2

The cylinder

2.1 Structures and functions

The cylinder block is the basic framework of a car engine. It supports and holds all the other engine components. Figure 2.1 shows a typical cylinder block without an integrated crankcase. Figure 2.2 shows the block with the upper part of the crankcase included. Figure 2.31 schematically illustrates the relative positions of the cylinder, piston and piston ring. The cylinder is a large hole machined in the cylinder block, surrounded by the cylinder wall. The piston rapidly travels back and forth in the cylinder under combustion pressure. The cylinder wall guides the moving piston, receives the combustion pressure, and conveys combustion heat outside the engine. Figure 2.4 gives an analysis of the materials needed for a cylinder with high output power and summarizes the reasons why a specific material or technology is chosen to fulfil a required function. A more detailed description is given in Appendix B.

2.1 Air-cooled block. 2.2 Cast iron cylinder block (closed deck type) including a crankcase portion. 2.3 Tribological system around a cylinder bore (black portions). These are: the running surfaces between the piston pin and piston boss, between the cylinder bore and piston, and the piston ring groove and piston ring. 2.4 Functions of engine cylinders for high output power.

The black portions in Fig. 2.3 indicate the areas that are most exposed to friction. These parts need to be carefully designed not only from the viewpoint of lubrication but also tribology, as this has a significant influence on engine performance. Tribology can be defined as the science and technology of interacting surfaces in relative motion, and includes the study of friction, wear and lubrication. Combustion heat discharges at a very high rate and, if not diffused, the raised temperature can lead to tribological problems.

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.

2.5 Scuffing at bore surface; piston travelling vertically.

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 of only 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.

2.6 Bore designs in engine blocks.

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.

Table 2.1

Cylinder structures

Monolithic (linerless) (1) Cast iron integrated type. Monolithic block (typically, JIS-FC 200) with sand casting. The water passage is formed using expendable shell core. Laser or induction hardening is sometimes used on the bore surface to give durability. Low cost but heavy. Heterogeneous (dry liner) (2) Cast iron block enclosing cast iron liner. High-P cast iron liner is slip-fitted in JIS-FC200 block. Hard liner gives durability. Heterogeneous (cast-in liner) (3) Aluminum block enclosing cast iron liner. Liner is enclosed in block (typically, JIS-ADC12 die casting, JIS-AC4B shell molding) by casting-in with various casting methods. Better cooling performance than type (1). Heterogeneous (cast-in liner) (4) Aluminum block enclosing PM-aluminum liner. PM aluminum liner is enclosed in block (typically, JIS-ADC12 diecasting) by casting-in with high-pressure die casting. Better cooling performance than type (3). Heterogeneous (dry liner) (5) Aluminum block enclosing cast iron or hyper-eutectic Al-Si liner with press-fitting. Liner is inserted in block (typically, JIS-ADC12 die casting, JIS-AC4B shell molding) by press-fitting or shrunk-in. Accurate roundness at elevated temperatures. Quasi-monolithic (linerless) (6) Aluminum block with plated bore surface. Monolithic block having a coated bore by porous-Cr or Ni-SiC plating. The block material is typically JIS-AC4B shell molding or JIS-ADC12 high-pressure die casting. High cooling performance. Bore pitch can be shortened in multi-bore engines. Quasi-monolithic (linerless) (7) Aluminum block with metal-sprayed bore surface. Wire explosion or plasma spraying (steel base alloy) on the aluminum bore wall. Cooling performance is the same as (6). Monolithic (linerless) (8) Hyper-eutectic Al-Si block without coating. Low-pressure die casting using A390 alloy. The bore surface is either etched or mechanically polished to expose Si. The wear-resistant coating is necessary on the piston side. Quasi-monolithic (linerless) (9) Fiber or particle rein forced Al alloy composite. Preform of fibers (Saphire + carbon) or Si particle is cast into aluminum by squeeze die casting. The rigidity of the cylinder bore increases.

2.2 The cast iron monolithic block

The use of cast iron blocks in Table 2.1 has been widespread due to low cost as well as formability. Figure 2.2 shows a V6 block used for a car engine. The block is normally the integral type where the cylinders and upper crankcase are all one part. The cylinders are large holes that are machined into the block. The iron for the block is usually gray cast iron having a pearlite-microstructure, typically being JIS-FC200 (Table 2.2). The microstructure is shown in Fig. 2.7. Gray cast iron is so called because its fracture has a gray appearance. Ferrite in the microstructure of the bore wall should be avoided because too much soft ferrite tends to cause scratching, thus increasing blow-by.

Table 2.2

Chemical compositions (%). JIS-FC200 is a flake graphite cast iron having a strength of 200 MPa. JIS-AC4B and ADC12 are aluminum alloy for castings. A 390 is a hyper-eutectic Al-Si...