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Optimized Energy Management for 48V Mild Hybrid Drives

48 Volt

6. August 2019 | Engineering Service, Featured Article

48 Volt

When looking at the current drive developments and market forecasts, 48V technology is gaining considerable significance in the automotive industry. This technology is an important part of many automotive manufacturer’s electrification strategies. With moderate technical effort, vehicle fleet CO2 reductions can be achieved in the short term. At the same time, 48V electrification offers significant potential for the reduction of emissions in real operation (real driving emissions – RDE). Given the many functions, such as brake energy regeneration, load point optimization, engine stop sailing, as well as electrification options for charging, driving dynamics, air conditioning, and exhaust systems, it is already foreseeable that the performance and energy reserves of competitive 48V systems will be limited.

The comparison with high-voltage hybrid systems in Figure 1 demonstrates that the operating range of 48V mild hybrid ­systems is clearly moving toward the system limits. The growing number of 48V components additionally increases the dynamics of torque requirements and the variances in terms of operating strategy. This comes with interactions, dynamic framework conditions, and a high system complexity that stretch rule-based operating strategies to their limits. The use of predictive energy management is very promising, since the available electrical energy and power is ideally distributed within the 48V on-board circuit, allowing for ideal operation of 48V systems designed to save costs and resources.

Fig. 1: Comparison of the operating ranges of a high-voltage plug-in hybrid electric vehicle (PHEV) and a 48V mild hybrid electric vehicle (MHEV) in WLTC

Concept vehicle

In cooperation with RWTH Aachen University, FEV has developed a 48V mild hybrid concept vehicle. The vehicle is based on a Mercedes-Benz AMG A45 equipped with all-wheel drive and a seven-speed dual clutch transmission. The series vehicle is equipped with a turbocharged 2.0 l gasoline engine that has a specific output of 133 kW/l. This impressive output is achieved through the use of a large exhaust turbocharger (ETC) that, despite twin-scroll technology, significantly limits the maximum torque in the lower engine speed range and results in a noticeably delayed response. In this context, electrified charging and/or electric torque support can significantly improve elasticity, especially in the economical, lower speed range. The 48V mild hybrid powertrain is schematically represented in Figure 2. The central element is the belt starter generator (BSG) in the belt drive of the combustion engine (CE). The P0 topology enables a variety of hybrid functions such as regeneration, load point shifting, and electric torque support. Since the maximum power that can be transmitted with the belt is limited and there is a permanent connection to the combustion engine, the system is not intended solely for electric driving.

Fig. 2: 48V mild hybrid powertrain of the concept vehicle

There is also an electric compressor (EC) positioned in the charge air path, upstream of the intercooler. The EC reaches a maximum pressure ratio of 1.45 and can significantly increase the charge pressure, and thus the response behavior, in operating ranges with low exhaust enthalpy, regardless of the operating condition of the BSG. The concept vehicle is operated using a rapid control prototyping (RCP) development control device.

Rule-based operating strategy

A driving performance-oriented, rule-based operating strategy with priority-based power distribution controls the electric charging, as well as the electric torque support of the BSG (Figure 3). The operating strategy is made up of the torque-supporting functions in drive management and the overarching power distribution in electric power management. The electric charging is controlled through the pressure ratio between the desired and the current charge pressure in the intake manifold. As long as the waste gate (WG)-regulated ETC does not provide the desired charge pressure, the pressure is additionally increased in the air path through the EC. The required rotational speed is calculated using the compressor diagram of the EC and then limited in accordance with the available electric power.

Fig. 3: Driving performance-oriented rule-based operating strategy with priority-based power distribution

In contrast to electric charging, during which the drive power results from the additional air and fuel mass, the BSG directly converts electric energy into mechanical drive power that supports the combustion engine (Figure 2). The torque required by the BSG results from the difference between the current torque of the combustion engine and the driver’s needs. When the accelerator pedal is pushed, this difference is positive, so that the BSG temporarily replenishes the torque deficit. The BSG torque is then limited in accordance with the available electric power.

The electric power limits of the various individual 48V components are prescribed by the electric energy management. During an acceleration, the 48V battery must also power the cooling agent pump and the 12V system via the DC/DC converter, in addition to the EC and the BSG. It is therefore necessary to carry out a situation-based prioritization of the 48V components. The available battery discharge capacity is, in this context, prescribed by the battery management system (BMS). The available electric discharge capacity for the respective 48V components is then calculated depending on their priority and the actual power consumption of elements with a higher priority. In order to ensure reliable driving operation, the engine cooling and the 12V system have a high priority in this context. The remaining power is made available for the EC and the BSG in consideration of a calibratable power ratio.

Even though such rule-based approaches can be improved through further dependencies, there are principle-based disadvantages. For instance, the operating strategy merely reacts to the current system status and adjusts the parameters regardless of the expected load status. Since, however, the temporal behavior of torque build-up and the efficiency heavily depend on the load status, the selected operating strategy of the electrified drive (CE with ETC, EC, and BSG), and the electric system limits, this control is usually suboptimal.

Optimized Energy Management

Predictive optimization-based energy management strategies use dynamic route information from the electronic horizon for the long-term optimization of route guidance and the speed trajectory. Based on this information and adequate vehicle sensor systems for surroundings detection, hybrid management considers the electric power limits and load prediction to determine ideal trajectories for gear selection, drive torque, and charge strategies for a medium-term horizon. The predicted system values also enable the derivation of an expectable charge condition evolution of the electric energy accumulator, which adapts an energy weighting factor. This factor represents the importance of electric energy in the energy balance sheet and directly influences energy optimization in drive management (Equation 1).

Equation 1

ETot = ∑N k=0E Chem(kT) + ξE El(kT)

At the same time, the response behavior through the regulation of the drive torque, which is made up from the combustion engine torque and the electric torque (Equation 2), is optimized while complying
with the dynamic system limits of the ­ 48V system.

Equation 2

ΔMAntrieb = ∑N k=0M Antrieb, Soll (kT) − MVM(kT) − iRiemenMRSG(kT)

Nonlinear model predictive control (NMPC) relies on a real time-capable, simplified process model of the 48V mild hybrid powertrain. It works with a time horizon of a few seconds and includes time increments of hundredths or tenths of a second for the representation of the nonlinear system dynamics.

The NMPC will calculate the ideal parameter evolution for the WG and the EC, which influence the combustion engine torque through the air path, as well as the torque of the BSG, which can obtained through the addition of the belt drive. This way, both the differences in the temporal behavior of the charge air path and of the BSG torque and their impact on the overall efficiency of the electrified powertrain are taken into account in the optimization.

Results

The NMPC was more closely examined during a validated co-simulation of a B-segment 48V mild hybrid with turbocharged gasoline, electric compression, and P0 BSG. Figure 4 shows a comparison of the NMPC and the rule-based approach for a full-load run-up for various energy weighting factors ξ. An energy weighting factor of four is equivalent to an overall charging efficiency factor of 25 percent, while the electric energy in the limit case of zero, e.g. due to a high battery state of charge and an upcoming downhill drive, is free of charge. Due to the lack of forecasting, the rule-based operating strategy reacts identically in both cases, while the NMPC adjusts the parameters for the WG, the EC, and the BSG based on the situation in order to achieve a desired drive torque. Beyond that, the variation of the optimization parameters shows that the NMPC reduces the drive torque with increasing weighting of energy (h˜NMPR ↑), in order to reduce energy consumption. If the electric energy is free of charge (ξ=0), the drive torque is shifted to the BSG, while the EC builds up charge pressure with the WG open, in order to reduce charge change losses. In contrast to this, at ξ = 4, the NMPC only briefly provides support through the BSG in order to utilize the rapid dynamics of the electric machine and subsequently save electric energy.

Fig. 4: NMPC optimization for various energy weighting factors for a full-load run-up in 5th gear compared to a rule-based approach (weighting ratio energy/response behavior)

The operating strategy, in such an acceleration scenario, is always a compromise between response behavior and energy efficiency. The response behavior is described through the acceleration time and the energy savings through the inverse of the effective drive efficiency. With a variation of the electric power limitation, the framework conditions are changed.

Additionally, for each of these power trajectories, the prioritization of the rule-based strategy and the weighting ratio of the NMPC optimization were varied. It becomes clear that increasing energy savings are at the expense of the response behavior. However, the NMPC resolves the conflict of objectives significantly better and can describe both the energy consumption and the energy savings through the inverse of the effective drive efficiency. With a variation of the electric power limitation, the framework conditions are changed. The stronger the electric power limitation and the smaller the focus on the response behavior, the more the potential of the NMPC develops.

For more information about 48V mild hybrid drives visit 48v.fev.com
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Reliable Partner for Turnkey Vehicle Development

From a Single Source – Part III

5. July 2019 | Engineering Service

From a Single Source – Part III

Part three of the series “Turnkey Vehicle Development From a Single Source”.

You find the first part of the series here.
And the second part of the series here.

Within the last decade, FEV has become an engineering service provider capable of covering the entire service spectrum of vehicle development. Three articles take a closer look at vehicle modules, body shell, interior/exterior, light and sight and chassis modules. Vehicle properties, such as NVH, driving dynamics, passive and active safety, and fatigue strength are considered. Virtual and real-life testing are the development tools here. The activities are accompanied by various control tasks such as benchmarking with subsequent target setting, test and prototype planning, weight management and homologation.

FEV assumes responsibility for the complete scope of turnkey vehicle development, as well as for the development of individual modules and for the selective design and calculation scope of individual components. The fact that the development competence for powertrain, transmission and vehicle comes from a single source makes FEV an ideal development partner – also for electrified vehicles. FEV offers particular expertise in the conversion of conventionally powered vehicles into electric vehicles. The ideal results here can be achieved only through the closely integrated and parallel development of powertrain and vehicle. Some turnkey vehicle development tasks will be introduced in the following:

Chassis and driving dynamics

For vehicles of all types, the chassis establishes the connection to the road, making it the assembly that transfers the force and torque which affect the vehicle. The chassis’ key task is therefore to always guarantee this contact, as otherwise the transfer of force is not possible. The fact that the road is never smooth and straight, as well as still having various friction coefficients, makes this task so demanding. Overall, the chassis is responsible for driving safety, driving comfort, and dynamic vehicle behavior, which can be broadly divided into longitudinal dynamics (brakes), lateral dynamics (steering) and vertical dynamics (suspension/absorption). There are conflicts of objectives, particularly in the conflict area between driving comfort and driving safety, which have to be resolved in the field of chassis design.

FEV also covers the complete development process in chassis and driving dynamics development. FEV is able to integrate its experience and skills into projects – from the development of new concepts and target values to the construction of parts and modules, right through to the testing and final approval of prototypes. A team of trained drivers is on hand for the tuning of dynamic driving properties to assess and optimize the vehicle subjectively and objectively with the aid of corresponding measurement technology. The close proximity of FEV to the ATC Testing Center in Aldenhoven (Germany) represents a major advantage here. An increasing number of electronic systems are also finding their way into vehicles. In addition to the ABS and ESP systems legally in force today, many advanced driver assistance systems affect the road holding and stability of the vehicle via the chassis. This again underlines the importance of the chassis for modern vehicles, as well as provides an explanation for FEV’s active role in the development of these new systems.

Alternative drive concepts with one or more electric engines offer a means of developing new chassis concepts that were previously impossible due to the installation space in conventional combustion engines.

Passive safety development

For years, there has been particular focus on passive vehicle safety in terms of development and is also in the special interest of buyers, as life and health depend on it in the event of an accident. In addition to legal provisions that have to be met for the registration and maintenance of a car (homologation), there are consumer protection organizations, such as EuroNCAP (European New Car Assessment Programme), which go beyond the minimum legal requirement – for instance, to assess the passive vehicle safety of cars.

In the past, the Consumer Protection Rating became increasingly important, causing the requirements for achieving a high rating (five stars) to increase steadily. The speed at which new test methods, test equipment, and crash dummies are introduced presents huge challenges for vehicle manufacturers every year. Globally active vehicle manufacturers not only have to meet different legal requirements worldwide, they also have to achieve a top rating in the different regional consumer protection ratings – for example J NCAP in Japan, C NCAP in China, US NCAP in the US and Bharat NCAP in India.

The introduction of the pedestrian protection leg impactor (aPLI) is a representative example of the rate of change of EuroNCAP for the implementation of new test specimens. The decision in favor of this new impactor was made in February 2019, resulting in the development strategy for projects already started with SOP 2022 having to be changed. However, the leg impactor currently exists only in the form of a physical impactor, with a virtual development model for the simulation expected to become available for the first time in the second half of 2019.

Thanks to the knowledge and the network of experts at FEV, customers can make the right adjustments to their projects at an early stage in order to implement farreaching development strategies in their development projects as soon as possible. FEV also provides the functional design during development in the field of passive vehicle safety. In addition to virtual crash simulations, part, component, and overall vehicle tests are organized, performed and evaluated. FEV facilities can be used for part and component tests. The company is working together with longstanding partners in the field of overall vehicle crashes. The integration of safetyrelevant components, such as airbags, is also managed by safety experts – constant and close communication between the virtual functional design, system suppliers, and construction is a given here.

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