A further tightening of emission legislation with Euro7 is expected. FEV’s hypothesis for the key challenges of the next European emission legislation consist of the following major topics:
- A general reduction of the gaseous emission limits CO: 500 mg/km . HC: 50 mg/km . NOX: 35 mg/km
- Non-allowance of auxiliary emission strategies that can lead to high emissions
- Particle number emissions measured down to 10 nm instead of 23 nm
- Incorporation of further emission components limits for the lab tests
- Extension of the RDE legislation framework to incorporate further emission components and short driving trips
FEV has investigated how ultimately even a zero-impact combustion engine could be achieved, causing less emissions than those contained in the ambient air. In particular the following targets have been set:
- Emissions in WLTC
- NOX: 40 µg/m³ (corresponds to approx. 0.03 mg/km)
- PM (2.5): 25 µg/m³ (corresponds to approx. 0.02 mg/km)
Compared to today’s Euro 6d legislation, this means a reduction of NOX emissions by 99.9 percent and PM emissions by 99.2 percent.
FEV has developed extensive patented and patent-pending development methods in the field of simulation, as well as testing and aging of emission-relevant components, which make it possible to demonstrate high robustness and forecast accuracy at an early stage of development.
RDE emission simulation and identification of worst case cycles
Emission simulation at FEV is an essential pillar in the frontloading of development. Presented
for the first time in 2016 at the Vienna Engine Symposium, and further refined since then, this modular FEV simulation toolchain based on the GT-Suite software environment is now an essential part of FEV development activities. Engine raw emissions are modeled based on stationary and transient measurement data from engine and roller test benches. The simulation models of the exhaust aftertreatment follow a map-based approach. Still, discretization of the catalyst monoliths allows a good description of the warm-up behavior to take into account individual, temperature-dependent conversion rates. Figure 1 depicts the all relevant variables which are included in the calculation of the conversion.
Knowledge of which vehicle- and powertrain-specific cycles can lead to the highest emissions is essential for reliable compliance with all emission limits under RDE conditions. FEV has realized an abstraction of such real driving conditions. The result is a derivation of a concise number of parameters. This parameterization allows machine learning techniques to be applied to identify the worst case RDE cycles based on an analysis of a few hundreds of simulated cycles. This methodology has meanwhile been successfully applied in many development projects.
Catalyst and gasoline particle filter characterization
In the course of the development of FEV’S RDE emission simulation methodology, it was identified that initially catalysts could hardly be modeled with sufficient precision. The reason for this lies in the mostly limited measurement data available from catalyst manufacturers and OEMs. However, for a precise prediction of the emissions under RDE boundary conditions, knowledge of the conversion rate at highest space velocities and in a wide temperature range is of high importance. FEV there-fore developed its own equipment that can be used to characterize catalysts under exactly these conditions. The system shown in Figure 2 is designed and proven for exhaust gas mass flows up to those produced by turbocharged V12 engines to measure the conversion efficiency at high mass flows and cold temperatures, such as they occur in a full load acceleration shortly after an engine start.
Catalyst and gasoline particle filter aging
FEV has established a method for rapid aging of catalysts and GPFs, as well. For GPF aging, the burner test bench is modified allowing oil to be burned in order to generate ash. Different methods have been investigated and finally oil injection was chosen. FEV generated a cycle and oil dosing strategy that is able to reproduce similar aging characteristics as they are found during vehicle durability testing.
Exhaust aftertreatment concept to achieve zero-impact emissions
Five building blocks form the exhaust aftertreatment concept for achieving zero-impact emissions.
- Optimization of NOX raw emissions during cat heating
- Exhaust aftertreatment with readiness immediately after engine start
- HC emission adsorption
- Increase of total catalyst volume
- GPF with improved filtration efficiency
The individual building blocks are discussed below.
NOX optimized catalyst heating
NOX aw emissions can be optimized by an adaptation of cat heating calibration. For very retarded ignition timings, a high amount of fuel is required to generate an IMEP that matches the FMEP. This results in dethrottling and a lower rate of internal EGR. The cylinder peak temperature increases and remains on a high level over a longer period of time. As a result, the NOX emissions increase. To achieve a drastic reduction in NOX emissions, an optimized cat heating calibration would therefore use only a mild spark timing retardation. As a consequence, HC raw emissions would increase, and additional measures need to be implemented to address this.
Electrically heated catalysts
Two electrically heated catalysts are integrated upstream of the main catalyst (4 kW per disc, 8 kW in total). The metallic substrate heats up rapidly achieving light-off after a few seconds. However, an engine start followed by cold exhaust gas flowing across the electrically heated catalysts would drop their temperature below the level needed for sufficient conversion efficiency. Therefore, a secondary air pump is used to flow air across the electrically heated catalysts prior to the engine start in order to heat up the main catalyst as well. Figure 4 illustrates the heat up process of the final system configuration. The convective heat transfer can clearly be seen in the lower half of the diagram. As soon as the engine is started the higher exhaust mass flow leads to even better convective heat transfer but at the same time also a reduction in the temperatures.
Emissions can be further optimized by ensuring that the catalyst system maintains a high temperature level. In a hybrid engine, this can be supported by the operation strategy and re-activation of the electrically heated catalysts.
Emission adsorption before catalyst light-off
One way to achieve emission adsorption is by dedicated coatings. In order to achieve a high adsorption efficiency, low temperatures are necessary. This matches with the lower incoming exhaust gas temperatures due to advanced ignition timings during cat heating. A metal substrate is considered since this allows high thermal inertia and thus low temperature increase in the first seconds of engine operation and an even distribution of the secondary air mass flow to the inlet face of the electrically heated catalyst. With a temperature limit of 850 °C the adsorption catalyst dictates the position of the exhaust aftertreatment system to be not closed coupled which in turn has benefit regarding thermal aging. Figure 5 shows a comparison of cat heating with and without HC adsorption, in this case downstream of the catalyst.
For exhaust aftertreatment systems targeting at catalyst pre-heating with a burner instead of electrically heated catalysts, the adsorption of the burner emissions via a small carbon canister positioned downstream of the catalyst might be a good solu-tion as well.
Increased catalyst volume
The catalyst volume is increase by 30 percent compared to the Euro 6d-TEMP base-line which is already using a bigger catalyst volume compared to former Euro 6b/c levels. This includes the volume of electrically heated catalysts. As a consequence, the space velocity at rated power is reduced to values at which high conversion efficiency can be maintained even in aged conditions.
GPF with improved filtration efficiency
Best-in-class Euro 6c and Euro 6d-TEMP engines without GPF already achieve PM emissions in WLTC of only 0.12 – 0.28 mg/km. Compared to the zero-impact target of 25 µg/m³ (approx. 0.02 mg/km), there is the need for a further PM emission reduction by 83 – 93 percent. This can well be achieved with a second generation GPF.
Final results and outlook
The outlined exhaust aftertreatment system is finally assessed in combination with a 2.0 l 4-cyl. turbocharged GDI engine in a plug-in hybrid configuration. Figure 6 shows the final exhaust aftertreatment system.
Extensive DoE investigations have been performed in order to achieve the zero-impact emission level while minimizing the fuel consumption penalty that arises from the electric pre-heating of the catalysts. Figure 7 depicts the correlation between the electrical pre-heating energy and all resulting gaseous emissions. Valid points fulfill the zero-impact target of NOX emissions lower than 40 µg/m³ as well as a balanced SOC of the battery at the end of the cycle. The optimum for meeting the zero-impact target at best possible fuel consumption is found slightly below 0.4 kWh. HC and CO emissions remain well below FEV’s anticipated Euro 7 limits. But, due to the concept, those emissions are not as drastically reduced as the NOX emissions.
The final results for the optimal operation strategy are depicted in Figure 8. The remaining NOX emissions – although hardly visible – mainly result from the first seconds after engine start. The oxygen storage capacity of the catalyst is completely filled at that time and initial rich operation is required to purge the catalyst before full NOX conversion efficiency is achieved. In the remaining part of the WLTC, NOX emission slips remain minimal. The electrically heated catalysts are re-activated for short intervals during the cycle to ensure the temperatures stay on a sufficiently high level at all times. Fuel consumption increases by 4.3 percent compared to the Euro 6d-TEMP baseline.
The zero-impact emission concept presented here is extremely biased towards achieving minimal NOX emissions. For the fulfillment of “just” the Euro 7 emission limit, several conceptual adaptations are possible, e.g. reduction of the number of electrical heated catalysts from two to one. Moreover, the adsorption catalyst could be eliminated, allowing the entire catalyst system to be re-located back to a closed coupled position.
The potential to achieve a simultaneous reduction in both NOx and CO2 emissions via fitting of a pre-turbine exhaust aftertreatment system (PT-EATS) in combination with a mild-hybrid concept was investigated via simulation. The main engine and hybrid system hardware were specified and thereafter, the operating strategies for recuperation and turbocharger control were determined to enable the system to meet a defined tailpipe NOX emissions value of 40 mg/km with a conformity factor (CF) of 1 over all real-world driving cycles. The performance and drivability of the demonstrator are defined to be equivalent to the base vehicle.
The main concept was to locate the exhaust aftertreatment system (EATS) directly downstream of the exhaust manifold, but upstream of the turbine as shown in Figure 1, so that the best CO2 reduction potential and the best aftertreatment performances can be achieved simultaneously.
The engine hardware and PT-EATS were designed and optimized via simulation to identify the best layout of the catalysts and to quantify the potential benefits for CO2 and NOX emissions reduction. The 48V system, made up of a belt starter generator (BSG) with the associated control components, an electric assisted turbocharger (e-TC) and the 48V battery as well as PT-EATS, were integrated to the existing engine model. The simulations optimized EATS component sizes to achieve successfully the integration within the engine bay. The e-Turbo was dimensioned in GT Power and moreover the EGR strategy was optimized to meet the extremely low engine-out NOx emission targets. Furthermore, the recuperation potential was established by using the simulation model. The original exhaust manifold was rotated 180 °C to enable the integration of the turbocharger and a larger EGR cooler was inserted to allow EGR to be used during full-load operation. Additional design modifications were made to the intercooler bracket, the water lines and air lines to allow the complete packaging within the engine bay of the chosen J-Segment demonstrator vehicle.
Placing the aftertreatment system upstream of the turbine results in an altered enthalpy and thermal inertia profile over the turbocharger compared to a conventional arrangement (Figure 2). The introduction of a 48V electric system to the vehicle enables the incorporation of a pre-turbine aftertreatment system via the integration of an e-TC which compensates the loss of pressure and temperature caused by the increased thermal inertia of the PT-EATS.
In the early phases of operation, the temperature before turbine is significantly lower than without the PT-EATS, owing to the increased thermal mass, but as the exhaust system heats up, a thermal lag and overall temperature offset is seen as a result of the higher thermal mass upstream of the turbine. The heat loss profile over the PT-EATS leads to a calculated cumulative enthalpy loss of ~ 4 percent over a WLTC (Figure 2). In order to maintain the boost pressure levels in such low enthalpy phases, the electric turbocharger generates additional boost pressure, it is also used to recuperate excess energy whenever possible.
The recuperation potential of the system was investigated at two part load operating points, shown in Figure 3. The comparison of the brake specific fuel consumption for 2 recuperation strategies was investigated and shown in Figure 3. Recuperation at the turbocharger via VGT is compared with the extraction of the same power over the BSG via an operating point load shift. The latter strategy shows a more energy-efficient path by up to 3.3 percent, as closing the VGT increases the pump losses, as seen in Figure 3, right. It should be noted however that increasing recuperation increases fuel consumption as additional power is needed to generate the same effective power.
The sizing of the e-TC was considered, as a larger turbine could reduce fuel consumption via optimized pumping losses, but, as typical passenger car driving scenarios are not significantly impacted by pumping loss based fuel consumption penalties, there was minimal benefit to upsizing the e-TC as the electrical boost required during transient operation would be increased so a smaller turbine was chosen. The key criteria in determining the size of the e-machine used is the transient response behavior of the vehicle. An acceleration from a standstill to 100 km/h was simulated with different sizes of e-TC to see which could achieve a comparable acceleration behavior to the base vehicle (8.7 s in the sprint to 100 km/h). These simulations are seen in Figure 4. Illustrating that without electrical boost support, the acceleration time increases to 13.0 s, confirming that an e-Turbo is required. An increase in the E-machine power above 11 kW showed no significant reduction in response time (9.0 s to 9.4 s) as acceleration was limited by the electrical machine speed to 180000 min-1.
Regarding the potential to reduce the cost and complexity of the EGR circuit, the use of an HP-EGR-only strategy was investigated which would consist of recuperating excess exhaust gas energy, while controlling the VGT position to achieve the required back pressure to drive higher EGR rates at comparable boost pressure. The results showed, that including the LP-EGR path reduces the required electrical energy over the WLTC by about 30 percent, therefore both HP- and LP- EGR are used. As the PT-EATS volume was found to show only minor impacts on the fuel consumption, compared to the EGR strategy influence, Figure 5, the EATS volumes were chosen to fill the available package space. All the above variables were combined to create an optimised air path strategy. Combining LP and HP-EGR, with a comparatively small turbine and an 11 kW electric motor allows for the lowest possible boost requirement during transient driving conditions. The additional energy requirement for this configuration over a WLTC was approx. 52 Wh when omitting recuperation at the BSG or e-Turbo.
Air path control
The electrical VGT turbocharger requires a dedicated control strategy to optimize the different operating states. For this concept configuration, the electric machine is mainly used for transient support during boost pressure build-up and recuperation during deceleration or in overrun operation. The conventional boost pressure control for the VGT was extended with an advanced model-based control for the power, respectively torque of the electric machine. In this approach the torque of the electric machine is calculated based on the difference between desired and actual turbine torque. An additional e-boost control factor is introduced to balance and adjust the responsiveness of the model-calculated torque demand to the e-machine against the electric energy consumption. The fuel consumption penalty and NOx engine-out emissions as a function of the e-boost control factor are shown, in Figure 6 for the WLTC.
When applying small e-boost control factors, the electric machine only supports during very high differences between desired and actual turbine torque, while for higher control factor values, the e-machine supports for smaller deltas in turbine torque. As such, the NOx engine-out emissions are reduced at higher e-boost control factor values, whereas the fuel consumption increases significantly in consequence of the increased electric power demand. These trends were combined to determine the target operation area.
Overall hybrid strategy optimization
The additional benefits of the 48V mild-hybrid system architecture shown in Figure 7 were evaluated. A 48V belt starter generator replaces the conventional 12V generator, a 48V battery with a capacity of 0.5 kWh, and the electrically supported electrical VGT turbocharger were integrated with the 12V on-board power supply provided via a bidirectional DC/DC converter. When optimizing the control of the electrical VGT turbocharger, a priority manager governs available power for the different consumers, based on the current state of the electric system. The simulation model uses a higher-level energy management strategy to ensure reliable supply for the on-board 12V network under all operating conditions and simultaneously maximizes the potential of the various 48V components to balance transient support during boost pressure build-up and recuperation potential at engine overrun operation and high enthalpy flow upstream turbine.
The balance of the electrical energy within the 48V system over the WLTC is shown in Figure 8. Recuperation takes place almost exclusively by the BSG, whereas the energy consumption is split in roughly equal parts between the supply of the consumers in the 12V network and to support the electrical boosting. As the recuperated energy exceeds the electrical energy consumption, approx. 30 percent is used to charge the 48V battery.
The second part of this paper in an upcoming magazin article will detail the PT–EATS system optimization and the overall system performance over key RDE cycles.
Legislation and public debate on reducing CO2 emissions in the transport sector have centered almost exclusively on nationwide use of battery electric vehicles (BEVs). There are areas of application, however, for which pure battery electric powertrains are not a suitable solution, e.g. for long-distance and heavy-goods transport. Due to their high energy density, liquid and gas fuels will remain the fuel of choice in these areas for a long while yet. This is particularly true for Europe, which will still remain heavily dependent on importing chemical sources of energy. In the future, some of these energy sources will be produced in regions where renewable energy sources are consistently available in large quantities. E-fuels – synthetic fuels made from renewable electricity and carbon dioxide (CO2) – represent a very attractive option here for powering mobility using a closed carbon cycle.
Significant reduction of CO2 emissions
Through the Paris Agreement, as well as the social policy goal of combating climate change, all sectors are committed to significantly reducing their CO2 emissions. The electric power supply sector is supposed to become entirely CO2 neutral by 2050. The transport sector is meant to reduce its carbon footprint by at least 80 percent as compared to 1990 – this despite an ever-increasing volume of transport. All conceivable climate-friendly solutions for passenger and goods transport also urgently need to be implemented. Some of these goals are reflected in the increasingly strict CO2 limits for newly registered vehicles in most countries. On December 17, 2018, the European Parliament and the Council of the European Union decided to reduce the fleet CO2 limits for newly registered vehicles again. CO2 emissions from cars are supposed to be reduced another 37.5 percent between 2021 and 2030. This translates to 59 g/km, which is equivalent to about 2.5 l of fossil fuel. When it comes to greenhouse gas reduction in the private transport sector in particular, one technical solution pops up more than most in the public and media debate: BEVs. The number of BEVs will indeed rise dramatically over the next decade. The starting level is still relatively low, however, with BEVs, plug-in vehicles, and range-extender vehicles currently making up roughly 2 percent of newly registered cars. The market penetration of BEVs depends heavily on various local legal parameters and government subsidies, and thus varies significantly in global terms.
The majority of all cars sold in 2030 (close to 90 percent) will still have a combustion engine for various reasons. The most frequent reasons cited for deciding not to buy an electric vehicle are the purchase price, the limited range, long charging times, and inadequate development of the charging infrastructure. In addition to this, only 2.5?–?5 percent of Europe’s vehicle fleet is replaced each year. As such, it will take several more years for BEVs and fuel cell vehicles to have significant market penetration. Relying solely on the growth of the electric fleet to achieve the ambitious CO2 targets is thus not an option. Instead, other effective technologies also need to be used. Using CO2-neutral synthetic fuels (e-fuels) can help in addition to electrifying powertrains and improving the efficiency of combustion engines.
It can be presumed that electrification will provide the largest contribution to CO2 reduction. In this context, electrification means not only pure electric vehicles, but also fuel cell vehicles, hybridizing combustion-powered powertrains, and powertrains with range extenders. An additional 24 percent reduction can be achieved by increasing efficiency through weight, friction, and enhanced aerodynamics as well as changing the modal split (shifting goods to rail). The remaining 31 percent needs to be covered by CO2-neutral fuels, as seen in Figure 2.
Looking at the energy situation in Europe and particularly in Germany, it is clear that the task of creating a system based 100 percent on renewable electricity poses a major challenge. Since renewable power (especially in Europe) is highly volatile and is not easy to store, grid expansion and necessary reserve capacities could involve very high investment costs in the future. The goal of using 100 percent renewable electric energy also means Germany would have to increase its production of renewable energy at least threefold compared to today’s production.
In addition to energy consumption, there is also significantly higher demand for energy for various industrial applications, the heating of buildings, and transport networks, currently total 2,600 TWh per year in Germany. In order to meet these needs using energy from renewable sources, Germany will have to start importing renewable energy on a large scale (Figure 3). Since some of the transmission paths are long, directly importing electric energy is only technically feasible to a certain extent, however. As a result, electric energy harnessed overseas using solar and wind power will need to be converted into chemical sources of energy by means of power-to-fuel. For regions with shorter distances between the production site and consumers, hydrogen or substitute natural gas could also be used as a carrier and transported via pipeline. Conversion into methanol or even Fischer-Tropsch products is a more sensible approach for more remote production facilities. Overall, Germany needs to import up to 29 percent of its energy requirement in the form of power-to-X (PtX) by 2050.
Using CO2-neutral sources of energy is the most efficient way to reduce its carbon footprint. As drop-in fuels, they can also reduce the carbon footprint of existing vehicle fleets. Due to their molecular structures, many prospective PtX fuels have different chemical and physical properties. The candidates that best meet the key criteria (energy density, fuel availability and established production paths, compatibility with the existing fleet) are Fischer-Tropsch fuels and longer-chain alcohols for diesel engines, as well as methanol-to-gasoline (MTG) and methanol for gasoline engines. It is possible to use hydroformylated Fischer-Tropsch fuels containing long and medium-chain alcohols to produce an e-fuel for diesel engines that is compatible with the current EN 590 standard and can thus be mixed into the existing fleet at any ratio. As shown in Figure 4, these fuels also allow for a significant reduction of soot and/or NOx.
Ethanol could be a highly promising candidate for gasoline engines. In 2018, some
110 million tons were synthesized and traded, primarily for the chemical industry. Due its very high knock resistance and good lean-burn characteristics, methanol can be used to significantly improve the efficiency of gasoline engines. As a result, e-fuels can be used to achieve similar tank-to-wheel efficiencies as in fuel cell vehicles. Figure 5 shows the increase in efficiency for engine measures implemented thus far and how the efficiency target of 50 percent could be achieved in the future.
Some countries, such as China, are massively promoting the use of methanol. In Europe, the methanol content in gasoline is currently limited to 3 percent v/v in EN 228, even though most fuel system materials are already certified up to 15 percent v/v. In addition to its direct use as fuel, methanol is also very well suited as a starting material for other fuels. For instance, the methanol-to-gasoline (MTG) process can be used to produce a gasoline-equivalent synthetic fuel that can also be mixed with conventional fuel in large quantities.
Areas of application for e-fuels
In the future, Germany will need to depend heavily on PtX imports. Cost will be the primary factor for the expansion of various technologies, however. Since local circumstances have a critical impact on the availability of renewable energies, the costs of synthesis also vary quite dramatically around the world. Figure 6 shows how greatly fuel production costs depend on the costs of power.
In many countries, e.g. in the Middle East and North Africa (MENA), the costs of synthesis will drop to below 1 euro/l diesel equivalent by 2030 due to low electricity rates. Although the potential for e-fuels has also been discussed outside of Europe, no large-scale PtX plan is currently in the works since legislation still does not acknowledge any CO2 reduction through e-fuels. As a result, market players still do not see a substantial enough business case to invest in e-fuels. A quick market introduction would be possible if one stakeholder were to profit from producing, marketing, or using e-fuels. A certificate trading system could also be introduced to allow carmakers to purchase CO2-neutral fuel and the corresponding certificates. By mixing it into the existing infrastructure, the fuel would be used by all customers.
The CO2 savings resulting from the use of e-fuels would then be counted towards the CO2 emissions of the manufacturer’s vehicle fleet. Another option would be to redesign the energy tax by lowering it on renewable sources of energy and gradually increasing the costs for CO2 emissions from fossil fuel combustion. The stakeholders addressed could be the petroleum industry or carmakers. This would yield a sustainable business model with the urgently needed investment security.
All technically feasible options will need to be used to achieve a quick reduction in CO2 emissions. However, the powertrain systems for heavy-load and long-distance vehicles cannot be electrified to the same extent as those for light commercial vehicles and cars. Goods transport will still need to rely on liquid or gas chemical sources of energy. As such, Europe will also rely on substantial imports of chemical sources of energy. The tank-to-wheel accounting currently gives a strong preference to electromobility – which will contribute enormously to lowering fleet CO2 emissions – over alternative technologies.
E-fuels have not been counted toward fleet emissions yet. Legislation thus requires urgent revision. There are various ways to make synthetic fuels more attractive, e.g. a tax on CO2 or carbon from fossil sources. Another option is to count e-fuels toward fleet emissions using a certificate trading system. Regardless of the political instrument used, synthetic fuel absolutely must be compatible with the existing fleet in order to quickly achieve market penetration.
The hybridization of powertrains is an important step toward efficient and clean mobility. In particular, the possibility of shifting the operation of the combustion engine to ranges with a higher efficiency level and representing purely electric driving modes is one of the main advantages of hybrid drives. This shifting of the load point can be further optimized on the basis of route data that includes the expected vehicle speed as well as the road gradient, and is considered to be the state of the art with regard to modern hybrid drives.
Combined with the development of predictive and automated driving functions, further potentials can be tapped. The key factor for an actual reduction of the energy requirement under real driving conditions is a precise forecast of the future development of a traffic situation. This forecast can be based on a multitude of potential sources, such as sensor data, high-resolution maps, and vehicle communication, whereby all the data is fused into a comprehensive environmental model.
Based on the information from this model, the longitudinal guidance of the vehicle and the powertrain control can be optimized. In cooperation with the Institute for Combustion Engines of RWTH University Aachen, Germany, FEV has developed a function structure that is capable of using a multitude of potential data sources. This creates a solution space for predictive speed profile optimization. This speed profile can then be used in order to optimize the operation of torque distribution between the hybrid components.
The function structure was integrated in a hybrid prototype vehicle constructed jointly with DENSO. A robust, real-time model predictive control algorithm is used in order to optimize the longitudinal guidance of the vehicle.
The HYBex3 concept vehicle
The HYBex3 (”HYBrid power exchange 3 modes“) vehicle was developed in order to determine the impact of a cost-effective DHT transmission concept on the driveability of the vehicle and test it under real conditions. It was developed jointly with DENSO AUTOMOTIVE Germany. The base vehicle is a MINI Cooper with a turbocharged 100 kW three-cylinder combustion engine. The serial transmission was replaced with the hybrid transmission to be examined, which was specially developed for the application case. The powertrain topology is equivalent to a mixed hybrid equipped with two electric engines (EE) in a P2/P3 layout. The P2 machine is located between the electrohydraulically powered clutch and the two-stage spur gear component. The synchronization elements are also actuated electrohydraulically. The P3 machine is positioned at the transmission output and therefore has a fixed transmission ratio to the wheel.
Various operating modes can be represented with this DHT transmission. For purely electric driving, the combustion engine is stopped and the clutch is opened. Electric engine P2 can therefore be operated in both transmission stages. In addition to a high starting torque in the first gear, this enables a maximum vehicle speed of 140 km/h in the second gear.
In hybrid operation, serial or parallel driving is possible. In parallel operation, one of the two gear sets is engaged. In serial operation mode, the transmission is shifted to neutral. The combustion engine is then exclusively connected to electric engine P2 while electric engine P3 operates the wheels. All gear changes are synchronized entirely electrically, so that the friction clutch can remain closed even in hybrid operation. The serial operation in the low speed range and the parallel operation at higher speeds enable a significant increase of the system efficiency level.
The operating strategy provides for the combustion engine being operated at a very low dynamic and the implementation of fast load changes by the electric path. The transmission ratios enable a significant reduction of the rotational speed of the combustion engine, without compromising the overall dynamic of the powertrain. The operating strategy was optimized with a Design of Experiments. For this purpose, the parameters of the stop-start strategy of the combustion engine were optimized simultaneously with the parameters of the battery charging strategy. For the final parameterization, a compromise between the layouts for different driving cycles was selected.
The distribution of the torques of the two electric engines, both in parallel operation and in fully electric driving, is determined by an online optimization patented by FEV. The search algorithm varies the torque distribution until the energetically optimal case is found. In doing so, both the battery limits and the power limits of the electric engines are taken into account for the current situation.
The function structure developed for predictive longitudinal dynamic control is designed in such a way that a multitude of data sources, optimization routines, and powertrain structures can be represented in said function structure.
The first step is an aggregation and fusion of the available data into an environmental model, followed by a prediction of the traffic situation. This enables an optimization of the speed profile. On the basis of that, an acceleration control of the vehicle is carried out. The planned speed profile can also be used in order to adjust the charging status strategy. If the desired charging performance is determined, the torque distribution between the powertrain components is carried out on the basis of said performance and the wheel torque requirement.
The precise forecast of the current traffic situation requires the aggregation of all available data. This includes, for instance, RADAR sensors, LIDAR sensors, or optical cameras that traffic participants can identify with the help of image recognition techniques. Usually, these sensors indicate the type (passenger car, truck, pedestrian, etc.), the relative positions and, potentially, the relative speed of the detected objects.Further information can be obtained from the on-board navigation systems, which indicate speed limits, road gradients and curvatures as well as, potentially, intersection data for the most probable path of the vehicle via an “electronic horizon”. If the navigation system is connected to the internet, data on average speeds along the planned route and traffic jams can be provided.
Additional data can be obtained through the future connection of vehicles using 5G or ETSI ITS G5. This Vehicle-to-everything (V2X) communication should include, among other things, the positions, direction, and speeds of other vehicles, as well as the layout of intersections and the status of traffic light systems. The vehicle communication can therefore provide data that goes beyond the horizon detectable via on-board sensors.
Since the same object can therefore be detected multiple times by various data sources, the data aggregation must also include a functionality for data fusion. This is especially advantageous for hardware setups with different types of sensors, e.g. a RADAR sensor and camera sensor. The RADAR sensor can precisely define the distance to and the relative position of a vehicle driving ahead, but cannot determine the lateral position of the vehicle in relation to the road markings. In contrast, the camera sensor can only provide estimates regarding the relative speed and the distance, but can precisely determine whether the detected object is in the same lane as the vehicle under consideration. After the fusion of several data sources, an aggregated object list is created, which only contains valid and relevant data for all detected objects, and generates a corresponding environmental model.
Before an optimization of the vehicle trajectory can be carried out, there must be a forecast of the development of the current situation. This forecast is based on the relevant objects that the environmental model provides. The first step is the determination of the speed limit along the prediction horizon. Based on that and the current condition of detected vehicles driving ahead, the speed and position trajectory of these vehicles is forecast.
On the basis of this, a solution space is spread out in which the downstream optimization algorithm can operate. The function structure developed by FEV and the Institute for Combustion Engines enables the implementation of different algorithms to this end. Depending on the requirement, simple, rule-based approaches, as well as model predictive control or discrete dynamic programming methods can be represented.
Application in the vehicle
To test the function structure, a real time-compatible model predictive control (MPC) was implemented in the rapid prototyping control unit of the HYBex3 concept vehicle and various test scenarios were carried out. In a first demonstration, the functionality and real-time compatibility of these scenarios for a predictive adjustment of the HYBex3 concept vehicle was proven. With an efficient implementation of the MPC using the qpOASES tool, an optimization of the speed curve for a horizon of 10 s can be carried out within less than 100 µs.
In the future, the modular design of the function structure can be used to expand the forecast horizon of the vehicle – for instance, with traffic lights ahead – or to represent predictive, automated driving functions such as Predictive Cruise Control (PCC).
Legislative requirements for emissions and fuel consumption reduction are driving OEMs to develop innovative powertrain and vehicle technologies. In addition to continued development of new technologies with conventional internal combustion engines (ICE), there is an increasing trend toward electrification. These trends make it essential to develop relevant means of assessing the NVH performance of electric drive tunits (EDUs). These components do not generate the amplitudes of noise and vibration observed from internal combustion engines (ICE). As such, the methods used for NVH assessment and target development of IC engines are not sufficient for electric machines: While the objectives of ICE-based NVH development are reduction and refinement of source excitations, EDU-based NVH development focuses on the elimination of potential objectionable noise behavior in the context of ever-changing or missing masking noise content. For example, there is a reduced background noise for masking tonal noise in the absence of a running internal combustion engine.
The expectation for interior noise content from ICE-based vehicles (i.e., “powertrain presence”) depends highly on the vehicle class and target demographic; while luxury cars target low interior noise content, performance vehicles demand some level of powertrain noise feedback (with an emphasis on development of the desired “brand character”). Conversely, the tonal noise typically associated with EDUs is universally considered annoying; hence the goal is to minimize perception of this content in the vehicle. This becomes challenging, given that the reduced overall noise content available to hide (mask) this tonal noise content is lower on electric vehicles than ICE-based vehicles. Figure 1 below illustrates the difference in typical noise levels observed in ICE-powered vs. electric vehicles (EV) in the form of FEV scatterbands. Clearly, significant reduction in overall noise levels on EV are evident, especially at low-to-mid vehicle speeds.
To predict the perceptibility of tonal noise content in-vehicle, masking band analysis can be used. As shown in the figure below, the order content can be compared to surrounding 3rd octave levels to determine how much noise is available in adjacent frequencies to mask the tonal noise. If the order level (of whine noise) is high relative to the corresponding 3rd octave band noise levels, this is an indication that there is insufficient background noise to mask the order, resulting in a perceptible, and hence, objectionable whine noise. Also shown below is a masking surface which illustrates the masking content for various orders over the operating range of an example vehicle. At higher vehicle speeds, wind noise is more prominent; this results in more masking content and an associated reduction in perception of whine noise.
NVH issue root-cause analysis & mitigation
Increased trends in electrification and associated technologies have posed new challenges in NVH development. In addition to minimizing tonal noise content in the vehicle’s interior, there are multiple potential NVH issues related to transient instabilities (e.g., gear rattle or other driveline issues). FEV utilizes a structured approach, with extensive experience in 8D analysis and Design-of-Experiments (DoE) to address such problems. As part of this root-cause analysis, FEV utilizes a combination of industry-standard methods (e.g., Ishikawa diagrams), as well as FEV developed tools and processes. FEV’s Vehicle Interior Noise Simulation (VINS) is an example of a unique methodology that can be effectively utilized in the support of root cause analysis with complex noise issues. The VINS process is a unique time-domain transfer path analysis which provides insights into noise sources and transfer paths which contribute to sound quality issues under steady-state or transient conditions. Any noise issues identified at the vehicle’s interior can be broken down to identify contributions of various structureborne and airborne noise paths. The critical noise paths can be further decomposed to identify any potential opportunities for improvement (mount isolation, attachment point stiffness, vibroacoustic sensitivity, acoustic attenuation, etc.). Because the results generated are in the time-domain, advanced analysis methods or subjective evaluations (listening studies) can be used for assessment of the overall simulated noise or individual path contributions. Figure 2 schematically shows the integration of the VINS methodology in a structured 8D root-cause analysis process.
Component-level EDU NVH assessments
FEV has established standard testing procedures for quantifying radiated noise, sound power, and vibration at the component-level to facilitate assessment of source-level inputs to support electric vehicle NVH development. Analogous to ICE-based powertrain NVH testing, overall EDU radiated noise levels are typically assessed based on average radiated noise, measured at a distance of 1 m from the EDU (e.g., using SAE J1074 standard). Additionally, it is common practice for electric machines and EDUs to augment these assessments with measurement of sound power, utilizing a hemispherical or parallelepiped array (e.g., IS0 3744 or 3745 standards). Structureborne excitations can be assessed by measurement of vibration at the EDU mounting locations (i.e., interface points between the EDU and vehicle).
Comparison of average overall sound pressure levels between ICE-based powertrains and EDUs in the figure below illustrates that noise levels radiated from EDUs are significantly lower than those observed from ICE powertrains. As such, assessment of individual orders excited by the electric machines and/or gear meshing frequencies is more relevant (than overall noise levels) for quantification of EDU NVH performance. An example of order content relative to overall radiated noise levels is illustrated below. This comparison provides information regarding the contribution of orders to overall noise levels. Additional investigation of the frequency content of the component noise levels can provide insights into perceptibility of this noise in a test cell environment. However, component level data analysis alone does little to predict the perceptibility of these orders by the customer in-vehicle. For this, a vehicle-centric data analysis approach is required, as described below.
Vehicle-centric EDU NVH target development
Derived from the VINS methodology, FEV has developed an additional process for interior noise prediction, called dBVINS. Unlike VINS (which utilizes vehicle-specific noise transfer functions), the dBVINS process predicts interior noise based on a combination of source data (noise and vibration, as measured in the test cell) and standardized vehicle noise transfer functions. These “standardized” noise transfer functions are based on median vehicle noise sensitivity performance, derived from the extensive database of vehicles assessed by FEV. By standardizing the transfer functions, the interior noise relevant NVH performance of a given component (e.g., EDU) can be judged based on component-level tests from a NVH test bench. This allows for direct comparison of the expected interior noise performance of different EDUs or design variants of a development EDU. Specific to EDU development, this process allows for prediction of relevant order content at the vehicle interior. Comparison of this order content to the masking noise levels discussed above provides insights into the potential perceptibility of tonal noise issues by the customer. Appropriate design changes using a combination of CAE (e.g., MBS/FEA) and test-based approaches (e.g., calibration changes, NVH countermeasure development) can be employed to improve the component-level NVH performance of the EDU, utilizing such a vehicle-centric approach.
Dipl.-Ing. Thomas Körfer, Group Vice President Light-Duty-Diesel, about the development of the new 3.0L Duramax Diesel Engine
The trend towards battery electric vehicles will continue and likely even accelerate in the future. These vehicles will make a significant contribution to meet future targets for fleet fuel consumption and emissions. To be commercially successful, these new vehicles require modern and intelligent solutions for their powertrain, including battery and drive unit.
The optimal drive unit concept has to be developed based on an evaluation of performance, efficiency, and cost on a system level, including all powertrain components, such as a battery, inverter, electric motor, and transmission. This is what FEV and YASA have done with a high-performance D-class passenger car application in mind. The result is a drive unit concept with exceptional power density and efficiency based on YASA’s unique axial flux motor technology and an innovative 2-speed transmission concept by FEV.
Figure 1 shows an external view of the drive unit and its main specifications. With a peak of power of 300 kW and a weight of less than 85 kg, it provides an outstanding power density of 3.5 kW/kg on system level. The maximum axle torque of 6.00 Nm even exceeds typical wheel slip limits for both front and rear-wheel drive applications and ensures superior acceleration performance on a vehicle level.
Electric motor and inverter
The YASA motor is an axial-flux permanent magnet machine and was chosen because of its high power density (up to 15 kW/kg for custom motor designs), high efficiency (especially at part loading) and low-cost manufacturing. In the YASA motor, the oil coolant is in direct contact with the copper windings, providing very efficient and even cooling over each winding.
YASA controllers feature similarly differentiated high-density performance. This is achieved by the use of some of the YASA motor’s proprietary direct oil cooling technologies that provide for very efficient cooling, and substantially reduce the need for heavy and costly heatsinks and power semiconductor packaging. When integrated with a YASA motor, the motor and controller share the same oil cooling circuit, further improving the standard integration benefits of reduced volume, mass and interconnection complexity.
Based on the above-mentioned findings, a powershift capable 2-speed concept was developed. Figure 3 shows different views of the drive unit.
The 2-speed functionality is realized based on a Ravigneaux planetary gearset. Figure 4 explains the topology of the transmission. The planetary gearset is arranged coaxially to the electric motor. The small sun (SS) does serve as the input, and the ring (R) serves as the output to the intermediate shaft and differential. Two brakes B1 and B2 are used to realize two speeds. Brake B1 is connected to the carrier and paired with a one-way clutch (OWC), B2 is connected to the large sun gear (SL). Despite being mechanically more complex than architectures in a simple planetary gearset, this architecture has a number of technical benefits. As shown in the table “Clutch relative speed matrix”, the delta speed at open brakes is always below the input speed at the small sun, an important quality for minimum drag losses. At the same time, the torque reaction at the brakes is favorable as shown in the table “Clutch torque matrix”. Brake B2 only has to react less than half of the input torque. Brake B1 has to react 1.5 times the input torque but is supported by the one-way-clutch. This allows to size the brake itself smaller, further reducing drag losses. As opposed to clutches, brakes avoid the use of rotary joints or engagement bearings to actuate the gearshift. In addition, the thermal capacity of brakes can be scaled via the thickness of their (stationary) steel lamellae without negatively affecting rotary mass moments of inertia. The exclusive use of brakes was, therefore, an important criterion in the selection of the concept. Both brakes are actuated via an existing series-production, on-
demand actuator from LuK. The unit, also known as HCA (hydrostatic clutch actuator), operates with a brushless electric motor for each gearshift element, which actuates a hydraulic master piston via a spindle. Because of the leakage-free seals, this system is very efficient. Alternatively, electromechanical actuation concepts could be used thanks to the good axial accessibility of the brakes.
Figure 5 summarizes the functions of both the brakes and the one-way clutch. It also mentions an additional advantage of the arrangement, including the one-way clutch: In 1st speed during drive (power-on) condition, B1 can be opened and the reaction torque at the carrier will only be provided by the one-way clutch. Then, the power-on upshift, which is most critical in terms of shift comfort, can be performed by only closing brake B2. This type of shift is easier and more robust than any conventional power shift which typically includes the simultaneous control of two shift elements. The same advantage applies to a power-on downshift when only B2 has to be opened. At zero rpm of the carrier, the one-way clutch will automatically lock-in and engage 1st speed.
Cooling and lubrication concept
As mentioned previously, the electric motor and inverter do share one common oil cooling circuit. Using a dedicated EDU oil which fulfills the requirements of both the electrical and mechanical components, also allows for the transmission to be integrated into that cooling circuit. As of today, such a fluid is not yet readily available, however, several oil suppliers have confirmed that it can be successfully developed within a standard series development of 3 years duration. The obvious advantage of such a highly integrated cooling and lubrication circuit is less complex and more cost-effective, as only one pump, one cooler, and almost no external hosing would be required. In addition, the interfaces to the vehicle would be simplified considerably. Alternatively, separate oil circuits can be used for the electric motor/inverter and transmission. In this case, oils are readily available and can be tailored even better for the requirements of each circuit. The development risk will be reduced, but the complexity and cost of the overall system will be increased.
Figure 6 explains the variant with one common cooling and lubrication circuit. An electric oil pump draws oil out of the transmission sump and feeds it via an oil/water heat exchanger to the inverter. From there, the oil flows through the electric motor and subsequently back into the transmission, where the volumetric flow is divided. One part is fed into the main shaft of the planetary gear set, from where it not only lubricates the gears and bearings but also cools the brakes as required. The remainder is not drained into the sump but buffered in a storage tank inside the transmission. From here, further components are lubricated via various channels, including the gear meshes and the bearings of the intermediate shaft.
An intelligent oil pump control strategy allows to vary the level of the storage tank and thus the oil level in the transmission, which makes a large contribution to a reduction in churning losses and thus an increasing inefficiency. Figure 7 shows two internal views of the transmission including the integrated oil reservoir. A park lock system is arranged on the intermediate shaft and can be actuated by a stand-alone, electro-mechanical park-by-wire actuator.
The presented 2-speed drive unit does use a high powered, dense yet modular combination of an axial flux electric motor and a coaxially arranged inverter. The transmission is based on a Ravigneaux planetary gearset with two brakes as the shift elements. Together, with a one-way clutch, this arrangement is both favorable in terms of drag losses, as well as controllability and shift comfort. The brakes are actuated on-demand for minimum energy consumption. The electric motor, inverter, and transmission do optionally share a single, common cooling and lubrication circuit which reduces complexity and simplifies the interfaces of the drive unit to the vehicle. With a peak power of 300 kW and a weight of less than 85 kg, the drive unit provides an outstanding power density of 3.5 kW/kg on system level. The maximum axle torque of 6,000 Nm even exceeds typical wheel slip limits for both front- and rear-wheel drive applications and ensures superior acceleration performance on vehicle level.
The drive unit concept presented in this article has been jointly developed by YASA and FEV. The motor and inverter technology described in this paper is owned by YASA Limited, a UK-based developer and manufacturer of electric motors and inverters. The 2-speed transmission concept described in this paper is owned by FEV, an independent provider of powertrain and vehicle engineering services.
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.
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.
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.
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).
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.
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.
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.
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
Philip Griefnow, RWTH Aachen University
Prof. Jakob Andert, RWTH Aachen University
Dr. Georg Birmes, FEV Europe GmbH
The cylinder deactivation on a diesel engine has showed potentials on the one hand side to further reduced pollutant emissions, while on the other hand to gain some fuel economy in parallel. This has been demonstrated by several investigations in the past. Nevertheless, a static deactivation of half of the cylinders is limited by their operation range. An additional dynamic deactivation of several cylinders delivers further degrees of freedom that could provide an extension of the cylinder deactivation operation range.
The authors have used different simulation tools such as 1D steady-state engine process model and transient mean value model to represent the possibilities of a dynamic cylinder deactivation on diesel engine applications.
A state-of-the-art diesel engine for passenger cars (PC) and medium duty (MD) truck applications have been used for the investigation program.
For the PC applications a 2.0 l 4-cylinder diesel engine with a single stage boosting system and a compression ratio (CR) of 15.5 has been considered. Further engine applications have been an advanced exhaust gas recirculation (EGR) system (uncooled high and cooled low pressure EGR path) and a 2000 bar fuel injection system (FIS). It has been decided to investigate two different vehicles, a C segment vehicle, as well as a compact SUV. Those have been equipped with a 7- and 8-speed dual clutch transmission (DCT). The exhaust aftertreatment system has installed a closed-coupled DOC, SDPF as well as a passive underfloor SCR. All EATS components have been used as aged system. The cycle investigations have considered the standard WLTC and a RDE operation.
The MD truck has been powered by a 7.7 l 6-cylinder diesel engine. The air path has a standard wastegate turbine (WG) boosting system together with a cooled HP-EGR system installed. The combustion system has considered a 2,400 bar FIS and a CR of 17.7. A state-of-the-art EATS based on closed-coupled DOC, DPF and SCR has been installed. For the MD truck application, the WHTC has been considered.
1D engine process simulation model
The commercial 1D engine process simulation software GT-SUITE has been used to investigate the thermodynamic reactions of the different exhaust gas heating strategies. The 1D engine model has considered the entire engine configuration, such as the boosting system, the air and exhaust path, the EGR path (high pressure and low pressure) and combustion chambers. The burn rate of fuel combustion has been implemented through profile arrays from several engine operation points of the entire engine operation range. Those have been generated by a standard 0D approach of cylinder pressure analysis of steady-state experimental engine measurements. The entire EGR control of the model has been modified from a mass flow control to an oxygen concentration control. The fuel injection pattern and rail pressure as well as boost pressure set points have been kept constant.
This 1D model can operate in the entire map range, and allows simulation throughout the entire engine operation range. Standard PID-controllers have been used to control components like EGR valves or turbocharges in order to regulate EGR rates or boost pressure under steadystate investigations. Finally, a sub-model for engine-out emission predictions had been added to the engine model. This uses the physical correlation approach of in-cylinder O2-concentration to predict engine-out NOx and soot emissions. Thus, transient effects on emission production have been considered, which usually occur at dynamic engine operation. In addition, HC and CO emissions have been implemented by steady-state maps which dependent on engine speed and load. The approach describes the standard at FEV and has been used in the past. To obtain an accurate result, the 1D model has been validated to surrogate data. The accuracy of boost pressure showed a deviation of maximum 1 percent. The calibration level of the emission models were more challenging and provided a maximum deviation of 5 percent.
Map calibration for considered heating strategies
To investigate the exhaust heating potentials of the different exhaust heating strategies within the mean value powertrain model (MVPM), the baseline engine-out maps have to be adjusted, based on the results of the 1D model simulations. For this purpose, differential and factorized maps have been generated and added incorporated into the base engine maps. Together with the differential and factorized maps, a new engine calibration with a specified exhaust heating strategy has been considered.
Mean value powertrain model
The FEV Complete Powertrain Simulation Platform, a precursor of FEV’s advanced VCAP calibration platform was utilized in this study. The powertrain model has integrated five main sub-models for boundary/ambient conditions, vehicle settings, transmission, engine and the aftertreatment system. The boundary/ambient condition sub-model described the different road conditions, emission test cycles and different driver behaviors. Inside the vehicle model the rolling resistance as well as road influence, aerodynamics and gravity were considered to model vehicle longitudinal dynamics. The main transmission and driveline components were modelled with ideal torsional systems, subjected to a distinct efficiency at different oil temperatures. Based on those sub-models, the main objective was to calculate the required inputs for the engine, mainly actual engine speed and load request. The engine model provided than on the specific operation point the corresponding engine out conditions, which were described by calibration maps at different coolant temperature.
Selective cylinder deactivation by Dynamic Skip Firing
Dynamic Skip Fire (DSF) is an advanced cylinder deactivation technology. A DSF-equipped engine has the ability to selectively deactivate cylinders on a cylinder event-by-event basis in order to match the torque demand at optimum fuel efficiency while maintaining acceptable noise, vibration and harshness (NVH). To illustrate this concept, Figure 1 shows an example of DSF operation in a four cylinder engine. A varying torque request is shown in green, which results in cylinders being fired (red) or skipped (grey). The combined firing pulse train for all four cylinders is in blue. As torque demand increases, the density of firing cylinders also increases. When torque demand is zero or negative, no cylinders fire. This is termed DCCO, or deceleration cylinder cutoff.
Evaluation of simulation results
The evaluation process has been substituted into two tasks. The first task has dealt with the steady state simulation investigations of the different heating strategies by means of 1D engine process models. Whereas the second task has focused on transient cycle investigation.
Analysis of steady state 1D engine process simulation results
The 1D steady-state investigation have been obtained for partly loaded operation. Those investigations have been done underfour different fire density (FD) levels, where 1 indicates full cylinder operation. A FD of 0.25 is equal to a single cylinder operation out of this 4-cylinder engine. The steps in between are defined as 0.75 and 0.5.
The engine operation at a FD below 1 has led to an anomalous turbocharger operation due to the changed exhaust gas dynamics. Therefore, reduced boost pressure levels have been achieved and resulted in a limitation of the maximum engine load operation. Figure 2 shows a schematic of maximum engine operation loads that can be achieved at different FD levels.
Since the deactivation of one or more cylinders, the load at the remaining fired cylinders have been increased to hold a constant engine power output. The increased inner load has provided a higher exhaust temperature at a higher engine efficiency. Figure 3 summarizes the relative simulation results at a FD = 0.5 of engine efficiency improvement by BSFC and absolute exhaust temperature increase in the lower part load area. It can be seen, that FD of 0.5 has provided a fuel consumption benefit of 15 percent in average in the shown operation area. At the same time an exhaust temperature increase of almost 130 K at 3 bar of BMEP has been achieved in comparison to a 4-cylinder operation.
Additionally to the mentioned advantages other effects have occurred by a steady-state cylinder deactivation. On the one hand a reduction of the exhaust mass flow rate has obtained by deactivating cylinders. Hence, also a lower emission engine out mass flow rate has been achieved. While this has delivered, on the other some degrees of freedom to lower the steady-state EGR calibration to keep the same NOx engine-out mass flow rate compared to a 4-cylinder operation.
Evaluation and assessment of transient MVPM simulation results
To determine the impact of DSF on relevant cycles, the WLTC and RDE were simulated for the PC application, and the WHTC was simulated for the MD application. Figure 4 shows transient results of C segment and compact SUV application over WLTC. It depicts the fire density, exhaust temperature upstream SDPF as well as the cumulated tail-pipe (TP) NOx emission.
The WLTC begins at an ambient temperature of 23 °C. A minimum coolant temperature limit of 60 °C is imposed to represent hardware constraints, and effectively eliminates DSF operation until 140 seconds. The exhaust temperature traces upstream SDPF have showed only slightly increase after cold start and warm-up phase, due to the thermal mass of the DOC. Afterwards, an exhaust temperature increase by around 20 K has been achieved under DSF operation at segment C vehicle. That increased exhaust temperature has improved the NOx conversion of SDPF and dropped the TP NOx emission down to 43 mg/km. It represents a reduction by
4.4 percent compared to the 4-cylinder operation of segment C. Additionally, these improved results have been achieved with a benefit in CO2 emission by 1.5 percent.
The results of compact SUV have showed a lower NOx reduction potential by DSF operation. This heavier vehicle application has led to a higher engine operation with an increase exhaust temperature level. Furthermore, the DSF operation has been reduced based on the higher load request. Thus, only a slightly exhaust temperature increase has entered the SDPF. Nevertheless, an improvement in CO2 emission by around 1 percent has been obtained.
Figure 5 summarizes the simulation results of WLTC and RDE. The results under RDE have provided than additional improvements at the trade off between NOx and CO2 emissions.
Figure 6 shows the simulation results of the MD truck application under cold stared WHTC. It can be seen, that the activation of DSF has increased the exhaust temperature upstream SCR by 10?–?30 K in a wide range of the cycle. Thus, an improved NOx
conversion has occurred and provided a tailpipe reduction by 15 percent compared to base configuration. Also fuel consumption benefit has achieved of around 1.6 percent due to the dynamic cylinder deactivation.
Figure 7 shows the summary results of MD truck in weighted WHTC. The weighting factors consider a distribution of 14percent cold started WHTC and 86 percent hot started WHTC.
The investigations have shown a tailpipe BSNOx improvement of around 30 percent in parallel to BSFC benefit of 1.6 percent.
The increasing tightening of global emission legislations promotes the further development of gasoline engines with the aim of clean engine operation under all real driving conditions. At the same time, performance requirements are growing. Gasoline engines compete increasingly with electrical components for package volume, and the displacement of high performance engines is reduced to lower the CO2 emissions. This article covers the trade-off between increasing specific power and switching to Lambda = 1 throughout the engine map.
Why Lambda = 1 throughout the engine map?
Components in the exhaust gas flow of gasoline engines are currently protected from excessive thermal stress at high performance by mixture enrichment (Lambda < 1). At the same time, such an operating strategy is linked to the cross-influences:
- The fuel consumption at high engine output is disproportionately high.
- The CO engine-out emissions are increased considerably by the mixture enrichment, and outside of the operating window with Lambda = 1, the three-way catalyst only provides very low conversion rates.
- CO emissions under RDE conditions are not limited by the Euro 6d legislation, but they are measured and recorded (“monitoring”).
- Apart from the monitoring of CO in the homologation process, non-government organisations also record CO emissions under RDE conditions.
- Since the introduction of RDE Package 4, so-called AES (Auxiliary Emission Strategies which influence emissions as e.g. mixture enrichment) can only receive a time-limited approval.
The switch to Lambda = 1 leads to a loss of performance and reduces the specific power of current representative technology packages of gasoline engines to ~ 65 kW/L. It results in the increasing introduction of technological measures which improve the specific power at Lambda = 1. These include:
- Integrated exhaust manifold (iEM)
- High temperature-resistant turbocharger turbines
- Miller cycle combined with corresponding boosting procedure as variable turbine geometry (VTG) or electrical turbocharger (eTC)
- Cooled exhaust gas recirculation (cEGR)
- Variable compression ratio (VCR)
For volume segments from 85 to 100+ kW/L can well be achieved. The development of drive systems for high performance vehicles allows more freedom with regards to cost and applicable technology. FEV has investigated the following question: “Are 200 kW/L at Lambda = 1 possible?”
Combustion process for 200 kW/L at Lambda = 1
The realization of the specific power of 200 kW/L at Lambda = 1 requires a break-up of the conflict of interests between supercharging and knock tendency. Water injection in the intake port represents the key technology. The reduction of the mixture temperature associated with the high evaporation enthalpy of water at the end of compression allows for a significant increase of the efficiency of the high-pressure cycle. Figure 3 shows a variation of the water-fuel ratio (WFR) at a speed of 7800 min-1 and stoichiometric engine operation. With the selected compression ratio of 9.3:1 the brake mean effective pressure (BMEP) can be increased with the growing water share at only a slight delay of the center of combustion to 30.8 bar, so that the value of 200 kW/L is achieved at a WFR of 55 percent. An absolute boost pressure of approx. 3.3 bar is required, which can be supplied with a single-stage compressor.
The position of the water injector in the intake port has been optimized with the help of 3D CFD simulations. For the distance that is furthest away from the valve, the wall film share is too high, because the water can wet the largest area. For water injection closer to the valve, the share decreases significantly, whereby the improvements for a distance of less than 60 mm are minor.
An analysis of the temperature distribution in the combustion chamber shows that the 60 mm position is preferable to the
30 mm position despite the same mean temperature.
With respect to the high mass flow rate and boost pressure demand, the requirement of a low throttle effect of the intake valves is in contrast to the objective of a high charge motion.
Figure 5 shows how 3D machined valve seat rings are used to achieve a high charge motion with simultaneously increased flow coefficient.
Design for high mechanical and thermal stress
An engine design for a specific power of 200 kW/L must withstand high thermal stress and high mechanical load. The turbine wheel is manufactured from MAR 246 and withstands a maximum temperature of 1,050 °C. In addition to the exhaust gas turbocharger turbine, the exhaust valves are exposed to particularly high thermomechanical stress. Therefore, sodium-cooled exhaust valves are used. An optimized solution is used which directs the sodium into the valve disc and at the same time largely maintains its structure.
The aluminium cylinder block is a rigid closed-deck design with a bed-plate and cast iron cylinder liners. An aluminium spray coating guarantees a good connection between cylinder and crankcase. The high thermomechanical stress with the corresponding pronounced cylinder deformation is addressed with free form honing.
High performance boosting and periphery
The system is equipped with an exhaust gas turbocharger on each cylinder bank. The turbine is equipped with a variable turbine geometry without wastegate. The use of the entire exhaust gas mass flow for the generation of the compressor drive power lowers the turbine pressure ratio and therefore also the pressure upstream of the turbine. This means that lower gas exchange losses and exhaust gas temperatures can be reached at rated power.
Secondly, the added hot wastegate mass flow downstream of the turbine with the associated inhomogeneous thermal stress on the catalyst due to insufficient mixing is eliminated. The compressor is equipped with a variable trim, the turbocharger with an electric motor on the shaft to improve the transient behaviour.
Powertrain architecture and electrification
The high performance engine is embedded in the drive system. It consists of:
- Internal combustion engine 600 kW
- Electric motor EM1 30 kW (peak 90 kW) in P1 hybrid architecture
- 7-speed double-clutch gearbox
- Electric motor EM2 55 kW (peak 160 kW) as electric drive unit (EDU)
- High voltage battery 120 kW and 4.0 kWh
The combustion engine and the electric motor EM1 power the rear axle. The electric motor EM2 is configured as an electric drive unit. For reasons of weight reduction, the high voltage lithium-ion battery is designed as a small unit with a capacity of 4.0 kWh. At the same time, it delivers an output of 120 kW at a high C-rate of 30. The torque characteristics of all three engines are shown in Figure 10.
In high-speed range, the combustion engine is the dominant drive source. It delivers more than 85 percent of the total system power of 710 kW. The maximum speed is reached in the sixth gear and is limited to 350 km/h. Acceleration from 0 to 100 km/h is achieved without gear change in less than three seconds and is traction limited by the high torque at the rear axle. The operating strategy of the hybrid powertrain is illustrated using the example of the Nuerburgring race track (Figure 12). During braking and before a curve, the energy is recuperated. The acceleration out of a curve is supported by boosting with the EDU (EM2) at the front axle. All engines drive the vehicle on straight sections at full power demand.
The cooling concept used here in the overall vehicle and the breakdown of the heat fluxes for a system power of 710 kW. The high temperature circuit (HT) of the engine cooling system needs to dissipate 232 kW. For this purpose, it uses two radiators integrated in the side pods. The transmission oil cooler transfers an additional 18 kW to the environment. The cooler for the low temperature circuit of the electric motor EM1 is located in the left rear wheel housing. The heat of the battery is transferred to a cooling circuit via an intermediate water circuit. The cooling circuit transfers the heat (6 kW) to the environment. A second condenser provides for the cooling need of the passenger cabin cooling. The heat of the cooling water of the air-water charge air cooler is transferred to the environment (in total 80 kW) through two low temperature coolers.
Emission control concept for Euro 7
The tightening of global emission legislations promotes the aim of low emissions operation under all driving conditions:
1 The restriction of the permissible particle number emission to 6 x 1011 PN/km x CF under RDE conditions which was introduced with Euro 6d-TEMP.
2 The auxiliary emission strategies which receive less and less acceptance, and the discussion about the introduction of conformity for the pollutant CO under RDE conditions.
3 The significant reduction of the emission limits for gaseous pollutant to ~ 50 percent of the currently applicable Euro 6d-TEMP limits with the simultaneous restriction of CF = 1 expected with Euro 7, and the stricter focus on shorter driving distances after a cold start (< 10 km).
Figure 14 shows the exhaust gas aftertreatment system. The illustrated system is designed for one bank, and is mirrored for the second bank. The exhaust gas aftertreatment is equipped with one adsorber catalyst with a volume of 1.5 L per bank. Its ceramic substrate has a high heat capacity and stores HC emissions after a cold start until the light-off of the main catalyst has been reached. For the main catalyst, a metallic support material with low heat capacity and high heat conductivity has been chosen to reduce the light-off time. The volume of the main catalyst is 3.5 L per bank without adsorber catalyst and without particulate filter. Two electrically heated discs per bank have been integrated into the main catalyst. A coated particulate filter (4WC) with a volume of 4.0 L follows downstream of the catalyst.