Vehicle electrification is an essential element in meeting future legislation regarding gasoline and Diesel engine pollutant and CO2 emissions. In this context, 48 volt architectures show particular potential since – in contrast to complex, full-hybrid systems with combinations of electric and conventional combustion engines – they do not involve the complete redevelopment of the entire powertrain. A particularly interesting piece of technology within a 48 volt electric system is an electrically driven compressor (E-Charger). FEV has used the AMG A45 to demonstrate what a series-compatible solution might look by advantageously integrating an additional 48 volt E-Charger into the A45. SPECTRUM sat down with FEV experts to discuss the project and the diverse potential that lies in the electrification.
Dr. Thewes, as the Manager for Thermodynamics, you played a decisive role in the project. Why did you choose the A45 AMG as the demonstration vehicle?
Thewes: The AMG A45 is a vehicle that compellingly shows the successes that can be achieved through uncompromised downsizing. The specific power output of 140 kW/l remains unmatched in this segment. At the same time, we are seeing a clear development trend toward even higher specific power outputs approaching 200 kW/l. To date, such a turbocharged engine has represented a compromise: the turbo lag of the engine is relatively pronounced. For everyday use – particularly when operated in a sporty driving style – this is not a significant factor and is effectively compensated for, thanks to an excellent transmission concept. However, we see considerable potential to comprehensively counteract this disadvantage through engine optimization and to further improve driving dynamics.
Could you briefly explain the basic principle of 48 volt E-Charging?
Thewes: When integrating a 48 volt E-Charger, an additional electrically-operated charger is integrated on the fresh air side. Due to the knocking tendency of the gasoline engine, integration in front of the intercooler is the preferred method. Although the E-Charger itself can be installed in front of or behind the exhaust turbocharger, the position chosen here is more favorable from a thermodynamic point of view for the design of the exhaust turbocharger compressor. The E-Charger supports the existing turbocharger for spontaneous load requirements. As soon as the desired load requirement is set through the turbocharger or when the E-Charger cannot provide an additional positive pressure ratio to achieve the load requirement – for instance, with high rpm – the E-Charger is circumvented through a bypass valve in order to minimize pressure loss.
What are the advantages realized at the AMG A45?
Thewes: The additional electrically-operated assembly supports the twin-scroll turbocharger optimized for maximum performance at low rpm and highly dynamic load-cycle changes. As a result, the maximum torque of 450 Nm is already available at 1,600 rpm, 650 rpm earlier than in the production version. In this way, the elasticity and the responsiveness increase and the number of gear changes can be minimized. For instance, the acceleration from 80 to 120 km/h improves – depending on the selected gear – by up to 54 percent.
Could these advantages not be achieved through a bi-turbo engine, two-stage charging, or variable turbine geometry?
Thewes: All those technologies are based on exhaust turbochargers and are, therefore, still dependent on available exhaust energy. The turbine power, which is converted into charge pressure via the shaft and the compressor, consists of exhaust mass flow and temperature. Both are often not sufficiently available at the low load operating points from which one accelerates. Here, electric or mechanical chargers have a clear advantage. FEV will be making some innovative proposals in the future for two-stage charging as well. The important thing is that good solutions are found to securely and holistically comply with the world’s most stringent emission thresholds, despite the increase in the number of exhaust turbochargers and the consequent increase in thermal inertia in the exhaust system upstream of the catalyst. This is a clear advantage for a solution with an E-Charger.
>> IN THE AMG A45, THE ACCELERATION FROM 80 TO 120 KM/H IMPROVES – DEPENDING ON THE SELECTED GEAR – BY UP TO 54 PERCENT
Mr. Körfer, electric charging – as well as the use of a turbocharger – are topics that don’t only apply to gasoline engines. What is your experience with 48 volt chargers in Diesel powertrains?
Körfer: We have already carried out partial electrification of the powertrain with the help of a 48 volt E-Charger in various applications and intensively examined and assessed the additional potential and expanded degrees of freedom. The improvements that can be achieved strongly depend on the existing underlying application. Ultimately, a unique compromise can be reached between reduced CO2 and pollutant emissions, on the one hand, and increased driving dynamics and performance behavior, on the other hand, for all the applications. The optimizations range from about 15% for the reduction of CO2 emissions to about 30% for the increase in driving performance, both with adequate powertrain adjustments. The “increase in comfort” factor also describes another important parameter, certainly in the luxury vehicle category. 48 volt technologies offer further potential especially in combination with modern automatic transmissions. Thanks to a significantly increased low-end torque and the resulting improvements in engine response, downspeeding can be realized by the use of a wide gear ratio. This improves fuel consumption and driving pleasure – especially under real life driving conditions. To this end, the FEV Group has developed a new and innovative DCT transmission family which combines high robustness and ride comfort with extremely good transmission efficiency, thus enabling very attractive configuration of the whole system.
>> FOR DIESEL ENGINES, THE OPTIMIZATIONS RANGE FROM ABOUT 15% FOR THE REDUCTION OF CO2 EMISSIONS TO ABOUT 30% FOR THE INCREASE IN DRIVING PERFORMANCE
To what extent do the possibilities that apply to Diesel engines differ from those that apply to gasoline engines?
Körfer: The application for the gasoline engine is mainly geared toward supporting the fundamental downsizing trend and enables the improvement of driving performance in different vehicle classes under the premise of minimized CO2 values. The advantage of a 48 volt E-Charger for the diesel engine is its wide range of applications with positive effects on various requirement characteristics. Based on our impression of the recently-formulated RDE requirements, we initially expect the usage to be mainly for emission reduction, combined with moderate consumption improvements in the certification cycles. The new, highly-dynamic operating conditions require adequate air management by the engine in order to achieve harmonized engine emission behavior and enable optimized conversion rates of the DeNOX catalysts. In this context, the 48 volt E-Charger, combined with innovative model- and rule-based function algorithms, offers a very strong potential.
What are the main differences of the integration of a 48 volt charger in a gasoline or a Diesel engine?
Körfer: For the integration of a 48 volt E-Charger in a modern Diesel engine, the first difference would surely be the diesel engine’s complex air system with high- and low-pressure EGR. This leads to increased demands for control quality as well as for the related complex modeling of the cylinder filling model. In addition, the component loading is increased due to the recirculated exhaust gas, which contains a high proportion of water and corrosive particles that inevitably lead to a significantly increased material stress. There is still a lot of work to be done in order to achieve a robust, production-capable application that makes optimal use of its versatile potential.
Dr. Hülshorst, as FEV Vice President for Electronics & Electrification, could you briefly outline possible scenarios for the E-integration of a 48 volt E-Charger?
Hülshorst: Generally speaking, in addition to the coexistence of 12 volt and 48 volt subnets, a conversion of the entire vehicle electrical system to 48 volts is conceivable. Suppliers now offer a wide range of 48 volt auxiliary equipment. From our point of view, however, the realistic solution is two parallel wiring systems. The high current load, such as air conditioning, heated catalysts or steering and stability control, are operated with the powertrain in the 48 volt subnet. Given that higher voltage is synonymous with decreased current, it is possible to lay out wire gauges, electric motors, transistors, and circuit breakers more efficiently or to make them smaller, which saves space and weight.
In contrast, simple, low power and especially established components continue to be operated with 12 volts. Here, the 12 volt network is fed by a DC-DC converter, which provides the 12 volt lead battery with energy.
Other than flagship projects such as the AMG A45, which is anything but a conventional vehicle, where do you see the future potential of electric charging?
Thewes: In the case of gasoline engines, the use of an additional 48 volt E-Charger promises – especially for luxury vehicles, which are increasingly equipped with four or six-cylinder engines – increased ride comfort. The compromises that result from the downsizing and – similar to our AMG example – must be compensated by the transmission concept, negatively impact ride comfort and background noise. An additional boost with the 48 volt E-Charger can help in this case, which is an important aspect for premium class vehicle buyers. Furthermore, the E-Charger can be used in concepts with specific powers lower than 140 kW/l to implement, for example, comprehensive Miller cycle processes in combination with externally-cooled EGR to achieve very fuel-efficient gasoline engine concepts. We have already discussed this option for three-cylinder gasoline engines; for smaller engines, a 12 volt E-Charger may even be sufficient.
Körfer: For Diesel engines as well, we currently see the main introduction starting at the upper middle class, meaning the D/E segment. The primary reason is the overall cost situation. In addition, there will probably be a few applications in the heavy C segment, meaning SUVs and MPVs. While in the case of the latter applications, the emission-related aspects are supported, the performance and comfort attributes also play a greater role in premium applications.
Hülshorst: In addition to electrified turbochargers, the layout of future vehicle electrical power network structures will be defined by future legislative provisions and consumer comfort expectations. These include, for instance, expanded start-stop, a sail function, safety-related assistance systems, pedestrian protection, or a defrosting front windshield. These features will require increasing electrification which, in turn, increases the electrical need of the electrical network, since many functions must be maintained during motor idling, such as the electric steering, the electric brake system, air conditioning, or the vehicle distance radar. In short, dual on-board power networks will become standard in the upper and middle vehicle classes. The electrically-supported turbocharger will then just be one of several components to be powered by the high-voltage network, and, therefore, will not represent a special requirement for the future electrical system. Beyond that, a sensible final development stage is the targeted recuperation of energy.
In our experience, when using technologies such as belt-driven starter generators, we can aim for up to ¼ of driving performance in sailing mode — and accordingly achieve better fuel-efficiency.