Thursday, June 29, 2017 Detailed Auto Topics
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With all automobile engines, the camshaft and crankshaft must rotate in precise synchronization. Early engines used timing chains or gears. Later designs use a timing belt. Today the timing chain is back, and it is not without problems.

Mechanical engine timing

They design the crankshaft of an engine, to control the precise position of the pistons in the cylinder. Pistons moving up and down in the cylinders makes, the crankshaft rotate. The camshaft operates the intake and exhaust valves, allowing air and fuel in and exhausts out. To produce power, these valves must open and close in a precise relationship to the piston position.

Mechanical engine timing, 4 stroke engine

For instance, on the intake stroke, of a four-cycle engine, the intake valve opens as the piston moves down the cylinder. This draws the fuel and air mixture in, which powers the vehicle. Mechanical engine timing is the relationship of the valve position to the piston position.

When the crankshaft and camshaft get out of time with each other, we have greatly diminished engine power. If the connection fails, the engine will stop running and severe damage may occur. This can happen if the timing chain or belt slips or breaks.

Timing belts and chains

Engineers may use chains, gears or belts to drive the camshaft and maintain valve timing. Each method offers advantages and drawbacks. Timing belts resist elongation and maintain precise valve timing for their life. The timing belt does have an age and mileage limitation, and we have to replace them, increasing maintenance costs.

A timing chain may last longer than a belt, but is more susceptible to a lack of maintenance. Improper oil viscosity, going too long between oil changes and inferior oil filters can each cause timing chain issues. Timing chains are also generally more damaging when they fail. Failure is often more expensive to correct than with a belt, due to collateral damage. Our Detailed Topic, All About Timing Belts give far more detail on the timing belt. The remainder of this article concentrates on timing chains.

Interference and non-interference engines

Designs vary, and some engines provide adequate clearance to prevent the valves from striking the pistons, when a timing failure occurs. We call these engines freewheeling or non-interference engines. To gain performance, they build many designs to closer tolerances. When failure occurs on these engines, the valves will strike the pistons. They call these engines interference design.

Manufacturers normally publish information on which design they use, with timing belts. When they employ a timing chain, they often omit this information due to the infrequency of chain breakage. The terms are a bit misleading and may lull owners into thinking they are safe. Under the wrong conditions, any engine may suffer damage, if valve timing fails. Timing chains can also cause significant damage without breaking. Addressing any symptom of timing chain problems, is best, to prevent a catastrophe.

Methods of driving the camshaft

Designers use many methods to connect the crankshaft to the camshaft and keep them in time with each other. With an overhead valve engine, the job is simple.

Early overhead valve engine design

The camshaft on this design is in the engine block. A sprocket on the crankshaft drives the chain, which rotates the camshaft. Because these components are in close proximity, such a chain is short and guides and tensioners are often unnecessary. This design is simple, robust and gives few problems. Overhead valve engines are also less common today. The trend is toward the more complex overhead cam design.

Many engines today use the overhead cam design. This may be a single overhead cam (SOHC) or dual overhead cams (DOHC) design. With the overhead cam engine, the camshaft(s) are in the cylinder heads and directly operate the valves. The distance between the camshaft(s) and the crankshaft is much greater. These engines use a far more complex arrangement or chains, guides and tensioners.

Typical overhead cam engine

As a timing chain gets longer, it may develop harmonic movement. This causes the chain, between the sprockets, to move back and forth. Engineers add chain guides to help control movement of longer timing chains. These guides may be fixed or movable, depending on design. Most engines use a fixed guide on the side of the chain that drives the cam. The idle side of the chain is often fitted with a movable guide and tensioner. A tensioner extends, pushing the guide against the chain, to keep it from moving.

Types of timing chains

Manufacturers most often use two primary designs for timing chains. Both designs offer advantages and have their short comings.

Roller and silent type timing chains

Steel rollers, which act much like a bearing, give the roller chain its name. This design lowers friction and wear on the sprockets. These chains are expensive to manufacture and are very dependent on proper lubrication and a clean work environment. Grit or debris, trapped in the rollers, will cause rapid wear. Roller chains also produce noise, especially if not properly dampened with guides and a tensioner.

Components of a roller chain

Each outer link connects two pins that serve as axles for the rollers. Roller supports slip over the link pins and act as a bearing surface. Hardened rollers cover the support pins and contact the sprocket surface, when we operate the chain. The rotating action of the rollers serves as a bearing. This design gives good service, but is complex. Roller chains can also produce noise when turning under a load.

Another design is the silent chain. This designs lowers cost and reduces chain noise. Most modern engines use silent chains. With the silent chain, pins hold the links together. Pivot points alternate between links. This produces a much simpler flexible design. The silent chain slides against the sprocket, rather than rolling.

Chain elongation

As a timing chain wears, slack develops in the pivots. This slack causes the chain to elongate and changes its pitch. Timing chain breakage is rare, but elongation causes many problems.

Timing chains elongate with use and valve timing changes

Mechanics often call timing chain wear, stretch. Stretch is technically the wrong term for what occurs. The links of the timing chain do not stretch, but wear causes each link to move apart.

Chains wear in the links and pins, and slack allow them to elongate

Wear occurs in the rollers and supports on a roller chain and in the pivot pins and links of a silent chain. If we measure the metal link of a worn chain, it will be the same as an unworn link. The slack is in the pins and holes through which they pass. Sprockets and guides also wear and this increases camshaft timing problems. The next article in this series covers the effect of retarding the camshaft timing.

Timing Chains part II





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