What are the differences between a timing belt and a timing chain?

Article by Christian A., on August 23, 2018

The engine is probably one of the most complicated elements of your car. Its workings are more complicated than you may think. It features many parts, components and accessories, one of which is the timing belt or the timing chain.

The timing belt and the timing chain aren’t the same thing, but they perform the same function -- mechanical engine timing.

They don’t coexist simply because they do the same work, which means the engine has either a timing belt or a timing chain. But before this article discusses the difference between a timing belt and a timing chain, it will talk about their importance.

Importance of timing belt or timing gear

A great number of engines nowadays are four-stroke (or four-cycle) types. This means that the piston does four distinct strokes – intake stroke, compression stroke, power stroke and exhaust stroke -- to complete the combustion process (operating cycle). During this cycle, the crankshaft rotates twice (720 degrees) to bring the piston to the: bottom dead center (BDC) during the intake stroke; top dead center (TDC) on compression stroke; BDC on power stroke; and TDC exhaust stroke.

During the combustion process, the intake and outtake valves open or close depending on the stroke type. In the intake stroke, the intake valve opens to allow the air-fuel mixture to enter the cylinder, while the outtake valve is closed. In the compression stroke, both valves are closed to trap and compress the air-fuel mixture with the piston, readying it for ignition.

Both valves remain closed in the power stroke, and the compressed air-fuel mixture is ignited. In the power stroke, the resulting movement of the piston to BDC position applies torque to the crankshaft to initiate new rotations. Then in the exhaust stroke, the outtake is opened to allow the exit of the gases resulting from the ignition (exhaust gases).

While the rotation of the piston is effected by the crankshaft, the opening and closing of the intake and outtake valves is controlled by the camshaft. During the combustion process, the camshaft rotates only once (360 degrees).

To ensure the piston goes up and down (BDC and TDC positions) at right precise moment as the intake and outtake valves open and close, the crankshaft and the camshaft have to be in sync with each other (2:1 ratio). The timing belt or the timing chain ensures this mechanical timing is implemented all the time by connecting the crankshaft to the camshaft. Incorrect timing could result to the engine’s inability to burn fuel.

The timing belt or timing chain is important in the operation of interference engines. An interference engine is a type of four-stroke engine in which a valve in open position extends into the same space where the piston may also extend. Because of a risk of collision between the valve and the piston, there is a need to sync their movements, as effected by a timing belt or a timing chain.

Timing belt vs. timing chain

While it may seem that timing belts were first to be on engines, this is totally wrong. Timing belts were first used in the 1960s while timing chain has been used ever long before that decade.

Timing belts – basically toothed belts -- were first used as alternatives to timing chains. However, timing belts were soon adopted in many engines. A timing belt offers a quieter operation than a timing chain. Made from rubber and containing fiber glass, a timing belt wears out through time and is susceptible to breaking. A broken timing belt can cause major damage to the engine, although not as much as that of a broken timing chain.

Most carmakers recommend that a timing belt should be replaced after 60,000 miles. To keep the proper tension of the timing belt, it is built around hydraulic belt tensioners.

On the other hand, a timing chain is basically a roller chain, just like that of a bicycle chain. The primary advantage of a timing chain is that it is very durable, and may live as long the car itself. The problem with a timing chain is that it produces noise during operation. If it happens that a timing chain breaks, it may cause expensive damage to your engine, and could result to an engine replacement.

Because it may last the lifetime of the car, a timing chain is hardly replaced. If there is a part that needs to be replaced, it is the plastic guides that run the chain. Nonetheless, these plastic guides are more than two times more durable than a timing belt, as they wore out after 150,000 miles. The tension of the timing chain is kept by tensioners controlled by the engine’s oil pressure.

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Topics: tips, engine

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