[汽车构造发动机中英文课件5.ppt

Chapter 5 pistons, 活塞环\活塞销) The piston Function: piston transfers to the crankshaft the force created by the burning fuel mixture . This force turns the crankshaft thin, circular, steel bands fit into grooves around the piston to seal(密封) the bottom of the combustion chamber. These bands are called piston rings. The grooves into which they fit are called ring grooves. Requirements: (1) be strong enough (2) as light as possible Composition : (1)head : is top part of piston; (2)skirt :the thin part of the piston;(3)ring lands :most pistons have three ring grooves (环槽)at the top. The section between the ring groves are called ring lands. Material: most pistons are made from cast aluminum. However, aluminum pistons expand when they get hot. This can cause problems. The fit between the piston and the cylinder is important. If the piston does not fit snugly((贴身,贴紧),, it will leak and be noisy. Pressure,oil,and gas may slip around its sides. If the piston fits too snugly,it will not slide freely in the cylinder. While the piston is cold when the engine first starts running, it soon heats up. Therefore, the fit of the piston within the cylinder must work at two temperature extremes. Tapered(锥形的) and Cam-ground (以凸轮为基础制造的,即椭圆形的)pistons See fig5-1 The hotter a metal becomes,the more it expands .thus, the top of the piston expands more than the bottom. To ensure a good piston fit,the piston is tapered.that is , the top is a few thousandths of an inch (a few hundredths of a millimeter) smaller than the bottom.as a result, the top is the same size as the bottom only during engine operation. See fig 5-2 Diameter of the piston must be changed. Because as the piston heats up, it expands more where it is thick than where it is thin. You saw that the piston is thick through the piston boss area and thin at other points. A cold piston is slightly oval(橄榄形),or elliptical(椭圆形) When the piston heats up, the differing rates of expansion of its parts will cause the piston to