What function does a knock sensor perform in an engine?

The correct answer identifies the primary role of a knock sensor, which is to measure the engine's vibration. This sensor is crucial in modern engines as it detects vibrations caused by engine knocking or pre-ignition. When the engine operates, the timing of the fuel ignition is critical for optimal performance and efficiency. A knock sensor translates the vibrations from these events into an electrical signal that is sent to the engine control unit (ECU).

The ECU uses this information to adjust the ignition timing, allowing the engine to prevent knocking, which can cause damage and reduce performance. By effectively managing these vibrations, the knock sensor helps ensure smoother engine operation, better performance, and improved fuel efficiency.

The other options pertain to different functions unrelated to the knock sensor, such as measuring fuel efficiency or monitoring exhaust emissions, which are handled by other components within the engine management system. Engine temperature is typically measured by coolant temperature sensors, not by a knock sensor. Thus, the primary and correct role of the knock sensor is indeed to measure the engine's vibrations related to knocking.