Dome PistonDesign and Manufacture of a Dome

Shaped Piston Head

April 10, 2015

List of all Slides and their Related Information Presented

Slide Information Presented

1 Group number, members, and project title

3 Overview of dome shaped piston head

4 Design focus for manufacturing process and final product

5 Detailed list of piston components

6 Detailed explanation of the expansion of the piston shape during operation

7 Piston rings and there functional purpose

8 List of all steps performed during the manufacturing process

9 Detailed description of casting and its related characteristics

10 Detailed description of forging and its related characteristics

11 Description of the heat treatment process and purpose

12 CNC Turning list of operation performed and characteristics of the CNC lathes

13 Drilling and grinding applications listed and described

14 Explanation of deburring and coating processes and their effects on the piston

15 Description of reaming and finish boring processes and matrices relating their individual pros and cons

16 Final inspection description

17 Performance and cost matrix related to the forging process

18 Performance and cost matrix related to two casting types

19 Performance and cost matrix related to coatings used on the piston head and skirt

20 -21 Manufacturing improvements and recommendations related to casting and forging

22 Cost and performance assessment matrix addressing each manufacturing process

23 Identifying the best technique and supporting information

24 Member contributions related to subject matter and percentage

25-26 List of all reference cited throughout the power point

• Increase in volume from the dome shape increases compression ratio. [15]

CR= (.25πb2 s+Vc )/Vc

Where: b=cylinder bore, s= piston stroke, Vc =clearance volume

• Dome shape deflects the inlet charge up toward the spark plug. [15]

• Flat portion decreases quenching effects. [15]

Dome Piston with Flat Portion

FOCUS REVIEW

• Improving the manufacturing process of the dome piston.

• Maximize life span based on manufacturing techniques (i.e. Forging and Casting).

• Identify cost benefits between casting and forging.

• Limit failure modes through the manufacturing process.

Crown Top of piston Combustion gases exert pressure on the

piston crown

Ring Land Sealing surface and support piston rings [8]

Relief cut into side profile where thepiston rings sit [8]

Ring Grooves Grooves used to retain piston rings [8]

Recessed area located around perimeter of the piston [8]

Skirt Portion of the piston closest to the crankshaft that

helps align the piston as it moves inside cylinder bore [8]

Wrist Pin boss Connects the small end of the connecting rod to the

piston by a wrist pin [10]

Aluminum expands so there must be an allowance between the piston diameter and the cylinder to account for the expansion, allowing the piston to move freely [13]

Top of piston (crown) experiences more heat than the bottom (skirt) so the crown will expand more[13]

Top diameter needs to be smaller than diameter of bottom of skirt, providing tapered shape[13]

The partial skirt lightens piston, thus increases the speed range of the engine and reduces the contact area between the cylinder wall, which decreases friction [14]

The skirt is also elliptical shape at room temperature

As the piston heats up, the pin bore area expands more than thinner areas of the piston making the piston shape become circular[ 13]

This circular shape matches the cylinder bore and improves sealing and efficiency[13]

Compression ring

Piston ring located in ring groove closest to the piston head [8]

Prevents oil from reaching combustion chamber[10]

Seals the combustion chamber from any leakage [10]

Wiper ring

Middle ring

Provides a consistent film of oil to lubricate the running of the compression ring [8]

Wipes away excess oil from cylinder wall

Combustion gasses which pass the compression ring are stopped by the wiper ring [8]

Oil ring

Piston ring located in the ring groove closest to the crankshaft [8]

Thin slots cut in ring to allow flow of excess oil back to oil basin [8]

Casting or Forging

Heat Treatment

Machining process

CNC turning

Drilling, slotting, and grinding

Deburring and Coating

Reaming or Finish boring

Cast pistons are made from an aluminum/silicon alloy (Hypereutectic ) Hypereutectic pistons have a lower coefficient

of thermal expansion than pure aluminum, so tighter tolerances can be set.[2]

Hypereutectic pistons are more brittle than pure aluminum.

Alloy is heated to 700°C , collected with a ladle and poured into mold, and cooled.[5]

Permanent molds are used and are typically made of cast iron.

The mold itself is expensive, but it is very durable so it can be reused for a long period of time.

Once the mold is made the cost per piston is very low.

High production rate

Forged pistons are mechanically shaped.[3]

A aluminum bar stock is cut into slugs that are then heated in an oven to about 425 °C

A mechanical or hydraulic press and die are preheated to the same temperature.[9]

The press applies 2,000 lbs of force to the heated slug shaping it to the desired shape.[9]

The forged piston then air cools slowly for roughly an hour.[9]

Forged pistons are more ductile and dense than cast pistons. [2]

Forging eliminates porosity. Tools used are expensive, develop wear more quickly, and are

slower compared to casting tools. More expensive final product Tend to go into plastic deformation when overloaded Engines > 500 hp will always have forged pistons

Diesel engines and race cars

Piston is placed into an oven twice

First time is at a higher temperature to strengthen the material [10]

Second is at a lower temperature to stabilize the material [10]

Controlled heating and cooling to change physical and mechanical properties of the piston, but maintain the same shape.

Increases strength and hardness

Simplifies machining processes

For the heated treated piston, CNC will perform the following: Facing Piston is cut down to desired bore diameter Oil ring grooves are cut Boring

CNC lathes are very accurate and are capable of holding tight tolerances.

The cycle is programmed in G-Code which tells the lathe to move to certain (X, Y, Z coordinates) at specific spindle speeds and feed rates.

CNC lathes are expensive and the computer training to operate them is intensive.

CNC lathes are easily programmable and processes can be repeated.

Drilling Drill Press

All oil holes (i.e. gudgeon pin, bosses, and oil rings)

Slotting Milling machine

Piston skirt or in oil ring groove

Grinding Only the skirt of the piston is grinded.

The skirt is usually cam grinded, ensures the expansion of the piston will be uniform in the bore of the engine.

Deburring tool is used to remove any unwanted material on piston surface

Piston is cleaned to remove oil, dirt, residue, etc.

Coating is typically sprayed on Dry film lubricants

Reduce friction, reduces scuffing, extends bore life[4]

Good safety margin[4]

Applied to piston skirts[4]

Thermal barriers Transfer heat and prevent hot spots on piston

face[4]

Results in less fuel needed for desired power[4]

Applied to piston crowns[4]

Oil shedding Increase cooling efficiency by not allowing oil

to coat certain surfaces[4]

Applied to piston bottoms[4]

Final process

Reaming or finish boring makes existing holes dimensionally more accurate and improves surface finish. [5]

Piston is placed in bath of oil and reamed at different size to reach desired size. [5]

A typical tolerance is about 0.4Ra. [5]

Pros Cons

Multiple cutting edges, so tool life is longer

Requires coolant

Can hold tighter tolerances

Time consuming

Pros Cons

More flexible and forgiving

Can’t hold tight tolerances

Precise hole location is less critical

One cutting tooth, so tool life is lower

Piston is cleaned of any residues left over from the manufacturing process [5]

Fitted with appropriate wrist pin [5]

Stamped with the piston’s overall size and any other manufacturer’s markings [5]

Manufacturing Process

Pros Cons CostAverage Lifespan

Forging

•Stronger than cast pistons [3]

•Less porosity and closer alloy

grains due to not using a cast.[3]

•Greater dimension stability [3]

•Dissipates heat better and can

with stand greater operating

temperatures [3]

•The dense, stretched, and strained material makeup of a forged

piston doesn't heat up to the operating

temperature as quickly as a cast piston. [2]

•They are more likely to cold seize.

•The high pressures required for the

operation increase wear on the dies per

run, which are costly to replace.

Higher than cast

pistons.

2X Lifespan of Cast Pistons

Manufacturing

ProcessPros Cons Cost

Average

Lifespan

Casting Types

Cast Aluminum

Piston with

Steel Struts

•Help control flow of heat

from combustion process

•Lightweight reduces

force required to initiate

and maintain

acceleration. [9]

•Can be embedded into

the piston assembly to

help control piston

expansion

•Lower strength than forged

pistons

•Contains neither grain flow

or directional strength[10]

•Fractures easier under

detonation and has few

options for compression and

rod length [2]

Less costly

than forged

pistons.[3]

1/2 Lifespan of

Forged Pistons

Hypereutectic

Piston

•Stronger than 100%

aluminum piston [3]

•Hypereutectic pistons

expand less than ordinary

cast aluminum alloys, and

CNC machining of the

piston profile allows

piston-to-bore clearances

to be reduced. [3]

•Reduces heat transfer. [3]

•Require close control of

melting and cooling process

because alloy separation may

occur. [1]

•Not easily modified.

Less costly

than forged

pistons.

1/2 Lifespan of

Forged Pistons.

Greater lifespan

than cast

aluminum with

struts.

Coatings Pros Cons Life-span

0.002" Ceramic Thermal Barrier

Coating

•Holds heat inside combustion

chamber reducing dissipation

through the piston that can weaken or

burn the metal.• Thin coating does not effect

clearance. [6]

Increases the cost

Increases

0.008" Tungsten-Molybdenum

Disulfide Polymer Matrix

•Reduces friction between piston

skirt and cylinder wall.[6]

Increases the cost

Increases

Forging■ Increase material resistance to

cracking by properly aligning the grain flow with the crack propagation direction during the extrusion process. [12]

■ Heat the press and die to the same temperature as the slug (425 C) so the slug is not cooled when put into the press.[9]

■ Allow roughly formed piston to air cool after it has been heat treated in an oven after pressed. This will allow for the molecular structure of the piston to reach a lower energy state and be therefore more uniform molecular.[11]

Result: Increases the strength of the forged piston

Casting

■ As the ratio of silicon to aluminum is increased the more brittle they become, as well as the coefficient of thermal expansion reduces and the piston expands less. A ratio of 16%- 19% is recommended.[3]

• Less than 12% Piston will expand too much.

• 25% and on Piston will be excessively brittle and loose strength. [ 3]

Result: Reduces piston expansion under during operation and increased strength without sacrificing too much ductility.

Forging

■ Apply adequate force (2000 tons) to the heated slug of aluminum in the press

Result: Decrease porosity, and compact grain flows.

■ Heat treat the pressed piston twice.

• First time at higher temperature to ensure the entire volume is heated to the same temperature.

• Second time to a lower temperature to allow for the molecular structure to stabilize. [11]

Casting

■ Repeatedly measure the dimensions of the cast for accuracy after a specific number of casting cycles.

Results: Dimensionally consistent casts

■ Reduce the amount of hydrogen gas in the molten aluminum prior to injecting the cast. Use high quality aluminum.

■ Use a vacuum while pouring the molten Al alloy to limit the porosity of the cast due to gas bubbles. [10]

Result: Decreases the porosity of the casted piston, thus increasing strength.

Manufacturing Processes for Dome Shaped Piston Head

Caste

d

Fo

rge

d

Rea

min

g

Fin

ish B

orin

g

He

at T

rea

tme

nt

No

He

at T

rea

tme

nt

Dry

Film

Lu

brica

nt

Th

erm

al B

arr

ier

Oil

Sh

ed

din

g

Drilli

ng O

il H

ole

s

Grin

din

g o

f P

isto

n S

kirt

Drilli

ng P

isto

n S

kirt

or

Oil

Rin

g

Gro

ove

0.0

02

" C

era

mic

Th

erm

al B

arr

ier

Coa

tin

g

0.0

08

" T

ungste

n-M

oly

bd

enu

m

Dis

ulfid

e P

oly

me

r M

atr

ix

CN

C T

urin

g L

ath

es

Manufacturing Process Influence on Piston

Quality and Cost Correlations

Cost 1 3 1 2 1 1 2 2 1 2 1 1 2 2 2

Equipment Cost 3 3 3 3 3 1 2 2 2 2 3 3 3 3 3

Manufacturing time 1 2 3 2 2 1 2 2 1 2 2 2 2 2 3

Strength 2 3 1 1 3 1 1 1 1 1 1 1 1 1 1

Thermal Expansion 3 2 1 1 2 1 2 3 2 2 1 1 3 2 1

Ductility 1 3 1 1 3 1 1 1 1 1 1 1 1 1 1

Friction 1 2 1 1 1 1 3 1 2 1 1 1 1 3 1

Porosity 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Density 2 3 1 1 2 1 1 1 1 1 1 1 1 1 1

Precision of Piston

Dimensions 2 3 3 1 1 1 2 2 1 3 2 1 2 2 3

Lifespan of Piston 2 3 1 1 3 1 3 3 3 3 3 3 3 3 2

Ranking: 8% 10% 7% 6% 7% 4% 7% 6% 5% 7% 6% 6% 7% 7% 7%

Forging is the Best Technique of the Manufacturing Process.

• Largest influence on ductility of the piston head.

• Greater dimensional stability effectively decreasing machining time

• Produces the greatest strength resulting from the grain flow in the extruded material

• Eliminates porosity in the material

• Extruded product dissipates heat better than casting and can withstand greater operating temperatures

• Forged pistons have a greater life span

[1] “Aluminum Piston Manufacturing Process.” Cast and Alloys. 05 May 2015 <http://www.cast-alloys.com/products/aluminium_piston_manu_process.htm>

[2] “Cast and Forged Pistons.” Tech Speak. 05 May 2015 http://www.hoon.tk/tech_tips/pistons.html

[3] “Cast, Hypereutectic or Forged Pistons.” Probe Industries. 05 May 2015 <http://www.probeindustries.com/Articles.asp?ID=144>

[4] “Coating Pistons.” Tech Line Coatings, Inc. 05 May 2015 <http://techlinecoatings.com/articles/Coating_Pistons_Article.htm>

[5] “How Pistons are Made” JP Pistons. 05 May 2015 http://www.jp.com.au/Made.html[6] “Piston Selection Guidelines.” Federal Mogul Technical Education Center. 05 May 2015

<http://fme-cat.com/docs/1104.pdf>[7] “Ultimate Ford FE Engine Piston Guide.” DIY Ford. 05 May 2015

<http://diyford.com/ultimate-ford-fe-engine-piston-guide/>[8] “Piston and Piston Rings.” Univsersity of Windsor. 03 May 2015

<http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20and%20Piston%20Rings.htm>

[9] “Piston Manufacturing Process.” Thomas McNish. 03 May 2015 <http://www.ehow.com/how-does_5502005_piston-manufacturing-process.html>

[10] “Casting Defects and Design Issures.” Prof. J.S. Colton. 03 May 2015 <http://www-old.me.gatech.edu/jonathan.colton/me4210/castdefect.pdf>

[11] “Heat Treatment of Ferrous Metals.” 03 May 2015 <http://avstop.com/ac/apgeneral/heattreatmentofferrousmetals.html>

[12] “Failures Related to Metal Working.” Fudan University. 04 May 2015<http://jpkc.fudan.edu.cn/picture/article/348/1b/ee/6dce0ae740cf8673b53e4e96abb8/7aa78636-dda1-46b9-859b-95db3cb616f8.pdf>

[13] “Piston Design.” University of Windsor. 05 May 2015 <http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20Design.htm>

[14] “Piston Assembly” SweetHaven Publishing Services. 05 May 2015 <http://www.waybuilder.net/sweethaven/MechTech/Automotive01/?unNum=2&lesNum=3&modNum=1>

[15] “Piston Dome.”Performance Trends Inc. 02 May 2015 <http://performancetrends.com/Definitions/Piston-Dome.htm>