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The Science and Technology of Materials in Automotive Engines Part 5

Tham khảo tài liệu 'the science and technology of materials in automotive engines part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 88 Science and technology of materials in automotive engines 2 mm c 4.1 a Piston rings for a four-stroke engine. Top and second rings two rings on the left and assembled three-piece oil control ring on the right . b Disassembled three-piece oil ring. c Magnified view of the spacer. There is also a one-piece oil ring. The piston ring 89 Cylinder Piston head Oil return hole Lubricatin oil film 4.2 Phenomena taking place around piston rings. Combustion pressure - Heat flow g second ring middle and oil control ring right . The oil control ring consists of three individual pieces two side rails and a spacer the corrugated sheet Fig. 4.1 c . Figure 4.4 shows the two rings in a two-stroke petrol engine. The second ring is shown with the expander located inside . The expander supports the second ring described later in Fig. 4.9 adding tension without a significant increase in total weight. To obtain the same tension with a one-piece ring the thickness needs to be increased which in turn makes the ring much heavier. Some diesel engines use more than three rings. In order to obtain high revolutions and quick response by reducing the weight of moving parts fewer rings are preferred.3 However for more powerful engines with high cylinder pressures such as diesels a greater number of rings is required to obtain sufficient durability in sealing. 4.2 Suitable shapes to obtain high power output Figure 4.5 illustrates a piston ring both before and after it expands into the ring groove. Figure 4.6 shows a ring installed in the ring groove. The piston with rings is inserted into the cylinder bore. The ring then expands from its initial diameter d1 and is forced tightly against the cylinder bore wall Fig. 4.5 . The ring width is called h1 and the radial wall thickness a1 Fig. 4.6 . The distance m is defined as the gap when the ring is uncompressed. The gap s1 also referred to as the closed gap or end clearance is the minimum gap obtained when the ring is installed in the cylinder bore.