A Poly-V belt, more commonly called a "serpentine" or "drive" belt, connects and runs several components in an engine at the same time.
Use in Engines
Usually connected to the crankshaft of an engine, a Poly-V belt runs through a series of pulleys. These pulleys connect to engine accessories such as the alternator and the air conditioning system, rotating them in order to make them function.
The name "Poly-V" refers to the ribbing running along the belt. This ribbing gives the belt a better grip on the pulleys for which they are designed. "V" and "Micro-V" belts also exist and perform a similar function.
To function properly, a Poly-V belt must be set at the correct tension. If the belt tensioner of the car is old, or the belt has become worn and stretched, the engine will begin to function poorly. A high, whistling sound often accompanies this. When you hear this sound, it is time to tighten or most likely replace the belt.