Super Clean Electrified Diesel: Towards Real NOx Emissions ... · In this context, in 2016...

27th Aachen Colloquium Automobile and Engine Technology 2018 857

Super Clean Electrified Diesel: Towards Real NOx Emissions below 35 mg/km Dr.-Ing. Giovanni Avolio, Dipl.-Ing. Rolf Brück, Dipl.-Ing. Jürgen Grimm, Dr.-Ing. Oliver Maiwald, Dr.-Ing Gerd Rösel, Dr.-Ing. Hong Zhang Continental Automotive GmbH, Regensburg and Lohmar, Germany

Summary

Air quality in the city centre is since decades one of the most important environmental topic for its big impact on the health consequences that it has for the citizens.

In this context, the vehicles equipped with combustion engine are under criticism as one of the contributors to the deterioration of the air quality. As a consequence, a lot of discussions are taking place by the different city’s administrations, worldwide, in order to implement legislative countermeasures to limit or completely ban the use of vehicle with combustion engines inside the city. In particular, the diesel engines as one of the strong contributors of NO2 emissions are challenged by potential bans.

Nevertheless the diesel keeps its benefit in terms of efficiency, representing a key technology in order to respect, in combination with diesel-like fuel (OME) produced by renewable sources or energy, the future target of CO2 emissions reduction (-15 % in 2025 and -30 % in 2030 with respect to 2020 target).

In this frame the Super Clean Electrified Diesel (SCED) program, launched by Continental in 2016 and arrived at the second step of development, is aimed to offer a technological platform that includes cost effective and integrated solutions to address the future post EU6d targets and the new stringent requirements proposed by NGOs (Non Gouvermental Organization) on the topic of real driving emissions. More in detail aim of the SCED demonstrator is to reach tail pipe NOX emissions level below the 35 mg/km in all real driving conditions.

To this scope, focus of the program was the improvement of the exhaust gas treatment system in terms of hardware optimization and development of new control functions offered by the combination of new sensing strategies.

This has been coupled with the further development of the 48 V system. In particular, it focusses the definition of control strategies aimed to properly manage the generator/motor phases of the electrical machine, keeping as targets the reduction of CO2, the temperature increase of the exhaust gas treatment (through an electrified catalyst) and the state of charge of the vehicle battery, as requested by the upcoming legislation.

This paper will describe the employment of technological bricks to upgrade a C-segment EU6b diesel vehicle in order to approach a potential "EU7" legislation scenario based on real NOX emissions below 35 mg/km.

858 27th Aachen Colloquium Automobile and Engine Technology 2018

1 Introduction

The introduction in September 2017 of the new EU6d-Temp legislation has clearly re-defined the way to develop passenger car application. The verification of the vehicle emissions in real driving conditions has obliged the automotive industry to offer and develop new component and system solutions capable to meet the current regulations in terms of NOX emissions.

In Figure 1 the results of study conducted by IIASA (International Institute for Applied System Analysis) for AECC (Association for Emission Control by Catalyst) are displayed.

Fig. 1: NOX emissions forecast by introduction of EU6d vehicle (sx) and NO2 emissions simulation by air quality monitoring stations in Europe in 2030 (dx) [1]

The diagram reported on the left of Figure 1 shows the evolutions of NOX emission of on-road vehicles up to 2040 considering the introduction of EU6d vehicle with an average NOX emissions level of 120 mg/km. The diagram on the right shows, for different European cities, the consequent NO2 emissions level forecasted in 2030.

The analysis of the above diagrams suggests as the introduction of the EU6d vehicle strongly contribute to the NOX emissions reduction. Despite this in many cities, due to the higher concentration of vehicle or to the geographical position (e.g. Italian Po valley), the level of NO2 will stay at critical level [1].

Furthermore as showed by D'Anna [2] the NOX emissions are responsible of the secondary organic aerosols and their formation can increase during particular weather conditions characterized by air stagnation [3].

For this reason the abatement of the NOX emissions in all driving conditions is crucial for the overall improvement of the air quality.


In this context, in 2016 Continental started a development program called "Super Clean Electrified Diesel vehicle" based on a C-segment vehicle, equipped with a serial 2.0 l diesel engine and upgraded, at this time, with new exhaust after-treatment system (DOC + SCRF). Target of the activity was to evaluate the capability of diesel technology to meet the current and future possible requirements settled by the upcoming legislations.

In Figure 2 the NOX emissions obtained in different driving conditions are reported:

Fig. 2: NOX emissions in different real driving conditions obtained with a C-segment vehicle equipped with a 2.0 l diesel engine and upgraded DOC/SCRF exhaust gas after-treatment system

Figure 2 shows as the highway emissions, as well as, the ones during short city driving, including the engine warm-up phase, are at the limit or above the current legislation. And even quite above if we consider a possible future scenario of the "EU7" - legislation around 35 mg/km.

How challenging the achievement of NOX emissions during short city driving is easily understandable looking at the following Figure 3. Here the engine out and tail pipe NOX emissions, obtained with the above mentioned vehicle during the execution of the Phase 1 of the WLTP cycle, are reported.


Fig. 3: Engine out and tail pipe NOX emissions during Phase 1 of the WLTP cycle obtained with C-segment vehicle equipped with 2.0 l diesel engine

In the first 180 s, before the activation of the exhaust gas after-treatment system, 90 % of the total emitted emissions during the Phase 1 of the WLTP cycle are generated.

After the 180 s the after-treatment system is working at 95 % of efficiency abating the emissions coming out from the engine.

For this reason, it is very important to reduce the activation time of the after-treatment system as much as possible and do it independently from the engine operating. In the same way the engine technology has to be updated in order to limit the emissions during the first seconds after start.

Regarding these boundaries Continental has developed its product portfolio in order to enable to meet the target of 35 mg/km in all driving conditions.

In the following paragraphs the authors will provide detailed descriptions on the employed technologies to meet the mentioned ambitious target and an overview of the obtained NOX emissions in some selected real driving conditions.


2 Engine and Vehicle Demonstrator Description

In Figure 4 the powertrain and the engine system layouts employed in the Super Clean Electrified Diesel vehicle are reported [4]:

Fig. 4: Powertrain and engine system layout of the Super Clean Electrified Diesel vehicle [4]

As mentioned in [4] a C-segment vehicle EU6b homologated and equipped with a 2.0 l diesel engine has been modified and upgraded by Continental's engineers in order to approach the target of 35 mg/km NOX emissions in all driving conditions.

Below in the Table 1 and Table 2 the main features of engine and vehicle are reported:

Tab. 1: Super Clean Electrified Diesel vehicle: engine features.

Tab. 2: Super Clean Electrified Diesel vehicle features.

To this aim the serial injection system and the ECU have been replaced with the Continental piezo common rail system (PCRs5). The complete engine control management strategies have been internally developed by Continental.

Engine Features Description Architecture 4 cyl line - 4 valves / cyl.

Max. Torque / Power 380 Nm / 135 kW Boosting System Single Stage VTG Turbocharger

EGR System Uncooled HP and cooled LP

Vehicle Features Description Class C-segment Type Sedan

Architecture FWD Transmission 6 Gears - DCT


The serial LNT and close coupled DPF have been removed and replaced with new exhaust after-treatment configuration based on 48 V electrically heated catalyst mounted (EHC - EMICAT®) on DOC; a close coupled metallic SCR and ceramic SCRF; an additional underfloor SCR coupled to an ammonia slip catalyst finalizes the engine exhaust after-treatment line.

Two AdBlue® dosing units (RDU) have been respectively installed before the close-coupled SCR-SCRF and upstream the underfloor SCR.

SCR dosing units and EMICAT® control are managed by dedicated units developed by Continental. The related control strategies, based on the feedbacks coming from 3 NOX sensors have been also developed by Continental.

Concerning with the powertrain configuration, a 48 V belt starter generator has been installed on the engine. The related additional 48 V components, DC/DC converter and battery are also installed in the vehicle.

48 V control strategies and functions are integrated in the ECU.

3 Test Description

In order to demonstrate the capability of Continental technology system to approach the target of 35 mg/km NOX emissions in all driving conditions, three different test cases will be analyzed:

1. Complete WLTP cycle as representative of all driving conditions; 2. 5 km real city driving in Regensburg including the cold start at the beginning; 3. A real driving including: city, rural road (uphill) and highway.

Each test has been conducted activating or not the EMICAT® to evaluate its benefits in terms of NOX emissions.

4 Results

4.1 Analysis of Experimental Results in WLTP Cycle

In Figure 5 the NOX emissions measured at engine out, downstream SCRF and at tail pipe, obtained during the execution of the WLTP cycle, are plotted. Two test cases have been reported: the WLTP cycle has been carried out activating and not activating the EMICAT®.


Fig. 5: Engine out, after SCRF and tail pipe NOX emissions, vehicle speed and driven distance by the SCED during execution of the WLTP cycle

As showed in the above picture, the application of double SCR system with twin AdBlue® dosing units allows to reduce the tail pipe NOX emissions up to 21 mg/km. Additional reduction of the nitrogen oxides is offered by the activation of the EMICAT®. In this case an out-standing value of 13 mg/km is achieved, facing an overall exhaust system NOX conversion efficiency above 95 %.

The benefit of EMICAT® becomes clear looking at the results showed in the following Figure 6. Here, for the Phase 1 of the WLTP cycle, the temperatures evolution of both close coupled SCRF and underfloor SCR catalysts are plotted. The AdBlue® mass flow injected by the first (indicated as RDU#1) dosing unit, placed upstream the close-coupled SCRF, the employed power at EMICAT® and its cumulated consumed energy, have been also reported.


Fig. 6: SCRF and SCR temperatures, injected AdBlue® mass flow upstream SCRF, employed power at EMICAT®, cumulated energy consumed by EMICAT®, vehicle speed and driven distance by the SCED during execution of Phase 1 of the WLTP cycle

The activation of the EMICAT® allows to achieve the target temperature for the AdBlue® injection 150 s earlier compared to a test without the electrically heated catalyst being turned on. The benefit of EMICAT® on the achievement of target temperature for SCRF is even bigger (~ 200 s). This has of course a big influence on the NOX emissions, as can be observed looking at the following Figure 7. Here the nitrogen oxides emissions, measured at engine out, downstream SCRF and at tail pipe, obtained during the execution of the Phase 1 of the WLTP cycle, are plotted.


Fig. 7: Engine out, after SCRF and tail pipe NOX emissions, vehicle speed and driven distance by the SCED during execution of the Phase 1 of the WLTP cycle

As it can be observed, the implementation of the electrically heated catalyst improves the NOX emissions by more than 40 %. This leads to a final tailpipe value of 41 mg/km after only 3.1 km.

At this stage of development, the activation of EMICAT® brings a slight benefit in terms of CO2 (< 1 % in the WLTP, < 2 % in Phase 1 of WLTP) due to the faster activation of fuel consumption oriented engine calibrations. The electrical energy consumption absorbed by the EMICAT® has been measured in the WLTP cycle. Its value is in the range between 60 and 70 Wh. As already presented in [4], the total electrical energy available in the 48 V batteries coming from recuperation phases is in the order of 210 Wh, well above the mentioned 70 Wh consumed by the EMICAT®.

Further activities will be performed to improve the NOX / CO2 trade-off by better interaction from the combustion engine, the exhaust after-treatment and the 48 V mild hybrid systems.

Coming back to Figure 5 and comparing the NOX emissions tail pipe and after the SCRF, it is clear as the current single SCR system layout is capable to meet the current EU6d legislation limits (80 mg/km). It is not enough if we consider future potential "EU7" - legislation scenarios with a further reduction of NOX emissions (Continental internal target 35 mg/km). In this case the employment of a second SCR system in underfloor position coupled with a second AdBlue® dosing system is required. This


furtherly allows to keep the engine out NOX emissions at a reasonable high level, avoiding any penalties in fuel consumption.

Besides, the application of double SCR system allows to distribute the AdBlue® between the two catalysts avoiding, in the case of single SCR system, problem of overdosing with negative consequence on the system durability (deposit formation).

The diagram reported in Figure 8 shows for the close coupled SCRF and for the underfloor SCR the catalyst temperatures and the injected AdBlue® mass flow of the related injectors.

Fig. 8: SCRF and SCR temperatures, injected AdBlue® mass flow upstream SCRF and SCR, vehicle speed and driven distance by the SCED during execution of the WLTP cycle

The above picture shows, as in the WLTP cycle, a key role is played by the close-coupled SCRF. 90 % of the total injected AdBlue® quantity is delivered upstream the SCRF. The remaining 10 % is injected upstream the underfloor SCR. This, as showed in Figure 5, contributes to furtherly abate the NOX emissions in the last phase of the WLTP cycle. In this situations the underfloor SCR works in perfect conditions in terms of temperature and space velocity contributing to the further reduction of the NOX.


4.2 Analysis of Experimental Results in Regensburg City Driving

A 5 km route in the city of Regensburg has been selected and used to show the potential of Continental technologies to approach the limit of 35 mg/km of tail pipe NOX emissions.

As mentioned in the introduction, the NOX emitted in the first seconds represent more than 90 % of the measured final emissions. In these conditions, the challenges are represented by a faster activation of the SCRF system and by the reduction of the engine out emissions when the after-treatment system is not properly working.

How the EMICAT® can help in this scenario can be observed looking at the Figure 9. Here the NOX emissions, measured at engine out, after SCRF and tail pipe positions, the instantaneous electrical power provided to the EMICAT®, its cumulated consumed electrical energy, the vehicle speed profile and its driven distance are reported.

Fig. 9: Engine out, after SCRF and tail pipe NOX emissions, employed power at EMICAT®, cumulated energy consumed by EMICAT®, vehicle speed and driven distance by the SCED during execution of the Regensburg city driving


The activation of the EMICAT® in the first 110 s allows to convert earlier the NOX emissions, resulting in a final tail pipe value of 48 mg/km. A saving in NOX emissions, in comparison with the test case without EMICAT®, of 35 % has been detected.

Additional contribution (22 %) to total reduction of the NOX emission is also given by the second SCR placed in the underfloor position, as can be observed from the following Figure 10. Here the SCRF and SCR catalyst temperatures, the related injected AdBlue® mass flows, the vehicle speed and its driven distance are reported.

Fig. 10: SCRF and SCR temperatures an injected AdBlue® mass flow upstream SCRF and SCR, vehicle speed and driven distance by the SCED during execution of the Regensburg city driving

As showed by the diagrams reporting the temperature evolutions of the SCRF and underfloor SCR, the activation of the EMICAT® accelerates the warm-up phase of the two catalysts. This results, as observed by the analysis of the diagrams reporting the AdBlue® injections respectively upstream SCRF (indicated as RDU #1) and upstream the underfloor SCR (indicated as RDU #2), in an earlier injections (~50 s upstream the SCRF and 130 s upstream the SCR) with respect the case without EMICAT®. This confirms the benefit in tail pipe NOX emissions reported in the Figure 9 and analyzed before.


4.3 Analysis of Experimental Results in Real Driving

Finally an on-road test has been selected to demonstrate the capability of Continental's technologies to abate the nitrogen oxides emissions of diesel vehicles. In the Figure 11 some information is given.

Fig. 11: Description of real driving emissions in Regensburg

In Figure 12 the NOX emissions along the exhaust gas system line, the vehicle speed and its driven distance in the on-road test are plotted.

Fig. 12: Engine out, after SCRF and tail pipe NOX emissions, vehicle speed and driven distance by the SCED during execution of the real driving test


The analysis of the above picture highlights the potential of the additional SCR underfloor system to reduce the tail pipe NOX emissions (22 mg/km).

The split of AdBlue® injection between the two RDU, as showed in Figure 13, allows to achieve an overall NOX conversion efficiency above 95 %, keeping the AdBlue® consumption at a reasonable level (1.1 l / 1000 km).

Fig. 13: SCRF and SCR temperatures, injected AdBlue® mass flow upstream SCRF and SCR, vehicle speed and driven distance by the SCED during the execution of the real driving test

5 Conclusions

In the previous paragraphs, the authors have described how the technology package, based on 48 V mild hybrid system and electrified exhaust gas after-treatment, represents a clear solution to approach future possible scenarios in terms of reduction of NOX emissions.

The main results are summarized in the below Figure 14. Here the NOX emissions for different driving conditions, obtained with the Super Clean Electrified Diesel vehicle, are reported.


Fig. 14: NOX emissions in different real driving conditions obtained with the Super Clean Electrified Diesel vehicle

Comparing the above Figure 14 with the Figure 2 reported at the beginning of this article, the employment of the electrically heated catalyst substantially improves the NOX emissions in the city driving phase. This allows to approach the Continental internal target of 35 mg/km. Further development in the engine calibration optimization and in the management of the EMICAT® have to be performed to reduce further the NOX emissions.

At the same time the employment of twin SCR system (close-coupled SRCF and underfloor SCR) in combination with double dosing and three NOX sensors based control strategy, improves the NOX emissions in the driving conditions characterized by higher engine loads (uphill and dynamic driving).

6 Acknowledgments

The authors thank BASF in the person of Dr. U. Zink for the support in the selection and procurement of the catalyst employed in the Super Clean Electrified Diesel vehicle demonstrator.


7 Nomenclature

AAEC - Association for Emission Control by Catalyst ASC - Ammonia Slip Catalyst BSG - Belt Starter Generator CO2 - Carbon Dioxide DC - Direct Current DOC - Diesel Oxy Catalyst DPF - Diesel Particulate Filter ECU - Engine Control Unit EHC - Electrically Heated Catalyst LNT - Lean NOX Trap IIASA - International Institute for Applied System Analysis NGO - Non Governative Organisation NOX - Nitrogen Oxides NO2 - Nitrogen Dioxide OME - Oxymethylether RDU - Reductand Dosing Unit SCR - Selective Catalytic Reduction SCRF - SCR on Filter SCED - Super Clean Electrified Diesel WLTP - Worldwide Harmonized Light duty Test Procedure

8 References

[1] Bosteels, D., 2018: Diesel engines on pathway to low impact on local air quality In: 4th International Conference Diesel Powertrain 3.0.

[2] D'Anna, B., Marchand, N., Sartelet, K., 2018: Emission impacts from car exhaust on air quality, in the SIA Powertrain Rouen 2018 - International Conference and Exibition.

[3] Sullivan, A. P., Hodas, N., Turpin, B. J., Skog, K., Keutsch, F. N., Gilardoni, S., Paglione, M., Rinaldi, M., Decesari, S., Facchini, M. C., Poulain, L., Herrmann, H., Wiedensohler, A., Nemitz, E., Twigg, M. M., and Collett Jr., J. L., 2016: Evidence for ambient dark aqueous SOA formation in the Po Valley, Italy, in Atmos. Chem. Phys., 16, 8095-8108.

[4] Rohrer, S., Swigon, T., Avolio, G., Atler, F., 2018: Mildhybrid diesel powertrain with integrated energy and exhaust management to comply with "post-EU6d" emission standards In: 9th Emission Control Conference 2018.

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