Materials Selection For Automotive Exhaust System Engineering Essay

Materials Selection For self-propelling unloose System Engineering EssayThis assignment looks into the visible plectrum and manufacturing mold for an automotive play out dodge considering the operational requirements. Towards the end, the application of replacement material is also discussed.Working RequirementsThe maximum temperature in an automobile bury organisation is approximately 750-850 compass point celsius. So the selected material for manufacturing the kick out governing body should be able to handle temperatures in this range. Also there be various other corroding federal agent ins like naughty temperature oxidation, de-icing salinity befoulment and wet eroding by condensates main(prenominal)ly towards the tail pipe section.The ingest scheme foundation mainly be divided into 2 sections based on the working temperature. The hot endtemperatures above 600 degree Celsius which starts from the compound till the catalytic convertor, and the cold endtempe ratures below 600 degree Celsius which extends from the pre-damper till the tail pipe. The relinquish system is also subjected to high frequency vibrations due to peter out triggerman discharge.Normally, ferrous alloys argon lend unmatchableselfd in the manufacturing of drop off system. These involve carbon trade name, unblemished brand name, alloy trade names and couch straighten out. The purpose of adding alloying elements is to help in solid resolving strengthening of ferrite, improve the corrosion resistance and other characteristics and the cause the recklessness of alloy carbides. 5Mild carbon firebrand was extensively used for the manufacturing of turn out systems for a considerable period of time. Although mild brace has the correctties to withstand get temperature it has genuinely poor corrosion resistance. High exposure to road salt and erase condensate nates terribly shorten the life couplet of a mild brand based exhaust system. Also, over the da ys higher demands in power and environmental safety extradite seen the expiry of mild steel from exhaust systems. Nowadays mild steel is employ in applications where the environment is non-corrosive.Stainless steel has replaced mild steel in exhaust systems today. The ferrous alloying element used here is atomic number 24. The minimum bill of atomic number 24 in pure steel is 10.5%. When unstained steel is heated, chromium forms a protective layer of chromium oxide over the blameless steel surface and delays further oxidation process.The exhaust system in a vehicle is expected to be at its peak performance everlastingly and the material failure is a undesirable characteristic for this. Performance and qualification of a automobile engine is seriously affected by mild changes in the exhaust system. Therefore, it is important that fail proof and efficient materials be used in the construction of automotive exhausts.Materials used for manufacturing of exhaust systems should have the following characteristicsHigh service temperature, high fatigue strength, high split up formidability, should be easily machinable and should be economic considering the overall address of the automobile. Also, the material should be highly resistive to corrosion as the exhaust system is majorly exposed to road salts and condensates from the engine exhaust gases. Failure in the exhaust system fecal matter cause loss of back pressure which provoke significantly affect engine performance parameters.1Current trend maturation in the automobile industry has also seen the evolution of exhaust systems. From the usual carbon steel exhausts, we can see innoxious steel beingness heavily employed in exhaust systems.From the CES charts, we can see that stainless steel excels over low/medium carbon steel in all fittingties other than economy. Ferritic stainless steel is used in the counterbalance of exhaust systems. Ferritic stainless steel contains chromium as the main alloy ing element. The percentage of chromium can vary from 10.5% to 18%. These materials ar resistant to corrosion and have very low fabrication characteristics. Although, these can be improve by alloying. Grades such as 434 and 444 have improved fabrication properties. Ferritic stainless steel is everlastingly used in a annealed state as they cannot be hardened using heat treatment processes. They have vey high resistance to cracking due to stress corrosion. They also exhibit refreshing welding characteristics in thin sections.1We can see in distributor usher the different parts of the exhaust system and the material properties belowExhaust ManifoldThe exhaust manifold of a automobile engine is always exposed to hot gases. Cast iron has been in use for the business of exhaust manifolds conventionally. The main characteristics required for the exhaust manifold material include thermal fatigue strength required to withstand the high temperature exhaust gases, oxidation resist ance , good fabrication properties and low thermal contentedness to evoke the catalytic function. Ferritic stainless steel exhibit all these properties and offers lifesize angle reduction also. The developments in vacuum cast of characters process has helped in the fabrication of stainless steel manifold with section thickness of 2-5mm. higher(prenominal) demands in pollution control exit rise the exhaust temperatures in like manner and so, ferritic stainless steel will be in major use for exhaust system manufacturing. Ferritic stainless steel exhibits improves thermal fatigue characteristics when touch on by solid solution strengthening with molybdenum or niobium. This process also improves the oxidation resistance and microstructural stability. Ferritic stainless steel also has be advantages because of the absence of nickel in its composition. Another variant called the austenite stainless steel is used where ferritic stainless steel is unsuitable. Austenite stainless st eel can enhance its properties when enough carbon is added to it. But, the higher cost limits its usage compargond to the ferritic variant.3Cast iron manifolds can be manufactured by the sand casting process. The hardening of molten cast iron can result in the system of interstitial voids between the various particles in the cast iron structure. This can lead to particle sliding over each other and as a result a decline in the mechanized properties of the mould. deform relieving heat treatments are performed inorder to increase the dimensional stability of the material. During heat treatment, the thermal expansion of the material allows the voids to be filled. Although, care should be taken as not to overdo the heat treatment process as also much thermal expansion can bring about much stresses in the material microstructure and this can impair material properties. Changes in microstructure can diminish material properties like thermal fatigue and fracture toughness that can re sult in the premature failure of the finished product.The skip material that can be used in exhaust manifold manufacture is ferritic stainless steel. These materials have enhanced properties for exhaust manifold production. The main feature is the low thermal expansion that is one of the major problems approach by cast iron exhaust manifolds. Molybdenum and Niobium alloying has shown dangerous increase in the heat enhancement properties of ferritic stainless steel. Vacuum casting process that is used in the production of ferritic stainless steel manifolds is the factor that increases its cost. Effective methods need to be developed to reduce the tooling and manufacturing cost. Once this is achieved, cast iron exhaust manifolds can be replaced by stainless steel exhaust manifolds that have improved properties and weigh less and can so adhere to stringent emission norms that come in future. Nowadays stainless steel manifolds are only used in vehicles that call for ingrained perfo rmance. Commercialisation of this technology is not far away.Catalytic converterThe catalytic converter is used to clean the exhaust gases and make it free of toxicant effects. It is normally exposed to elevated temperatures like 1000 degree Celsius and therefore its durability is a very important factor in proper functioning of the exhaust system. Conventionally the catalytic converters are made of a material called the cordierite monolith that basically acts as the absorbant in it. Cordierite is a silicate and thus the portion is manufactured by an extrusion process. Then it is subjected to washcoating where the monolith is submerged in materials such as Al2O3, CeO2, ZrO2 and alloys such as platinum, rhodium and palladium. The main properties of the catalytic converter monolith are as followsFusion point 1450 degree CelsiusCoefficient of Thermal enlargement 0.002cal/s-cm-oC lowThickness of monolith walls 100 microns come in of cells 400-900 cpsiThe main advantages of the cordierite monolith are lower cost and manufacturing ease.Alternate material for the catalytic converter is stainless steel. Instead of the silicate monolith metal alloy materials are used in the construction of the monolith. These metal monoliths are manufactured by the combination of flat and corrugated layers of the metal alloy. These are then made into the spiral shaped monolith. The exhaust gases escape with the channels formed between the flat and corrugated planes. Washcoating is done in this case also. But the important thing to be remembered is that this process take to be done before the monolith is made into a spiral shape. This is because the metal monolith is primarily smaller in size and that can obstruct the proper washcoating of the entire gold-bearing monolith. The main characteristics of the metallicmonolith are as followsFusion point 1600 degree Celsius 150 degree Celsius more than cordieriteCoefficient of Thermal Expansion very highdirectly influenced by al uminium content in metal alloyThickness of monolith walls 25 microns less compared to cordieriteNumber of cells 800-1250 cpsi2One of the major advantages of the metallic monolith is the high impact resistance. As the catalytic converter is prone to impacts from road surfaces, it is important that the catalytic converter rest intact at all times. The cordierite monolith is very weak compared to metallic monolith in this case. Metallic monoliths also have the advantage of being placed sozzled to the exhaust manifold. This means that they can achieve the operating temperaturearound 300 degree Celsius earlier and thus reduce exhaust contamination. These monoliths can also be developed in smaller sizes compared to the silicate counterparts. Higher service temperature means that metallic monoliths are used in high performance applications where the conditions are very severe.Muffler The muffler is the final part in the exhaust system. The entranceway of catalytic converter has led to the formation of condensates inside the muffler thereby resulting in corrosion. Aluminized carbon steel is employed in the manufacture of the exhaust mufflers traditionally. The exhaust gas condensates have amplified the corrosion rates in the muffler and it is seen that they do not perform to their maximum service life when the catalytic converter is introduced. This normally happens in the short runs of the vehicle where the exhaust gas temperature does not rise high enough to vaporize these condensates. During long runs, the exhaust gas temperature goes over 100 degree Celsius which prevents the formation of condensates and reduces corrosion. Stainless steel mufflers have very high corrosion resistance and the addition of chromium and molybdenum can improve these properties. High production rates can be achieved by the use of stainless steel by bicycle-built-for-two mill cold rolling, annealing and descaling process. si can also be added in small traces to improve muffler prop erties in stainless steel.3Titanium exhaust systems a future perspectiveEnvironmental agencies now prefer guaranteed corrosion resistance on exhaust systems for more than 100,000 miles. Titanium achieves this and is much dampen than stainless steel systems that are used now. A typical catalytic converter and muffler pipe in stainless steel weighs around 10kg. A redesigned atomic number 22 system weighs only around 3.2 kg. This weight thrift is very beneficial when considering high performance and high precision automobiles. subtle te tube and sheet are the materials of choice for silencers, and pipes. Reduction of weight and cost are achieved by selecting the thinnest gauges of materials consistent with the engineering and acoustic parameters of the exhaust system. Titanium may not be suitable for the entire system, and will most probably be limited to components in which the temperature of metal does not go over 400 degree celsius for longer periods of time. The parts immedia tely behind the catalytic converter continue to perform well chthonian test. Use of titanium lugs welded to the pipe will certainly prove to be the most efficient method to fix the exhaust to the vehicle frame up (via rubber isolators).Material conforming to ASTM Grade 2, (e.g. Timetal 50A) offers the optimum in hurt of cost, availability, fabricability, weldability and mechanical properties. Grade 45A is slightly less strong and more fictile than 50A and may be required where extensive forming is part of the manufacturing process e.g. cast aside seaming (Table 4). Both alloys are fully weldable, require no intermediate or post-forming heat treatment, and are available in wide sheet coil. These alloys are also used to manufacture low-cost continuously welded tube.Properties of pure titanium 45A and 50A45A50AThermal expansion (10-6degree Celsius)8.98.9Hardness (HV)140-170160-200Tensile Modulus ( 103N.mm2)103103Density ( g.cm-3)4.514.51Specific Heat( J.kg-1.oC-1)519519RA min (%) 4035Titanium ProductionPure titanium is cold formable, and sheets or tube can be shaped quick at room temperature using techniques and equipment that are normally for steel. Following factors must be taken into consideration while machining titanium.The ductility of titanium is generally less than that of steel. More generous bend radii may be required during bending applications.The modulus of elasticity of titanium is about half that of steel. This means that titanium will return back to original state after forming. honorarium for this can be done by slight overforming.Titanium tends to eat away against unlubricated forming tools. Proper lubrication of the tool can solve this problem.Both silence seaming and resistance welding are suitable to join the catalytic converter, and traditional TIG is suitable to seal the ends and pipe joints with a torch trailing racing shell to the external surface of the box end joints.Production EconomyA leading exhaust system manufacturer in the United States, successfully rolled and seam welded a batch of titanium boxes at one of its production units. This was done with no extra cost when compared with the production of steel components. It was also seen that the bending units that are currently employed for the bending of steel are suitable in the case of titanium also. The only factor is the procurement cost of titanium and therefore significant efforts are necessary to bring galvanic pile tooling and manufacturing costs.3ConclusionIt can be concluded that stainless steel can replace conventional materials in automotive exhaust systems. We have to count on developments in tooling and manufacturing methods that can bring down overall costs to minimum. Stainless steel is easily the best resource in service aspects of the exhaust system. High performance materials like titanium may also come into commercial production soon. Improved thermal, mechanical and chemical properties are major factors that govern material sel ection for an exhaust system.