Project-6
Design of a Piston
National Institute of Technology, Rourkela
Machine Element Design Practice-II
2018-19
Name: Swarnendu Ganguly
Roll no: 116me0442
ME-1(Group 6)
Problem Statement and Solution
Problem : Design a piston for a four stoke petrol engine with the following given specifications :-
1. Brake Power (Bp ) = 8 kW
2. Rated Speed (N) = 950 RPM
3. Mean Effective Pressure (Pm ) = 0.8 MPa
4. Mechanical Efficiency (ηm ) = 80%
Assume Maximum Pressure is 10 times the mean effective pressure
⋆Nomenclature :-
Bp = Brake Power, kW N = Rated Speed, RPM
Pm = Mean Effective Pressure, MPa ηm = Mechanical Efficiency
D = Inner Diameter of Cylinder, mm L = Length of Piston, mm
H = Amount of Heat Conducted, W tR = Thickness of Ribs, mm
C = Ratio of Heat Absorption Pmax = Maximum Pressure, MPa
k = Thermal Conductivity Factor, W/m/ C ◦
σt = Allowable Tensile Stress, MPa
Te = Temperature at Edge of Piston Head, C ◦
Tc = Temperature at Center of Piston Head, ◦ C
HCV = Higher Calorific Value, kJ/kg m = Average Consumption of Fuel, kg/kW/s
b = Radial Width of Piston Rings, mm σult = Ultimate Tensile Stress, MPa
fs = Factor of Safety µ = Coeficient of Friction
z = No. of Rings I = Area Moment, mm4
th = Thickness of Piston Head, mm h = Axial Thickness of Piston Ring, mm
h1 = Top Land Width, mm h2 = Ring Land width, mm
σb = Bending Stress, MPa G1 = Gap b/w free ends before Assembly, mm
G2 = Gap b/w free ends after Assembly, mm M = Bending Moment, Nmm
P = Gas Force, N do = Outer Diameter of Piston Pin, mm
di = Inner Diameter of Piston Pin, mm t3 = Thickness of Piston Barrel at Top End, mm
t4 = Thickness of Piston Barrel at Lower End, mm Pb = Allowable Bearing Pressure, MPa
ls = Length of Skirt, mm (Pb )1 = Bearing Pressure at Small End, MPa
l1 = Length of Piston Pin in Bush, mm Pw = Allowable Radial Pressure, MPa
y = Distance to Neutral Axis, mm
1
(a) Piston Parts (b) Schematic of Piston
Figure 1: Piston
⋆Material Selected and Design Data :-
Sl no. Component Material Design Data
1 Piston Grey Cast Iron ASTM 60 σult = 500 MPa, k = 46.6 W/m/◦ C
2 Piston Rings Grey Cast Iron FG 200 σt = 100 MPa
3 Piston Pin 50C8 Carbon Steel σb = 84 MPa
⋆Assumed Data[1] :-
µ = 0.1 Pb = 0.6 MPa
m = 9.45 × 10−5 kg/kW/s Pw = 0.04 MPa
fs = 5 σb = σt
σult
σt = Tc − Te = 220 ◦ C
fs
HCV = 47000 kJ/kg C = 0.05
l1 = 0.45D (Pb )1 = 25 MPa
⋆Calculation of Piston Head Thickness :-
On the basis of strength criterion, the thickness of the piston head is given by according to Grashoff’s
formula[2] :-
r
3Pmax
th = D (1.1)
16σb
So, we get the thickness as, th = 12.25 mm.
On the basis of heat dissipation criterion, the thickness of the piston head is given by[2] :-
H
th = × 103 (1.2)
12.56k(Tc − Te )
2
The amount of heat conducted through the piston is given by :-
H = [C × HCV × m × Bp ] × 103 (1.3)
We get the heat conducted as, H = 1775.56 W.
Putting this in the equation (1.2) we get the thickness as, th = 13.789 mm.
Comparing the thickness obtained from both the criterion and choosing the greater one, which is the
thickness obtained based on the heat dissipation criterion, we take the thickness of the piston head as,
th = 14 mm.
⋆Design of Piston Ribs and Cup :-
Piston Ribs are provided only when the obtained piston head thickness is greater than 6 mm. Since our
obtained piston head thickness is 14 mm which is greater than 6 mm. Piston ribs are to be provided with
no. of ribs between 4 to 6. The thickness of the ribs is given between the follwing limits :-
th th
≤tR ≤ (1.4)
3 2
(4.67 mm) ≤tR ≤ (7 mm)
So, let us take the no. of ribs as 4 having thickness a of, tR = 5 mm.
Piston cup is provided if the ratio of stroke length to the bore diameter is less than 1.5. Since out stroke
length is 200 mm and the bore diameter being 100 mm the ratio 2 which is greater than 1.5 and hence no
cup is required.
⋆Design of Piston Rings :-
The radial width[2] of the compression ring is given by :-
r
3Pw
b=D (1.5)
σt
So, we get the radial width as, b = 3.5 mm.
The number of rings is usually between 3 to 4 so let us take the number of rings as, z = 4.
The axial thickness is given as :-
0.7b ≤h ≤ b (1.6)
(2.45 mm) ≤h ≤ (3.5 mm)
So, the axial thickness is taken as, h = 3 mm. Also the minimum axial thickness is given by :-
D
hmin = (1.7)
10z
Hence, we get the minimum axial thickness as, hmin = 2.5 mm. Since it is less than our chosen axial
thickness our design is satisfactory.
3
The top land width is given as :-
th ≤h1 ≤ 1.2th (1.8)
(14 mm) ≤h1 ≤ (16.8 mm)
So, let us take the top land width as, h1 = 15 mm.
The ring land width is given as :-
0.75h ≤h2 ≤ h (1.9)
(2.25 mm) ≤h2 ≤ (3 mm)
So, let us take the ring land width as, h2 = 2.5 mm.
The gap between the free ends before assembly and after assembly is given as :-
3.5b ≤G1 ≤ 4b 0.002D ≤G2 ≤ 0.004D (1.10)
(12.25 mm) ≤G1 ≤ (14 mm) (0.2 mm) ≤G2 ≤ (0.4 mm)
So, let us take the gap between free ends before and after assembly as, G1 = 13 mm and G2 = 0.3 mm.
⋆Design of Piston Barrel :-
The thickness of the piston barrel[2] at the top end is given as :-
t3 = 0.03D + b + 4.9 (1.11)
So, we get the top end thickness as, t3 = 11.5 mm.
The thickness of the piston barrel at the lower open end is given by:-
0.25t3 ≤t4 ≤ 0.35t3 (1.12)
(2.275 mm) ≤t4 ≤ (4.025 mm)
So, let us take the lower end thickness as, t4 = 3 mm.
⋆Design of Piston Skirt :-
The length of the piston skirt[2] is given by the following equation :-
2
πD
µ Pmax = Pb × D × ls (1.13)
4
So, we get the piston skirt length as, ls = 100.50 mm.
The total piston length is given by :-
L = top land + length of skirt + length of ring section (1.14)
So, we get the total piston length as, L= 135 mm.
The total length of the piston should between the following limits :-
D ≤L ≤ 1.5D (1.15)
(100 mm) ≤L ≤ (150 mm)
4
Since our obtained length is between the above limits our design is satisfactory.
⋆Design of Piston Pin :-
The outer and inner diameter of the piston pin[2] is determined with the help of following equation :-
2
πD
Pmax = (Pb )1 × do × l1 (1.16)
4
di = 0.6do (1.17)
So, we get the outer and inner diameter as, do = 56 mm and di = 34 mm.
⋆Bending Consideration :-
The bending stress[2] can be calculate using the following equations :-
My
σb = (1.18)
I
PD
M= (1.19)
8
π (d4o − d4i )
I= (1.20)
64
do
y= (1.21)
2
So, we get the bending moment as, M = 785398.16 Nmm and the area moment as, I = 417152.45 mm4
and y = 28 mm and the bending stress as, σb = 52.71 MPa which is less than the allowable bending
stress of 84 MPa and hence our design is satisfactory.
⋆Result and Discussion :-
A piston for a internal combustion 4 stroke petrol engine was designed having a total length of 135 mm,
head thickness of 14 mm and four ribs with a width of 5 mm. The piston ring radial width was taken to
be 3.5 mm and no. of rings as 4. The piston pin outer and inner diameter were obtained as 56 mm and
34 mm with a bending stress of 52.71 MPa.
⋆References :-
[1] K. Lingaiah, Machine Design Data Book, 2nd Edition, New Delhi: Tata McGraw-Hill
Education, 1994
[2] V.B Bhandari, Design of Machine Elements, 3rd Edition, New Delhi: Tata McGraw-Hill
Education, 2010
5
14.00
15.00
A
3.50
11.50
3.00
135.00
2.50
60.00
5.00
56.00
100.50
50.25
3.00
SECTION A-A A
SCALE 1 : 3
0 . 00
10
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
TITLE : Piston Head
MATERIAL : Grey Cast Iron ASTM 60
THIRD ANGLE PROJECTION
(All Dimensions are in mm)
MACHINE ELEMENT
DESIGN PRACTICE - II ME-I(GROUP-6)
SCALE : 1:3 ME-381
13
.0 0
56.00
3.00
34.00
3.50
.0 0
R 50
100.00
Piston Ring Piston Pin
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
TITLE : Piston Ring and Piston Pin
MATERIAL : Piston Ring - Grey Cast Iron FG 200 & Piston Pin - 50C8 Carbon Steel
THIRD ANGLE PROJECTION
(All Dimensions are in mm)
MACHINE ELEMENT
DESIGN PRACTICE - II ME-I(GROUP-6)
SCALE : 1:5 ME-381
3
ITEM NO. PART NUMBER MATERIAL QTY.
1 Piston Head Grey Cast Iron ASTM 60 1
2 Piston Pin 50C8 Carbon Steel 1
3 Piston Ring Grey Cast Iron FG 200 4
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
TITLE : Bill Of Materials
MATERIAL : As Mentioned
ISOMETRIC EXPLODED VIEW
(All Dimensions are in mm)
MACHINE ELEMENT
DESIGN PRACTICE - II ME-I(GROUP-6)
SCALE : 1:3 ME-381
