Design of Piston
Design of Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms.
It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.
Function of Piston-
The Piston Performs The Following Functions-
1. Receive the thrust produced by the combustion of the gas in the cylinder and transmit it to the connecting rod.
2. Piston reciprocates in the cylinder as a gas-tight plug generating suction, compression, expansion and exhaust strokes.
3. Piston forms a guide and bearing to the small end of the connecting rod and to take the side thrust due to the obliquity of the rod.
Types of Pistons-
1. “Lo-Ex” alloy Pistons
2. Invar Strut Pistons
3. Autothermic Pistons
4. Bi-Metal pistons
5. Specialloid Pistons
6. Wellworthy Pistons
1. “Lo-Ex” alloy Pistons- Lo-Ex is the name of a light alloy for piston, signifying low expansion with temperature. It contains the following:
Silicon 11 to 13%
Nickel 0.7 to 2.5%
Magnesium 1%
Copper 1%
Aluminium 86.3 to 82.5%
The coefficient of expansion is actually only about 2% less than that of pure aluminium.
2. Invar Strut Pistons- In this types of pistons, Invar is an alloy containing 36% nickel and 64% iron. It has a negligible coefficient of expansion, 000000063 per °C). Invar struts have been put in the piston connecting the piston pin bosses and the skirt and so proportioned that the resultant expansion of the piston is nearly the same as that of the cylinder.
3. Autothermic Pistons- This types of pistons contain low expansion steel inserts at the piston pin bosses. These inserts are so moulded that their ends are anchored in the piston skirt.
4. Bi-Metal pistons- This types of pistons are made from both steel and aluminium.It consists of a steel skirt and pin basses.
5. Specialloid Pistons- Specialloid production covers a wide range of pistons for zero, automobile petrol engines and diesel engines as used for commercial vehicles, industrial stationary, rail traction, marine main propulsion and auxiliary purposes.
6. Wellworthy Pistons-Wellworthy Ltd of Great Britain produces high duty pistons will hall have cast iron inserted ring carriers for the top piston rings. These inserted carriers are recently applied to only one ring in diesel engine piston. The wear in the top groove is thereby reduced to a minimum as compared with that which it would have been in the unprotected light alloy.
Quality of Piston-
The Piston Must Possess The Following Qualities
1. Rigidly to withstand high pressure.
2. Lightness to reduce the weight of the reciprocating masses and so enable higher engine speeds.
3. Good heat conductivity to reduce the risk of detonation so allowing higher compression ratio.
4. Silence in operation.
5. Material having low expansion and provision to allow for different expansion rates of cast iron cylinder block and an aluminium piston.
6. Correctly formed skirt to give uniform bearing under working conditions.Material for Piston
Materials of Piston-
The material used for pistons is aluminium alloy. Aluminium pistons can be either cast or forged. Cast iron is also used for the piston. Cast iron is a universal material in the early years because it possesses excellent wearing quality, coefficient of expansion and general suitability in manufacture.
Parts of Piston-
1. Head or crown -The piston head or crown may be flat, convex or concave depending upon the design of combustion chamber. It withstands the pressure of gas in the cylinder.
2. Piston pin -It is also called gudgeon pin or wrist pin. It is used to connect the piston to the connecting rod.
3. Skirt -The skirt acts as a bearing for the side thrust of the connecting rod on the walls of cylinder.
4. Piston rings -The piston rings are used to seal the cylinder in order to prevent leakage of the gas past the piston.
The piston rings are of the following two types :
1. Compression rings or pressure rings- The compression rings or pressure rings are inserted in the grooves at the top portion of the piston and may be three to seven in number. These rings also transfer heat from the piston to the cylinder liner and absorb some part of the piston fluctuation due to the side thrust.
2. Oil control rings or oil scraper- The oil control rings or oil scrapers are provided below the compression rings. These rings provide proper lubrication to the liner by allowing sufficient oil to move up during upward stroke and at the same time scraps the lubricating oil from the surface of the liner in order to minimize the flow of the oil to the combustion chamber.
1. It should have enormous strength to withstand the high gas pressure and inertia forces.
2. It should have minimum mass to minimize 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 disperse 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.
Design of Piston-
1. Piston Head or Crown
The thickness of the piston head (tH ), according to Grashoff’s formula is given by
Where-
p = Maximum gas pressure or explosion pressure in MPa
D = Cylinder bore or outside diameter of the piston in mm
σt = Permissible bending (tensile) stress for the material of the piston in MPa. It may be taken as (35 to 40 MPa) for grey cast iron, (50 to 90 MPa) for nickel cast iron and aluminum alloy and (60 to 100 MPa) for forged steel.
Thickness of piston also is given by
Where -
H = Heat flowing through the piston head in kJ/s or watts,
k =Heat conductivity factor in W/m/°C. Its value is 46.6 W/m/°C for grey cast iron, 51.25 W/m/°C for steel and 174.75 W/m/°C for aluminum alloys.
TC = Temperature at the centre of the piston head in °C,
TE = Temperature at the edges of the piston head in °C.
The temperature difference (TC–TE) may be taken as 220°C for cast iron and 75°C for aluminum.
The heat flowing through the position head (H) may be determined by the following expression, i.e.,
H = C × HCV × m × B.P. (in kW)
where C = Constant representing that portion of the heat supplied to the engine which is absorbed by the piston. Its value is usually taken as 0.05.
HCV = Higher calorific value of the fuel in kJ/kg. It may be taken as (45 × 103 kJ/kg) for diesel and (47 × 103 kJ/ kg) for petrol
m = Mass of the fuel used in kg per brake power/sec
B.P. = Brake power of the engine per cylinder
The thickness of the ribs may be takes as tH /3 to tH /2.
2. Piston Rings
The radial thickness is given by
Where -
D = Cylinder bore in mm,
pw = Pressure of gas on the cylinder wall in MPa. Its value is limited from 0.025 MPa to 0.042 MPa.
σt = Allowable bending (tensile) stress in MPa. Its value may be taken from 85 MPa to 110 MPa for cast iron rings.
The axial thickness (t2) of the rings may be taken as 0.7 t1 to t1.
The minimum axial thickness (t2) may also be obtained from the following empirical relation:
t2 = D/ 10 nR
Where nR = Number of rings.
Width of top land b1 = tH to 1.2 tH
Width of other ring lands b2 = 0.75 t2 to t2
The gap between the free ends of the ring is given by 3.5 t1 to 4 t1. The gap, when the ring is in the cylinder, should be 0.002 D to 0.004 D.
3. Piston Barrel
The maximum thickness (t3) of the piston barrel may be obtained from the following empirical relation :
t3 = 0.03 D + b + 4.5 mm
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.
4. Piston Skirt
Maximum side thrust on the cylinder due to gas pressure (p)
Maximum side thrust on the cylinder due to gas pressure (p)
........(1)
Take µ = 0.1
Side thrust due to bearing pressure on the piston barrel ( Pb)
R = Pb × D × l ………..(2)
(Taking Pb = 0.45 N/mm2)
Find out the length of skirt from equation 1 & 2.
Total length of the piston
L = Length of the skirt + Length of the ring section + Top land = l + (4 t2 + 3b2) + b1
5. Piston Pin
Load on the piston due to gas pressure or gas load
Load on the piston pin due to bearing pressure or bearing load = Bearing pressure × Bearing area
= Pb1 × d0 × l1 ……………. (2)
From equations (1) and (2) the outside diameter of the piston pin (d0) may be obtained.
Where d0 = Outside diameter of the piston pin in mm
l1 = Length of the piston pin in the bush of the small end of the connecting rod in mm. Its value is usually taken as 0.45 D.
Pb1 = Bearing pressure at the small end of the connecting rod bushing in MPa. Its value for the bronze bushing may be taken as 25 MPa.
The inside diameter of the pin (di) is usually taken as 0.6 d0.
Let the piston pin be made of heat treated alloy steel for which the bending stress ( σb )may be taken as 140 MPa. Now let us check the induced bending stress in the pin.
Maximum bending moment at the centre of the pin
We also know that maximum bending moment (M)
Find out the value of σb which is always less than σb for alloy steel or pin materials.
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