DE VELO PMENT Crankshaft Driv e
Lightweight Potential of Crankshafts with Hollow Design AUTHORS
© Georg Fischer Automotive
In a passenger car, the crankshaft makes up 12 % of the engine weight. Usually, crankshafts are made of forged steel. GF Automotive is a worldwide supplier producing hollow crankshafts using casting technique for 20 years. Using casting instead of forging allows weight reductions between 15 and 20 % and cost reductions of up to 15 %. Being used only in low-
Dipl.-Ing., MBA Ilias Papadimitriou is Technical Expert in the field of Powertrain Components in the Product Management and Development department at Georg Fischer Automotive AG in Schaffhausen (Switzerland).
and medium-torque engines so far, a new study of GF Automotive illustrates that hollow crankshafts can also be used in high-torque engines.
MOTIVATION
Ing. Kurt Track is responsible for Special Projects at Georg Fischer Eisenguss GmbH in Herzogenburg (Austria).
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Combustion engines are not only part of conventional powertrains; they are part of hybrid electric or plug-in hybrid electric vehicles, too. To optimize the emissions, the efficiency and the fuel consumption, it is necessary to invest furthermore in innovations and the development of combustion engines. Moreover, there is still potential for optimization in the areas of combustion, friction, aftertreatment and lightweight design. The trend towards lightweight components will intensify in the future. For hybrid powertrains, weight reduction will be an important aspect due
to the additional weight of the electric engine and the battery. Lightweight design for the crankshaft can be reached due to the design of the counterweight which is limited because of the direct influence on the balancing ratio and NVH issues. Another potential is the weight reduction without changing the balancing ratio, accordingly the design of cavities in the connecting rods and main bearings of the crankshaft. The design of cavities is state of the art for Formula 1 crankshafts, but its application for passenger car engines is limited due to the high machining costs. However, this lightweight potential can be
used when the hollow construction is made of high-strength cast iron. This method is well established and drives to lower costs for a high volume production because of the lower material and process costs [1]. The study shows this potential for a 1.6-l four-cylinder diesel engine achieving a weight reduction of 20 %. METHOD FOR DEVELOPMENT
The development is based on a 1.6-l diesel engine. The crankshaft, originally made of 38MnSV5, weights 12 kg and makes up 12 % of the total engine weight. The development of the crankshaft using hollow design includes the following steps: design, simulation of filling and solidification, multi-body simulation, fatigue analysis, production of the prototypes, fatigue tests. DESIGN
The crucial point during the design development is the maximization of cavity volume, FIGURE 1, taking the production limits for cavity core parts into account. A lower wall thickness risks ruptures of the core during the production as well as during casting. Additionally, a highly inhomogeneous wall thickness could lead to different cooling behaviours and inhomogeneity of the mechanical material characteristics. Here, the process stability is of major importance whereas expert experience allows to approximate the process limits.
FIGURE 1 Design of hollow crankshaft and four-part core (© Georg Fischer Automotive)
SIMULATION OF FILLING AND SOLIDIFICATION
The filling and solidification simulation includes the simulation of the casting parameters such as flow profile, flow rate and cooling down of the melted iron. The main focus during the evaluation of the results is on the identification of locale gas porosities and their elimination through the modification of the casting gating system. The casting process is mathematically described by the continuity equation, the Navier-Stokes equation, the single-phase flow of the liquid metal and the function of the volume fraction. The homogeneous solidification, FIGURE 2, and cooling are central design criteria for the casting system. In order to achieve the required material quality in terms of mechanical properties, the last solidi MTZ worldwide 01|2018
FIGURE 2 Solidification simulation (© Georg Fischer Automotive)
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DE VELO PMENT Crankshaft Driv e
FIGURE 3 AVL/Excite model (screenshot) and FE models (© Georg Fischer Automotive)
fication should take place in the gating. This is a challenge for the design of the gating system. SIMULATION OF MULTIBODY DYNAMICS
methods are inductive hardening and cold rolling. In this project, a cold rolling method suggested for cast iron crankshafts was performed. The cold rolling leads to local high plastic deformation of the material and thereby increases the fatigue strength of the
llets [2]. To optimize the effect of fi cold rolling, a new cast iron alloy based on the GJS family called GJS CS 800 HY with a high yield strength of 555 MPa and ultimate strength of 825 MPa has been developed. The cold rolling treatment was provided by the company
The multibody simulation indicates the stress in a crankshaft, FIGURE 3. The results of the bearing forces show a typical behaviour of a four-cylinder diesel engine. The comparison of the bearing forces, FIGURE 4, between the hollow crankshaft and the forged crankshaft does not show any significant differences. The same force levels result from the same balancing ratio. The comparison of the bending moments, FIGURE 4, shows a higher level of 46 Nm for the hollow-casted crankshaft at the bearing near to flywheel. This difference is related to the low elastic modulus of cast iron compared to forged steel. In order to prevent any wear on the bearing it might be necessary to modify the bearing profile. The stress distribution of the crankshaft was calculated and evaluated for the engine speeds between 500 and 4500 rpm in steps of 250 rpm for a total engine cycle (720°) in steps of 2 °CA. MECHANICAL PROPERTIES OF THE MATERIAL
The highest strength areas of the crankshaft are typically the fillets. Therefore, it is necessary to perform some local surface treatment. The typical treatment
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FIGURE 4 Forces and bending moments in the main bearings (© Georg Fischer Automotive)
FIGURE 5 S/N curve GJS CS 800 HY, bending R = -0.5 (© Georg Fischer Automotive)
Hegenscheidt MFD, a manufacturer of cold rolling machines with profound knowledge in this area.
for a better comparability of the results for different materials and fillets. SAFET Y FACTORS
FATIGUE STRENGTH MEASUREMENT
The fatigue strength estimation is necessary to calculate the safety factors. The fatigue strength measurement of cold rolled fillets was performed on crank throw cuts of the crankshaft in a dynamic test bed. The strength values are determined by the S/N curve for the fillets and different cold rolling forces. The results show a significant increase of the fatigue strength depending on the specific cold rolling force, FIGURE 5. The similar effect should be expected for crankshafts made of steel. The specific cold rolling force was introduced
The safety factors which are calculated based on the Haigh diagram and the Goodman line are between 1.5 and 2.0, which are typical factors for released crankshafts. TRIBOLOGY
The parameters influencing the friction of hydro-dynamic bearings are the bearing geometry, the oil distribution, the physical properties of lubricants, material properties of the bearing shaft and the surface structure of both components. For the tribological evaluation of cast iron, the Stribeck curve was used for
FIGURE 6 Pin-on-disc labor investigations (© Georg Fischer Automotive) MTZ worldwide 01|2018
different materials and surface conditions. FIGURE 6 shows the results of laboratory tests for steel and different casting surface structures. The results show a low friction coefficient in the mixed friction area for ductile cast iron which results from the effect of oil pockets in the burr-free surface [3]. CONCLUSION
Despite the strong development towards lightweight components for engines, the lightweight potential has not been completely exhausted yet. The study shows that the design of a hollow crankshaft allows to reduce the weight by 20 %. Especially the geometry of the cavity, the material choice and the parameter of the cold rolling process have to be considered carefully. During the design development of the optimal geometry, special attention has to be given to the casting technique. Not only for technical but also for economic reasons, hollow crankshafts with high-strength cast iron are a good choice for the engines of the future. REFERENCES [1] Menk, W.; Kniewallner, L.; Prukner, S.: New P erspectives in Automotive Construction – Cast Crankshafts As an Alternative to Forged Crankshafts. In: MTZworldwide 68 (2007), No. 5, pp. 23-25 [2] Fomen, G.; Diefenbach, C.: Bewertung der Schwingfestigkeit von festgewalzten Kurbelwellen unterschiedlicher Werkstoffe. In: FVV final report Dauerfestigkeit II (2015), No. 1079, Issue 261 [3] Summer, F.; Grün, F; Schiffer, J.; Gódor, I.; Papadimitriou, I.: Tribological study of crankshaft bearing systems: Comparison of forged steel and cast iron counter parts under start/stop operation. In: Wear 338-339 (2015), pp. 232-241
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