by Jerry Hastings

Created on: October 24, 2008   Last Updated: November 05, 2008

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)

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