Design of IC Engine Components

1. DESIGN OF IC ENGINE COMPONENTS
DR.MAHALINGAM COLLEGE OF ENGINEERING AND TECHNOLOGY, POLLACHI.
P.KARUPPUSAMY AP / AUTO
R.VISHNURAMESHKUMAR AP / AUTO
DEPARTMENT OF AUTOMOBILE ENGINEERING
Reference:
1. A text book of MACHINE DESIGN – R.S. KHURMI
2. Design of MACHINE ELEMENTS – V.B. BHANDARI
Year / Sem : III / VI
AY : 2018 – 19

2. UNIT – III
DESIGN OF CONNECTING ROD
2

3. ENGINE CONNECTING ROD
 The connecting rod is the intermediate member between the piston and the
crankshaft.
Function of a Piston:
 To transmit the push and pull from the piston pin to the crankpin
 To convert the reciprocating motion of the piston into the rotary motion of the
crank.

4. Connecting Rod
 It consists of a long shank, a small end and a big end.
 The cross-section of the shank may be rectangular, circular, tubular, I-section or H-
section.
 Circular section is used for low speed engines
 I-section is preferred for high speed engines.

5.  The length of the connecting rod ( l ) depends upon the ratio of l / r, where r is the radius of
crank.
 The smaller length will decrease the ratio l / r. This increases the angularity of the
connecting rod which increases the side thrust of the piston against the cylinder liner
which in turn increases the wear of the liner.
 The larger length will increase the ratio l / r. This decreases the angularity of the
connecting rod and thus decreases the side thrust and the resulting wear of the cylinder.
 But the larger length of the connecting rod increases the overall height of the engine.
Hence, a compromise is made and the ratio l / r is generally kept as 4 to 5.
Ratio l / r

6.  The small end of the connecting rod is usually made in the form of an eye and is provided
with a bush of phosphor bronze.
 It is connected to the piston by means of a piston pin.
 The big end of the connecting rod is usually made split (in two halves) so that it can be
mounted easily on the crankpin bearing shells.
 The bearing shells of the big end are made of steel, brass or bronze with a thin lining (about
0.75 mm) of white metal or babbit metal.
 The wear of the big end bearing is allowed for by inserting thin metallic strips (known as
shims) about 0.04 mm thick between the cap and the fixed half of the connecting rod.
Small end and big end

7. Material Requirement
 should have adequate strength, stiffness and minimum weight.
Materials
 The carbon steel having 0.35 % carbon has an ultimate tensile strength of about 650
MPa when properly heat treated
 A carbon steel with 0.45 % carbon has a ultimate tensile strength of 750 MPa.
 These steels are used for connecting rods of industrial engines.
 The alloy steels have an ultimate tensile strength of about 1050 MPa and are used for
connecting rods of aero engines and automobile engines

8. Forces Acting on the Connecting Rod
1. Force on the piston due to gas pressure and inertia of the reciprocating parts,
2. Force due to inertia of the connecting rod or inertia bending forces,
3. Force due to friction of the piston rings and of the piston, and
4. Force due to friction of the piston pin bearing and the crankpin bearing.

9. Design of Connecting Rod
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.

10. Buckling of connecting rod
 A connecting rod is a machine member which is subjected to alternating direct
compressive and tensile forces.
 Since the compressive forces are much higher than the tensile forces, therefore, the
cross-section of the connecting rod is designed as a strut.
 A connecting rod, as shown in Fig., subjected to an axial load W may buckle with X-
axis as neutral axis (i.e. in the plane of motion of the connecting rod) or Y-axis as
neutral axis (i.e. in the plane perpendicular to the plane of motion).
 The connecting rod is considered like both ends hinged for buckling about X-axis and
both ends fixed for buckling about Y-axis.
 A connecting rod should be equally strong in buckling about both the axes.
1. Dimensions of cross-section of the connecting rod

11. 1. Dimensions of cross-section of the connecting rod
 Connecting rod should be designed in such a way that it is equally resistant to buckling
in either plane.
Ixx = 4 Iyy
I = moment of inertia of cross section (mm4)
I = Ak2
Akxx
2 = 4 Akyy
2
kxx
2 = 4 kyy
2
k = radius of gyration of cross section
kyy
2 = kxx
2 / 4
Ixx / Iyy = 4

12. 1. Dimensions of cross-section of the connecting rod

13. 1. Dimensions of cross-section of the connecting rod

14.  This shows that the connecting rod is four times strong in buckling about Y-axis than
about X-axis.
 If Ixx > 4 Iyy, buckling will occur about Y- axis
 If Ixx < 4 Iyy, buckling will occur about X-axis.
 In actual practice, Ixx is kept slightly less than 4 Iyy.
 It is usually taken between 3 and 3.5 and the connecting rod is designed for bucking
about X-axis. The design will always be satisfactory for buckling about Y-axis.
 The most suitable section for the connecting rod is I-section with the proportions as
shown.

15. 1. Dimensions of cross-section of the connecting rod
Schematic diagram of crank and connecting rod mechanism is as shown in figure

17. Dimensions of cross section are calculated for buckling load in the plane of rotation or
about XX axis

18. For a connecting rod made of mild steel and plain carbon steel
In a plane of rotation, both ends are hinged and equivalent length is equal to the actual
length of connecting rod.
The constant ‘a’ for steel material is
The critical buckling load,
Pcr = Pc * (fs)

19. Step by step procedure to find the dimensions of “I” section
(i) Calculate the force acting on the connecting rod
(ii) Calculate the critical buckling load by
Pcr = Pc * (fs)
(iii) Use Rankine’s formula, Calculate the value of “t”

20. (iv) Find the dimensions of “I” section using proportions.
B = 4 * t & H = 5 * t
(v) The width ‘B’ is kept constant throughout the length
(vi) At middle section, H = 5 * t
At small end, H1= 0.75 H to 0.9 H
At big end, H2= 0.75 H to 0.9 H

21. Tutorial: Dimensions of “I” section
Determine the dimensions of cross-section of the connecting rod for a diesel engine with the
following data:
Cylinder bore = 100 mm
Length of the connecting rod = 350 mm
Maximum gas pressure = 4 MPa
Factor of safety = 6

23. 2. Dimensions of the crankpin at the big end and the piston pin at the small end
Small End Big End

25. 2. Size of bolts for securing the big end cap
 The maximum force acting on the cap and two bolts consists only of inertia force at the
top dead center on the exhaust stroke.
 The inertia force acting on the bolts or cap is given by,
Pi = inertia force on the cap or bolts (N)
mr = mass of reciprocating parts (kg)
ω = angular velocity of crank or angular speed of the engine (rad/s)
r = crank radius (m)
L = length of the connecting rod (m)
n1 = ratio of length of the connecting rod to crank radius = (L/r)
θ = angle of inclination of crank from TDC

26. Mass of reciprocating parts
mr = mass of piston assembly + (1/3) * mass of connecting rod
Angular velocity of crank
ω =
2𝜋𝑁
60
N = crank speed (rpm)
Crank radius
r = l/2
The inertia force will be maximum at the top dead center position where (θ = 0).
When (θ = 0),
cos θ = 1 and cos 2θ = 1

27. Forces on cap and bolt
 The cap, also called keep plate, is subjected to
inertia force (Pi)max, as shown in Fig.(b).
 Treated as a beam freely supported at the bolt
centres
 Loaded in a manner intermediate between
uniformly distributed and centrally
concentrated load in which case the bending
moment is (W*l/6).

28. 𝑀 𝑏 =
(𝑃𝑖) 𝑚𝑎𝑥 ∗ 𝑙
6
Mb = bending moment acting on cap (N-mm)
l = span length or distance between the bolt centers (mm)
l = diameter of crank pin + 2 * [thickness of bush (3 mm)]+ nominal diameter of bolt (d)+
clearance (3 mm)
The thickness of the Cap (c) is obtained by
σ 𝑏 =
𝑀 𝑏 ∗ 𝑦
𝐼
𝐼 =
(𝑏 𝑐)(𝑡 𝑐
3
)
12
𝑦 =
𝑡 𝑐
2
bc = width of cap (mm); equal to length of crankpin Or big end bearing (lc) .
tc = thickness of big end cap (mm)

29. The following data is given for the cap and bolts, of the big end of connecting rod: .
Engine speed.= 1800 rpm
Length of connecting rod = 350 mm
Length of stroke = 175 mm
Mass of reciprocating parts = 2.5 kg
Length of crankpin = 76 mm
Diameter of crankpin = 58 mm
Thickness of bearing bush = 3 mm
Permissible tensile stress for bolts = 60 N/mm2
Permissible bending stress for cap = 80 N/mm2
Calculate the nominal diameter of bolts and thickness of cap for the big end.
Tutorial: Size of bolts for securing the big end cap

31. Determine the dimensions of cross-section of the connecting rod for a diesel engine with the
following data:
Cylinder bore = 100 mm
Length of connecting rod = 320 mm
Maximum gas pressure = 2.45 MPa
Factor of safety against buckling failure = 5
Tutorial:

32. Tutorial:
Determine the dimensions of small and big end bearings of the connecting rod for a diesel
engine with the following data:
Cylinder bore = 100 mm
Maximum gas pressure = 2.45 MPa
(l/d) ratio for piston pin bearing = 1.5
(l/d) ratio for crank pin bearing = 1.4
Allowable bearing pressure for piston pin bearing = 15 MPa
Allowable bearing pressure for crank pin bearing = 10 MPa

33. Tutorial:
The following data is given for the cap and bolts of the big end of the connecting rod:
Engine speed = 1500 rpm
Length of connecting rod = 320 mm
Length of stroke = 140 mm
Mass of reciprocating parts = 1.75 kg
Length of crank pin = 54 mm
Diameter of crank pin = 38 mm
Permissible tensile stress for bolts = 120 N/mm2
Permissible bending stress for cap = 120 N/mm2
Calculate the nominal diameter of bolts and thickness of cap for the big end.

34.  The small end of the connecting rod is subjected to pure translation motion while
the big end is subjected to pure rotary motion.
 The intermediate points on the connecting rod move in elliptical orbits.
 The lateral oscillations of the connecting rod induce inertia forces that act all along
the length of the connecting rod causing bending.
 This type of action is called whipping.
 The bending stress due to inertia force is called “whipping stress”.
3. WHIPPING STRESS

35. The mass of the connecting rod

36. σ 𝑏 =
𝑀 𝑏 ∗ 𝑦
𝐼

37. The following data is given for a connecting rod
Engine speed =1800 rpm,
Length of connecting rod = 350 mm
Length of stroke = 175 mm
Density of material = 7800 kg/m3
Thickness of web or flanges = 8 mm.
Assume suitable cross-section.
Calculate whipping stress in connecting rod
Tutorial: Whipping Stress

39. The following data is given for a connecting rod:
Engine speed = 1500 rpm
Length of connecting rod = 320 mm
Length of stroke = 140 mm
Density of material = 7830 kg/m3
Thickness of web or flanges = 6 mm
Assume the cross-section.
Calculate whipping stress in connecting rod
Tutorial: Whipping Stress

40. Tutorial: Design of Connecting Rod
The following data is given for a connecting rod
Cylinder bore = 85 mm
Length of stroke = 350 mm
Maximum gas pressure = 3 MPa
Factor of safety against buckling failure = 5
(l/d) ratio for piston pin bearing = 1.5;
(l/d) ratio for crank pin bearing = 1.25
Allowable bearing pressure for piston pin bearing = 13 MPa
Allowable bearing pressure for crank pin bearing = 11 MPa
Length of stroke = 140 mm
Mass of reciprocating parts = 1.5 kg

41. Tutorial: Design of Connecting Rod (contd.)
The following data is given for a connecting rod
Engine speed = 2000 rpm
Thickness of bearing bush = 3 mm
Material of cap = steel 40C8
Yield strength of cap material = 380 N/mm2
Factor of safety for cap = 4
Material of bolts = chromium molybdenum steel
Yield strength of bolt material = 450 N/mm2
Factor of safety for bolts = 5
Density of connecting rod = 7800 kg/m3

42. Calculate:
(i) dimensions or cross-section of connecting rock
(ii) dimensions of small and big end bearings;
(iii) nominal diameter of bolts for the cap
(iv) thickness of cap; and
(v) magnitude of whipping stress
Tutorial: Design of Connecting Rod (contd.)

45. Design a connecting rod for a high-speed IC engine using the following data:
Cylinder bore =125 mm
Length of connecting rod = 300 mm
Maximum gas pressure = 3.5 MPa
Length of stroke = 125 mm
Mass of reciprocating parts = 1.6 kg
Engine speed = 2200 rpm
Assume suitable data and state the assumptions you make.
Tutorial: Design of Connecting Rod