In a piston engine, either a timing belt (also called a cambelt) or timing chain or set of timing gears is 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.
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 pushrods), which in turn actuates the intake and exhaust valves. These valves allow the engine to inhale air (or an air/fuel mixture) and exhale the exhaust gasses.
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 chains were common on mass-production cars until the 1970s and 1980s, however since the 1990s timing chains have become more common due to the replacement intervals required when using timing 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, less noise and that belts traditionally do not require lubrication. The main disadvantage is that belts wear over time, therefore belt replacement is recommended at specific intervals. 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 2008 have used "wet timing belts", whereby the belt is lubricated by engine oil to reduce friction. In some engine designs the timing belt may also be used to drive other components, such as the water pump and oil pump.
A timing belt is typically made from rubber, although some belts are instead made from polyurethane or neoprene. The structure of the belt is reinforced with corded fibres (acting as tension members) and the toothed surface is reinforced with a fabric covering.
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. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Manufacturer-specification timing belts may stretch at high rpm, retarding the cam and therefore the ignition. Stronger aftermarket belts will not stretch and the timing is preserved. When designing the timing belt, a wider belt increases its strength however a narrower belt reduces weight and friction.
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. 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.
Toothed belts were invented in the early 1940s, for use in textile mills. The first known automobile engine to use a timing belt was the American 1954 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. This car won the Sports Car Club of America (SCCA) National Championship in 1956.
The 1962 Glas 1004 was the first mass-produced vehicle to use a timing belt. The 1966 Pontiac OHC Six engine was the first US mass-produced vehicle to use a timing belt, while the 1966 Fiat Twin Cam engine was the first mass-produced engine to use a timing belt with twin camshafts.
Metal timing chains have become more widespread in car engines produced since the 1990s, 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. Early symptoms of a timing chain system is becoming worn include a rattling noise from the front of the engine.
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.
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 these other materials can fail suddenly and without warning.
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, 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:
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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In earlier engines, camshafts were often gear-driven off the crankshaft. Later, powerplant designers developed chain drives in OHV (overhead valve) configurations that allowed some flexibility in the placement of the camshaft so that shorter pushrods could be used, for more performance and efficiency. Those engines with long chains sometimes tended to whip about and cause problems. The only alternative was a noisy and complicated multi-gear train until the cogged rubber synchronous timing belt was invented in 1945. Today the timing belt cam drive is used in distinguished automobiles such as Ferrari, Mercedes, Cadillac, Corvette, BMW, Alfa Romeo, Porsche, etc.
Example of an Audi belt that broke before the recommended replacement interval.