Timing Belt
Usually timing belts are found in smaller engines with smaller horsepower potential such as the 4 cylinder engines found in smaller front wheel drive cars. Larger engines still mostly use timing chains or gears. Large diesel engines use timing gears almost exclusively.
A timing belt in later model engines replaces timing chains or gears as a non slipping linking device between the crankshaft and the camshaft(s). Their appearance is similar to the readily visible serpentine belt found on most engines manufactured today, and used to drive the alternator, water pump, power steering pump, engine fan (unless the engine fan is electrically driven), AC compressor, etc. A serpentine belt uses pulleys with grooves that run around the circumference of the pulley and that matches grooves that runs along the length of the belt. A timing belt uses pulleys with grooves across the face of the pulley and cogs on the belt that matches these grooves. Timing belts are covered by protective covers and are not normally visible until the covers are removed.
Timing chains simply work like bicycle chains and sprockets. Timing gears work by meshing the gear teeth much like gear teeth in a manual transmission. Timing belts, on the other hand, work by fitting the cogs located on the pulley side of the belt into grooves cast or machined across the face of each pulley. These cogs and grooves provide precise timing just like a chain and sprocket or gear teeth does. Since this belt cannot slip when properly installed, the engine timing is therefore maintained for as long as the integrity of the belt is maintained.
Pistons in an internal combustion engine are attached to the crankshaft by piston rods. Spring loaded intake and exhaust valves are located in the cylinder head(s) and are opened or closed by the camshaft(s). Since piston position versus valves either opened or closed is a critical element in internal combustion engine operation, the use of timing belts, chains or gears is necessary to link the crankshaft to the camshaft(s) in a manner so that the timing of these different operations can be constantly maintained throughout the RPM range of the engine.
There is more than one cycle (up and down piston strokes) necessary for a four cycle engine to operate as designed. So that the pistons are at the proper height and the valves are open/closed at the precise time for combustion or exhaust, the use of a larger pulley, sprocket or gear on the crankshaft with smaller pulley(s), sprocket(s) or gear(s) on the camshaft(s) are used. The reason for the different sized pulleys, sprockets or gears is because the crankshaft rotates at approximately twice the speed of the cam in a single overhead cam engine. Either additional gears, a chain or a belt with cogs provide the link in between the crankshaft and the camshaft(s).
The plural connotations above are because an engine, depending on exactly how the engine was designed, can have either one or two camshafts, one or two cylinder heads and a timing belt, timing chain or timing gears. Engines in respect to camshaft(s) are known as a single overhead cam engine if it only has one camshaft while double overhead cam engines have two camshafts.
While timing chains and even gears can fail, timing belt failure is all too often commonplace especially when the belt replacement schedule is not kept up to date. Many unknowing people simply are not aware of the presence of such a thing inside their engine or unaware of the replacement schedule, so they naturally do not replace the timing belt when it is time to replace it. Premature timing belt breakage or stripping off of the cogs of the belt is often a problem too. In such a scenario, they will give no warning such as noise or very poor engine performance either.
Timing belts, unlike chains or gears, can be installed too loose or too tight. A too loose timing belt will likely cause the engine to run slightly out of time and performance will be not as good as it could be. If a timing belt runs too loose, it may also strip off the cogs of the belt over time or the entire belt may come off the pulleys. If it is run too tight, premature breakage is a real possibility. Both scenarios can produce significant costly engine damage upon failure of the timing belt unless the engine is designed in such a manner to where the pistons absolutely cannot contact the valves.
Because of possible early failure, I have found that a timing belt is a not so good linking device used to link the crankshaft to the camshaft(s) in a specific manner that will provide proper engine timing on certain engines. A good idea before purchasing any vehicle with an engine that has a timing belt, is to research that engine's capability for the pistons to NOT contact the valves upon timing belt failure.
