tailieunhanh - The Science and Technology of Materials in Automotive Engines Part 2

Tham khảo tài liệu 'the science and technology of materials in automotive engines part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The cylinder 13 The black portions in Fig. 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 ựm 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 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. 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 shows the air-cooled type and Fig. 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 14 Science and technology of materials in automotive engines 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 shows cutaway views of four different types of cylinder block .