How Your Car’s Timing Belt Works

In modern vehicles, the timing belt is a rubber belt (or, occasionally, a chain) responsible for keeping the crankshaft and camshaft synchronised with each other in order to ensure that valve timing is correct at all times. Incorrect valve timing can have devastating effects for your engine, causing the engine to either run poorly or not run at all. Mechanics will look at your timing belt during a service, and if it is worn out it, may need replacing immediately.

Understanding the engine

These days, timing belts are also used to control a vehicle’s water pump and power steering, but it’s traditional use is to control the valve timing of the engine. In a standard four-stroke engine, there are four key phases or “strokes”. The timing belt is responsible for regulating these strokes and ensuring that the valves open and close at the correct time. The four strokes are as follows:

• The intake stroke. In this stroke, the piston moves down and the intake valve should open to allow an air and fuel mixture to enter the cylinder. The exhaust valve should be closed at this stage.
• The compression stroke. During this stroke, all valves should be closed. The piston moves up, compressing the air and fuel mixture.
• The combustion stroke. Again, all valves should remain closed during this stroke. As the piston moves down, the spark plug ignites the compressed air and fuel mixture, generating power for the vehicle.
• The exhaust stroke. The intake valve should remain closed, but the exhaust valve should open, allowing any excess air and fuel still in the cylinder to leave the engine.

How does the timing belt work?

The timing belt rests on a crankshaft pulley and either one or two camshaft pulleys, depending on the make of the vehicle. When the crank turns, it also turns the crankshaft pulley. This forces the timing belt to move too. As the timing belt moves, it works to turn the camshafts. It is the camshafts that are responsible for controlling the position of the all-important valves.

If the crankshaft and camshaft are out of sync, then the valve timing will be off, causing the valves to open or close during the wrong engine stroke. This can be absolutely disastrous for the engine. Some of the results of incorrect valve timing may include:

• Bending the valve
• Bending a piston rod
• Destroying the piston

Common timing belt problems

Both the timing belt and the pulleys are moulded with “teeth” that fit with the teeth of the pulley, preventing the belt from sliding around as it moves. If any of these teeth wear out, the crankshaft and the camshaft can be thrown out of sync, causing the valve timing to be incorrect. Additionally, the timing belt may stretch or break if it becomes worn out over time, which will also mess with the valve timing of the engine.

For this reason, your manufacturer will have recommendations as to how often your timing belt should be replaced. Generally, around the 60,000km mark it is prudent to get a mechanic to replace your timing belt to ensure that the engine continues to run efficiently.