In the first segment of this four part (at least) series, we covered the components which make-up the short block of the internal combustion engine. Add to the short block assembly the top-end, which will be covered in this segment, and you will have the long block assembly. Essentially the top-end is comprised of the cylinder head, or heads, with the valve train and the camshaft/s. We will break down the internal components of the cylinder head assembly, as well as explain two different cylinder head designs; the overhead valve and the overhead cam. We will also briefly explain camshaft timing and its relationship to the four-stroke engine strategy.
Before explaining the difference in overhead cam and overhead valve engines, we must gain an understanding of the purpose of the camshaft. The camshaft, like the crankshaft, rotates in the engine block or cylinder head/s, in a series of journals which have been precisely line bored. Bearings are then pressed into the bored holes but we will explain the function of the bearings in the third segment, entitled Free Automotive Guide: Internal Combustion Engine-Lubrication System. The camshaft or camshafts as the case may be, is manufactured from steel or cast iron. It traverses from the front to the rear of the engine block and is made with a set number of lobes, precision machined to a specific height and shape. The exact height of each intake and exhaust lobe is matched on all of the cylinders and is measured as “lift.” The shape of each lobe is known as the “duration.” The lift determines how far a valve will be opened and the duration determines how long that it will remain open, before it is allowed to close. The amounts of fuel and air allowed to enter the cylinder during the intake stroke, as well as the rate at which the spent gases are allowed to escape during the exhaust stroke, are determined by the camshaft lift and duration. Usually there are two lobes per cylinder; the first is used to open and close an intake valve and the other operates the exhaust valve in like manner.
The camshaft timing is controlled by a timing chain/s or belt/s which is carefully positioned using gears or sprockets. A particular gear tooth on the camshaft is aligned with a precise gear tooth on the crankshaft to properly “time” the engine. Once the engine is started this chain or belt will keep the camshaft and crankshaft in sync. The crankshaft in a four-stroke automobile engine will accomplish two rotations per every single rotation of the camshaft in order to perform an intake stroke, compression stroke, power stroke, and an exhaust stroke. The valves are only opened during the intake stroke and the exhaust stroke.
The overhead valve engine is most commonly used in today’s rear-wheel drive cars and trucks, such as the Ford F-Series and Chevrolet Silverado and Tahoe lines. Not so many years ago, automobile and light truck production utilized the overhead valve design almost exclusively. The overhead valve design uses hydraulic or solid lifters with pushrods and rocker arms. In a V-type engine block, the camshaft is located near the middle of the engine and is chain driven by the crankshaft. Directly above the camshaft there are lifter galleys, wherein the lifters are positioned. The flat, solid end of each lifter sits upon its corresponding camshaft lobe, with the hydraulic end facing upward. Into the hydraulic end, one end of a pushrod is placed. The top end of the push rod is placed into a rounded recess in the rocker arm, which pivots on the surface of the cylinder head. The pushrod is hollow and rounded on each end.
Overhead valve engine cylinder heads are cast from either iron or aluminum. They are designed with a series of pivots on the top, upon which the valve rocker arms are attached. Either a stud and nut or bolt, with a pivot ball is used to actuate each valve, individually. On the bottom surface, the appropriate number of recessed areas forms the compression chambers; the remainder of the bottom surface must be extremely flat so that a sufficient seal can be formed to accommodate the demands of high-compression. Typically cylinder head surface and engine block deck height tolerances of only a few thousandths of an inch in variation are acceptable.
Very precise holes are drilled through the cylinder heads from top to bottom; one individual hole for each valve. The valves resemble a flower with a long stem and petals at the top; except that the valve has a distinctively shaped precision designed and manufactured head at the end. The precision head of the valve forms a tight seal with the valve seat which is cut into the combustion chamber which is in a recessed area on the bottom surface of the head. The stem faces upward, extending through the hole and protruding from the upper surface of the cylinder head. Upon the protruding valve stem, a sturdy valve spring and retainer are affixed. This spring, or group of springs, forces the valve to close as the camshaft allows. Two valves per cylinder is the most common design; one for the intake stroke and the other one for the exhaust stroke.
The cylinder head/s are bolted directly to the engine block using large bolts or studs, which must be tightened evenly to a specified degree of torque. The cylinder head is positioned on top of the engine block so that the valves for each particular cylinder, facing downward, are directly over the cylinder and piston. As the camshaft is rotated the lobes force the lifters gradually upward (according to lift and duration). This action forces the pushrods upward, pivoting the rocker arms upward on the inboard tip and downward on the outboard tip. This opens the valve in correct timing. As the lobe on the camshaft continues to rotate, the valve spring snaps the valve closed and makes the combustion chamber ready for the compression or power stroke. This process is repeated rapidly for each engine revolution. The average engine rotates at approximately 650 revolutions per minute (RPMs) at idle and can achieve 5,000 RPMs with normal daily operation.
Overhead camshaft engine designs have been used extensively in imported automobiles since the 1950s. The popularity of this particular design has grown over the last 20-years and is now widely used in front-wheel drive vehicles and especially imported cars. More often than not, overhead camshaft engines are configured using a timing belt, with a spring-loaded or hydraulic tensioner assembly. Although the trend towards integrated timing chains, which require far less diligent maintenance, is ever increasing particularly among high-line automakers, such as Honda, Toyota, and Nissan.
Single and dual overhead camshaft cylinder heads are, as the name implies, configured with the camshaft/s located in the top of the cylinder head. There are no pushrods required in this type of engine; the camshaft rotates in journals which are machined into the top of the cylinder head, instead of the engine block. In the overhead cam engine, valves, valve seats, and valve springs operate in much the same manner as the overhead valve engine, mentioned above. Typically single overhead cam cylinder heads are made from aluminum or cast iron and they use two valves per cylinder. Dual overhead cam cylinder heads normally utilize a four-valve-per-cylinder configuration. In this design camshaft lobes directly contact lifter or valve lash adjuster surfaces to open the valves at the appropriate time. The valves are then closed using single or multiple valve springs on each valve.
In both cylinder head designs, oil is pumped up from the sump and onto valve springs and camshaft journals to keep valve train components cool and lubricated.