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A hydraulic tappet, also known as a hydraulic valve lifter or hydraulic lash adjuster, is a device for maintaining zero valve clearance in an internal combustion engine. Conventional solid valve lifters require regular adjusting to maintain a small clearance between the valve and its rocker or cam follower .
In an internal combustion engine, a tappet (also called a 'valve lifter' or 'cam follower') [3] [4] [5] is the component which converts the rotation of the camshaft into vertical motion to open and close an intake or exhaust valve. The principal types of tappets used in automotive engines are solid, hydraulic, and roller. [6] [7]
The common valve spring system is satisfactory for traditional mass-produced engines that do not rev highly and are of a design that requires low maintenance. [1] At the period of initial desmodromic development, valve springs were a major limitation on engine performance because they would break from metal fatigue.
The valve timing of a diesel engine also depends on tappet clearance of the inlet and exhaust valves. If tappet clearance is less, then valve will open early and close late. [2] If tappet clearance is more, then valve will open late and close early. Tappet clearance is measured by an instrument called feeler gauge.
The stroke of each piston is caused by an eccentric drive shaft or an external eccentric tappet (e.g., stroke ring). When filling the workspace of the pumping pistons from "inside" (e.g., over a hollow shaft) it is called an inside impinged (but outside braced) radial piston pump (picture 1).
In contrast, MultiAir uses managed hydraulic fluid to provide variable valve control. Control of a MultiAir engine's intake valves works via a valve tappet (cam follower), moved by a mechanical intake cam, which is connected to the intake valve through a hydraulic chamber, controlled by a normally open on/off solenoid valve. [11]
The engines all employ hydraulic tappets which operate in aluminum guides that are machined into the crankcase. The tappets are built from four parts, a cam follower body, cup, cylinder, and piston and operate with clearances of 0.03 in (1 mm) to 0.11 in (3 mm). The pushrods are steel and feature pressed-in ball ends. [1]
The tappet bore where a hydraulic tappet would normally reside becomes the cylinder, and the retainer assembly becomes the piston. Pressurized air ( nitrogen ) is pumped into this cylinder which then causes the piston/retainer to rise to the top of cylinder, causing the valve to form an airtight seal with the seat.