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News Center > Knowledge |
| Piston and Piston Ring |
| 2007-09-22 09:22:38
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Piston is one of main parts of engine. Together with piston ring and piston pin, it consists of piston assembly. With cylinder head, it commonly constitutes combustion chamber and bears gas action force, and then transmits it to crankshaft by piston pin and rod. Thus whole working process of internal combustion engine is finished. Because piston will remain in high-speed, high-pressure and high-temperature bad working conditions and else smooth operation and durability of engine must be considered, piston is required to have enough intensity and stiffness, good heat conductivity, good head resistance, small expansion coefficient, small relative density (low weight), wear resistance, corrosion resistance and low cost. Due to so many requirements and some of them conflicted each other, it’s difficult to find a piston material that can meet all requirements. Now engine piston is commonly made of aluminum alloy featured by small density and good heat conductivity. However aluminum alloy have lots of disadvantages, such as large expansion coefficient and poor intensity at high temperature, so it can only be made up by rational structure design. Hence, the measurement for a good engine depends on both its materials and design rationality.
Piston head refers to the top of piston and ring groove. Piston top depends on the situation of combustion chamber totally. It utilizes flat topping or near flat topping design in favor of reducing contact area between piston and high-temperature air and makes stress distributed evenly. Most of gasoline engines use this kind of flat topping piston. Some engines improve combustion efficiency and reduce deflagration to the minimum extent in order to produce mixed gas, so piston top needs a complex shape in design. A dent can be designed as a part of combustion chamber. The piston groove can be called ring groove that is used in piston ring installation. The function of piston ring is sealing, namely guarding against leakage and preventing oil entering into combustion chamber.
Piston skirt refers to piston bottom, whose function is to keep piston vertical in cyclic motion. It’s also a guide to piston. Its shape is very special, especially for those light passenger cars. Designers often take structure and performance of engine into consideration and pay much attention to piston skirt so as to make engine compact in structure and smooth in operation. Piston pin boss is a holding part, by which, piston can connect rod with piston pin. It situates at the top of piston skirt. The advantage of piston pin boss of high-speed engine lies in that the pinhole might not be in the same center line plane with piston. It can deflect to some side a little bit, namely the direction of power stroke piston contacting cylinder wall. After piston reaching TDC and changing direction, the knocking to cylinder wall by piston will be reduced and engine noise will be improved, too.
There are thousands of parts in a car, such as big crankshaft, gearbox and small spring pad, bolt and nut. Each one has its parts. For example, piston ring-"little thing," from the shape, seems simple, light, and cheap, but its role is significant. Without it, vehicle can not move. If there is little wrong with it, vehicle will be out of run, maybe heavy on oil, maybe shortage of power. In fitting place of cylinder and piston unit, it’s piston that directly connects with cylinder wall and fills up the gap between them as well as makes combustion chamber sealed. Therefore, it is also the most easily worn-out engine part.
Generally, piston ring is composed of cast iron, flexible and multiple in shapes in cross-section. Its surface bone coating can increase running-in performance. When engine running, piston is expandable. So piston ring should have gap clearances. In order to stop leakage, the gap clearances of each piston ring should be staggered. There are 3~4 piston rings in one piston and they can be divided into compression ring and oil ring. Compression ring is mounted in the groove on the top of piston, which is used for stopping leakage, passing heat on piston head to cylinder wall and dispersing heat in piston. The function of oil ring is to prevent lubricants entering into combustion chamber and scraping excess lubricants back to oil pan, which is mounted in bottom of square groove. On the premise of ensuring sealing, the fewer the piston number is, the better the effect will. Fewer piston numbers will keep the minimum rubbing area, reduce power loss, and shorten piston’s height, as well as engine’s height. Now, high-speed gasoline engine generally has two compression rings and one oil ring. |
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