Manufacturing of Automotive Piston - Essay Example

Materials and Manufacturing Report Automotive Piston Before manufacturing a piston, it is important that one selects the right piston design that suits the engine that it is being designed for. The most common designs for pistons are the flat head, dished piston and domed piston, each of which has unique properties that enable it the purpose for which it was designed. Standard engines are equipped with the flat head piston but if one wants to increase or reduce the engine’s compression ratio, then one could use the dished or domed piston, respectively. Apart from altering the compression ratio, the other factor that is considered when selecting the appropriate piston design is the required efficiency. If one wants to improve the efficiency of a piston, one should select a piston design that creates turbulence and, in so doing, improves the mixing of fuel and air. Once one has the right design ready, the next step is to select the suitable materials that will be used to manufacture the desired piston. The decision on the materials to be used largely depends on the knowledge of the conditions in which the piston will be expected to operate and also, the properties that the manufacturer requires the piston to have. A piston basically functions as a moving plug within the bore of the combustion chamber. The piston, together with the combustion chamber, cylinder head, and the piston rings seal the combustion gas and after ignition, transfers the combustion pressure to the rotating crankshaft through the connecting rod and the piston pin. In two stroke engines, the piston serves as a valve that facilitates the transfer of gasses. When the engine is running, the piston is exposed to gasses at high temperatures and is expected to reciprocate at high speeds along the bore of the cylinder. The material used to manufacture a piston should therefore have high strength because enhanced strength will enable the piston not only to withstand the high gas temperatures but to work well within the high temperatures. If the manufacture desires to design an engine with a high power output, the manufacturer needs to produce the piston using a material that is light and durable. The piston should also be designed with a material that is capable of handling the increased stress on the piston which increases due to the piston’s light weight. As the engine is switched on, the combustion process heats up the piston and rapidly expands its diameter. This expansion in diameter is likely to interfere with the engine’s operation in more ways than one. Given that the cylinder block that encloses the piston is usually designed to have a high heat capacity and is water-cooled, it heats up slowly. The difference between in the heating patterns of the piston and the cylinder block affects the clearance between the two. If the running clearance between the cylinder bore and the piston is too narrow, the piston will instantly touch the bore and if the clearance is too wide, it will cause the gasses to blow-by the piston during compression. Moreover, when the engine bore is cold, the large running clearance will produce noise as the piston knocks the bore wall. The heating characteristics of the piston and the cylinder bore emphasize the need to manufacture the piston using a material that has a low thermal expansion coefficient. As noted by Madsen (2004), in the past, cast iron pistons were mainly used to operate petrol engines because using similar materials for the cylinder bore and the piston has the advantage of reducing the negative effects of working with materials that have different thermal expansion coefficients. The choice of using cast iron was also influenced by the presence of graphite in the cast iron which enabled the piston have excellent wear resistance as a good lubricant would. Even with these advantages, the weight of cast iron made the piston unsuitable for high revolution speeds which characterize high performing engines. The heavy cast iron piston increased the load on the crankshaft and the connecting rod and therefore made it necessary for these parts to be strengthened with thicker designs. Thicker designs meant using more metal and this increased the cost of production. Moreover, cast iron has a low thermal conductivity which disrupts the dissipation of heat in turn causing the air fuel mixture to ignite spontaneously. These disadvantages of cast iron are the reasons why the material is no longer used to manufacture pistons. Presently, light aluminium alloys are widely used in the manufacture of pistons. The first aluminium alloy pistons were manufactured at the beginning of the 1900s following the invention of the electrolyte smelting technology (Hillier 1991). Pure aluminium has a low melting point of 6600C and a thermal expansion coefficient of up to 23.5×10-6/0C. In order for the piston to withstand the hot combustion gasses and expand minimally when heated, it needs to be strengthened with an element that will make it have the desirable properties. Silicon is therefore added to the aluminium to form the alloy used to make convectional pistons. Silicon reduces the thermal expansion coefficient of the aluminium because Silicon has a lower thermal expansion of 9.6×10-6/0C. Silicon adds several advantages to aluminium when alloyed with aluminium and these include lowering the density of the alloy while increasing the weight of the alloy. Pure Silicon has the density of 2.33g/cm3 while that of pure Al is 2.67 g/cm3. The other advantage is that silicon raises the alloy’s wear resistance capabilities. Additionally, the silicon-aluminium alloy has proved to be very effective in preventing seizure of the piston ring to the ring groove. The other advantages are that silicon hardens the alloy given its Vicker’s hardness value that falls in the range of 870 to 1350 HV and also, makes the alloy have some ceramic properties because silicon is a semi metal (Levy 1995). The characteristics of the aluminium-silicon alloy therefore make it an excellent material for the manufacture of pistons. Various improvements have since been made on the original aluminium-silicon alloy to produce alloy compositions that suit different engine types. For instance, the JIS-AC8A and the JIS-AC9B, which have different percentages of silicon in them, are presently mostly used to manufacture pistons for the four stroke engines and the two stroke engines, respectively (Cantor Dunne & Stone 2003). The two stroke engine generates a higher power output compared to the four stroke engines and this also implies that they are likely to reach a higher temperature during operation. The higher temperature increases the chances of the engine experiencing seizure and to prevent this, more silicon is added to the aluminium used to make the pistons for two stroke engines to decrease their thermal expansion and make them seizure resistant. Pistons in the present day are either manufactured using the forging method or the casting process. The method selected to manufacture a piston highly depends on the type of engine that the piston shall be installed into. The pistons of standard engines are usually manufactured using the casting process. The casting process is cheaper compared to forging and can be used to manufacture pistons that will withstand the normal load on non-specialized engines. Apart from being cheaper than forging, the casting process of manufacturing pistons also results in the manufacture of a piston that has a low thermal expansion rate. The expansion rate of cast pistons is low because the grain structure of the cast alloy is usually less dense compared to that in other manufacturing processes. The resulting low expansion rate piston enables the manufacturers of the engine to tighten the piston clearance clearances therefore reduce the chances of a piston rattle or piston slap occurring when the engine is cold. If the engine is designed for high performance, then the more expensive forging process is more suitable a process for the manufacture of the required high strength piston. Forged pistons tend to be stronger due to the dense grain structure which enables the piston to remain cooler than cast iron pistons during operation. The expansion rate of the forged metal largely depends on the type of alloy used but generally, the forging process results in a piston that has a higher thermal expansion rate compared to that which is manufactured by casting. As a way of reducing the relatively higher expansion rate of pistons that are forged, several alloys with a low expansion rate have been developed. Automotive pistons are usually forged in three stages that include the hot extraction and hot upsetting processes (Mishra 2013). Hot extraction forging process is used to force the melted billet through the opening on the shaped die while the hot upsetting forging process is used to enlarge and shape sections of piston bar’s cross-section using a pair of dies. These forging processes are popular in many manufacturing processes of today given the several advantages that they present including the savings on materials and energy used for production, improvement of quality, and improvement of homogeneous qualities throughout the product. Even with these advantages, these processes are complicated given that the forger has to carefully inspect and control the processes to ensure that the final product possesses the right mechanical properties. In order to minimize the effects of temperature on the material, precise analysis and simulation of the processes is required. In the recent times, several computer aided engineering software have been designed to assist forgers to control the forging process. The other popular manufacturing method for pistons is that of hypereutectic casting which results in pistons that have higher silicon content that those manufactured using the aforementioned processes. Most cast pistons are manufactured with approximately 9.5 % silicon content while those in forged pistons range from 0.1 to 10%, depending on the aluminium alloy used. As had earlier been stated in this paper, silicon is added to the aluminium to increase its strength and to provide the alloy with lubricating properties. Hypereutectic cast pistons are manufactured from alloys that contain a silicon percentage of about 16-20% (Lingenfelter 1996). Aluminium can only dissolve about 12 percent silicon which is usually added to it when it is at its molten state and then dissolved. When the 12 percent silicon is added, the silicon remains dissolved in the aluminium even when the piston has cooled. If an extra amount had been added, the extra silicon does not dissolve but settles at the bottom of the alloy. Hypereutectic casting process entails controlling and monitoring the heating and cooling of the piston and this helps to maintain the un-dissolved Si spread out throughout the piston even after the piston has cooled. The advantages of hypereutectic casting include its ability to increase the piston’s resistance against temperature fatigue therefore enabling the manufacturers to make the piston thinner than those that are produced through the casting process. The strength of the piston produced using the hypereutectic process is more than that of a cast piston but is less than that of a forged piston. The main disadvantage of manufacturing pistons using this process is that the increased percentage of silicon reduces the piston’s ability to conduct heat away from the combustion chamber which implies that the piston would operate at a higher temperature compared to a piston that has been cast. The higher temperatures could easily result in a detonation despite the piston’s increased resistance to temperature fatigue. One the manufacturing process begins, the quality assurance team needs to pay close attention to the whole process and control the quality of the piston at each of the manufacturing stages. The quality control is important because this is what will ensure that the end product meets specific chemical composition, has the right mechanical properties, and possesses the correct micro structure as had been intended. The quality control measures that are commonly applied when manufacturing pistons include carbon equivalent determination, tensile strength test, hardness determination and chemical analysis to determine the other alloying materials in the piston. References Cantor, B. Dunne, F & Stone, I (2003). Metal and Ceramic Matrix Composites. London: CRC Press Hillier, V. (1991) Fundamentals of Motor Vehicle Technology Cheltenham: Nelson Thornes Levy, A. (1995), Solid Particle Erosion and Erosion-corrosion of Materials, Ohio: ASM International. Lingenfelter, J. (1996) John Lingenfelter on Modifying Small-Block Chevy Engines: High Performance Engine Building and Tuning for Street and Racing, New York: Pinguin Madsen, D. (2004) Print Reading for Engineering and Manufacturing Technology New York: Cengage Learning Mishra B. (2013) Review of Extraction, Processing, Properties, and Applications of Reactive Metals: 1999 TMS Annual Meeting, San Diego, CA, February 28 - March 15, 1999, Pennsylvania : John Wiley & Sons Read More