Design of Piston
BY Choudhary Ritesh, 24
Ashish Chaursasia, 18
Ashish Chaursasia, 18
The piston is a disc which reciprocates
within a cylinder and is either moved by or moves the fluid which enters the
cylinder. The piston of an I.C engine receives the impulse from the expanding
gas & transmits the energy through the connecting rod to the crank.
Piston design is a challenging engineering
problem which involves complex physics and requires satisfying multiple
performance objectives. Uncertainty in piston operating conditions and
variability in piston design variables are inevitable and must be accounted
for. The piston assembly can be a major source of engine mechanical friction
and cold start noise, if not designed properly.
MODEL
OF ENGINE CHOOSEN FOR STUDY:
BAJAJ
AVENGER-220 DTSI
http://www.hastingsmfg.com/ServiceTips/piston.htm
I. GIVEN DATA :
Displacement=219.89cc
Maximum
power=19.03 Ps @8400rpm
= 14 kw
II. SOLUTION:
Volume = π/4
d2 *l
219.89=π/4
d2*1.5 d {Assuming l=1.5d}
219.89=π/4
*1.5 d3
D=5.174cm=6cm=60mm
1. PISTON HEAD OR CROWN:
The thickness of the piston head or crown
is determined on the basis of strength as well as on the basis of strength as
well as on the basis of heat dissipation and the larger of the two values is
adopted.
(a) Thickness(tH) of piston head on the basis of
strength
Material Selection: selecting material for piston as aluminium alloy.
Permissible bending [tensile] stress
σ=50 to 90 MPa
th=σ
{taking p=45kg/cm2 f=500 kg/cm2}
tH=0.78cm=7.8
mm
2. HEAT FLOWING THROUGH THE PISTON HEAD:
H=C*HCV*m*BP
C= Constant representing that portion of
the heat supplied to the engine which is absorbed by the piston.
It varies from 5 to 20 %
HCV= higher calorific value of the
fuel 47*103 KJ/BP/hr for petrol
m= Mass of fuel used in kg/BP/sec
H= 0.05*47*103 *41.7*10-6 *14
=1.37 kW
{Taking c=5% m=0.15 kg/BP/hr= 41.7*10-6
kg/BP/sec }
(b)Thickness
of piston head on the basis of heat dissipation
tH=(1.37*103)/(12.56*174.75*75)
tH =0.0083m=8.32mm
Taking the larger of two values
tH=8.32 mm
k= thermal conductivity factor 174.75 W/M/
[Tc-Te]=temperature difference = 75
3. RADIAL RIBS:
Assumption:
(a) 3 compression rings
(b) 1
oil ring
Radial thickness of piston rings t1 = σ
T1=0.388=3.88mm
Taking pw=0.7 kg/cm2
σt =500 kg/cm2
t2=0.7t1=0.7*3.88=2.716 mm
The minimum axial thickness of the piston ring
tr =D/10nl=6/(10*4)=1.5 mm
Axial thickness of the piston
ring as already calculated [tr=2.7] is satisfactory.
Distance from the top of the
piston to the first ring groove,i.e. width of top land
b1=(1 to 1.2)tr
= 1.2*8.33=9.99≈1 cm
Width of the other ring lands
b2=(0.75 to 1)t2
=2.7 mm
The gap between the free ends
of ring
G1=[3.5 to 4]t1
G1=4*3.8=15.2 mm
Gap when the ring is in the
cylinder
G2=[0.002 to
0.004] D
G2=0.004*6=0.024 cm=0.24mm
4. Piston
Barrel
Radial depth of the piston ring
gloves
b=t1+0.4
=2.21+0.4=2.25mm
Maximum thickness of Barrel
t 3=0.03D+b+4.5
=(0.03*45)+2.25+4.5=8.1mm
Piston wall thickness towards the open end,
t 4=0.25 to 0.35 times t3
t4 =0.30*8.1=2.43mm
5.Piston
Skirt
Let,l=length of the skirt,mm
The maximum side thrust on the cylinder due
to gas pressure
R=µ*π/4*d2*p
=0.1*π/4*2025*4.414
=702.02N
Taking µ=0.1
P=45*9.81*(1/100)=4.414N/mm2
Side thrust due to bearing pressure on the
piston barrel (Pb)
R=Pb*D*l
=0.45*45*l taking Pb=0.45Mpa
=20.25l N
20.25l=702.02N
L=34.67mm
Therefore total length of the Piston
L=length of the skirt + length of the ring
section + top land
=l+[4(ti)+3b2]+b1
L =34.67+[4*1.5+3*2.7]+10=58.77mm
6.Piston
Pin
d0=outside diameter of pin,mm
l1=Length of pin in the bush of
the small end of the connecting rod,mm
Pb=Bearing pressure at the
small end of the connecting rod bushing in N/mm2
For bronze bushing=25 N/mm2
Load on the pin due to bearing pressure,
=Bearing pressure * Bearing
Area
=Pb1*d0*l1
=25d0*0.45*45 [l1=0.45D]
=506.25 d0 N
Maximum gas load =π/4 (D*D)*P
=π/4*2025*4.4
=6.998 KN
therefore 506.25 d0=6998 N
d0=13.824 mm ≈14mm
inside diameter of pin d1=0.6*d0
=0.6*13.824=8.3mm
d1=9 mm
Let the Piston pin be made of heat treated
alloy steel for which the bending stress (σb) may be taken as 180 NPa
To check induced bending stress
in pin
Maximum bending moment at center of pin
M=P*D/8
=7*45*10^3/8=39375
Also,the maximum bending moment
39375=π/32[d04-di4/d0]σb
93300= π/32[14^4-9^4/14] σb
σb=176.48 N/mm^2
Since the induced bending stress in the pin
is less than the permissible value of 180 Mpa.
Therefore, the dimensions for the pin as
calculated above (i.e.d0=14 mm & di=9 mm)are satisfactory.
Design
considerations that are considered
1. The piston must have the strength to
resist the impulse and inertia forces.
2. Ability to disperse the heat of combustion
and avoid thermal distortion.
3. Sealing the gas and oil
4. Sufficient bearing area to work for
large number of reciprocating cyles
5. Minimum weight
6. Smooth noiseless operation
7. Provide adequate support for piston pin
1)
Introduction to Piston:-
The
piston is a disc which reciprocates within a cylinder and is either moved by or
moves the fluid which enters the cylinder. 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
wall.
The
Crown:
The crown is the top most part of the
piston. It is subjected to high temperature in the combustion space and the
surface is liable to eroded/burn away. For this reason material for which is
made up of must be able to maintain high strength and resist corrosion at high
temperature.
The
Ring lands:
Are the reliefs cut into the side profile
of the piston where the piston rings sit. The ring lands are typically taller
than the ring thickness which allows the rings to move and rotate in the bore.
It
also allows combustion pressure to contact
the entire piston ring top face inside the ringland pressing it down (and out
in some designs) improving ring seal.
The
Skirt:
The piston skirt is the extension of the
side profile of piston which controls the piston movement in the bore
preventing it from wobbling around and controlling the angular forces present
on the piston walls from the angular rotation of the crankshaft.
The
Underside:
This part of the piston is exposed to the
crank case and houses the wrist pin (connecting the piston to the rod) and
exposed to the engine oil in 3 ways:
Oil collected by the oil retention ring
(the bottom most piston ring) is routed through holes in the side of the piston
to the underside to drain back into the crank case.
Oil sloshing around in the crankcase due to
the crankshaft counterweights dipping in and out of the oil sump as well as oil
forced up through the connecting rod up to lubricate the wrist pin (on forced
oil pins).
On engines equipped with oil squirters
under the piston, where oil is squirted on the underside to help cool the
piston mass for longevity or racing applications, which in some situations may
also allow for an overall thinner crown without sacrificing the strength of the
piston and while reducing the overall weight of the package.
PISTON NOMENCLATURE
PISTON
MATERIAL:
Materials used in piston are, for the most
parts, either aluminium alloy or some form of cast iron. Various alloys of cast
iron are used including the special form Meehanite. A few engines use malleable cast iron. Pin
carrier inserts are generally cast iron but sometimes are made of heat treated
steel forgings. A few large assembled pistons have a separate crown made of
either cast steel or a steel forging.
The
coefficient of expansion, the increase in size per degree of temperature
increase of aluminium is approximately twice that of cast iron. This fact must
be taken into account when determining minimum piston clearance.
The
heat conductivity, the rate of heat flow, of aluminium is approximately three
times that of cast iron. The result is that an aluminium piston has less
variation in temperature from top to bottom.
The density of cast iron is three times as
much as aluminium. This does not mean that an aluminium piston weighs only a
third as much as a cast iron piston because strength and heat transfer problems
dictate that the metal sections of an aluminium pistons be made proportionately
thicker.
Manufacturing:
Die
Casting of Aluminium
Casting is the beginning of the piston. At
the foundry the die is prepared by heating it to operating temperature for
approximately one hour. This process allows the die to readily accept the
molten material when it is poured.
Process starts by heating the material to 700 degrees Celsius. This is
well above the melting point of the aluminum, but below its boiling point. The
material is then scooped up with a ladle from the crucible (the pot that holds
the molten material). This is then poured into the die through the sprue. The
material is then allowed to cool before it is removed from the die and placed
into a bin of hot water. This water is used to facilitate a more even settling
of the hot metal.
After the castings have had time to cool
they are placed into a heat treatment plant overnight. This process tempers the
casting and ensures the piston will have improved qualities.
After it is removed from the heat treatment
the casting has its runner removed.
Process Parameters:
Dies used are 5 piece and three piece.
These dies are made from cast iron
Temperature: 700 degrees Celsius
Pin
Boring
At this stage of the piston manufacturing
process the casting has the gudgeon pin hole rough machined and the locating
bung machined. This process is where the casting is machined on the base to
allow placement of the casting in other machines. This is carried out on a
simple lathe.
The
pin bore
Pin boring is done in conjunction with the bung turning, as one casting
is removed from having the bung face machined it is placed on the pin borer.
The pin borer is only a rough machining
process which allows the reamer to enter the gudgeon hole later.
CNC
Turning
Turning of the casting is carried out on
CNC (Computer Numeric Control) machinery. This equipment is the most accurate
and fastest available for this application with very tight tolerances and
extremely fast spindle speeds.
The castings are placed in the lathe on a bung and held in place by a
solid rod through the gudgeon pin hole. A draw bolt is activated in the chuck
which draws the rod toward the chuck and holds the piston in place.
The lathe is then started and the machining cycle begun. This cycle is
programmed into the lathe in a basic language called G-Code (this code is not
the only one available). G-Code has basic commands to tell the lathe to move to
certain positions (X,Y,Z co-ordinates), at particular spindle speeds (eg S2500
means spindle speed 2500rpm), at particular feed rates (eg G01; rapid traverse)
and other commands such as M01 (repeat programmed) and others.
After the piston is machined it is removed
from the lathe and the part number stamped on the crown (top) of the piston.
Machine specifications:
Machine used : CNC Lathe ECONO CNC 26.
Maker : HMT
Height of centres : 260 mm
Swing over bed : 575 mm
Swing over cross slide : 340 mm
Distance between centers : 1000 mm
Speed range : 40-2040 rpm
Process parameters:
Rough turning:
Speed : 25 m/s
Feed : 0.5 mm per rev
Depth of cut : 0.4 mm
Drilling
The first stages of the finishing process
include drilling, slotting, valve and crank relieving.
Drilling
Drilling includes all oil holes in places
such as the gudgeon pin bosses and oil ring grooves.
Slotting
Slotting is where slots are placed in the
skirt or in the oil ring groove.
Valve relieving
This process is done on a mill and involves
setting the machine up for the process, choosing the correct cutter and
completing the job. Since there are so many different types of valve reliefs it
is impossible to have a specialised machine set up to do one job.
Crank relieving
Crank relieving is carried out on a
specialised machine which scallops the skirt of the piston to the required
shape and depth by using two opposed cutters placed on a common shaft.
Grinding
This process involves the final size being machined on the piston. The
grinder machines the skirt of the piston only and in the majority of cases is
cam ground. Cam grinding ensures the piston will "grow" evenly in the
bore of the engine. A perfectly round piston will expand unevenly during use
because of the uneven placement of material in the casting (gudgeon pin bosses
and ribbing used for strengthening).
Machine specifications:
Machine used : CNC Grinding machine -
PMT-AWH-100
Maker : PMT
Grinding length (max) : 1000 mm
Height of centres : 450 mm
Grinding diameter (max) : 450 mm
Grinding wheel diameter : 350 mm
Process parameters:
Rough Grinding:
Speed of wheel : 20 m/s
Feed of workpiece : 1 mm per min
Depth of cut : 0.05 mm
Finish grinding:
Speed of wheel : 40 m/s
Feed of workpiece : 0.5 mm per min
Depth of cut : 0.02 mm
Reaming
Final machining process for the piston is that of reaming. This process
involves the piston being placed in a bath of oil and reamed at different sizes
to reach the final size required. Since the pin boring process is only rough it
is necessary to ream the pin bore a number of times to achieve the surface
finish and size required. Reaming is not a fast process and is only partially
automated (there are automatic feeds on the reaming machines). Tolerances
achieved on the finished reamed surface is 0.4Ra.
Radial drilling machine is used for
performing reaming operation.
Machine specifications:
Machine used : Radial drilling machine -
KML -40A
Maker : KML
Drill capacity (in steel) : 40 mm
Drill depth : 180 mm
Taper spindle nose socket :MT-4
No. of speeds and range : 6 (45-660 rpm)
Drill power : 10 KW
Process parameters:
Drilling:
Speed : 20 m/s
Feed : 0.1 mm per rev.
7.Pin
Fitting and Final Inspection
At this stage the piston is cleaned, fitted
with the appropriate gudgeon pin, stamped with the pistons' oversize and any
other markings, and then sent to despatch.
8.Despatch
Finally, the piston is wrapped and placed
in the shipping container with the ring set and sent to the customer.
References:
http://www.jp.com.au/Made.html
http://www.superchargerperformance.com/supercharger-power-parts/introduction-to-piston-design-for-forced-induction-engines
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