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/stressMaterial/s

usedSizes

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

UNIT 2

Design of Piston

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.

Nomenclature

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.

Piston for I.C. engines

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.

Piston Pin

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.

Design of Crank shaft

UNIT 4

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 CrankshaftThe 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