TAILIEUCHUNG - Motorcycle Mechanics 2009 Part 7

Tham khảo tài liệu 'motorcycle mechanics 2009 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Unit 10 Four-Stroke Upper End spark plug is fired just before the piston reaching its top position top dead center or TDC . By the time the piston reaches TDC the fuel charge is burning well enough to insure complete and effective combustion on the power stroke Fig. 10-1 . Power Stroke. The power stroke is the one phase when the engine is generating power. This power results from the violently burning air-fuel mixture which forces the piston down the bore moving the rod that applys this force to the crankshaft Fig. 10-1 . Exhaust Stroke. After the power stroke the cylinder is filled with burned gases that must be expelled somehow. Another valve is opened to permit the exhuast gases to exit as the piston comes up on its last stroke in 4-phase cycle Fig. 10-If . The piston now begins another set of strokes so it can get to its next power stroke and do its job. Top-End Parts The cylinder is sometimes called the barrel or jug and it is bolted to the crankcase section. Most cylinders are made of aluminum fins with replaceable steel or iron liners in which the piston travels. Some cylinders are simply made of cast iron but these are a bit heavy. Other cylinders are made of aluminum with a chrome-plated bore but the chrome flakes off. This type is not easily rebored Fig. 10-2 . Cylinders are attached to the lower end cases either by long studs that also hold the cylinder head or by short studs that are attached to a flange around the bottom of the cylinder Fig. 10-3 . Either system seems to work but if the long studs are overtightened cylinder distortion and loss of power may result. Flanged Barrel Fig. 10-3 Typical cylinder hold-downs. Long-Stud Barrel Piston. In nearly all motorcycles on the road today the piston is an aluminum casting. A few of the high performance engines do have stronger forged aluminum pistons. Pistons are nearly perfectly round being only slightly oblong to compensate for different expansion rates at different places on the piston. The dimension .

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