Tappet

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Ford I4 DOHC engine, showing the tappet just under the cam lobe]

A tappet in mechanical engineering is a projection which imparts a linear motion to some other component within an assembly. Properly speaking, a tappet is only that part of a rocker arm which makes contact with an intake or exhaust valve stem above the cylinder head of an internal combustion engine. As the cam rotates it creates both a sideways and a downward force on the tappet. Without a tappet (and the cam acting directly on the valve) the sideways force would cause the valve stem to bend. With a tappet the sideways force is transferred to the cylinder head so only the downward force acts on the valve stem.

On most modern overhead cam engines the camshaft is directly over the valves, as pictured. In some other overhead cam engines, a rocker arm pivots on a fixed shaft while one projection of the rocker-arm rides on a cam of the rotating camshaft. In both cases this creates an oscillating linear motion, opening the valve.

The closing of the valve is typically accomplished by a compression spring placed between the valve collet (or retainer) and the cylinder head above the combustion chamber. In early engines the camshaft was located near the crankshaft and motion of the cam lobes was followed by cam followers (similar to tappets) and transferred up to the cylinder head and the rocker arm assembly by means of long pushrods.

Traditionally, the nominal distance (clearance) between the tappet surface and the valve's contact surface was maintained by means of an adjustment screw on the rocker arm. Today, this is typically accomplished by introducing shims into the space between the cam follower (tappet) and cam to create the necessary clearance, or by hydraulic adjusters.

This traditional mechanism for opening and closing valves, while tried and true, has its drawbacks. Valve clearances periodically require adjustment as the contact surfaces of both tappet and valve stem wear. Also the problem of valve float has for decades plagued high-rpm engines. This problem occurs when a valve spring cannot close the valve quickly enough at high engine rpm. While this has largely been solved with modern metallurgy, engines in the 20,000 plus rpm range can still exhibit valve float.

The Ducati motorcycle company partially solved these problems with their desmodromic valve train. The desmodromic principle attempts to minimize wear by minimizing clearances between contacting surfaces, and eliminating the 'tapping' action of valve actuation. The real advantage to a desmodromic valve however, is that it is positively closed by the mechanism instead of being allowed to close by spring action. Mercedes-Benz successfully built and raced desmodromic Grand Prix engines in the 1950s, but never put them into production.

In internal combustion engines of the future, the use of tappets in the mechanical opening and closing of valves may disappear completely in favor of electronically controlled linear actuators. Engines utilizing this technology already exist (though production costs are high). The advantages here are many; valves can be opened or closed dynamically, optimising power output and fuel consumption based on changing conditions and without respect to the mechanical limitations of a camshaft; valves can be opened or closed almost instantaneously, eliminating valve float; engines can be used as very effective downhill-brakes, much like the engine brake of a semi-trailer truck; the seals and bushings of a traditional valve train could potentially be eliminated, reducing the overall complexity of the engine.

Failure of tappets

Over time, tappets fail and may break. The result is that they no longer close and open the valves properly. This causes the affected cylinder to constantly misfire. The engine will be noticeably slower, the idle speed will be lower, sometimes the engine will shudder, and it may produce copious amounts of smoke (as opposed to the normal small amounts of slightly bluish smoke from petrol/gasoline engines and black smoke from diesel engines).

On a V8 engine, the failure of one tappet is rarely noticeable unless one is aiming for high acceleration.

On a V6 engine, the failure of one tappet is often noticeable, as the engine is not as responsive.

On a 4-cylinder engine the failure is extremely noticeable, causing reduced idle speeds and increased shuddering as the vehicle's engine attempts to keep running with a quarter of its engine power missing.

On many diesel engines the failure of a tappet may cause the affected valve to contact the top of the piston, causing major damage to both components and possible damage to the engine block and cylinder head.

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