Improving air quality while simultaneously reducing CO2 emissions requires a significant modification to drive systems. In 2021, newly approved vehicles will need to reach 95 g/km of CO2, and following this first milestone a further reduction of 15 percent per year is planned for 2025 to 2030. In order to achieve these goals, the electrification of the powertrain is critical. The hybridization on a 48 V basis, in this context, offers significant potential, while simultaneously avoiding extensive reconstruction.
>> THE REQUIRED REDUCTIONS CANNOT BE ACHIEVED WITH THE OPTIMIZATION OF CONVENTIONAL POWERTRAINS ALONE
The efforts of reducing greenhouse gas emissions results in stricter testing cycles for the determination of CO2 emissions. The expansion to real driving emissions (RDE) means a massive tightening of the requirements. In regards to the global efforts of CO2 reduction, it cannot be excluded that, in the future,, other climate-relevant exhaust gas components, such as CH4 and N2O,will also be subject to regulation. The study of the CO2 norm shows that the required reductions cannot be achieved with the optimization of conventional powertrains alone, but that a change in technology is necessary. This means that, in addition to the introduction of electrically operated vehicles (Battery Electric Vehicle – BEV), conventional drives must also be electrified.
The EU market is characterized, among other things, by compact vehicles that offer various versions on one platform. The compact car segment (C-segment) covers the classic second vehicle with short-distance traffic on one hand, and serves as family vehicle on the other. In light of this background, the potential analysis for partially electrified drives was carried out with regard to:
- Lowest emissions; especially in urban operation
- CO2 reduction in real operation
The basis was a EU6b vehicle with CO2 emissions of 100 g/km in the NEDC. For modifications in light of the above-mentioned goals, a targeted adjustment of the technical package was carried out:
A matrix-based selection of the components was carried out with regard to these objectives. The powertrain configuration examined represents a pMHEV concept with a 48V BSG, meaning that a Diesel engine with improved functional characteristics was partially electrified in a P0 layout.
The core elements of engine optimization are the new 2,500-bar CR system and a newly specified exhaust turbocharger with variable turbine geometry (VTG). In addition to an eCompressor and cooled multiple exhaust gas recirculation (M-EGR), the new engine version also has a 48 V BSG. By way of an immediate torque increase, the eCompressor enables a reduction in emissions in highly dynamic cycles and an improvement in responsiveness. Two configurations were investigated for the exhaust aftertreatment system:
- Topology A: Combined DeNOx system with a nitrous oxide catalytic converter installed close to the engine, DPF, and underbody SCR unit.
- Topology B: Electrically heated catalytic converter (E-DOC) with SDPF installed close to the engine and active underbody SCR system (incl. 2nd dosing)
In light of stricter air quality requirements, there was a focus on the optimization of urban operation. Since the BEV is considered to be locally emission-free, the “almost emission-free” attribute must be formulated for conventional vehicles to maintain market acceptance.
▶ NOx: ≤ 55 mg/km
▶ PM /PN: ≤ 3 / ≤ 5 x 1011 mg/km / #/km
▶ HC: ≤ 80 mg/km
▶ CO: ≤ 500 mg/km
With regard to CO2 reduction, a target range of 115 g/km was defined for real operation, derived from:
CO2,current:NEDC:100 ▶ CO2,current:WLTP:110 ▶ CO2,Target,WLTP:~100 ▶ CO2,Target,RDE: ≤115 g/km
The functional results were evaluated using RDE driving profiles from the extensive FEV database.
Holistic Powertrain Optimization Approach
The significantly increased complexity of a Diesel-hybrid powertrain requires a systematic optimization process that follows the flow shown in Figure 2.
At the start, the parameters of the operating strategy and the powertrain are reduced on a weighted basis, including catalytic converter volumes or temperature thresholds. This allows the system characteristics to be optimized simultaneously with the operating strategy.
A specific DoE approach is elaborated for each layout using these combinations. This leads to a statistical overall model and the identification of the key parameters. The optimizations are then evaluated and validated for benchmark driving cycles using FEV SimEx software, a virtual test environment for the powertrain which takes into consideration the EA system functionality. The model has a modular construction, so that layouts, cycles, and strategies are calculated flexibly and can then be used again for validation.
For implementation, a pMHEV concept with dual influence was selected through the targeted addition of electric components to the engine. The integration of the 48 V BSG was carried out to guarantee the following functions:
- Engine support and phlegmatization
- Start/stop functionality, especially combined with variable valve actuation
- Recuperation and optimized battery management in combination with an e-catalytic converter
In addition to this step, the inclusion of the following is essential:
- eCompressor for:
▶ Accelerated charge pressure for attractive driving behavior
▶ Reduction of NOx and PM peaks
- E-catalytic converter in an EA version
For a hybrid concept, the operating strategy is key for potential maximization. When stationary, the torque distribution between the engine and the e-machine is determined based on the torque requirement at the transmission input, the engine or transmission rotational speed, and the state of charge (SOC) of the battery. For heavy loads, the BSG system helps to avoid high NOx emissions during operation. In case of a low SOC, additional sets of parameters determine the conditions under which load shifting is required. This field is limited by defined efficiency increases and the NOx increase. In transient operation, BSG is also in control for a moment to avoid engine exhaust soot and NOx emission peaks.
For the cold-start phase or at low speed, a dedicated heating strategy is selected that includes a more aggressive load switchpoint and stronger transient support. This heating strategy is active until the EA system has reached an adequate conversion level. The operating strategy selection is carried out as a rule-based optimum between NOx emissions in low-load cycles and CO2 emissions in the overall cycle.
The achievement of challenging target values requires a systematic approach in the specification of the systems for the reduction of exhaust emissions. This is made possible by the application of an internal methodology with a high prediction share, such as FEV XiL. The use of virtualized routines enables the efficient management of high system complexities, while effort and expenses are reduced. At the start, requirements for emission-critical scenarios are identified and the scenarios are evaluated in a multi-dimensional DoE campaign. Then, the virtual system optimization geared toward robustness is carried out. Finally, the system behavior is validated extensively in a second DoE campaign.
>> FOR HEAVY LOADS, THE BSG SYSTEM HELPS TO AVOID HIGH NOx EMISSIONS DURING OPERATION
The high performance of the selected technology components is evident in Figure 3. Since the variable heating performance represents a powerful degree of freedom, the e-catalytic converter version has an advantage in future requirements.
Vehicle acceptance is also determined by driving and comfort attributes. In this context, the combination of additional drive torque via the BSG and the “multiplier function” for the engine torque from the electric compressor can provide impressive values. Combined with the model-based control algorithms, advantages in CO2 values and emissions can also be obtained in this way. As seen in Figure 4, an examination of various cycles under variable temperature conditions describes the CO2 potential of the powertrain configurations.
Benefits of the FEV Ecobrid
Even 48 V hybridization offers significant potential in emissions reduction. Multi-level DeNOx systems enable almost “zero NOx” emissions as soon as the catalytic converter start-up temperature is reached. The very low NOx emissions can be achieved with the selected combinations, but driving at a reduced speed remains the biggest challenge. Electrically heated catalytic converters offer a degree of freedom with additional reduction potential. A partially electrified diesel engine is, and will be in the future, an attractive drive concept, and makes a convincing argument against inner-city restrictions on the operation of vehicles with combustion engines.