The cylinder must maintain an accurate roundness and straightness of the
order of μm during operation. The cylinder bore wall typically experiences local wear at the top-dead-center point, where the oil film is most likely to fail, and scratching along the direction of travel of the piston. Figure 2.5 shows vertical scratching caused by scuffing. The grooves caused by scratching increase oil consumption and blow-by. In extreme cases, the piston seizes to the bore wall. The demand for higher output with improved exhaust gas emission has recently increased heat load to the cylinder even more. A much lighter weight design is also required.An engine generating high power output requires more cooling, since it generates more heat. Automotive engines have two types of cooling systems, air-cooled and water-cooled. Figure 2.1 shows the air-cooled type and Fig.2.2 the water-cooled type. Whilst an air-cooled engine may use a much simpler structure because it does not use the water-cooled system, the heat management of the cylinder block is not as easy. As a result, most automotive engines nowadays use water-cooled systems. It would be no exaggeration to say that the required cooling level for an individual engine determines its cylinder structure.
Figure 2.6 shows cutaway views of four different types of cylinder block structure. The monolithic or quasi-monolithic block (on the right) is made ofonly one material. It is also called a linerless block because it does not contain liners (described later). The bore wall consists of either the same material as the block or a modified surface such as plating to improve wear resistance. It is normally difficult for one material to fulfill the various needs listed in Fig. 2.4. However a liner-less design in multi-bore engines can make the engine more compact by decreasing inter-bore spacing.
The other designs in Fig. 2.6 (on the left) incorporate separate liners. A liner is also called a sleeve. A wet liner is directly exposed to coolant at the outer surface so that heat directly dissipates into the coolant. To withstand combustion pressure and heat without the added support of the cylinder block, it must be made thicker than a dry liner. A wet liner normally has a flange at the top. When the cylinder head is installed, the clamping action pushes the liner into position. The cylinder head gasket keeps the top of the liner from leaking. A rubber or copper O-ring is used at the bottom, and sometimes at the top, of a wet liner to prevent coolant from leaking into the crankcase. A dry liner presses or shrinks into a cylinder that has already been bored. Compared to the wet liner, this liner is relatively thin and is not exposed to the coolant. The cast-in liner design encloses the liner during the casting process of an entire cylinder block.
Table 2.1 lists various types of cylinder structures, their processing and characteristics. Cylinder blocks are normally made of cast iron or aluminum alloy. The aluminum block is much lighter. Various types of materials are combined to increase strength. In the following sections, we will look at the blocks of four-stroke engines. Those for two-stroke engines are discussed in the final section.
The science and technology of materials in automotive engines
Hiroshi Yamagata
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
WOODHEAD PUBLISHING LIMITED
Cambridge England
for more details and updates about automotive-technology-guide please visit.........
www.automotive-technology-guide.com
---or---
