Hydraulic lash adjusters
How a lash adjuster works
A valve train needs zero clearance and a way to keep it there while metal expands, wears and settles. The adjuster does both with engine oil and one check valve, and it does so on every cam revolution.
The problem it solves
Lash is either noise or a burnt valve
A valve train with mechanical clearance has to be set with a feeler gauge, and it drifts. Too much clearance and the train hammers on every lift, which is noise now and wear later. Too little, once the engine is hot, and the valve never quite seats; it runs hot, loses its seal, and burns. The adjuster removes the compromise by taking up whatever clearance exists, continuously, and giving it back when it is needed.
It manages that because engine oil is incompressible on the timescale of a valve lift and free to move on the timescale of a base circle. The part is a small hydraulic cylinder that is stiff for a few milliseconds and compliant for the rest of the revolution.
Inside the part
Six pieces, one fit that matters
- Body
- A cup, bored to size. The bore is honed, and it is what the plunger is matched to.
- Plunger
- A second cup inside the first, case hardened and superfinished on the outside. Its outside diameter against the body bore is the fit that decides the part.
- Low pressure reservoir
- Oil held inside the plunger, fed from the head gallery through a hole in the body wall and a hole in the plunger.
- Check valve
- A ball on a seat in the plunger floor, with a small spring and a retainer. It lets oil down into the high pressure chamber and stops it coming back.
- High pressure chamber
- The space under the plunger. When the ball is seated this oil is trapped, and trapped oil is a solid link.
- Plunger spring
- Pushes the plunger out whenever the load is off, which is what takes the clearance up.
One revolution, three states
Refill, rigid link, leak-down
- 01 Base circle: refill
- With the cam on its base circle the load is off. The plunger spring extends the plunger until every micron of clearance is gone, the check ball lifts, and oil flows from the reservoir into the high pressure chamber to fill the volume the plunger just vacated.
- 02 Lift: a rigid link
- The cam starts to lift and load arrives through the follower. The pressure in the chamber seats the check ball. Trapped oil cannot compress, so the adjuster becomes a strut and the valve opens with no clearance to take up.
- 03 Leak-down: by design
- During the lift a small quantity of oil escapes upward through the clearance between plunger and bore. The plunger settles by a few microns. That rate is the specification. It is set by the fit, and it is what the next base circle refills.
Leak-down is the whole design in one number. Too slow and the adjuster pumps up when the engine is hot and holds the valve off its seat. Too fast and it collapses under a high lift and the valve train becomes noisy. The fit that gives the right rate is measured in microns, and it is held on every part rather than on average.
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