Keywords

1 Introduction

Particularly in the automotive industry, process chains for manufacturing motor vehicles are highly complex. Furthermore, the individual processing steps are energy-intensive. Against the background of global warming, this process chains have the potential to increase resource efficiency in the manufacturing phase of powertrain components. In addition, the use phase is also essential for the overall energy requirements of vehicle components. Within the scope of the project, the cumulative energy demand (CED) [1] was used to calculate the energy saving potentials. Here, the component weight and energy losses (e.g. due to friction) in particular have an impact on the energy requirement. By using innovative manufacturing processes, new types of process chains for resource-efficient production and an energy-optimized use phase for powertrain components can be established. This enables significant CO2 savings in the automotive industry. The investigated components include cylinder liners, chassis components, drive shafts and vane pumps. In addition, a method for continuous ecological evaluation as well as energy- and resource-oriented control of production is provided. Finally, a digital demonstrator is implemented in order to collect and visualize the gained knowledge for the investigated processes.

2 Resource-Efficient Process Chains

2.1 Tribologically Optimized Cylinder Liner

Despite the increasing share of electric drives in passenger and freight transport, in certain fields the combustion engine still has to be used for the next years. In particular, this is the case for heavy-duty engines (e.g. shipping, mining or railroad transport). For the energetic optimization of cylinder liners, versatile approaches have been pursued in the past to produce an adapted geometry or topography. Examples are mold honing, coatings or microstructures.

Previous research work realized a reduced friction by form honing and microstructuring [2, 3]. However, shape honing is energy- and resource-intensive and microstructuring could not yet be integrated into a process chain. Therefore, the replacement of form honing by dry non-circular turning is investigated. Furthermore, the integration of microstructuring into the process chain is investigated and a new process chain containing the two processes is set up and shown in Fig. 1. This allows the energetic optimization of cylinder liners and their manufacturing chain.

Fig. 1.
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Process chain variation for the integration of non-circular turning and microstructuring

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The approach contains a piezoactuated hybrid tool for non-circular turning and microstructuring. A Non-circularly machined cylinder liner with applied micro dimples is shown in Fig. 2. The design of it is based on advanced simulation methodology [10].

Fig. 2.
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Non-circularly machined cylinder liner with applied micro dimples (left graphic from Alshwawra, following [10])

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The cylinder liner is examined at the Institute for Technical Combustion of the Leibniz University Hannover in an engine test rig. The operating range covers a cylinder pressure pmi = −2–16 bar and rotational speed of n = 600–1,600 min−1. The combination of non-circular turning and microstructuring can reduce friction by a maximum of 17%. However, the same maximum friction saving is also achieved by non-circular turning without micro dimples. But, there is a difference in the average over the test’s operating range. The average amounts to -9% (non-circular, no micro dimples) and -12% (non-circular, micro dimples). This difference shows that the mode of action of micro-lubrication pockets depends on the load point.

2.2 Process Chains for the Manufacturing of Chassis Components

In the context of large-scale production, laser structuring and automated repair welding of casting molds are being investigated as approaches for further increasing the resource efficiency of process chains. By introducing laser structures into the casting molds, chassis components can be manufactured with smaller wall thicknesses and up to 10% less weight [4, 5]. Thus, less energy is required to manufacture the components and a lower weight results in lower CO2 emissions during the use phase. The energy saving potentials for production and during the service life are determined on the basis of a reference component and its process chain. The insertion of structures into casting molds by means of laser structuring is being researched in order to optimize the application behavior of molds in automotive engineering. To evaluate the influence of the textures on the flow properties of the melt, mold inserts were laser-textured and flow path length casting tests were carried out. The textured patterns carbon and sharkskin were produced. The results are shown in Fig. 3. The laser structuring results in a significant increase in flow path length compared to unstructured reference molds. For the selected test setup, the flow path length increases by 39.3% for the carbon structure and 44.4% for the sharkskin structure. The significant increase in flow length makes it possible to reduce casting restrictions. As a result, the component weight of the considered subframes can be reduced by up to 10%. This leads to energy savings both in the operation of the component and in the manufacturing phase. The energy savings potential per year for a fleet of 130,000 vehicles is therefore 3,688 MWh. The savings potential can be scaled by the number of castings produced and the number of molds. This means that there is great potential for savings, which will have to be tapped in the future.

Fig. 3.
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Effect of Laser structuring on the melt flow behavior in casting molds

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An automated repair welding and finishing process of the molds offers considerable optimization potentials, e.g. related to the service life of the molds [6]. This reduces the repair costs of the molds and their wear behavior is optimized by targeted mechanical machining. A longer service life of the molds and thus an increase in resource efficiency is the result. The ensuing energy saving potentials are determined and presented on the basis of a case study. With the aid of an automated repair process for casting molds for a hinge bearing, consisting of milling, repair welding and fine machining in a single clamping set up it is possible to plan repairs with reproducible quality and to extend the service life of the casting molds. Automated repair welding of the casting molds is predicted to have the potential to increase service life by 20%. Taking into account the number of units produced for a hinge bearing and the lifetime gain due to repair welding, a forecast energy saving of up to 362.9 MWh is expected.

2.3 Process Chain for the Production of Energy Efficient Drive Shafts

Despite the steady advance of e-mobility, drive shafts are still an essential part of the powertrain. Nevertheless, the e-mobility is redefining the requirements for these components. For instance lightweight construction concepts are increasingly being used for all vehicle components in order to increase the range per charging cycle. For this purpose, a new, primary cutting process chain was defined for the production of shafts and is shown in Fig. 4. The new shaft is a three-part welding hollow component with different material thicknesses. This results in a weight reduction of up to 17% compared to conventional formed lightweight shafts. The weight compared to solid shafts is reduced up to 55% with similar energy consumption in production. A total of up to 2.5 kg can be saved per vehicle. Due to the lower material requirements, the total energy demand can be further reduced.

Fig. 4.
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New process chain for the three-part welded lightweight shaft

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The efficiency of electrically powered vehicles is essential for increasing their range. For this purpose, the use of microstructures in joint housing was investigated. The structure geometry and arrangement was determined by simulation and validated in a swivel bearing test rig. The structural arrangement with 79% reduced friction torque was then moved into the ball raceways of fixed joints housing by means of laser ablation. Afterwards, the structured components were examined on a drive shaft test bench. Within the WLTP test, a reduction in power loss of up to 20% could be determined for ‘run-in’ components. The subsequent endurance tests showed no increased wear of the structured components compared to the series components.

In addition to the energetic consideration of the use phase, the process chains of the drive shaft components were also adjusted and evaluated. An important approach is the elimination of hard machining for the ball hub and joint housing. With a milling process adapted to residual stresses and an inductive hardening strategy designed to minimize the distortion, dimensional deviations can be predicted and compensated during soft machining. The distortion compensation of the ball tracks is shown in Fig. 5.

Fig. 5.
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Distortion compensation of the ball tracks using the example of the ball hub

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The shortened process chain requires 12% less energy to manufacture a joint housing and 20% less energy to manufacture a ball hub. Fluctuations within the process chain are counteracted by an automated cascade control. The optimization of the drive shaft achieved in this way offers the potential, based on the annual production figures and mileage of the vehicles of the Volkswagen Group, to save about 3,200 MWh of energy per year in production and about 2,000,000 t of CO2 in the use phase.

2.4 Tribologically Adapted Vane Pump

The vane pump is required to provide the gear box with pressurized hydraulic oil. As it is operated at the engine’s rotational speed, the boundary conditions are demanding. The moving components inside the pump are subject to high pressures of up to pmax = 22 bar, rotational speed of up to npump = 9,000 min−1 and temperatures of up to θ = 70 ℃ [7]. One of the major losses in vane pumps is friction between the vanes and the inner cam ring. This tribological contact zone is aimed to be improved by the application of micro dimples. These are generated using an embossing process with a biaxial rolling embossing kinematic. Therefore, cemented carbide shafts were microstructured on the shell surface by grinding. The manufactured tool is shown in Fig. 6. As these negative micro dimples are on the outside of the cylindrical tool, a good accessibility is given. The tool is used to generate micro dimples on the inner surface of the cam ring.

Fig. 6.
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Tool microgeometry for rolling embossing

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Micro dimples with a width of wi = 30 µm and a depth of di = 15 µm have successfully been manufactured on the inner ring of a ball bearing. Due to the workpiece’s and tool’s elastic behaviour the micro dimples are smaller than the size of the embossing element, which has to be taken into account in the process. As a next step the adapted vane pumps are tested on a vane pump test rig. Here, the flow rate, the temperature and flow losses will be evaluated in comparison to vane pumps without micro dimples.

2.5 Energy-Efficient Production Planning

An additional savings potential, is offered by energy-efficient production planning. By optimizing the production system with regard to the target variable energy (e.g. kWh/part), it is possible to calculate ecological process parameters, which were able to demonstrate a further savings effect. Based on previous approaches to the ecological evaluation of machining processes, an approach was developed that enables the ecological evaluation and optimization of process chains [8]. For this purpose, the productivity of process chains must be taken into account. In line with the overall equipment effectiveness (OEE) [9], the productivity is assessed based on quality and capacity utilization. The scrap rate (SR) is selected as KPI for quality, the capacity utilization is assessed on the basis of idle times (EI), like production downtimes. The combination of productivity and CED as well as the consideration of a number m of produced products results in the following indicator CEDPPC.

$$ CED_{{PPC}} = \frac{1}{m}\left( {\left( {1 + SR} \right)\Sigma _{{j = 1}}^{m} \left( {\Sigma _{{i = 1}}^{n} CED_{{i,j}} + \Sigma _{{i = 1}}^{n} E_{{I,i,j}} } \right)} \right) $$
(1)

As part of the evaluation of the developed procedure, a tool for the ecological assessment and optimization of process chains was developed and implemented as a prototype in a real process chain of an axle journal. Based on online machine data (e.g. feed rate, spindle speed) and manual inputs (e.g. cycle time, roughness requirement), an ecological evaluation of the process is possible. After calculating the component's energy footprint and optimizing the process parameters, extensive information regarding actual and target data (e.g. including engagement conditions, process time, energy requirements) is stored in a component-related ecological database of the production system. The calculation of the energy shares is done according to [8]. With the application of the developed procedure, an additional saving of 8% could be achieved for through the energy-efficient planning of process chains. Figure 7 shows the energy savings achieved and the corresponding process parameters.

Fig. 7.
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Presentation of the test results for hard machining of the axle journal on the CTV 250 DF

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2.6 Digital Demonstrator

For final consideration and visualization of the knowledge gained and occurred advantages of the components, a digital demonstrator was created. Thereby, the efficiencies of the powertrain and the energy, which must be expend for production, were processed and displayed graphically. The savings of the developed components during the lifetime were worked out and used for a comprehensive consideration over the period. Comprehensive analyzes were conducted and provided for the consideration of energy consumption. In addition, for the demonstrator a calculation of the CO2 savings was implemented. The demonstrator was developed with MATLAB, since large data collections can be handled mathematically and a conversion to a stand-alone application can be realized. The MATLAB App Designer software is used to graphically process the results. The user has the opportunity to view the individual components in detail and to see the individual energy consumption from production. It is also possible to adjust the data sets and analyze the effects on the overall energy consumption. The boundary conditions of the expected service life can be adjusted. With the demonstrator, a tool was developed with which an overall view of the project and future technical innovations can be shown.

3 Conclusion

In this paper process chains for machining and manufacturing cylinder liners, casting molds, drive shafts and vane pumps were investigated and optimized. A method for energy-efficient production planning and a virtual demonstrator for visualization and output of the results were developed. These enable the process chains to be planned, controlled and monitored in terms of resource efficiency. In the project “Powertrain 2025 - Energy-efficient process chains for the production of a friction-, weight- and service life-optimized powertrain”, funded by the Federal Ministry for Economic Affairs and Climate Action, significant energy-saving potential was identified both in the production phase and in the service life phase of powertrain components. The research results and measures presented resulted in a weight reduction of the powertrain components of 4.5 kg per vehicle and a potential annual energy saving of 13,073 MWh.

With the method for energy-efficient production planning and the digital demonstrator, it is possible to evaluate and optimize the energy and resource efficiency of the process chains. This approach can be transferred and applied to further process chains.