Valve Train Components

Valve Train. There has emerged in recent years a trend in passenger car engines toward overhead camshafts (OHC) and double overhead camshafts (DOHC) while engines with camshafts located below [overhead valve (OHV)] continue to be used, particularly in large-displacement, V-block engines.

Engines with overhead camshafts are developed so valve trains can be engineered to withstand the high speeds required in higher-performance engines. DOHC concepts give the engineer the option of mutually independent timing for the intake and exhaust camshafts using camshaft shifters. OHV and OHC concepts are characterized by compact shapes and sizes and by economy in their manufacture.

For diesel engines for utility vehicles, one sees a trend toward four-valve concepts. Rocker arms or double rocker arms fitted with mechanical valve play adjustment and driven by pushrods and camshafts located below—as is the case in two-valve designs—affect valve lift.

OHC concepts are employed, in addition to OHV versions, in smaller utility vehicle engines that utilize engine braking effect, and hydraulic valve lifters are used increasingly to compensate for valve lash.

Direct Drive Valve Trains. This category embraces valve trains with hydraulic (Fig. 7-109) or mechanical valve lifters as well as so-called “bridge” solutions in which components, guided by columns, lift multiple valves by direct actuation with a single camshaft. A subgroup within the latter solution is represented by the bridge that interfaces with two hydraulic valve lifters (Opel direct-injection diesel engines).

Fig. 7-109. Hydraulic valve lifters

Direct drive always offers very good stiffness values with relatively modest masses in motion. This is the prerequisite for trouble-free valve train operation even at very high speeds (loss of contact force, premature valve seating). Thus, efficient, high-speed engines can be realized particularly through employing valve lifters.

In the interest of reducing the masses in motion, preference among mechanical valve lifters is given to those with graduated crown thickness (Fig. 7-110) or those with adjustment shims located at the bottom.

Fig. 7-110. Mechanical valve lifters with graduated crown thickness

For service work (adjusting valve play), rods with an adjustment shim at the top (Fig. 7-111) are preferable since with this version it is not absolutely necessary to remove the camshaft. Such units are, however, considerably heavier and require more installation space than the other version (at identical valve lift).

Fig. 7-111. Mechanical valve lifters with adjustment shim at the top

The basic body for the valve lifter is made of ductile steel. Aluminum is found in only two applications (Toyota Lexus V-8 and Jaguar V-6 and V-8). The shims are usually made of steel that can be hardened. When bodies made from deep- drawn sheet steel and small hydraulic elements (11 mm O.D.) are used, hydraulic valve lifters achieve very low masses that, at identical lobe contact diameters, are far lighter than mechanical valve lifters with shims at the top.

The sliding contact with the lobe requires careful machining at the camshaft—stone finishing following cam lobe grinding has proved to be the most favorable. Over and above this, the camshaft material has to be matched to the loading situation to avoid wear. The versions that have been found to be particularly advantageous are hard-cast camshafts and camshafts made of gray cast iron with a remelted surface. The valve lifters and shims should rotate in order to achieve uniform wear for the cam lobe contact surfaces.

This is achieved by shifting the cam in relation to the shim (toward the camshaft centerline) or with offsetting and an angular grind for the cam lobe at the point where the lobe contacts the valve lifter. Valve trains with valve lifters and the mechanical versions in particular offer the advantage of lower cylinder head height in DOHC designs. Valve lifters are found in many different applications, e.g., two- and four-valve gasoline engines and diesel engines.

Volkswagen uses a valve lifter incorporating a special hydraulic element designed to prevent increases in contact force while it sweeps the lobe’s circular segment in all of its pump-nozzle diesel engines.