Basic engine construction varies little, regardless of the size and design of the engine. The intended use of an engine must be considered before the design and size can be determined. The temperature at which an engine will operate has a great deal to do with the metals used in its construction.
The problem of obtaining servicing procedures and service parts in the field are simplified by the categorization of engines into families based on construction and design. Because many kinds of engines are needed for many different jobs, engines are designed to have closely related cylinder sizes, valve arrangements, and so forth. As an example, the GeneralMotors series 71 engines may have two, three, four, or six cylinders. However, they are designed so that the same pistons, connecting rods, bearings, valves and valve operating mechanisms can be used in all four engines.
Engine construction, in this chapter, will be brokendown into two categories: stationary parts and moving parts.
The stationary parts of an engine include the cylinder block, cylinders, cylinder head or heads, crankcase, and the exhaust and intake manifolds. These parts furnish the framework of the engine. All movable parts are attached to or fitted into this framework.
Engine Cylinder Block
The engine cylinder block is the basic frame of a liquid-cooled engine, whether it is the in-line, horizontally opposed, or V-type. The cylinder block and crankcase are often cast in one piece that is the heaviest single piece of metal in the engine. (See fig. 12-9.) In small engines, where weight is an important consideration, the crankcase may be cast separately. In most large diesel engines, such as those used in power plants, the crankcase is cast separately and is attached to a heavy stationary engine base.
In practically all automotive and construction equipment, the cylinder block and crankcase are cast in one piece. In this course we are concerned primarily with liquid-cooled engines of this type.
The cylinders of a liquid-cooled engine are surrounded by jackets through which the cooling liquid circulates. These jackets are cast integrally with the cylinder block. Communicating passages permit the coolant to circulate around the cylinders and through the head.
The air-cooled engine cylinder differs from that of a liquid-cooled engine in that the cylinders are made individually, rather than cast in block. The cylinders of air-cooled engines have closely spaced fins surrounding the barrel; these fins provide an increased surface area from which heat can be dissipated. This engine design is in contrast to that of the liquid-cooled engine, which has a water jacket around its cylinders.
Figure 12 - 9 Cylinder Block and components
Cylinder Block Construction
The cylinder block is cast from gray iron or iron alloyed with other metals such as nickel, chromium, or molybdenum. Some lightweight engine blocks are made from aluminum.
Cylinders are machined by grinding or boring to give them the desired true inner surface. During normal engine operation, cylinder walls will wear out-of-round, or they may become cracked and scored if not properly lubricated or cooled. Liners (sleeves) made of metal alloys resistant to wear are used in many gasoline engines and practically all diesel engines to lessen wear. After they have been worn beyond the maximum oversize, the liners can be replaced individually, which permits the use of standard pistons and rings. Thus, you can avoid replacing the entire cylinder block
The liners are inserted into a hole in the block with either a PRESS FIT or a SLIP FIT. Liners are further designated as either a WET-TYPE or DRY-TYPE. The wet-type liner comes in direct contact with the coolant and is sealed at the top by a metallic sealing ring and at the bottom by a rubber sealing ring. The dry-type liner does not contact the coolant.
Engine blocks for L-head engines contain the passageways for the valves and valve ports. The lower part of the block (crankcase) supports the crankshaft (the main bearings and bearing caps) and provides a place to which the oil pan can be fastened.
The camshaft is supported in the cylinder block by bushings that fit into machined holes in the block. On L-head in-line engines, the intake and exhaust manifolds are attached to the side of the cylinder block. On L-head V-8 engines, the intake manifold is located between the two banks of cylinders; these engines have two exhaust manifolds, one on the outside of each bank.
The cylinder head provides the combustion chambers for the engine cylinders. It is built to conform to the arrangement of the valves: L-head, I-head, or other.
In the water-cooled engine, the cylinder head (fig. 12-10) is bolted to the top of the cylinder block to close the upper end of the cylinders. It contains passages,
Figure 12-10-Cylinder head for L-head engine.
Figure 12-11.—Intake and exhaust manifolds.
matching those of the cylinder block, that allow the cooling water to circulate in the head. The head also helps keep compression in the cylinders. The gasoline engine contains tapped holes in the cylinder head that lead into the combustion chamber. The spark plugs are inserted into these tapped holes.
In the diesel engine the cylinder head may be cast in a single unit, or it may be cast for a single cylinder or two or more cylinders. Separated head sections (usually covering one, two, or three cylinders in large engines) are easy to handle and can be removed.
The L-head type of cylinder head shown in figure 12-10 is a comparatively simple casting. It contains water jackets for cooling, openings for spark plugs, and pockets into which the valves operate. Each pocket serves as a part of the combustion chamber. The fuel-air mixture is compressed in the pocket as the piston reaches the end of the compression stroke. Note that the pockets have a rather complex curved surface. This shape has been carefully designed so that the fuel-air mixture, compressed, will be subjected to violent turbulence. This turbulence ensures uniform mixing of the fuel and air, thus improving the combustion process.
The I-head (overhead-valve) type of cylinder head contains not only valve and combustion chamber pockets and water jackets for cooling spark-plug openings, but it also contains and supports the valves and valve-operating mechanisms. In this type of cylinder head, the water jackets must be large enough to cool not only the top of the combustion chamber but also the valve seats, valves, and valve-operating mechanisms.
The crankcase is that part of the engine block below the cylinders. It supports and encloses the crankshaft and provides a reservoir for the lubricating oil. Often times the crankcase contains a place for mounting the oil pump, oil filter, starting motor, and generator. The lower part of the crankcase is the OIL PAN, which is bolted at the bottom. The oil pan is made of pressed or cast steel and holds from 4 to 9 quarts of oil, depending on the engine design.
The crankcase also has mounting brackets that support the entire engine on the vehicle frame. These brackets are either an integral part of the crankcase or are bolted to it so that they support the engine at three or four points. These points of contact usually are cushioned with rubber that insulates the frame and the body of the vehicle from engine vibration and therefore prevents damage to the engine supports and the transmission.