttle controls air flow into the engine, the carburetor, while an older technology, mixes air and fuel before they enter the combustion chamber rather than injecting it, and the radiator is part of the cooling system, unrelated to fuel delivery. These distinctions highlight the specialized function of the fuel injector in modern fuel delivery systems.