What function does a nozzle serve in a turbojet engine?

In a turbojet engine, the nozzle plays a critical role in directing exhaust gases to produce thrust. After the combustion process occurs, the high-speed exhaust gases expand through the nozzle, which is designed to convert the thermal energy of the gases into kinetic energy. This acceleration of the exhaust gases creates thrust according to Newton's third law of motion— for every action, there is an equal and opposite reaction.

The shape of the nozzle, often convergent or convergent-divergent, allows for the control of the flow rate and velocity of the exhaust gases. By carefully designing the nozzle, engineers can optimize engine performance and efficiency, ensuring maximum thrust is produced as the gases exit the engine.

Other options reflect functions that are important in the overall operation of a turbojet engine, but they do not pertain specifically to the role of the nozzle in producing thrust. For instance, regulating fuel flow is vital for combustion but is managed by the fuel control system rather than the nozzle itself. Cooling of engine components is achieved through various cooling methods and not directly through nozzle operation. Lastly, mixing fuel with incoming air occurs prior to combustion in the combustion chamber, which is distinct from the nozzle's function.