Lubricating Oil Management. Engineering Design Details

Motor oil under pressure, used to lubricate the cylinder head, is generally delivered by the oil pump in the engine block, through transfer ports in the head gasket. The oil passes through lateral bores or special supplementary lines to the points served, such as the camshaft bearings, hydraulic valve lifters, hydraulic valve play compensating elements, camshaft shifters, or oil spray nozzles above the cams.

The pressurized motor oil supply for the cylinder head is managed by the cross sections of the supply tubing and specially provided choke points to keep the oil volume to an absolute minimum. To keep the hydraulic valve play adjusters and the camshaft shifters from running dry, check valves are provided in the lines supplying these elements.

Multivalve cylinder heads, because of their greater number of lubrication points, are more difficult to coordinate and involve greater oil requirements. A more powerful oil pump is often required where camshaft shifters are employed. In spite of this, it has been possible in recent years to keep the total oil volume, even in multivalve engines, within reasonable limits. This goal was met with higher precision in machining to minimize play, through more precise tuning of the oil circuit and through technical calculations.

The oil flows back to the sump through return bores of appropriate size, located between the cylinder head and the engine block. These returns are situated at the lowest possible point, which depends in part on the engine’s attitude when mounted in the engine compartment.

The rotation of the camshafts in some cases slings the oil so severely that it foams. Accordingly, sufficient cross sections are also provided in the area below the camshafts to ensure draining toward the engine block. Particularly in boxer or V-block engines, it is necessary, because of the installation attitude for the cylinder head, to engineer the design to ensure sufficiently large drain cross sections.

Engineering Design Details. The cylinder head bolts are normally bolts with collars. Here the collar, because of the surface pressure to be transferred between the bolt contact surface area and the cylinder head, is broader than the bolt head itself. In monolithic cylinder heads, this can impose limitations on the camshaft arrangement.

The diameter of the tool used to tighten the bolts or the outside diameters of the bolts themselves thus determines the locations of the camshafts if the latter are to remain in place inside the cylinder head while the cylinders are being installed. In some cases the cylinder heads are made in two or more sections, and the valve timing elements are borne by one or two separate cast components. In this case the design of the lower cylinder head section is simpler, as is the casting technology. Because of cost considerations monolithic cylinder heads are used in the majority of all passenger car engines.

Depending on the combustion process, appropriate space must be provided in the cylinder head to accommodate spark plugs, glow plugs, or injection nozzles and the diameters of the tools used to install and remove them. Wherever possible, spark plugs should be selected that use commonplace thread diameters and wrench sizes. In diesel engines or direct-injection gasoline engines, the arrangement of the cylinder head components is tight, particularly where a multivalve concept is used. It is for these reasons that the number of valves per combustion chamber is limited to four.

The space required for these components can be modeled by assigning parameters using 3-D CAD when defining the basic layout for the cylinder head. This makes it easy to depict potential geometric arrangements. The wall thicknesses required around these components in the rough cylinder head casting reduce the overall installation space for the valve assembly or the camshafts. The cross sections required for cooling are also limited in the same fashion.

Modern multivalve engines incorporate camshaft shifters in the cylinder head. The systems in mass production are all located on the camshaft drive and are driven by the crankshaft by either a timing belt or a timing chain. Suitable oil supply lines have to be built into the cylinder head to serve the shifter. This is simpler when a cylinder head is developed from scratch. The space required by the shifter is not particularly great for the vane-type system normally used today. With these shifters the shift angle for the camshafts can be rotated steplessly in relation to the crankshaft.

The diameters of the camshaft drive gears determine the minimum clearance between the camshafts. Particularly where the camshafts are driven directly by the crankshaft, this distance has great influence on the cylinder head design. Often, and in multivalve engines, too, the camshafts are driven by intermediate gearing. When using camshaft shifters, however, drive directly at the end of the cylinder head is the most economical.

In this camshaft drive concept the clearance between the camshafts is of appropriate size or an intermediate gear is used between the crankshaft and the camshaft. The most widely used arrangement is with the camshaft drive at the forward end of the engine, i.e., at the end opposite the clutch. Drives centered between the cylinders are seldom used in passenger car engines, while they are being seen more frequently in motorcycle engines. Drives at the clutch end of the engine are also unusual.