Timing Belts | Chains

Timing Belts | Chains

A timing belt, timing chain, or cambelt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft(s) so that the engine's valves open and close at the proper times during each cylinder's intake and exhaust strokes. In an interference engine the timing belt or chain is also critical to preventing the piston from striking the valves. A timing belt is usually a toothed belt—a drive belt with teeth on the inside surface. A timing chain is a roller chain.

Many modern production automobile engines use a timing belt to synchronize crankshaft and camshaft rotation; some engines, particularly cam in block designs, used gears to drive the camshaft, but this was rare for OHC designs. The use of a timing belt or chain instead of a gear drive enables engine designers to place the camshaft(s) further from the crankshaft, and in engines, with multiple camshafts a timing belt or chain also enables the camshafts to be placed further from each other. Timing chains were common on production automobiles through the 1970s and 1980s, when timing belts became the norm, but timing chains have seen a resurgence in recent years. Timing chains are generally more durable than timing belts—though neither is as durable as gear drive—however, timing belts are lighter, less expensive, and operate more quietly.

What Is Engine Timing?

For the engine to operate at its best, each cylinder’s fuel/air mixture must be fired just as the piston reaches the top dead center ( TDC ). It takes a certain amount of time for the spark plug to ignite the mixture and build up the combustion. This time remains roughly the same, no matter how fast the engine is running. The timing mechanism is set up to fire the plug a short time before the TDC. But because the engine motion operates the mechanism, this time would typically decrease as the engine runs faster, and the plug would fire too late.

It is estimated that there are approximately 10,000 moving parts within the average internal combustion-powered vehicle. Put that way. It’s a genuinely mesmerizing engineering feat to get all those components to talk to each other and mesh together to form machines that we know and love. And in terms of the heart of the beast – the engine – timing is an essential factor. Since the precise movement of camshafts, valves, pistons, and crankshafts is an integral part of the internal combustion process, there is no room for error given the rate and violence components interact with each other.

Why Is Engine Timing Important?

To understand the importance of engine timing, let's understand what's going on in a standard four-stroke engine’s cylinders. First, the piston inside the cylinder is forced down, and the mixture of air and fuel enters through the opening of the inlet valve. Once the piston reaches BDC (bottom dead center), it begins its journey back to the top of the cylinder (top dead center) with the inlet valve shutting down and compressing the air/fuel mixture.

In a four-stroke cycle, the crankshaft must rotate a full two rotations (or 720 degrees) to complete the engine cycle, rotating a full 360 degrees each time the piston moves from the TDC to the BDC and back. And in a car with a red line of about 7500 rpm, the engine completes this reciprocation about 125 times a second. 

To hold this extremely accurate series of events together, a timing belt or chain is used to connect the engine's vital components to keep everything in sync. The timing belt is a thick, toothed belt that runs around the camshaft sprockets, the water pump pulley, and the crankshaft sprocket and rotates in time with the crankshaft at the bottom of the engine block.

This means that the water pump increases and decreases the coolant flow rate in tandem with any engine speed changes, allowing more coolant to circulate the engine block when the engine is working hard. The final component of this timing system is the timing belt tensioner, which acts as a spring-loaded prod on the side of the timing belt, keeping it at a set tightness to stop the belt from slipping or jumping over the teeth of the sprockets with which it is meshing.

This timing system is synchronized with the ignition by alignment markers or timing marks on the valve cover, cam, and crank sprockets. Using small dashes, numbers, or lobes on the sprockets, the timing system can be aligned so that – once the engine is fired – the rotation of the timing belt synchronizes the camshafts opening their respective valves to the reciprocating crankshafts of the pistons, along with the timing of the ignition. The manufacturer shall place these timing marks to set the crankshaft’s angle (within its 360-degree range) for ignition to occur.

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