Development of Lightweight Engine Mounting Cross Member

Transcription

1 Development of Lightweight Engine Mounting Cross Member Nitin Babaso Bodhale Team Lead Tata Technologies Ltd Pimpri Pune , India. Jayeshkumar Raghuvanshi Sr. Team Lead Tata Technologies Ltd Pimpri Pune , India. Vilas Bijwe Head-NVH CAE Tata Motors Ltd Pimpri Pune , India. Abbreviations: NVH- Noise Vibration and Harshness, UV- Utility Vehicle Keywords: Cross member, Optimization, Engine Mount, Attachment Stiffness Abstract Engine is a major source of excitation for noise and vibrations in vehicles. It is desired to have higher attachment stiffness at engine mounting points for better isolation. In UVs, these mountings are on long members and cross members. In current automotive scenario, it is a need of every automotive OEMs to reduce weight of the car and improve the performance simultaneously. This paper discusses such a development of engine mounting cross member design, which meets the weight and NVH performance requirements. The development process started with identifying design space considering various constraints. This design space is optimized for finalizing cross member mounting locations and design layout, using Altair Optistruct tools. Then optimized design is converted into feasible design. On this design, further optimization iterations are carried out to make it light weight with the performance as constraint. This methodology from identifying design space to arriving at light weight, feasible design is explained. Introduction Typically, rear wheel drive vehicles have North-South oriented powertrain. Powertrain mounting scheme consists of 3 mounts, 2 on front side and 1 on rear side. Rear engine mount mostly lie on centerline of vehicle through additional supporting cross member. To achieve desired isolation at engine mounts (for better NVH), engine mount locations need to have higher dynamic stiffness. Third engine mount location packaged at the middle of cross member is generally prone to lower dynamic stiffness. This is so due to wider span of cross member and its bolting scheme with long member. Heavy or bulky design of cross member is required to achieve desired dynamic stiffness for such mounting scheme. Fig.1 shows such conventional C-in-C type rectangular section cross member with 8 mounting bolts to long members. This design concept was heavy in weight, difficult to assemble and it was short of targeted dynamic stiffness at mounts location. To overcome these drawbacks in conventional design, Altair topology and gauge optimization techniques are deployed to Fig.1: Conventional Design 1

2 arrive at optimized design concept. The concept received through topology is then further engineered to make it light weight and manufacturing feasible. This simulation based optimization approach is explained in details in following sections. Process Methodology To arrive at new design shapes, Altair Optistruct Topology tool is used. Below steps are followed to evolve the optimized design shape. Initially design optimization is done at component level. Final design concept performance is confirmed at Trimmed Body Level. Step 1: Identification of Design Space Outcome of any topology optimization depends on the initial design space. Here design space is identified considering various constraints like packaging, ground clearance etc. As shown in Fig. 2, top design space is limited due to gearbox and bottom design space is constrained because of vehicle ground clearance requirement. Other aggregates like exhaust and fuel pipe routing etc. are also needed to provide clearance from the cross member. Hatched region in the Fig. 2 shows the available design variable for optimization. Exhaust Pipe Max Possible width Long Member Rubber Mount Location Engine+ Gearbox Packaging constraints Gearbox Fig. 2: Available Design Space Ground Clearance Line Front View Top View Step 2: Setting up Boundary condition Initially, cross member is constrained at 8 locations (4 on each side) where it gets supported on vehicle long member. Two rigid elements are created at each bolting faces of the cross member. Independent nodes of these rigid elements are connected with bolts and bolt ends are constrained in all direction as shown Fig.3. These constrained faces are also part of the design space for optimizing the end mounting conditions. Step 3: Applying Design Constraints Component level dynamic stiffness target at mount location in each of the X, Y and Z directions is established. The variation stiffness performance from component level to trimmed body level is also accounted while establishing the component level dynamic stiffness target. Displacement at mount attachment location in each of the 3 directions is considered as design constraints for the component level optimization. Step 4: Evolving geometry concepts through topology Using Altair Optistruct, topology design optimization is carried out at component level using following optimization parameters. Objective = Min Volfrac (to minimize volume and hence the weight) Load case = FRF (to calculate drive point displacement in all directions) 2

3 Design variable = Hatched volume as shown in Fig.2 (to define design space) Constraints = X, Y and Z Directional Displacements at engine mount location Optistruct iterated the design through 30 optimization cycles and produced the final optimized design satisfying all design constraints. Fig. 3 shows the design concept evolved through topology optimization. Topology optimization has given the optimized shape of cross member as well as indicated the possibility to remove the vertical bolts (2 on each side). This resulted in a light weight design with 4 bolts mounting scheme instead of initial 8 bolts mounting scheme. A frame type structure is generated through this optimization, which needs to be simplified for making it manufacturing feasible. Fig. 3: Topology solution Step 5: Converting Optimized concept into feasible design Above optimised design solution is not manufacturing feasible due to its odd shape and material distribution. This design concept is further engineered to make it feasible. Two members running parallel to each other, broadly capturing the outline of the topology solution is worked out. For supporting engine mount, lateral connecting members are added in the central region of cross member span at engine mounting location. With the help of this simplification, cost of manufacturing got reduced significantly. At this stage, there are no lateral stiffeners in between these two main members. So one more loop of optimization is needed to identify shape and number of lateral stiffeners between 2 main load bearing members. Fig. 4: Optimization output to feasible design Step 6: Identifying the feasible Lateral Support through Topology After finalizing the profile of main load bearing members as base structure, volume between these two members is used as design variable for identification of lateral stiffeners. With earlier topology optimization parameters, second loop of optimization is iterated. Final optimization output of this loop is then engineered 3

4 with steel plates, which can be easily welded to main members in area highlighted by topology optimization. Fig. 5 explains the step wise approach deployed in second loop. Thickness of all steel plates and tubes are taken as 2.5 mm for initial design, which needs to be further optimized to reduce the weight of the structure. Main load bearing members with Middle Design space Topology Topology Optimization Solution highlighting the area where material is needed Feasibility Main members with Steel Plates welded at required area Fig. 5: Second loop of topology and feasibility Step 7: Gauge Optimization In this step cross member assembly is optimized for thickness of different components. Gauge optimization has been carried out using below parameters. Objective = Min Mass Design variable = Thicknesses of components Load case = FRF Constraints = X, Y and Z Directional Displacements This size optimization resulted in thickness output for all parts of the cross member assembly, which is light weight and meets the performance. Final performance at trimmed body level is studied after finalizing the thickness of each component of cross member assembly. Dynamic stiffness and point mobility results before and post optimization are discussed in next section. Result Discussion Dynamic stiffness and Point Mobility performance: Plots in Fig. 6 show that dynamic stiffness and drive point mobility have improved significantly in all directions. Baseline design was failing to meet the target in X and Z direction but optimized design meets those targets. Table 1 summarizes percentage of improvement in dynamic stiffness and mobility for optimized design, which is 20% lighter in weight. Performance/Design Parameters Dynamic Stiffness Drive Point Moblity Table 1: Performance Improvement Direction % Improvement X 39.0 Y 26.0 Z X 49.5 Y 33.6 Z 59.2 Mass Reduction

5 Dynamic stiffness and point mobility curves: Fig. 6: Dynamic stiffness and Point mobility curves Initial Design Optimized Design Benefits Summary Optimized design concept evolved through multistep topology optimization and gauge optimization using Optistruct resulted in following benefits: 1. Improved point mobility and dynamic stiffness performance to meet NVH requirements % weight reduction in cross member assembly design. 3. Reduction in mounting bolts from 8 to 4; hence resulting in lower material and assembly cost 4. Reduction in manual simulation iterations for design improvement. Challenges Major challenge was engineering the optimized design concept into manufacturing feasible design. And this needs the additional skill of design and knowledge of manufacturing processes. If Optistruct can do this using some algorithms or by taking some additional input parameters, process can be completely automated. 5

6 Conclusions During the development process, optimization played a major role. After deriving shape and bolting locations through topology optimization further weight reduction is achieved using gauge optimization. Optimized design gives stiffness improvement of 132% in Z direction and meets targets in other two directions because of its unique design. It also leads to significant mass saving and ease in assembly. The above exercise explains the strategic approach for design and development of cross member with the help of Altair Optistruct tool. ACKNOWLEDGEMENTS The authors would like to acknowledge Altair s Technical Support Team. REFERENCES [1] OptiStruct User s Manual-Altair Engineering. 6