Automotive Piston Usage - Essay Example

AUTOMOTIVE PSISTON By Student’s name Course code and name Professor’s name University name City, State Date of submission Introduction A connecting rod is a high performance internal combustion engine part that is meant to transmit translational motion from the piston to the crankshaft. The types of loads that connecting rods are exposed to are cyclic by nature and comprise of compressive and dynamic tensile forces. Due to these forces, connecting rods are designed to withstand high centrifugal forces and bending stresses due to pull and thrust on the engine piston. A schematic view is shown in figure 1 below to create more understanding on the features that form the structure of a connecting rod. The main objective of this activity is to carry out an analysis of the materials for purposes of design, manufacturing and assembly of the connecting rod. Improvements are also identified with respect to the procedures instilled in the production of these important parts. Figure 1: A schematic illustration of a connecting rod (Schreier, 1999). Designing a Connecting Rod A connecting rod has three main zones as shown in the above schematic illustration. These zones include; the piston pin end, the big end and the centre shank. It is evident from the illustration that the connecting rod is just but a pin jointed strut whose weight is more concentrated on the big end. Due to the nature of the environment that the connecting rod is exposed to, it is necessary to ensure that it can withstand failure due to fatigue. According to Thomas et al. (2011), it is important to note that the forces that usually act on the connecting rods are caused by gas loads and inertia forces that arise from engine operations. Fatigue is therefore applied as the main force of consideration when coming up with designs for the connecting rod –inertia forces held constant while forces due to gas loads are varied. Fatigue in a material is known to be caused by manufacturing defects, poor dimensional detailing and errors or omissions brought about by poor calculations with respect to loads encountered. Stress usually concentrates between the center shank and the small end because of huge changes in the cross sectional areas. This calls for careful design as failure statistics found in Thomas et al. (2011) show that the failure rates during the design stages lie at about 80%. While some of the design processes are elaborated in the manufacturing section, it is important to note that the design has to take into consideration the residual forces due to manufacturing. It is therefore at the design stage that the manufacturing process should be decided upon. The process parameters that are involved in the manufacturing processes have to be analyzed in order to enhance the fatigue life of the connecting rod. Modern design procedures have gone a step further to allow for design stresses in a bid to come up with improved fatigue performance between the shank and the big end. Table1 below shows the design specifications that should be achieved while coming up with the choice of material and process for connecting rods. Table 1: The design requirements of a connecting rod (Ashby, 2005) Function Engine connecting rod Constraints Must not fail due to elastic buckling Must not fail due to high cycle fatigue Stroke to determine the length Objectives Mass minimization Free variables Cross sectional area Material choice Materials Selection This process hugely involves choosing materials that match the case in the design section above. A summary of the design characteristics that are required in the connecting are given in table 1 above. The choice of material however has to fit correctly into the manufacturing process, production economies of scale, technical constraints and technical requirements. The economies of scale involve batch size and raw material cost that is used in the production process. The technical requirements are related to the weight of the connecting rod and minimum fatigue requirements for each section (Chattopadhyay, 2010). According to Ashby (2005), the potential material choices for the connecting rods include aluminium, beryllium, magnesium and titanium based on the constraints of high cycle fatigue and elastic buckling. In a study carried out by Ashby et al. (2008), materials such as high strength aluminium, magnesium and high tensile steel have been singled out due to fracture toughness and high cycle fatigue constraints. In the same study, a refined search has shown that, “aluminium reinforced with silicon carbide, boron or alumina fibres, beryllium alloys and a number of high performance carbon reinforced composites” are fit for this component. Material performance requirements therefore have to be associated with the possible modes of failure and the manufacturability. Initial material screening followed by catastrophic fracture assessment is necessary when coming up with the correct material through the elastic modulus verification. Materials with high elastic modulus are considered to withstand high levels of fatigue as compared to those with low elasticity. The manufacturing processes that are used for producing these components include die casting, drop forging and powder forging. Manufacturing Processes 1. Sand Casting Sand casting is a process of producing components in which molten metal is subjected to a mould in order to form a given shape. This method has been used since 1962 for production of connecting rods when the General Motors Company incepted the production of Buick V-6 engine. Over 50 million connecting rods were produced through this method by using the pearlitic malleable iron material of up to 428 cubic inches in volume. The difference in cross sectional requirements forced General motors’ to adapt the forging systems that were in existence during that time in order to come up with a sustainable solution. In so doing, the I-beam cross section and the general radii of the bearing were considerably increased to fit the requirements at hand (Chattopadhyay, 2010). The green sand cat connecting rods were basically annealed to 1750°F for up to 18 hours while being air cooled and then reheated to 1600°F and quenched with oil to achieve a martensitic microstructure after which the resulting product was tempered by up to 4 hours at around 1180°F. The properties of the resulting connecting rod were 100ksi for tensile yield strength, 80ksi for yield strength and 2% elasticity. This process was considered as economic and competitive due to savings that came from resultant durability and excellent machining qualities (Chattopadhyay, 2010). 2. Conventional Forging Figure 2: A layout of the production process for connecting rod using conventional forging. This process of manufacturing connecting rods has been used as a standard method of choice for a very long period of time. The forging dies used in production of connecting rods involve progressive steps that are meant to achieve the final shape from a billet which is the starting form. Each new step involves exerting a new impression to the last one to achieve the desired form, although heating is involved for purposes of straightening and ensuring weight balance is achieved. The above layout shows the processes that entail to this production process. This process leads to a waste of up to 30% of the initial material due to trimming and capping in order to achieve the ultimate goal (Chattopadhyay, 2010). Microalloyed steel is one of the most common materials that have been used in the drop forging process. This material is high in fatigue stress with a composition of 0.3 % carbon steel added with calcium, phosphorous, sulphur and vanadium. From this combination, 26% more fatigue strength has been achieved creating a competition to the old materials used in drop forging. Such properties do not need a material to be heat treated in order to achieve better qualities. Another advantage of this method is that it bestows more strength to the material without necessarily adding weight to the connecting rod. In a patent obtained by Olaniren and Stickels (1992), crackable wrought forged connecting rods’ production was made feasible through a controlled process. However, the ductile ferrite content had to be increased in order to achieve a fully perlitic microstructure for cleavage fracture at room temperature to be possible. 3. Powder Forging The process of powder forging has been utilised during the 1970s as an extension for sinter powder metallurgy and conventional press in which a densified perform is hot forged using a single blow. This process is however carried out in enclosed dies that are heated to high temperature that do not involve a flash at all. The first type of powder forging is called hot upsetting whereby the preform is exposed to a large amount of lateral material flow. The second method is hot repressing in which material densification occurs in the direction of pressing also known as hot recoining or restriking (Chattopadhyay, 2010). Figure 3: The process layout for powder forging. This process offers advantages such as the reduced machining operations and weight control capabilities. It is however sad to realize that inasmuch as this technology produces superior connecting rods, it has proven expensive from attempts made in the past. The motivation by most manufacturers to adopt it is due to the desire for lighter connecting rods with reduced harshness and vibrations. Conclusion This study successfully carries out the analysis materials, design procedure, manufacturing processes and assembly of the connecting rod. It is important to realize that the main design objectives that are usually aimed at by the manufacturers mainly regard the elasticity, high cycle failure and mass minimization as the main constraints. The processes that have been used in the production of connecting rods are sand casting, drop forging and the latest and most superior being powder forging. The environmental impacts reduce in this sequence due to increase in technological efficacy and sustainability that new technologies pose to engineering processes. List of References Ashby, M.F. (2005) Materials Selection in Mechanical Design, Oxford: Butterworth- Heinemann. Chattopadhyay, S. (2010) 'Selection of Material, Shape, and Manufacturing Process for a Connecting Rod', American Society for Engineering Education, pp. 1-12. Olaniran, M. and Stickels, C. (1992) Machinable, strong, but crackable low ductility steel forging, USPTO: Ford Motor Company:United States. Schreier, L. (1999) Tension and Compression in Connecting Rods, 26 April, [Online], Available: http://emweb.unl.edu/Mechanics-Pages/Luke- schreier/unzip/Tension%20and%20Compression%20in%20Connecting%20Rods%20VI. htm [19 April 2014]. Thomas, T.G., Srikari, S. and Suman, M.L.J. (2011) 'Design of Connecing Rod for Heavy Duty Applications Produced by Different Processes for Enhanced Fatigue Life', Sastech Journal, vol. 10, no. 1, May, pp. 1-7. Read More