Basic automobile design

Basic Automobile Design
Prepared by ,
Chirag Bhangale

Syllabus
1.Introduction and Design Consideration
2.Design of Piston.
3.Design of Connecting Rod.
4.Design of Crank shaft

Introduction and Design Consideration

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.

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.

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. 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.

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.

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.

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.

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

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

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

Design different parts of piston
 Piston Head or Crown
 Piston Rings
 Piston Barrel
 Piston Skirt
 Piston Pin

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.

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.

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 %.

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.

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.

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.

 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.

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.

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.

 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.

The thickness of the piston head (tH ), according to
Grashoff’s formula is given by

Piston Rings
Radial ribs
•The radial ribs may be four in number.
• The thickness of the ribs varies from tH / 3 to tH / 2

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

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.

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.

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.

Design of Connecting Rod
UNIT 3

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.

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,

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.

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

4. Thickness of the big end cap

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.

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).

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.

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.

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

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.

Centre crankshaft at dead centre

(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).

(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.

 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

Basic automobile design

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