What Is an Engine Pulley?

Dan Cavallari

An engine pulley is a wheel mounted to the engine of an automobile or other vehicle that guides or drives a belt, sometimes known as a serpentine belt. This belt is part of a system that powers peripheral devices within the engine compartment, such as the air conditioner, power steering, and fan of the vehicle. An engine pulley is often also used in conjunction with a timing belt — the serpentine belt takes the place of a timing belt in many vehicles — which helps time the opening and closing of the engine's valves.

Idler pulleys maintain tension on belts.
Idler pulleys maintain tension on belts.

The serpentine belt system in a vehicle will feature several pulleys that have different functions. One engine pulley may be responsible for powering an air conditioning unit, while another engine pulley or set of pulleys may be responsible for engine timing. Some types of pulleys, often known as idler pulleys, simply keep tension on the belt and help guide the belt properly through the serpentine system. Idler pulleys generally are not connected to any peripheral devices, and they can be adjusted in several directions to adjust the tension on the serpentine belt. Adjusting the tension must be done carefully to prevent breakage as well as loose belts that can dismount the pulleys.

Automobile and motorcycle engines typically use pulley systems, such as those that work with the alternator.
Automobile and motorcycle engines typically use pulley systems, such as those that work with the alternator.

It is quite common to see an engine pulley that is grooved around its circumference. This groove is intended to help secure the belt in place as it spins around the pulley. The belt itself will be designed to work with the grooved pulley, creating a system that reduces slippage or the dismounting of the belt. A well-tensioned belt that is properly run through the pulleys will not slip much at all, and it will not need regular tensioning as the belt is designed to resist stretching.

Another type of engine pulley is the harmonic balancer, which is attached to the crankshaft. This pulley is designed to help reduce vibration caused by the interaction between combustion and crankshaft response to this combustion. Without the harmonic balancer, the crankshaft may experience strain beyond its means caused by the vibration of combustion, leading to damage to the crankshaft and possible complete failure.

All the pulleys in an engine compartment need to be made from high-quality metals that will withstand regular use as well as exposure to high temperatures on a regular basis. They must also be designed to work with specific belts to prevent excess or premature wear of the belt systems.

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