Lightweight Crankshafts

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SAE TECHNICAL PAPER SERIES

2006-01-0016

Lightweight Crankshafts Alan P. Druschitz, David C. Fitzgerald and Inge Hoegfeldt INTERMET Corporation

Reprinted From: From: New SI Engine Engine and Component Design 2006 (SP-2004)

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2006-01-0016

Lightweight Crankshafts Alan P. Druschitz, David C. Fitzgerald and Inge Hoegfeldt INTERMET Corporation

Copyright © 2006 SAE International

ABSTRACT The automotive industry continues to look for opportunities to reduce weight and cost while simultaneously increasing performance and durability. Since the introduction of aluminum cylinder blocks and heads, very few “innovations” have been made in powertrain design and materials. Cast crankshafts have the potential to produce significant weight savings (3-18 kg) with little or no cost penalty. With the advent of new, high strength, cast ductile iron materials, such as TM MADI (machinable austempered ductile iron), which has the highly desirable combination of good strength, good toughness, good machinability and low cost, lightweight crankshafts are posed to become a high volume production reality. An extreme demonstration of a lightweight crankshaft is the current use of a cast MADI crankshaft in the 1100 HP Darrell Cox subcompact drag race car. This paper provides examples of lightweight crankshaft designs and a comparison of machinability, fatigue performance and vehicle performance of regular cast ductile iron, regular cast austempered ductile iron, cast MADI and forged steel.

BACKGROUND The automotive industry continues to look for opportunities to reduce weight and cost while simultaneously increasing performance and durability. After the introduction of aluminum cylinder blocks and heads, very few “innovations” have been made in powertrain materials. Some notable exceptions are titanium connecting rods, intake valves and pushrods in the Z06 Corvette and compacted graphite iron bedplates in the DaimlerChrysler 4.7 liter engine [1]. The crankshaft is a relatively heavy component (15-45 kg) and has received little “light-weighting” attention. Due to the difference in material density alone, a cast ductile iron crankshaft would weigh ~10% less than a forged steel crankshaft of identical design. Further, castings can be produced with weight reducing features that would require the machining of forged products, such as hollow pins and mains and hollowed-out

counterweights. The use of forged steel crankshafts results in not only higher weight, but it also results in higher total component cost due to higher material, production and machining costs. However, the recent trend in automotive crankshaft design has been to replace cast ductile iron with heavier forged steel. Often, the decision to switch is based on an anticipated NVH benefit due, presumably, to the higher elastic modulus of steel. The other reason often cited is the need for higher strength. However, a number of cast ductile irons have strength equal to or better than many forged steels typically used for automotive crankshafts. In the early 1990s, Sumitomo Metal Industries presented data comparing the NVH performance of forged steel crankshafts to cast ductile iron crankshafts [2]. In their study, a forged steel crankshaft produced, at best, 2-3 dB less noise at a distance of one meter from the right side of the cylinder block. No mention of torsional vibration dampers was made in their study. In 1999, a study was published that concluded that a conventional, cast ductile iron crankshaft with a suitably designed damper was optimal for cost, weight and NVH [3]. This study used a production, Ford, Duratec, 2.5 liter, V-6 engine, which had an aluminum cylinder block and forged steel crankshaft. Ricardo, Inc. measured and reverse engineered the production crank and then developed two new crankshaft designs; a direct copy of the production, forged steel design and a four counterweight, hollow pin, lightweight design. Intermet Corp. designers took the preliminary crankshaft designs and produced final casting designs. Intermet designers also demonstrated potential reductions in machining by developing the lightweight version as a cheekless and topless design. The Intermet Research Foundry developed the material specification and produced prototype castings. Kellogg Crankshaft Company machined the crankshafts and Hegenscheidt-MFD performed fillet rolling as would be used in production. Ricardo, Inc. performed NVH testing on the production engine and predicted the performance of the cast ductile iron crankshaft designs with and without dampers. Using Ricardo’s predictions, dampers were developed by Simpson Industries, Inc. (now a part of Metaldyne). The


program was brought to a conclusion when Ricardo, Inc. performed NVH testing on the engine after being rebuilt with the new cast ductile iron crankshaft-damper combinations. This NVH testing revealed the sound pressure levels were as good or better than the production forged steel crankshaft-damper combination at less than 4500 rpm and that bedplate and engine mount accelerations were the same or lower than the production forged steel crankshaft-damper combination throughout the rpm range tested. Recently, a new high strength and easy to machine cast ductile iron, MADI, was developed to match the strength of forged steel [4,5,6]. This paper contains static and dynamic property data, component performance and machinability data and preliminary vehicle test data for crankshafts produced from this new material and compares this data to crankshafts produced from regular cast ductile iron, regular cast austempered ductile iron and forged steel. Lightweight crankshaft design concepts are also present. PROCEDURES LIGHTWEIGHT CRANKSHAFT DESIGN

CRANKSHAFT FATIGUE (RIG) TESTING Hegenscheidt-MFD Corp. has estimated that fatigue life can easily be improved by 70-100% based upon crankshaft undercut geometry, material and deep rolling force but the improvement is material dependent. Typically, metals with high ductility and high work hardening rate show a greater improvement in fatigue strength after fillet rolling. To determine the influence of deep fillet rolling and to compare crankshafts produced from MADI to current production crankshafts, sections of crankshafts were tested on a crankshaft magnetic resonance bending fatigue test rig. VEHICLE TESTING MADI crankshafts are currently being evaluated on the street and on the race track in 450-500 HP street cars and the 1100 HP Darrell Cox sub-compact drag race car. These crankshafts were cast at the Intermet Archer Creek Foundry, heat treated at a high volume, production-intent heat treat source, fully machined on the production line at Macimex (Mexico) with no changes in tooling or machine tool speeds and feeds and assembled into 2.4 liter engines by Darrell Cox Racing.

Lightweight crankshaft designs were developed and shown to have the potential to provide 10-50% reduction in weight (2-20 kg).

RESULTS & DISCUSSION

MATERIAL EVALUATION

Lightweight crankshaft designs have been developed, modeled and shown to have the potential to provide 1050% reduction in weight (2-20 kg). Using a V-8 crankshaft as an example, the weight savings of simple and aggressive designs were calculated. The typical “solid” design weighed 23.6 kg (52.0 lbs) and is shown in Figure 1. First, a simple “cope/drag” lightweight counterweight design was developed that produced a weight savings of 2.9 kg (6.5 lbs or 12.5%). Next, a simple “cored” pin and main design was developed that produced a weight savings of 3.1 kg (6.8 lbs or 13%). Next, an aggressive “cored” pin, main and counterweight designs was developed that produced a weight savings of 11.8 kg (25.9 lbs or 50%). The model of the simple “cope/drag” is shown in Figure 2, the model of the “simple” hollow main design is shown in Figure 3 and the model of the “aggressive” lightweight crankshaft design showing hollow pins, mains and counterweights is shown in Figure 4.

Various OEM cast and forged crankshafts were purchased, sectioned and the hardness and tensile properties determined. A variety of crankshafts (for 1.8 liter to 5.7 liter engines) were produced in MADI at the Intermet Archer Creek Foundry for internal evaluation and evaluation by various North American and European OEMS. The MADI castings were produced on the production green sand molding line and no changes were made to the production tooling. Internal evaluation included hardness, tensile properties at room temperature in the as-heat treated condition, after aging o o at elevated temperature (150 C, 300 F) and upon exposure to various liquids, coefficient of thermal expansion, machinability and in-vehicle testing. OEM evaluation included machining trials on production machine lines and laboratory component fatigue tests. MACHINABILITY TESTING Machinability testing was performed by the Machine Tool Systems Research Laboratory at the University of Illinois at Urbana-Champaign, which has extensive machine tool and instrumentation facilities for conducting machinability studies, and on OEM production machine lines. For the OEM production machine line trials, no changes were made to the tooling or the equipment speeds and feeds.

LIGHTWEIGHT CRANKSHAFT DESIGN


Figure 1. Model of a Standard, “Solid”, V-8 Crankshaft Design. The weight for this V-8 crankshaft design was 23.6 kg (52.0 lbs).

Figure 4. Model of the Aggressive Lightweight V-8 Crankshaft Design. Note the hollow pins, mains and counterweights. The potential weight savings for this V-8 crankshaft design was 11.8 kg (25.9 lbs or 50%).

The lightweight features in two of these cast crankshafts were produced using cores. No cores are required to produce the design shown in Figure 2. The cores required to produce these features are shown in Figures 5 and 6.

Figure 2. Model of the Simple “cope/drag” Lightweight V-8 Crankshaft Design. Note the lightweight counterweights. The potential weight savings for this V-8 crankshaft design was 2.9 kg (6.5 lbs or 12.5%).

Figure 5. Model of the Core Required for the Simple Lightweight V-8 Crankshaft Design.

Figure 3. Model of the Simple Lightweight V-8 Crankshaft Design. Note the hollow mains. The potential weight savings for this V-8 crankshaft design was 3.1 kg (6.8 lbs or 13%).

Figure 6. Model of the Core Required for the Aggressive Lightweight V-8 Crankshaft Design.


A very aggressive lightweight, cast, austempered ductile iron, V-10, crankshaft that had a weight savings of 20.5 kg (45.1 lbs or 45%) showing hollow pins, mains and counterweights is shown in Figure 7.

Table I. Comparison properties.

Crankshaft

Figure 7. Fully Machined Casting of an Aggressive Lightweight V-10 Crankshaft Produced in Grade 1 ADI. Note the hollow pins, mains and counterweights. The weight savings for this V-10 crankshaft was 20.5 kg (45 lbs or 45%).

MATERIAL EVALUATION The material property requirements of lightweight crankshafts are more demanding that those for a standard crankshaft design. Therefore, higher strength cast ductile irons were required. At first, Grade 1 ADI was investigated, but due to the poor machinability of Grade 1 ADI, a new machinable, austempered ductile iron (MADI) was developed. The mechanical properties of MADI are superior to typical “crankshaft” cast ductile irons (D5203, D5506, DC Hi-Hard) and comparable to many forged steel crankshafts. A comparison of the mechanical properties of MADI, regular cast ductile iron, regular austempered ductile iron and forged steel crankshafts are shown in Table I. Steel, ADI and MADI exhibit beneficial work hardening that results in an increase in yield strength when prestrained. This is a desirable characteristic since cast crankshafts are typically fillet rolled (cold worked) to increase fatigue strength. Five percent pre-strain increased the yield strength of these materials by 5060%. The effect pre-strain on the tensile properties of steel, ADI and MADI are shown in Table II.

1 2 3 4

of

crankshaft

Material

UTS

mechanical

Average (unless noted) Hardness YS Elong

(MPa)

(MPa)

(%)

(BHN)

DC 1.8 liter DC 2.4 liter To ota 2.2 liter 1 Toyota 2.5 liter Ford 2.5 liter General Motors

forged steel for ed steel for ed steel forged steel forged steel for ed steel

778 963 806 851 776 2 850

462 648 508 553 456 2 580

12.4 9.5 12.9 7.3 10.9

233 273-288 242 259-273 248 248-302

D5203 5.7 liter D5506 5.7 liter 3 DC “Hi-Hard”

cast DI cast DI cast DI

610 707 783

402 407 438

2.7 6.8 4.0

225 225 253

MADI 1.8 liter 4 MADI 2.4 liter MADI 4.7 liter MADI 5.7 liter

cast DI cast DI cast DI cast DI

796 834 774 748

488 506 502 492

12.0 15.3 12.9 10.4

249 258 246 244

ADI 4.7 liter cast DI 1002 634 6.5 ADI 5.7 liter cast DI 1009 644 9.9 ADI 5.7 liter cast DI 1076 748 8.9 Titan Racing (1500 HP, drag race car) crankshaft minimums DaimlerChrysler production 2.4 liter “high-hardness” ductile crankshaft Darrell Cox Racing (1100 HP, drag race car) crankshaft

312 298 325

iron

Table II. Effect of pre-strain on the tensile properties of steel, ADI and MADI. Material

Pre-Strain (%)

UTS (MPa)

YS (MPa)

Elong (%)

steel

0 1 3 5

950 945 949 971

633 703 865 949

11.6 8.9 7.3 4.2

ADI

0 1 3 5

1032 953 1016 1009

627 760 919 1001

8.4 4.8 4.7 1.0

MADI

0 1 3 5 7

748 756 759 798 792

492 585 673 735 764

10.4 9.9 9.0 9.1 4.9

MADI and ADI can exhibit beneficial age hardening with little or no loss of ductility when exposed to powertrain temperatures for extended periods of time. For the TM MADI and ADI chemistries and heat treatment cycles used by Intermet, an increase in yield strength of 13% and 38%, respectively, after 2000 hours of exposure at o o 150 C (300 F) was noted. Beyond 2000 hours, the improvement is negligible. This finding demonstrated that MADI and ADI powertrain components will maintain their properties throughout the product life cycle. The o o effect of exposure at 150 C (300 F) for up to 4000 hours is shown in Table III.


o

o

Table III. Effect of exposure to 150 C (300 F) on the mechanical properties of MADI and ADI. Average YS Elong

Exposure Time (hrs)

(MPa)

(MPa)

(%)

Hardness (BHN)

MADI

0 1000 2000 3000 4000

795 839 869 871 869

489 544 554 552 546

11.9 11.6 13.0 11.0 10.9

249 257 276 279 278

ADI

0 1000 2000 3000 4000

991 1075 1179 1166 1171

639 812 879 887 897

5.6 6.8 6.0 5.0 4.5

314 341 363 366 374

Material

UTS

There is no published data showing a negative effect of “automotive” liquids on the mechanical properties of forged steel. However, MADI and ADI exhibited a loss of ductility in constant strain rate tensile tests when simultaneously in contact with water and ADI exhibited a loss of ductility when in contact with virtually all “automotive” liquids [7]. Currently, the mechanism for this apparent “environmental embrittlement” is not known although it has many features similar to liquid metal embrittlement. This “environmental embrittlement” had no effect on the yield strength of either MADI or ADI. Since automotive components are not typically stressed beyond their yield strength in service, this effect may be a “non-issue”. However, component fatigue tests should be run in the presence of automotive liquids to clarify this issue. The effect of exposure to liquids on the mechanical properties of MADI and ADI are shown in Table IV. Table IV. Effect of exposure to liquids on the mechanical properties of MADI and ADI [7].

Liquid pure water 3.5% salt water mineral oil synthetic motor oil gear oil power steering fluid brake fluid diesel fuel

Loss of Ductility MADI Grade 1 ADI 37% 32% none none none none

9% none

71% 74% 56% 53% 60% 53% 47% 53%

The coefficient of thermal expansion (CTE) is an important material characteristic that needs to be accounted for in today’s high performance engines. Notable failures in race engines have occurred because this was not properly accounted for, e.g., titanium connecting rods are known to adhere to steel crankshafts causing catastrophic failure if the bearing clearances are not adequate. This occurs because the CTE of titanium is less than the CTE of steel, therefore, the bearing clearances are reduced as the engine components heat-up. For gray iron cylinder blocks, cast ductile iron and steel are ideal choices for crankshafts

because the CTE of gray iron, ductile iron and steel are similar. However, today’s aluminum cylinder blocks have a much greater CTE than ductile iron or steel and therefore a significant mis-match exists. The mis-match is even worse for magnesium cylinder blocks. MADI and ADI have higher CTE’s than pearlitic ductile iron or steel and, therefore, are advantageous in maintaining proper bearing clearances as the engine heats-up. This has been previously noted in a publication describing the use of a cast austempered ductile iron crankshaft in the Tuscan Speed Six sports car [8] and later in a Ford Motor Company patent [9]. The coefficients of thermal expansion for gray iron, steel, pearlitic ductile iron, MADI, Grade 1 ADI, aluminum and magnesium are shown in Table V. Table V. The coefficients of thermal expansion for gray iron, steel, regular ductile iron, MADI, Grade 1 ADI, aluminum and magnesium. Coefficient of Thermal Expansion Material Ti-6Al-4V gray iron forged steel pearlitic ductile iron* MADI* Grade 1 ADI* aluminum magnesium * determined from samples

o

ppm/ C

o

ppm/ F

8.8 4.9 10.1 5.6 10.8-11.3 6.0-6.3 12.2-12.4 6.8-6.9 14.4-15.3 8.0-8.5 17.3 9.6 21.6 12.0 25.9 14.4 sectioned from components

MACHINABILITY TESTING Grade 1 austempered ductile iron has desirable strength but poor machinability, therefore, MADI was developed as a “comprise” material that has the combination of good strength and good machinability. Crankshafts (and numerous chassis components) have been successfully machined and assembled on OEM production machine lines set-up for regular ductile iron (ferritic and pearlitic) with no changes in tooling or equipment speeds and feeds. No material related problems have been encountered during any of the production machine line trials to-date. These trials were run after the Machine Tool Systems Research Laboratory at the University of TM Illinois at Urbana-Champaign demonstrated that MADI was easier to machine (milling, drilling and turning) than regular cast ductile iron of the same hardness [6]. For milling, the forces were similar for MADI, ferritic and pearlitic ductile iron and the insert wear for MADI was less than that for pearlitic ductile iron of the same hardness. For drilling, a central composite design of experiments was performed and mathematical models developed that correlated speed and feed with tool forces and surface roughness. Drill wear and product surface roughness were similar for MADI and pearlitic ductile iron of the same hardness. For turning, a central composite design of experiments was performed and mathematical models developed that correlated speed and feed with tool forces and surface roughness. Forces and product surface roughness were similar for MADI


and pearlitic ductile iron of the same hardness. Also for turning, the insert wear for MADI was less than that for pearlitic ductile iron of the same hardness and much less than that for Grade 1 ADI. CRANKSHAFT FATIGUE (RIG) TESTING Component (rig) fatigue testing has shown that MADI is superior to regular cast ductile iron with and without fillet rolling. The data for D5203 was from regular production validation testing. Due to all of the initial MADI test samples “running-out” (no failure after 10 million cycles), additional castings were produced and machined so that further testing at higher loads could be run to determine the true capability of MADI. Component (rig) fatigue data is shown in Table VI. Table VI. Component (rig) fatigue data for regular ductile iron (production D5203) and MADI 5.7 liter crankshafts.

Bending Moment, Condition 1184 N-m, fillet rolled 1184 N-m, not fillet rolled

Figure 8. 1100 HP Darrell Cox, Mopar, Sub-Compact Drag Race Car with MADI Crankshaft (foreground) Driven by Mike Crawford.

Cycles to Failure D5203 MADI 1 million immediate

>10 million 9-20,000

SUMMARY AND CONCLUSIONS 1.

Lightweight crankshaft designs have been developed and shown to have the potential to provide significant reductions in weight (2-20 kg, 1050%).

2.

Significant weight savings at little or no cost penalty are possible through the use of cast ductile iron crankshafts.

3.

Cast MADI crankshafts are capable of meeting the everyday needs of 450-500 HP street cars and the extreme demands of 1100 HP drag race cars.

VEHICLE TESTING MADI crankshafts have performed without incident in 450-500 HP street cars and the 1100 HP Darrell Cox sub-compact drag race car. The street cars have been driven daily for over six months and the drag race car raced the entire 2005 season. The drag race car runs 060 mph in less than three seconds and can obtain speeds of nearly 200 mph in approximately eight seconds on a quarter mile track. During the 2003 and 2004 race season, broken crankshafts were a problem. During the 2005 race season, there were numerous catastrophic events, such as, compressed combustion chambers, compressed piston skirts, bent piston wrist pins, the torque converter cover friction welded to the flex plate and a baseplate-cylinder block separation, but, no broken crankshafts. The 1100 HP Darrell Cox subcompact drag race car with a MADI crankshaft on the track in Rockingham, NC is shown in Figure 8.

TM

TM

4. MADI has mechanical properties similar to many forged steel automotive crankshafts and thus 10% weight savings (difference in density between forged steel and cast ductile iron) are easily obtainable. 5.

TM

Cast, high-strength, MADI crankshafts have been successfully machined on production machine lines set-up for regular cast duc tile iron.

ACKNOWLEDGMENTS The Intermet Archer Creek Foundry has cast over 27 metric tons (60,000 lbs) of MADI since 2002 and cast all of the MADI crankshafts described in this report. The Machine Tool Systems Research Laboratory at the University of Illinois at Urbana-Champaign performed the “laboratory” machinability testing. Macimex (Mexico) machined and fully assembled the MADI crankshafts used for vehicle testing on their production machine line. The assistance of Darrell Cox Racing in assembling, testing and evaluating MADI crankshafts in his street and race cars is gratefully acknowledged.


REFERENCES CONTACT INFORMATION 1.

Warrick, et al, “Development and Application of Enhanced Compacted Graphite Iron for the Bedplate of the New Chrysler 4.7 Liter V-8 Engine,” SAE Technical Paper #1999-01-0325, Society of Automotive Engineers, Warrendale, PA (1999).

2.

“Comparison of Characteristics Between Forged and Cast Crankshaft”, presentation by Sumitomo Metal Industries, Ltd. (1992).

3.

Druschitz, et al, “Influence of Crankshaft Material and Design on the NVH Characteristics of a Modern, Aluminum Block, V-6 Engine,” SAE Technical Paper #1999-01-1225, Society of Automotive Engineers, Warrendale, PA (1999).

4.

Druschitz, et al, “Machinable Austempered Ductile Iron,” U.S. Patent Application No. 60/408,174 (Provisional Application September 4, 2002), International Publication No. WO 2004/022792 A2. TM

5. Druschitz, et al, “MADI : Introducing A New, Machinable Austempered Ductile Iron,” SAE Technical Paper #2003-01-0831, Society of Automotive Engineers, Warrendale, PA (2003). TM

6.

Druschitz, et al, “Machinability of MADI ,” SAE Technical Paper #2005-01-1684, Society of Automotive Engineers, Warrendale, PA (2003).

7.

Druschitz, et al, “Effect of Liquid Environments on the Tensile Properties of Ductile Iron,” SAE Technical Paper #2004-01-0793, Society of Automotive Engineers, Warrendale, PA (2004).

8. Brandenberg, K.R., et al, “An ADI Crankshaft Designed for High Performance in TVR’s Tuscan Speed Six Sports Car,” SAE Paper No. 2001-010408, Society of Automotive Engineers, Warrendale, PA (2001). 9. Mayer, K.M., “Crankshaft for an Internal Combustion Engine Disposed in A Motor Vehicle,” U.S. Patent No. 6,761,484 B2 (July 13, 2004).

Dr. Alan P. Druschitz Director of Materials R&D INTERMET Technical Center 939 Airport Road Lynchburg, VA 24502 (434) 237-8749 phone (434) 237-8752 fax [email protected] David C. Fitzgerald Director of Product Engineering and Design INTERMET Technical Center 939 Airport Road Lynchburg, VA 24502 (434) 237-8703 phone (434) 237-8752 fax [email protected] Inge Hoegfeldt Technical Director Europe INTERMET Europe Unterturkheimer Strasse 39-41 D-66117 Saarbrucken Germany (49) – (0) 681-92740-11 (49) – (0) 681-92740-19 [email protected]

Lightweight Crankshafts

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