Hydraulic Valve Play Compensation. Mechanical Valve Play Adjustment

For many years now one goal of engine builders has been to keep the adjustment and service work of the engine to a minimum. Thus, it is hardly a surprise that the first engines with hydraulic—and thus automatic—valve play compensation were produced well before World War II. These were, however, large-displacement engines that ran at moderate speeds. Higher engine speeds were attained in the 1970s in the Mercedes Benz V-8 engines with hydraulic screw-in elements (cam follower system).

A further milestone reached in the 1970s was the introduction of hydraulic valve lifters in the V-8 engine used in the Porsche 928. Today hydraulic valve play compensation is employed in all engine classes and even in high-speed engines such as those used by Ferrari and Porsche.

The hydraulic elements consist of an outer casing in which a plunger with an integrated check valve is installed. These two parts can slide one inside the other and, at the contact surface, form a leak gap only a few micrometers wide. A spring on the inside keeps the two components apart.

During the valve stroke the valve spring and mass forces impose load on the hydraulic element. High pressure is developed in the space defined by the casing and the plunger (with the check valve closed). A small amount of oil escapes through the very narrow gap and is passed to the reserve space inside the plunger.

In the following phase, while contact is made with the lobe’s circular segment (valve closed), the inside spring pushes the hydraulic element apart until the valve play is once again fully compensated. The differential pressure thus arising causes the check valve to open; the amount of oil required for compensation can flow in. Thus, the length of the hydraulic element can change in both directions.

The advantages of hydraulic compensation for valve play include
- Simple mounting of the cylinder head (no measurement or adjustment work since the hydraulic element compensates for all tolerances)
- Freedom from service requirements
- Constant timing at all throttle settings and at all times (no need to adjust time to account for thermal effects or wear in valve train components)
- Low noise level (thanks to low opening and closing ramps at the camshaft and low opening and closing speeds).

Achieving this places certain demands on the oil circuit (oil pressure, foaming). It is also necessary to observe close shape tolerances when machining the circular lobe segment. The elements could become compressible in the event of a deficiency in the oil supply (air in the high pressure chamber), which would result in insufficient valve lift and consequently would induce noise or changes in dynamic response at high engine speeds.

The hydraulic element recognizes loss of contact force as valve play, and this could result in an undesired lengthening of the element, with the result that the valves would not close completely.

Mechanical Valve Play Adjustment. Valve play is adjusted with
- Screws
- Adjusting shims of graduated thicknesses
- Valve lifters with graduated crown thickness (only for valve trains incorporating valve lifters)

Common to all three options is finite adjustment precision, which needs to be taken into account in the design of the lobe ramps for opening and closing the valves. It is necessary to measure and adjust for valve play when mounting the cylinder head. The increase in valve play resulting from wear at valve train components can be corrected by adjustments made during service work; changes in play resulting from temperature development in the engine cannot be automatically corrected.

The effects enumerated here harbor the potential for a wide spread in the amount of play and necessitate steep ramps with great opening and closing speeds. This wide spread implies critical changes in timing and thus has negative effects on exhaust gas quality; rapid closing causes valve train noise.

The advantages of mechanical valve play adjustment (compared to comparable hydraulic valve train components) include
- Greater stiffness
- Lower friction losses (by eliminating friction at the lobe’s circular segment and through modified valve spring characteristics)
- Lower component costs.