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Basic automobile design

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automobile design

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Basic automobile design

  1. 1. Basic Automobile Design Prepared by , Chirag Bhangale
  2. 2. Syllabus 1.Introduction and Design Consideration 2.Design of Piston. 3.Design of Connecting Rod. 4.Design of Crank shaft
  3. 3. Introduction and Design Consideration
  4. 4. Introduction and Design Consideration  General Considerations in Machine Design 1.Type of load and stresses caused by the load. The load, on a machine component, may act in several ways due to which the internal stresses are set up. 2. Motion of the parts or kinematics of the machine. The successful operation of any machine depends largely upon the simplest arrangement of the parts which will give the motion required. The motion of the parts may be : (a) Rectilinear motion which includes unidirectional and reciprocating motions. (b) Curvilinear motion which includes rotary, oscillatory and simple harmonic. (c) Constant velocity. (d) Constant or variable acceleration.
  5. 5. 3. Selection of materials.  It is essential that a designer should have a thorough knowledge of the properties of the materials and their behavior under working conditions.  Some of the important characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity, etc 4. Form and size of the parts.  The form and size are based on judgment. The smallest practicable cross-section may be used, but it may be checked that the stresses induced in the designed cross-section are reasonably safe.  In order to design any machine part for form and size, it is necessary to know the forces which the part must sustain. It is also important to anticipate any suddenly applied or impact load which may cause failure.
  6. 6. 5. Frictional resistance and lubrication.  There is always a loss of power due to frictional resistance and it should be noted that the friction of starting is higher than that of running friction.  It is, therefore, essential that a careful attention must be given to the matter of lubrication of all surfaces which move in contact with others, whether in rotating, sliding, or rolling bearings. 6. Convenient and economical features.  In designing, the operating features of the machine should be carefully studied.  The starting, controlling and stopping levers should be located on the basis of convenient handling.  The adjustment for wear must be provided employing the various takeup devices and arranging them so that the alignment of parts is preserved.
  7. 7. 7. Use of standard parts.  The use of standard parts is closely related to cost, because the cost of standard or stock parts is only a fraction of the cost of similar parts made to order. 8. Safety of operation.  Some machines are dangerous to operate, especially those which are speeded up to insure production at a maximum rate.  Therefore, any moving part of a machine which is within the zone of a worker is considered an accident hazard and may be the cause of an injury.  Its, therefore, necessary that a designer should always provide safety devices for the safety of the operator.  The safety appliances should in no way interfere with operation of the machine.
  8. 8. 9. Workshop facilities.  A design engineer should be familiar with the limitations of his employer’s workshop, in order to avoid the necessity of having work done in some other workshop.  It is sometimes necessary to plan and supervise the workshop operations and to draft methods for casting, handling and machining special parts. 10. Number of machines to be manufactured.  The number of articles or machines to be manufactured affects the design in a number of ways.  If only a few articles are to be made, extra expenses are not justified unless the machine is large or ofsome special design.  An order calling for small number of the product will not permit any undue expense in the workshop processes, so that the designer should restrict his specification to standard parts as much as possible.
  9. 9. 11. Cost of construction.  The cost of construction of an article is the most important consideration involved in design.  In some cases, it is quite possible that the high cost of an article may immediately bar it from further considerations.. 12. Assembling.  Every machine or structure must be assembled as a unit before it can function.  Large units must often be assembled in the shop, tested and then taken to be transported to their place of service.  The final location of any machine is important and the design engineer must anticipate the exact location and the local facilities for erection.
  10. 10. General Procedure in Machine Design: the general procedure to solve a design problem is as follows : 1. Recognition of need.  First of all, make a complete statement of the problem, indicating the need, aim or purpose for which the machine is to be designed. 2. Synthesis (Mechanisms).  Select the possible mechanism or group of mechanisms which will give the desired motion. 3. Analysis of forces.  Find the forces acting on each memberof the machine and the energy transmitted by each member. 4. Material selection.  Select the material best suited for each member of the machine.
  11. 11. 5. Design of elements (Size and Stresses).  Find the size of each member of the machine by considering the force acting on the member and the permissible stresses for the material used.  It should be kept in mind that each member should not deflect or deform than the permissible limit. 6. Modification.  Modify the size of the member to agree with the past experience and judgment to facilitate manufacture.  The modification may also be necessary by consideration of manufacturing to reduce overall cost. 7. Detailed drawing.  Draw the detailed drawing of each component and the assembly of the machine with complete specification for the manufacturing processes suggested 8. Production.  The component, as per the drawing, is manufactured in the workshop
  12. 12. Requirement Model (Rough idea) Creation How a design is born marketability Availability of FUNDS Available material Manufacturing resources Analysis Market survey Aesthetic Ease of handling Safety Economical Recyclability Force/stress Material/s used Sizes
  13. 13. Mechanical properties: • STRENGTH – resist externally applied loads without breaking or yielding • STIFFNESS – resist deformation under stress • ELASTICITY – regain original shape once the force is removed • PLASTICITY – property which retains deformation (required forging etc) • DUCTILITY – ability to be drawn into a wire by a tensile force • BRITTLENESS – sudden breaking with minimum distortion • TOUGHNESS – resist fracture due to high impact load • CREEP – deformation under stress and high temperature • FATIGUE – ability to withstand cyclic stresses • HARDNESS – resistance to wear, scratching, deformation, machinability etc
  14. 14. UNIT 2 Design of Piston
  15. 15. Design different parts of piston  Piston Head or Crown  Piston Rings  Piston Barrel  Piston Skirt  Piston Pin
  16. 16. PISTON  Piston is considered to be one of the most important parts in a reciprocating engine in which it helps to convert the chemical energy obtained by the combustion of fuel into useful (work) mechanical power.  The purpose of the piston is to provide a means of conveying the expansion of gases to the crankshaft via connecting rod, without loss of gas from above or oil from below.  Piston is essentially a cylindrical plug that moves up & down in the cylinder. It is equipped with piston rings to provide a good seal between the cylinder wall &piston.
  17. 17. FUNCTIONS 1. To reciprocate in the cylinder as a gas tight plug causing suction, compression, expansion and exhaust strokes. 2. To receive the thrust generated by the explosion of the gas in the cylinder and transmit it to the connecting rod. 3. To form a guide and bearing to the small end of the connecting rod and to take the side thrust due to obliquity of the rod.
  18. 18. MATERIALS  The materials used for piston is mainly Aluminium alloy.  Cast Iron is also used for piston as it possesses excellent wearing qualities, co-efficient of expansion. But due to the reduction of weight, the use of alluminium for piston was essential.  To get equal strength a greater thickness of metal is essential. Thus some of the advantage of the light metal is lost.  SAE has recommended the following composition.  SAE 300 : Heat resistant aluminum alloy with the composition, Cu 5.5 to 7.5 %,Fe 1.5 %, Si 5.0 to 6.0 %, Mg 0.2 to 0.6 %, Zn 0.8 %, Ti 0.2 %, other Elements 0.8 %.
  19. 19. MATERIALS  Advantages: i. Maintain mechanical properties at elevated temperature ii. Heat conductivity about 4.4 times cast iron iii. Specific gravity 2.89  SAE 321 : Low expansion Alloy having the composition, Cu 0.5 to 1.5 %, Fe 1.3 %, Si 11 to 13 %, Mn 0.1 %, Mg 0.7 to 1.3 %, Zn 0.1 %, Ti 0.2 %, Ni 2 to 3 %, other Elements 0.05 %.  Y – Alloy: (Developed by National Physical Laboratory, London.) it is also called alluminium alloy 2285. This alloy is noted for its strength at elevated temperatures. Also used f%or cylinder heads. Composition of Cu 4%, Ni 2%, and Mg 1.5.
  20. 20. Piston (properties) characteristics  It should be silent in operation both during warm-up and the normal running.  The design should be such that the seizure does not occur.  It should offer sufficient resistance to corrosion due to some properties of combustion Ex : Sulphur dioxide.  It should have the shortest possible length so as the decrease overall engine size.  It should be lighten in weight so that inertia forces created by its reciprocating motion are minimum.  Its material should have a high thermal conductivity for efficient heat transfer so that higher compression ratios may be used with out the occurrence of detonation.  It must have a long life.
  21. 21. Nomenclature
  22. 22. Piston Ring  Provide seal between cylinder wall and piston  Rings ride on a thin film of oil  Conduct heat from the piston out to the cylinder and the fins  Materials: Piston rings are made of fine grained alloy cast iron. This material possesses excellent heat and wears resisting quantities.  The elasticity of this material is also sufficient to impact radial expansion and compression which is necessary for assembly and removal of the ring.
  23. 23.  Types of Piston Rings: There are two types of piston rings. 1. Compression rings or Gas rings. 2. Oil control rings or Oil regulating rings.
  24. 24. Piston Pin  Piston pin or gudgeon pin or wrist pin connects the piston and the small end of the connecting rod. Piston pin is generally hollow and made from case hardening steel heat treated to produce a hard wear resisting surface.
  25. 25. Design Considerations for a Piston In designing a piston for I.C. engine, the following points should be taken into consideration : 1. It should have enormous strength to withstand the high gas pressure and inertia forces. 2. It should have minimum mass to minimise the inertia forces. 3. It should form an effective gas and oil sealing of the cylinder. 4. It should provide sufficient bearing area to prevent undue wear. 5. It should disprese the heat of combustion quickly to the cylinder walls. 6. It should have high speed reciprocation without noise. 7. It should be of sufficient rigid construction to withstand thermal and mechanical distortion. 8. It should have sufficient support for the piston pin.
  26. 26. Piston for I.C. engines
  27. 27.  The piston head or crown is designed keeping in view the following two main considerations, i.e. PISTON HEAD OR CROWN 1. It should have adequate strength to withstand the straining action due to pressure of explosion inside the engine cylinder, and 2. It should dissipate the heat of combustion to the cylinder walls as quickly as possible.
  28. 28. The thickness of the piston head (tH ), according to Grashoff’s formula is given by
  29. 29. Piston Rings Radial ribs •The radial ribs may be four in number. • The thickness of the ribs varies from tH / 3 to tH / 2
  30. 30. The radial thickness (t1) of the ring may be obtained by considering the radial pressure between the cylinder wall and the ring. From bending stress consideration in the ring, the radial thickness is given by
  31. 31. Piston Barrel  It is a cylindrical portion of the piston. The maximum thickness (t3) of the piston barrel may be obtained from the following empirical relation : t3 = 0.03 D + b + 4.5 mm where b = Radial depth of piston ring groove which is taken as 0.4 mm larger than the radial thickness of the piston ring (t1) = t1 + 0.4 mm Thus, the above relation may be written as t3 = 0.03 D + t1 + 4.9 mm The piston wall thickness (t4) towards the open end is decreased and should be taken as 0.25 t3 to 0.35 t3.
  32. 32. Piston Skirt From equations (i) and (ii), the length of the piston skirt (l) is determined. In actual practice, the length of the piston skirt is taken as 0.65 to 0.8 times the cylinder bore. Now the total length of the piston (L) is given by L = Length of skirt + Length of ring section + Top land The length of the piston usually varies between D and 1.5 D.
  33. 33. Piston Pin
  34. 34. Numerical  Design a cast iron piston for a single acting four stroke engine for the following data:Cylinder bore = 100 mm ; Stroke = 125 mm Maximum gas pressure = 5 N/mm2 ; Indicated mean effective pressure = 0.75 N/mm2 ; Mechanical efficiency = 80% ; Fuel consumption = 0.15 kg per brake power per hour ; Higher calorific value of fuel = 42 × 103 kJ/kg ; Speed = 2000 r.p.m. Any other data required for the design may be assumed.
  35. 35. Design of Connecting Rod UNIT 3
  36. 36. Design of Connecting Rod  In designing a connecting rod, the following dimensions are required to be determined: 1. Dimensions of cross-section of the connecting rod, 2. Dimensions of the crankpin at the big end and the piston pin at the small end, 3. Size of bolts for securing the big end cap, and 4. Thickness of the big end cap.
  37. 37. 1. Dimension of I- section of the connecting rod Flange and web thickness of the section = t Width of the section, B = 4t depth or height of the section, H = 5t Let A = Cross-sectional area of the connecting rod, l = Length of the connecting rod, σc = Compressive yield stress, WB = Buckling load, Ixx and Iyy = Moment of inertia of the section about X-axis and Y-axis respectively, and kxx and kyy = Radius of gyration of the section about X-axis and Y-axis respectively. According to Rankine’s formula,
  38. 38. The buckling load (WB) may be calculated by using the following relation, i.e. WB = Max. gas force × Factor of safety The factor of safety may be taken as 5 to 6.
  39. 39. 2. Dimensions of the crankpin at the big end and the piston pin at the small end
  40. 40. 3. Size of bolts for securing the big end cap
  41. 41. 4. Thickness of the big end cap
  42. 42. QUESTION  Design a connecting rod for an I.C. engine running at 1800 r.p.m. and developing a maximum pressure of 3.15 N/mm2. The diameter of the piston is 100 mm ; mass of the reciprocating parts per cylinder 2.25 kg; length of connecting rod 380 mm; stroke of piston 190 mm and compression ratio 6 : 1. Take a factor of safety of 6 for the design. Take length to diameter ratio for big end bearing as 1.3 and small end bearing as 2 and the corresponding bearing pressures as 10 N/mm2 and 15 N/mm2. The density of material of the rod may be taken as 8000 kg/m3 and the allowable stress in the bolts as 60 N/mm2 and in cap as 80 N/mm2. The rod is to be of I- section for which you can choose your own proportions. Draw a neat dimensioned sketch showing provision for lubrication. Use Rankine formula for which the numerator constant may be taken as 320 N/mm2 and the denominator constant 1 / 7500.
  43. 43. Design of Crank shaft UNIT 4
  44. 44. Crankshaft  A crankshaft (i.e. a shaft with a crank) is used to convert reciprocating motion of the piston into rotatory motion or vice versa.  The crankshaft consists of the shaft parts which revolve in the main bearings, the crankpins to which the big ends of the connecting rod are connected, the crank arms or webs (also called cheeks) which connect the crankpins and the shaft parts.  The crankshaft, depending upon the position of crank, may be divided into the following two types : 1. Side crankshaft or overhung crankshaft, as shown in Fig. (a), and 2. Centre crankshaft, as shown in Fig. (b).
  45. 45. Material and manufacture of Crankshafts •The crankshafts are subjected to shock and fatigue loads. Thus material of the crankshaft should be tough and fatigue resistant. The crankshafts are generally made of carbon steel, special steel or special cast iron. •In industrial engines, the crankshafts are commonly made from carbon steel such as 40 C 8, 55 C 8 and 60 C 4. In transport engines, manganese steel such as 20 Mn 2, 27 Mn 2 and 37 Mn 2 are generally used for the making of crankshaft. •In aero engines, nickel chromium steel such as 35 Ni 1 Cr 60 and 40 Ni 2 Cr 1 Mo 28 are extensively used for the crankshaft.
  46. 46. Design Procedure for Crankshaft The following procedure may be adopted for designing a crankshaft. 1. First of all, find the magnitude of the various loads on the crankshaft. 2. Determine the distances between the supports and their position with respect to the loads. 3. For the sake of simplicity and also for safety, the shaft is considered to be supported at the centres of the bearings and all the forces and reactions to be acting at these points. The distances between the supports depend on the length of the bearings, which in turn depend on the diameter of the shaft because of the allowable bearing pressures.
  47. 47. 4. The thickness of the cheeks or webs is assumed to be from 0.4 ds to 0.6 ds, where ds is the diameter of the shaft. It may also be taken as 0.22D to 0.32 D, where D is the bore of cylinder in mm. 5. Now calculate the distances between the supports. 6. Assuming the allowable bending and shear stresses, determine the main dimensions of the crankshaft
  48. 48. Design of Centre Crankshaft •When the crank is at dead centre. At this position of the crank, the maximum gas pressure on the piston will transmit maximum force on the crankpin in the plane of the crank causing only bending of the shaft. • The crankpin as well as ends of the crankshaft will be only subjected to bending moment. Thus, when the crank is at the dead centre, the bending moment on the shaft is maximum and the twisting moment is zero.
  49. 49. Centre crankshaft at dead centre
  50. 50. (b) Design of left hand crank web The crank web is designed for eccentric loading. There will be two stresses acting on the crank web, one is direct compressive stress and the other is bending stress due to piston gas load (FP).
  51. 51. (c) Design of right hand crank web The dimensions of the right hand crank web (i.e. thickness and width) are made equal to left hand crank web from the balancing point of view.
  52. 52.  Design a plain carbon steel centre crankshaft for a single acting four stroke single cylinder engine for the following data: Bore = 400 mm ; Stroke = 600 mm ; Engine speed = 200 r.p.m. ; Mean effective pressure = 0.5 N/mm2; Maximum combustion pressure = 2.5 N/mm2; Weight of flywheel used as a pulley = 50 kN; Total belt pull = 6.5 kN. When the crank has turned through 35° from the top dead centre, the pressure on the piston is 1N/mm2 and the torque on the crank is maximum. The ratio of the connecting rod length to the crank radius is 5. Assume any other data required for the design. Question

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