Measures such as catalytic heating and 48-V hybridisation support diesel engines in order to reach the future RDE target values, even under unfavourable conditions, as an analysis of AVL shows.
PILLAR OF CO2 REDUCTION
In addition to conventional emissions legislation, many countries have defined fleet CO2 targets in an effort to meet the global climate protection objectives. For CO2, the fleet target to be reached in Europe by 2020/2021 is defined in the NEDC as 95 g CO2/km. (1) shows the NEDC-certified CO2 values plotted versus vehicle weight for different technologies. It is noticeable that selected, currently available diesel models already meet the 2021 targets today.
Also, an analysis of customer-relevant real-world consumption shows that a shift from diesel toward gasoline-powered engines increases volumetric fuel consumption in l/100 km by as much as 30 to 40 % . Starting with the status quo, this paper uses RDE studies to examine the subject of diesel engine emission compliance, which has recently fallen under heavy criticism. It also explores the diesel engine’s future potential with a particular focus on urban operations. Taking into account load spectrum, driver influence and ambient conditions, the future diesel engine is to be defined by its technical features and further potential. The paper concentrates on the challenging combination of aggressive driver and low ambient temperatures in urban as well as in extra-urban operation. Proof that the diesel can be clean and efficient across the entire use area will thus help to keep the diesel on the market to support CO2 reduction.
RDE BASELINE MEASUREMENTS
To provide a basis for further considerations, the measurements in the context of the current definition were recorded for moderate and aggressive driving styles by calculating the product of positive acceleration and vehicle velocity, (2). The defined maximum limit was exploited to the fullest. The test vehicle was a mid-sized passenger car with a curb weight of 1700 kg. The exhaust line consisted of a 2.0-l LNT and a SCR-coated 2.7-l DPF positioned close to the engine with a downstream SCR brick in the same canning. There is also an ammonia slip catalyst in under-floor position. The tests were conducted on a system aged to full useful life. The 2.0-l diesel engine was equipped with a temperature-controlled high-pressure EGR system (HP EGR).
The test drives were conducted with a special focus on urban areas at an ambient temperature of around 0 °C. A moderate driving style delivered a conformity factor (CF) of 0.84 for the entire distance, while the value for city areas reached 1.30. Considering the values achieved by a moderate driving style, the results are undoubtedly attractive and clearly within the currently discussed limits. Yielding a value of 1.39, the aggressive/dynamic test drive produced an acceptable value for the entire cycle, while the value for the city part was significantly increased. Although remaining just within the limit that takes effect in autumn 2017, this value does show that there is still need for further improvement in order to take the diesel engine out of the firing line.
As shown in the figure, inner-city operations deliver a temperature profile that remains below the critical threshold for exhaust aftertreatment for a very long time. Also during extra-urban operation, the temperature drops significantly after the motoring phase, which, in the case of aggressive driving, is even followed by a standstill. This can be seen rather clearly at around 5200 s. The acceleration that follows the motoring/standstill phase produces a peak in the raw NOx emissions, which is also clearly reflected in the tailpipe emissions.
With respect to engine starting conditions, the test drives represented worst-case scenarios for exhaust aftertreatment. The LNT was fully loaded while the SDPF and the SCR were without NH3 load. Although this scenario may seem rather unusual, certain specific driving manoeuvres do occur that can lead to such an unfavourable loading state in the aftertreatment components. In summary, the assessment of the current status reveals that the analysed concept basically delivers very good emission values; in view of the more dynamic city operations and future requirements, however, there is still room for improvement.
To conduct a comparative assessment of various different technology elements, the paper turns its attention to the virtual environment. (3) shows a comparison between a measurement and a simulation based on a dynamic-driving RDE cycle at an ambient temperature of 0 °C. To include a proper representation of the system interactions, the entire system (consisting of drivetrain, exhaust aftertreatment and vehicle) was transferred to the model-based simulation environment . A closer look at the embedded table reveals a maximum deviation of 4 % for the cumulated values, indicating a sufficiently high simulation quality.
MINIMUM EMISSIONS UNDER RDE CONDITIONS
To represent low raw emissions while maintaining attractive fuel consumption levels in the high-load range, i.e. in high dynamic operation, the base engine was complemented by a low-pressure EGR system and a water-cooled charge air cooler positioned close to the intake manifold. In combination with a temperature-controlled high-pressure EGR, this allowed EGR application over the entire engine map. The boundary condition for the reduction of NOx raw emissions was to maintain the same DPF regeneration interval as was done with the base system. (4) shows the achieved emission results compared with the baseline (HP EGR), for which the simulated values served as basis. The figure shows that even with an aggressive/dynamic driving style, emissions can be achieved that have a CF below 1.0. This means that even with this rather conventional approach, it is still possible to meet the target values set for 2020 and beyond .
Further optimisation potential can be identified regarding pollutant conversion in low-load operation and after a cold start . Especially in the first 500 s after engine start, the exhaust temperature is still too low for adequate NOx conversion. As a result, aggressive catalyst heat-up measures are fundamental to ensuring minimum emissions, even with different driving profiles and ambient conditions. (5) compares the exhaust temperature increase obtained with early exhaust valve opening (EEVO) versus that obtained with the use of an electrically heated catalyst (e-cat).
The latter has the inbuilt advantage that, regardless of the engine operating point, the energy goes straight to the exhaust aftertreatment system with minimal losses. Apart from that, the higher load of the alternator leads to a shift in the engine operating point, which, in turn, has a favourable impact on the combustion temperature . Moreover, in order to further optimise the trade-off between emissions and consumption, the battery can be used to perform a balancing between the engine load and the corresponding raw emissions, the condition of the aftertreatment components and the battery’s state of charge. Certainly, the applied EEVO strategy is likewise shown to cause a significant temperature increase. The noticeable reduction in HC emissions resulting from the impact on the combustion process additionally improves the performance of the catalyst.
This delivers a temperature increase comparable to that of a 12-V e-cat with an output of 1.5 kW. By comparison, a 48-V e-cat with a power of 4 kW exhibits a much better performance, which allows the catalyst light-off to occur considerably faster. Accordingly, the following investigations were performed with the 4-kW e-cat, since, in addition to the clearly better performance, it makes it easier to represent the modularity for a global product strategy.
EMISSION AND CONSUMPTION REDUCTION BY MILD HYBRIDISATION
The topic of mild hybridisation with 48 V is expected to gain further importance in the near future. Not only will it enhance the efficiency of the entire drivetrain but it will also address customer-relevant requirements such as driving pleasure and, depending on the architecture, also partially electric driving . The analyses below are based on an existing 48-V architecture in P0 arrangement (belt integrated 10-kW e-motor), and focus on emission-related topics. This solution can be implemented without having to make major changes to the base engine.
During dynamic driving, LNT regenerations today are typically interrupted by requested torque changes. The e-machine is able to absorb the dynamics in a certain range, thereby reducing the emission peaks as indicated in (6). A uniform LNT loading curve is reached along with fully completed regenerations, resulting in reduced consumption. In addition, motoring the combustion engine by e-motor use at the beginning of the cycle with simultaneous activation of the e-cat supports aftertreatment system heat-up. This allows the exhaust system to warm up even before the engine starts producing emissions.
Advanced hybrid architectures, such as P2 (e-motor between combustion engine and transmission) or P4 (electric axle) configurations, enable further extended electric driving options in which, with a full battery, the temperature-critical light-load range is almost entirely avoided. When the battery is low, the load for the internal combustion engine can be correspondingly increased in order to charge the battery in parallel . This raises the complexity of the operating strategy by the requirement of a correspondingly adapted energy management system.
SUMMARY AND OUTLOOK
Already in the first step, an EGR system adjustment did set the CF to the range of 1. Especially due to the implementation of a low-pressure EGR system, this led to a reduction in fuel consumption by 2.4 %. Aggressive catalyst heating by means of an electrically heated catalyst lowered the CF by another 30 %. This leads to an improvement in aftertreatment efficiency already shortly after engine start. While this heating measure does entail a rise in fuel consumption, the values remain below the initial value. The use of a 48-V system delivers an excellent CF in the range of 0.5 with significantly reduced consumption, both for an aggressive/dynamic driving style across the entire cycle as well as for city operation. All of this is achieved at an ambient temperature of 0 °C with worst-case conditioning. With a moderate driving style, even the baseline is within the range of CF 1.
For ensuring continued high-speed driving in specific vehicle segments, the addressed configuration also opens up the option of moving the second SCR brick to an under-floor position and adding a second Adblue dosing unit. The added cost is very moderate, since the required urea infrastructure already exists. Due to the achieved reduction of exhaust temperatures, this specific under-floor configuration ensures the safe handling of long-distance, high-load driving.
These investigations show that a modern diesel engine will remain capable of combining efficiency with environmental compatibility in a cost-effective manner, making it an attractive option for customers. Added electrification will not only improve customer-relevant attributes, such as driving pleasure, additional features and consumption reduction, it will also optimise emission stability even further. Against the background of the booming SUV segment, the diesel engine will continue to play a major role in the reduction of the CO2 fleet emissions.
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DR. KLAUS HADL is Development Engineer Exhaust Aftertreatment at AVL List GmbH in Graz (Austria).
DIPL.-ING. (FH) BERNHARD ENZI is Product Manager Powertrain Systems Passenger Cars at AVL List GmbH in Graz (Austria).
DIPL.-ING. STEFAN KRAPF is Manager Passenger Car Diesel Development at AVL List GmbH in Graz (Austria).
DIPL.-ING. MICHAEL WEISSBÄCK is Deputy Vice President Powertrain Systems Passenger Cars at AVL List GmbH in Graz (Austria).