How a Timing Belt Works

Automotive combustion engines require either a timing chain or belt to keep the camshaft to crankshaft correlation correct as part of the design of the engine. A timing belt is more common in overhead camshaft engines and is constructed of a fiber reinforced rubber belt with vertical cogs vulcanized to the inside. When a timing belt fails it will remove the cogs that keep the engine in-time which will stop the engine from running. Much like a timing chain there are timing marks on the crankshaft and camshaft of the engine which denote the position of each when the belt is installed. In the image below you can see how a typical timing is configured on a Volkswagen engine.

This image was taken at a car show in Los Angeles, California. The timing belt is routed at the crankshaft, through the water pump, onto the fuel injection pump and finally around the camshaft, the belt slack and tension is controlled by the timing belt tensioner.
timing belt

The timing belt tensioner is used to hold pressure on the belt which is spring loaded, oil pressure activated or set manually and when released facilitates the removal of the timing belt.
timing belt tensioner

The timing belt cogs (teeth) are what drive each accessory, both the smooth side and the cog side is used depending on the item, a water pump which does not need to be "set" in time with the engine is driven on the smooth side.
timing belt cogs teeth

Depending on design there can be an auxiliary timing belt which is smaller and can be used to drive a water or oil pump as in the image below.
oil pump timing belt


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