Plastic versus Steel: An Automotive Fuel Tank Case Study Using the ...

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Plastic versus Steel:

An Automotive Fuel Tank Case Study

Using the 2013 GM Cadillac ATS Platform

Eric Neuwirth

Spectra Premium Industries


Outline

• Case Study Review ― Design Requirements ― Design Overview ― Forming Analysis ― Manufacturability ― Fuel Capacity / Grade Venting Studies ― Mass ― Pressure / Vacuum Cyclic Fatigue

• Summary ― Conclusions ― Additional Opportunities


Design Requirements

• Atmospheric (non-pressurized) system

• Steel fuel tank requirements

― must fit existing 2013 Cadillac ATS package space, while maintaining appropriate clearances

― must be formable using commercially available steel grades

― must be manufacturable using standard equipment

― must meet or exceed fuel volume of existing plastic fuel tank

― must have a mass that is equivalent to or less than the mass of existing plastic fuel tank

― must meet applicable fuel tank pressure/vacuum cycling durability requirements for an atmospheric system:

― 12,000 PV cycles + 50% safety factor

o Pressure: 14.9 kPa

o Vacuum: 7.0 kPa


Design Requirements

• Based on the design assumptions listed on the previous slide, two

different steel tanks have been designed:

– Volume-Maximizing Steel Tank

o Seeks to maximize usable fuel volume to a level greater than that of

the plastic fuel tank while still maintaining a mass less than that of

the plastic fuel tank

– Volume-Equivalent Steel Tank

o Seeks to achieve a usable fuel volume equal to that of the plastic

fuel tank while achieving a mass significantly less than that of the

plastic fuel tank


Design Requirements

Steel fuel tank must fit existing

2013 Cadillac ATS package space,

while maintaining appropriate clearances.


Design Overview:

Volume-Maximizing Steel Tank


Design Overview:


SECTION at Y=zero


Design Overview:



Design Overview:

Volume-Equivalent Steel Tank


Design Overview:


INTERNAL VAPOR MANAGEMENT STEEL : GREEN PLASTIC : PURPLE


Forming Analysis

Steel fuel tank must be formable

using commercially available steel grades.


Forming Analysis



Forming Analysis:


Top Shell Thinning at 0.7 mm nominal


Forming Analysis:


Top Shell Forming Limit Diagram


Forming Analysis:


Bottom Shell Thinning at 0.65 mm nominal


Forming Analysis:


Bottom Shell Forming Limit Diagram


Forming Analysis



Forming Analysis:


Top Shell Thinning at 0.67 mm nominal


Forming Analysis:


Top Shell Forming Limit Diagram


Forming Analysis:


Bottom Shell Thinning at 0.65 mm nominal


Forming Analysis:


Bottom Shell Forming Limit Diagram


Manufacturability

Steel fuel tank must be manufacturable

using standard equipment.


Manufacturability

The following slide shows the relevant Contour II design guidelines, as published by welding

equipment manufacturer Soutec.

Both the Volume-Maximizing tank and the

Volume-Equivalent tank adhere to these guidelines.


Manufacturability


Manufacturability

Andritz Soutec AG Contour II

Fuel Tank Welding Machine


Fuel Capacity / Grade Venting Studies

Steel fuel tank must meet or exceed

fuel volume of existing plastic fuel tank.



Grade Venting Requirements

When filled to capacity, the fuel tank assembly must be capable of

venting when the vehicle is inclined up to 30% in the four primary

orientations, and up to 27% in the four secondary orientations,

accounting for thermal expansion of the fuel. The fuel tank capacity

used shall be the “Customer Fill Fuel Capacity” plus an additional

4% for fuel expansion for grades less than or equal to 6%, and 2.2%

for fuel expansion for grades greater than 6% up to 30%.

The fuel tank assembly shall be designed to address either a failed

FLVV or GVV with the vehicle on these grades.


Fuel Capacity / Grade Venting Studies:


30% STANDARD GRADES

FRONT UP REAR UP

RIGHT SIDE UP LEFT SIDE UP




0

180

45

90

135

315

270

225

27% COMPOUND GRADES




LEVEL




Usable Volume* Vapor Space Limiting

(%) (degrees) (gallons) (%) Factor

0° Front Up 30 16.7 19.9 7.4 GVV

45° Right Front Up 27 15.1 20.1 6.4 X-connector

90° Right Up 30 16.7 18.9 12.0 X-connector

135° Right Rear Up 27 15.1 19.9 7.2 X-connector & GVV

180° Rear Up 30 16.7 19.4 9.6 GVV

225° Left Rear Up 27 15.1 19.7 8.2 X-connector

270° Left Up 30 16.7 18.8 12.5 X-connector

315° Left Front Up 27 15.1 19.8 7.9 X-connector

n/a Level 0 0 19.6 8.6 GVV

FLVV Shutoff Height (maximum) 19.1

* Usable volume shown is net of a 0.5-gallon contingency (design safety factor) to account for the effect of unusable fuel.

Usable volume shown accounts for 2.2% thermal expansion of fuel on non-zero grades.

Usable volume shown accounts for 4.0% thermal expansion of fuel at level (zero grade).

GradeOrientation

GR

AD

E V

EN

TIN

G




(gallons) (liters)

Volume-Maximizing Steel Tank 18.8 71.2

Production Plastic Tank* 16.5 62.5

Steel Tank Advantage 2.3 8.7

* Production plastic tank volume provided by General Motors Product Engineering. Advertised volume is 16.0 gallons (60.6 liters).

Usable Fuel Volume




Cut-out for serviceable sub-side module

But what if the sub-side module were serviceable?

If the steel tank sub-side module were serviceable, the steel tank usable volume would be reduced by only 0.87 gallons (3.3 L), resulting in a steel tank usable volume of 17.9 gallons (67.8 L), which is still 1.4 gallons (5.3 L) more useable fuel than the plastic tank capacity.




30% STANDARD GRADES FRONT UP REAR UP

RIGHT SIDE UP LEFT SIDE UP




0

180

45

90

135

315

270

225

27% COMPOUND GRADES




LEVEL




Usable Volume* Vapor Space Limiting

(%) (degrees) (gallons) (%) Factor

0° Front Up 30 16.7 17.5 8.0 GVV

45° Right Front Up 27 15.1 17.8 6.8 X-connector

90° Right Up 30 16.7 16.6 12.9 X-connector

135° Right Rear Up 27 15.1 17.5 8.0 X-connector & GVV

180° Rear Up 30 16.7 17.3 8.9 GVV

225° Left Rear Up 27 15.1 17.5 8.4 X-connector

270° Left Up 30 16.7 16.5 13.7 X-connector

315° Left Front Up 27 15.1 17.5 8.2 X-connector

n/a Level 0 0 17.4 8.9 GVV

FLVV Shutoff Height (maximum) 16.5

* Usable volume shown is net of a 0.5-gallon contingency (design safety factor) to account for the effect of unusable fuel.

Usable volume shown accounts for 2.2% thermal expansion of fuel on non-zero grades.

Usable volume shown accounts for 4.0% thermal expansion of fuel at level (zero grade).

GradeOrientation

GR

AD

E V

EN

TIN

G


Mass

Steel fuel tank must have

a mass that is equivalent to or less than

the mass of existing plastic fuel tank.


Mass:


(pounds) (kg)

Plastic Fuel Tank 17.5 7.92

Heat Shield 1.1 0.50

Total Assembly,Plastic Fuel Tank

18.6 8.42

(pounds) (kg)

Steel Fuel Tank 16.9 7.66


Total Assembly,Volume-Maximizing

Steel Fuel Tank16.9 7.66

(pounds) (kg)

Mass Savingswith Steel

1.7 0.76

Mass

Mass

Mass

Despite the 2.3-gallon (8.7-liter) usable fuel advantage of the Volume-Maximizing steel tank, the mass of the steel tank is still 1.7 pounds (0.76 kg) less than the mass of the production plastic tank.


Mass:


Flangeless Alternative

If a less conventional welding method were used which would eliminate the need for a weld flange, the mass impact of removing weld flange would be a further reduction of 1.1 lbs (0.5 kg), resulting in a total mass improvement of

2.8 lbs (1.26 kg) compared to the production plastic fuel tank: Top Shell = 3.64 kg (Δ = -0.26 kg)

Bottom Shell = 3.52 kg (Δ = -0.24 kg)

Total = 7.16 kg (Δ = -0.50 kg)


Mass:


(pounds) (kg)

Plastic Fuel Tank 17.5 7.92


Total Assembly,Plastic Fuel Tank

18.6 8.42

(pounds) (kg)

Steel Fuel Tank 15.5 7.03


Total Assembly,Volume-Maximizing

Steel Fuel Tank15.5 7.03

(pounds) (kg)

Mass Savingswith Steel

3.1 1.39

Mass

Mass

Mass

In the case of the Volume-Equivalent steel tank, the mass benefit is even greater. This fully functional design saves 3.1 pounds (1.39 kg) compared to the production plastic tank.


Pressure / Vacuum Cyclic Fatigue

Steel fuel tank must meet

pressure / vacuum cycling durability requirements

for an atmospheric system:

12,000 PV cycles + 50% safety factor

Pressure: 14.9 kPa

Vacuum: 7.0 kPa


Pressure / Vacuum Cyclic Fatigue:





Loading Details

Load Optistruct Equation Location

Hydro 7.23213e-6*(898-z) All elements below Z height of 898 mm

Positive Pressure 14.9 kPa All internal elements

Negative Pressure 7 kPa All internal elements

Pre-load Z=3 mm Strap ends




Material Properties

Shell Top Bottom

Name EDDS EDDS

Young’s Modulus 210,000 MPa 210,000 MPa

Yield 152 MPa 152 MPa

UTS 306 MPa 306 MPa

Optistruct Fatigue Parameters

Sf' 607 607

b -0.116 -0.116

c -0.437 -0.437

Ef' 0.125 0.125

n' 0.234 0.234

K' 832.0 832.0

Nc 2.0E+08 2.0E+08




Shell Fatigue Life

(cycles)

Top 17,975

Bottom 19,382




Fatigue - Top




Fatigue - Bottom




Displacement (+14.9 kPa)




Displacement (-7 kPa)




Stress - Top Shell (+14.9 kPa)




Stress - Bottom Shell (+14.9 kPa)




Stress - Top Shell (-7 kPa)




Stress - Bottom Shell (-7 kPa)




PRT-00001567/AA.036




Loading Details

Load Optistruct Equation Location

Hydro 7.23213e-6*(898-z) All elements below Z height of 898 mm

Positive Pressure 14.9 kPa All internal elements

Negative Pressure 7 kPa All internal elements

Pre-load Z=3 mm Strap ends




Shell Top Bottom

Name EDDS EDDS

Young’s Modulus 210 000 MPa 210 000 MPa

Yield 150 MPa 150 MPa

UTS 270 MPa 270 MPa

Optistruct Fatigue Parameters

Sf' 405 405

b -0.087 -0.087

c -0.58 -0.58

Ef' 0.59 0.59

n' 0.15 0.15

K' 445 445

Nc 2.0E+08 2.0E+08

Material Properties




Shell Fatigue Life

(cycles)

Top 36,285

Bottom 20,453




Fatigue - Top




Fatigue - Bottom




Displacement (+14.9 kPa)




Displacement (-7 kPa)




Stress - Top Shell (+14.9 kPa)




Stress - Bottom Shell (+14.9 kPa)




Stress - Top Shell (-7 kPa)




Stress - Bottom Shell (-7 kPa)


Summary: Conclusions

• Two different steel fuel tanks – a volume-maximizing tank and a volume-equivalent tank – have been designed to fit the existing 2013 Cadillac ATS package space, while maintaining appropriate clearances.

• These steel fuel tanks

– are both formable using commercially available steel grades.

– are both manufacturable using standard equipment.

– have a usable fuel volume that exceeds the usable fuel volume of the existing plastic fuel tank by up to 8.7 L (2.3 gallons).

– are as much as 1.39 kg (3.06 lbs) lighter than the existing plastic fuel tank, not including the additional mass avoidance with the flangeless alternative.

– both meet the fuel tank pressure / vacuum cycling durability requirements specified for an atmospheric system, including a 50% safety factor.


Summary: Additional Opportunities

• The results presented here are a work-in-progress. It is possible to

continue to improve the 2013 Cadillac ATS steel fuel tank designs with the

following goals:

– Achieve nominal gauge of 0.6 mm through further topography

optimization.

– Design slosh baffles that are also structural, thereby allowing a

further reduction in shell gauge.

– Further reduce mass through the use of alternative steels such as

advanced high strength steels or stainless steels.


For More Information

Visit: www.autosteel.org

@SMDISteel

www.facebook.com/SMDISteel Rich Cover Program Manager, SASFT +1 (248) 762-7732 [email protected] Eric Neuwirth Spectra Premium Industries

+1 (248) 207-5509

[email protected]

http://www.autosteel.org/

mailto:[email protected]

mailto:[email protected]


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