How Does My Car’s Timing Belt Work?

Timing in any situation is important. If you’re playing an instrument, you must make sure the beats played in a measure are equal to the beat of the song. When you miss a beat by playing too quickly or too slow, it can affect the integrity of the song. If you’re playing in a band or orchestra, you won’t be in sync with your bandmates. Similarly, your vehicle’s timing belt is essential to your engine for a successful combustion process. What  does a timing belt do in a car? The timing belt is what keeps the crankshaft and camshafts turning so that the valves open and close to create the combustion process, all in perfect harmony. 

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What Does a Timing Belt Do in a Car?

The timing belt has notches that turn the camshaft in coordination with the crankshaft. Crankshafts are intended to operate at half the speed of the camshaft. For example, two turns of the crankshaft are equal to just one turn of the camshaft. In addition to keeping time, timing belts are tasked with powering the water pump, intermediate and balance shafts, and the oil and injection pumps. Timing belts need tension in order to perform properly with the use of a timing belt tensioner. Vehicles today are equipped with automatic timing belt tensioners that do not require adjustment; however, older model vehicles would require adjustment from time to time if the belt became loose. Timing belts that are loose are problematic as they force the timing out of range. 

How Does a Timing Belt Work?

As part of the combustion process, the intake valves open to allow air and fuel into the cylinders and then close during the compression and combustion phase. The pistons then travel upward to compress the mixture. Once ignition occurs, the pistons are forced back down and move back up to dissipate the exhaust gases through the now opened exhaust valves. The process continues over and over as the vehicle runs. The timing of this process is important. If one of these operations is off, it can affect the efficiency of the engine resulting in a loss of power, misfires, and in some cases an inoperable engine. So where does the timing belt fit into this process? The timing belt is connected to the engine uses a pulley to turn the crankshaft as well as the camshaft to control the opening and closing of the valves. 

Timing Belt or Timing Chain?

Initially, vehicles were manufactured with timing chains and they connected the crankshaft and the camshaft but were overhead valves, not overhead camshaft engines. In the s timing belts were introduced and widely used because of the introduction of the overhead camshafts. Now, vehicle manufacturers are turning back to timing chains, though many continue to utilize timing belts as well. Timing belts and timing chains perform the same task but do have minor differences. Timing belts are made of rubber and timing chains are well, metal chains. Timing belts are quieter and less expensive than chains, however, chains do last longer and rarely require replacement. A timing belt, on the other hand, will require replacement somewhere between 60,000 to 90,000 miles. The location of the timing belt and chain differ as well. Timing belts are located on the exterior of the engine whereas timing chains are inside the engine to receive lubrication from motor oil. Refer to your owner’s manual to determine whether your car uses a belt or chain for timing. 

When Good Timing Goes Bad

It’s important to have the timing belt inspected to ensure the belt is free from any damage such as cracks, gouges, oily residue, or other signs of wear. A timing belt that is in poor condition is at risk for breaking, which can cause the engine to quit running altogether. When a timing belt becomes loose and detaches, the camshaft stops turning, leaving valves in the open position. This is troublesome because the crankshaft continues turning, making the pistons move up and downward and possibly colliding with the open valves resulting in a bevy of damaged engine components. If you notice any damage to the timing belt or experience any of the symptoms below, be sure to take your vehicle to an automotive service center right away for replacement:

• Check Engine Light On 
• Rough idling
• Car Won’t Start
• Misfires

When it’s time to replace the timing belt, also consider replacing the water pump at the same time. The two are located so closely together that though you will be paying for the cost of the water pump, you’ll save on labor fees as opposed to replacing it later. It’s important to note that water pumps have about the same life expectancy of timing belts, so replacing them at the same time is a smart choice. Replace your timing belt every 60,000 miles to ensure your engine continues to work in perfect harmony. 

In a piston engine, either a timing belt (also called a cambelt) or timing chain or set of timing gears is a perishable component used to synchronize the rotation of the crankshaft and the camshaft. This synchronisation ensures that the engine's valves open and close at the correct times in relation to the position of the pistons.

Design

In most piston engines, the camshaft(s) are mechanically connected to the crankshaft. The crankshaft drives the camshaft (via a timing belt, timing chain or gears), which in turn actuates the intake and exhaust valves.[1] These valves allow the engine to inhale air (or an air/fuel mixture) and exhale the exhaust gasses.[2]

The most common devices to transfer the drive are toothed rubber belts, metal timing chains or a set of gears. The teeth of the belt/chain/gears mesh with both the crankshaft and camshaft(s), thereby synchronising their motion.

In many older overhead valve engines, the camshaft is located in the block near the crankshaft, therefore a simple gear system is often used to drive the camshaft. Overhead camshaft engines mostly use timing belts or timing chains, since these are better suited to transferring drive over larger distances. Timing belts were common on mass-production cars until the s and s,[citation needed] however since the s timing chains have become more common due to the replacement intervals required when using timing belts.

Toothed rubber belts

The phrase "timing belt" usually refers to a rubber toothed belt. The advantages of timing belts are typically a lower cost, reduced friction losses,[citation needed] less noise and that belts traditionally do not require lubrication.[4] The main disadvantage is that belts wear over time, therefore belt replacement is recommended at specific intervals.[5][6] Replacement of the engine's water pump at the same time is often recommended, since the water pump is also subject to wear and easily accessed during the replacement of the timing belt.

Timing belts are typically located in front of the engine and are often behind a cover for protection against dust and debris. However a few engines since have used "wet timing belts", whereby the belt is lubricated by engine oil to reduce friction losses by 30% and thus reduce fuel consumption by 1%.[7] In some engine designs the timing belt may also be used to drive other components, such as the water pump and oil pump.

Construction

A timing belt is typically made from rubber, although some belts are instead made from polyurethane or neoprene.[8][9] The structure of the belt is reinforced with corded fibres (acting as tension members)[10] and the toothed surface is reinforced with a fabric covering.[11]

For more information, please visit Automotive Timing Belts.

Rubber degrades with higher temperatures, and with contact with motor oil. Thus the life expectancy of a timing belt is lowered in hot or leaky engines. Also, the life of the reinforcing cords is also affected by water and antifreeze, so it is important that belt that can be exposed to water is able to drain the water away quickly.

Older belts have trapezoid shaped teeth leading to high rates of tooth wear.[citation needed] Newer manufacturing techniques allow for curved teeth that are quieter and last longer.

Manufacturer-specification timing belts may stretch at high rpm,[citation needed] retarding the cam and therefore the ignition.[12] Stronger aftermarket belts will not stretch and the timing is preserved.[13] When designing the timing belt, a wider belt increases its strength however a narrower belt reduces weight and friction.[14]

The usual failure modes of timing belts are either stripped teeth (which leaves a smooth section of belt where the drive cog will slip) or delamination and unraveling of the fiber cores. Breakage of the belt, because of the nature of the high tensile fibers, is uncommon.[15] Often overlooked, debris and dirt that mix with oil and grease can slowly wear at the belt and materials advancing the wear process, causing premature belt failure.[16]

History

Toothed belts were invented in the early s, for use in textile mills.[17] The first known automobile engine to use a timing belt was the American Devin-Panhard racing car, used an engine converted from pushrods to overhead camshafts through the use of a toothed belt made by the Gilmer Company.[18][19] This car won the Sports Car Club of America (SCCA) National Championship in .[20]

The Glas was the first mass-produced vehicle to use a timing belt. The Pontiac OHC Six engine was the first US mass-produced vehicle to use a timing belt,[21][22] while the Fiat Twin Cam engine was the first mass-produced engine to use a timing belt with twin camshafts. Carmakers began to adopt timing belts in the s and compared to timing chains are less expensive, smaller, lighter, quieter, isolate harmonics of the crankshaft from the valve train, require less power than chains and can potentially function without lubrication. Timing belts are usually made of Neoprene or HNBR.[23][7]

Timing chains

Metal timing chains have become more widespread in car engines produced since the s, due to the lack of the regular maintenance task of replacing a rubber timing belt. While the chains themselves are subject to minimal wear, lubrication of the chain or failure of the tensioner and chain guides can cause maintenance issues. By contrast to rubber-based timing belts which give no indication of snapping while one is driving, early symptoms of a timing chain system becoming worn exist, and these include a rattling noise from the front of the engine.[24]

Most pushrod engines, where the crankshaft and camshaft are very close together, use a short chain drive rather than a direct gear drive. This is because gear drives suffer from frequent torque reversal as the cam profiles "kick back" against the drive from the crank, leading to excessive noise and wear.[citation needed]

Timing gears

Timing gears are used in various overhead valve engines, due to the close proximity of the camshaft to the crankshaft.

Fiber or nylon covered gears, with more resilience, are often used instead of steel gears where direct drive is used. However, commercial engines and aircraft engines use steel gears only, as the other materials can fail suddenly and without warning.[25]

Effects of failure

Failure of the timing system will prevent an engine from running. Many modern cars use interference engines, which can suffer catastrophic damage in the event of timing system failure,[26] since the loss of synchronization between the crankshaft and the camshaft will cause the valves to collide with the pistons.

Warning signs that a timing chain needs to be replaced include:[27]

• Later model VVT applications will generate engine codes and check engine lights prior to engine rattle
• Older, pre-VVT applications will generate engine rattle

Aside from the belt/chain itself, also common is a failure of the tensioner, and/or the various gear and idler bearings, causing the belt/chain to derail. Furthermore, in engines where the timing belt drives the water pump, failure of the water pump can cause the pump to seize, which can break the timing belt or chain. For this reason, timing belts and chains are often sold as part of a kit with the water pump, tensioner, and idler pulleys so that these parts can be replaced to prevent timing belt or chain failure due to failure of these parts.

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See also

• Gilmer belt
• Interference engine
• List of auto parts
• Pushrod
• Serpentine belt