Lifter / Lash Adjuster: Description and Operation

5. Hydraulic Lash Adjuster
The hydraulic lash adjuster is located between the camshaft and valve stem. The top surface of the hydraulic lash adjuster is always in contact with the cam face. The cam directly pushes the lash adjuster to open or close the valve.
The engine oil flows through the cylinder head and goes into the lash adjuster so as to always maintain zero valve clearance.
1) Action when the valve starts to lift
When the cam begin to push the lash adjuster, the bucket and plunger are pushed down. At the same time, the body is pushed up by the reaction from the valve stem. This causes the high pressure chamber to compress, increasing the oil pressure in the high pressure chamber.
2) Action while the valve is lifted
As long as the lash adjuster is pushed by the cam1 the oil pressure in the high pressure chamber is held high. The oil in the high pressure chamber leaks through a very small clearance between the plunger and body.
Since the high pressure chamber is compressed in a very short time, almost no change occurs in the oil quantity inside the high pressure chamber.
Accordingly, the bucket, plunger and adjuster body work as an integral unit to push down the valve stem to open the valve. The passage for supplying oil from the cylinder head to the lash adjuster is closed during this period, and no oil flows into the lash adjuster.
3) Action when the valve stops lifting
When the cam completes its lash adjuster pressing stroke, the passage for supplying oil from the cylinder head to the lash adjuster opens, allowing oil to flow from the cylinder head into reservoir 2 of the lash adjuster Since the pressure in the high pressure chamber is lower than the pressure in reservoir 2, the check ball is pushed down by the pressure in reservoir 2.
Accordingly, oil flows into the pressure chamber until the oil pressure become equal between the high pressure chamber and reservoir 2. Under the action of this oil pressure, the body is pressed against the valve stem and the bucket against the camshaft.