lowest real driving emissions-v9 final...for next generation of 48 volt electric drive systems up to...

28th Aachen Colloquium Automobile and Engine Technology 2019 1

Lowest Real Driving Emissions: Solutions for Electrified Gasoline Engines Dr. Gerd Rösel, Dr. Erwin Achleitner, Friedrich Graf, Paul Rodatz, Peter Senft CPT Group GmbH Regensburg, Germany

Rolf Brück, Holger Stock Continental Emitec GmbH Lohmar, Germany

Summary

The future worldwide reduction of gaseous emissions, particulates and fuel consump-tion requires the electrification of the powertrain, especially for Real Driving Emission (RDE) legislation. Optimum interaction of 48 V high-power hybrid with the combustion engine are the key technology to reduce CO2 emissions.

The main sources of pollutants are cold start and highly dynamic engine operation. Therefore, an optimized injection system for 350 bar with optional extension to 500 bar fuel pressure and 48 V electrification are the measures to reduce engine out emissions. This updated injection system will be the furthermore enabler to reduce CO2 by knock resistance at high compression ratio engines at low particulate number raw emission.

For a highly efficient pollutant reduction, the full operation of the aftertreatment system after start of internal combustion engine is required. The electrically heated catalyst with an active purge pump will bring aftertreatment to light off even if combustion en-gine will not be active. The combination of HC trap and electrically heated catalyst has been presented as an option to further reduce hydrocarbon emissions. The active purge pump enables additionally a stoichiometric or lean mixture in the catalytic con-verter at low temperatures when the combustion engine operated with rich air/fuel ratio or during desorption of the HC trap and during gasoline particle filter regeneration.

On the road to unlimited clean mobility the presented technologies applied in a system approach allow the full operation of aftertreatment system close to cold start to deal with RDE short travel distances.

2 28th Aachen Colloquium Automobile and Engine Technology 2019

1 System Configuration to Meet RDE Emission Targets

The challenges to fulfill future emission requirements are not only set by legislation requirements but more and more by the public perception. The fear of critical media reports or of bad ratings in environmental benchmarks are of at least equal importance in the definition of future exhaust system configurations.

A clean vehicle shall not only satisfy the emission standards in the respective emission cycle defined by the real driving emission legislation, but the emission standard must be kept in all driving condition (refer to figure 1). These conditions may vary from stop and go in a traffic jam to sharp accelerations after a short stop at a service station of the highway or a high-speed drive on the German Autobahn with high payload. Emis-sions shall be reduced to a zero-impact level on road and not just for the shake of type approval tests.

Fig. 1: Main challenges for EU7 and beyond

An introduction of a post EU6d legislation is expected in the timeframe of 2023 to 2025. The EU7 emission limits, are expected to drop to at least China 6b level. In case of NOx, even 30 mg/km is foreseen for an approval by the Green NCAP consortium to reach a five-star rating. The particle number limit shall include all particles down to 10 nm (compared to 23 nm with Euro6d). In addition, the unlimited pollutants like NH3 and N2O might be limited.

RDE conformity factors for NOx are anticipated to decrease to 1.0. However, this does not mean that the uncertainties of PEMS devices like zero-drift will completely disap-pear until the introduction of a post EU6 legislation. Nevertheless, a conformity factor other than 1.0 will be hard to justify, therefore, uncertainties in the emission measure-ment need to be considered when defining the engineering target.

More ambitious and tougher to comply with is the expected modification of the test procedure to reflect real driving conditions. Currently the cold start emissions need to


be considered in the first phase of the RDE test. The Cold phase has 16 km distance for RDE test which is expected to reduce to 8 km or even below. Therefore, emissions emitted prior to the readiness of the exhaust aftertreatment or particulates on a fresh regenerated GPF are averaged over a shorter distance which leads to higher total tailpipe emissions.

After introduction of post EU6d emission challenges, the CO2 fleet emissions must be decreased by 15% in 2025 and by 37.5% in 2030 compared to the WLTP fleet emission levels in 2021. These reduction targets are already defined and published in EU regu-lation 2019/631.

The optimization target for the overall system design must reduce the fuel consumption while meeting the legislation emissions limits. For the very challenging CO2 emission regulation a further fuel consumption reduction beyond P0 Battery Starter Generator (BSG) system will be required. Figure 2 shows a P2 hybrid system configuration that meets these requirements.

Fig. 2: Electrified Gasoline Powertrain System Configuration

In restricted city areas, the vehicle should be able to run only by electric drive, which is possible with a cost effective 48 V high power system. If the Internal Combustion Engine (ICE) must be started due to low battery State-Of-Charge (SOC), the start emissions of the ICE must be very small or with other words should not impact the environment. This could be achieved by preheating the exhaust system with an EMICAT® before ICE start, [1]. It is only possible to bring a part of the catalyst above light off temperature before ICE start in an acceptable time duration with a 48 V system. The Active Purge Pump (APP), see reference [2], was initially developed to fulfill the evaporative emissions legislation requirements. APP will be an enabler to increase the catalyst volume which is above light off before ICE start and reduce the emissions during warm up.


The hybridization variant shown in figure 2 has a high power 48 V P2. The advantage of this concept is, that the combustion engine can be decoupled from the powertrain and shut down when it is no longer needed for delivering driving torque. That eliminates drag losses in hybrid modes such as recuperation, coasting (open powertrain), sailing (driving electrically at constant speed) and electric driving. The clutch has been in-stalled before and after the electric motor which is typical for P2 hybrids. The "C0” friction clutch has been installed on the engine side which will help to start the ICE with the electric machine. Coupling with the transmission has been realized via the clutch “C1”.

Compared to other hybrid concepts such as P3 and P4 hybrids, the P2 hybrid concept has an advantage, that the speed of the electric motor can be kept in the optimum efficiency range of the electric motor by carrying along the gear ratios.

The connection between the 48 V and 12 V on-board power supply has been estab-lished via a bidirectional DC/DC converter with a nominal output of 3 kW with efficiency more than 96%. With the absence of 12 V generator, the primary task of DC/DC con-verter is to supply the 12 V continuously to consumers and charge the 12 V battery. Reciprocally, the DC/DC converter also ensures that the 48 V network has been pow-ered by the 12 V battery, although this is restricted to very short time intervals due to the limited discharge capacity of the lead acid battery.

The goal of above described system will be the robust achievement of future EU7 and China 7 emission legislation even in short real driving conditions in combination with CO2 reduction.

2 Electrified Gasoline Engine

2.1 Electric Power

The current generation of 48 Volt electric drive addressing mainly the P0 architecture has been designed to provide up to 60 Nm and 15 kW peak power. With such systems, the CO2 reduction on WLTC cycle has been limited to maximum 10 % depending of the component sizing, vehicle size, transmission type and others powertrain charac-teristics. The following hybrid functions are available: Start/Stop, recuperation, torque boost, coasting, electric creeping and reduce emission for Gasoline or Diesel based internal combustion engine. And thanks to the combination of 48 Volt system and the electrically heated catalyst – EMICAT® [3] [4].

To get a first impression, a chassis dyno test bench study with a P0-system has been completed to analyze the impact on emissions and energy needed.

Torque assistance by boosting with the electric motor shows in figure 3, during accel-eration of the vehicle reduces the thermal energy before the catalyst. This results in a slower temperature rise in the catalyst and a longer time period until the catalyst is fully above light off.


Fig. 3: Influence of torque assistance during the first seconds of the FTP test cycle

Fig. 4: Emission improvement and energy consumption with electric power

Figure 4 shows the emission improvement with torque assistance impact of an 11.7 kW P0 Battery Starter Generator (BSG) during vehicle take off in the first seconds of an FTP test cycle. Due to the 104 kJ torque boost of the electric motor in the first minute after engine start less mechanical work of the combustion engine is required with 32% reduced thermal energy for catalyst warm up. The 300 °C at the end of the catalyst are reached after 1 minute with the BSG. HC + NOx emissions are reduced by 6 %. CO2 reduction of 11 % is possible in the first minute, when the CO2 equivalent of the electric energy consumption of the BSG has been considered. The light-off over the complete length of the three way catalyst is 14 seconds delayed with the BSG. By that high space velocities due to fast accelerations with the ICE in the first minute would cause


an emission increase until the complete TWC is above light off temperature. The longer time duration until the complete catalyst is above light off and the overall lower tem-perature level inside the catalyst increases the N2O emission, which must be avoided for future emission regulations.

2.2 “48 Volt High Power” Electric Drive Component

For next generation of 48 Volt electric drive systems up to 20 % CO2 benefits are ex-pected and electric driving features on a C segment vehicle with P2 – P4 architecture are requested. Assuming an acceleration of 1.5 m/s2 up to 30 km/h in full electric mode and electric driving up to 55 km/h, power increase as well as an enhanced efficiency (>90 %) are key factors. Further, best in class engine restart in full electric mode re-quires 30 kW (mechanical) for 5 seconds, full electric city driving 20 kW for 20 seconds and 15 kW continuous electric power at 2200 rpm crankshaft speed. Diesel cold crank and eboost are estimated 70 Nm for 5 seconds. This boost results in additional crank-shaft torque of 180 - 210 Nm as a function of the speed ratio of el. machine coupling.

Taking the new requirements defined above as inputs, Continental has developed an electric motor so called ‘48 Volt High Power’ Electric Drive, see figure 5. This electric motor embodies a very high-power density and high efficiency.

Fig. 5: “48 V High Power” (30 kW) characteristics

Figure 6 shows, the electric driving mode simulation of the 48 Volt P2 mild hybrid ve-hicle on a WLTP cycle. The first start of combustion occurs only after 12.75 minutes. Indeed, a very significant section (94 % of time) of WLTC can be driven without the use of the combustion engine.

These results underline the usage of a ‘48 Volt High Power’ Electric Drive in a P2 con-figuration providing extended electric driving capabilities which are very relevant espe-cially in urban conditions.


In a next step, a P2 architecture could be enhanced as a Plug-In Hybrid Electric Vehicle (PHEV) by upscaling battery and adding an on-board charger. As an outlook, this con-figuration could have the potential as a Low-Emission vehicle with weighted CO2 emis-sions below 50 g/km (new EU Type approval 2017/1151, WLTP, does not require pure electric driving for PHEV).

Fig. 6: Extended electric driving capabilities with 48 V and 30 kW hybrid

For further emission reduction and fulfillment of the emission limits at shorter driving distances and with increased electric driving range of high power hybrids, additional measures for exhaust aftertreatment like active heating are required.

The engine and engine calibration were improved before getting into the aftertreatment system.

2.3 Injection System

The multiple injections and variable penetration for the various engine displacements will be necessary for EU7 applications. Hence, the existing XL5 injectors were evolved to a new XL5.1 which provides the high injection precision that will be needed for future challenges.

The newly developed seat with reduced sack volume has been promoting stepless designs for a wide range of engine applications with reduced penetration keeping low tip sooting. For very low static flows, the stephole design has been be applied to control the penetration.

For injections up to 8 pulses at a high accuracy of the injected fuel masses has been mandatory. This has been achieved by an optimized shot to shot performance of the


XL5.1 injectors with an enhanced low injection amount capability which comes in com-bination with the control COSI functions.

Based on this optimized 350 bar fuel injection system architecture the PN emissions are significantly reduced for RDE requirements like -7 °C cold start and full load. During cold engine operation the wall wetting will be the primary source of PN emissions and wall wetting leads to oil dilution.

The multiple injections with significant proportion of injected fuel during the compres-sion stroke are the key factors to reduce the wall wetting. Hence, the backpressure and higher temperature during the compression phase will be reducing the wall im-pingement of the fuel droplets. To provide sufficient fuel preparation due to reduced evaporation time, the injected fuel must be distributed by multiple injection strategies during the compression phase as shown in figure 7. The main objective of multiple injection strategy will be injecting higher fuel amount as late as possible into the com-pression stroke for better PN emissions. This supports low real driving emissions in case of poor fuel qualities or GPF during its low efficiency phases (fresh regenerated).

Fig. 7: Injection strategy of a 350 bar fuel system at cold engine operation and high load

The improvement of oil dilution has been mandatory for the future electrified power-trains with its significantly higher ratio of engine-off and therefore more cool and cold start events of the powertrain. The wall wetting during cold starts causes an increase of the fuel content in the engine oil. In a conventional engine the fuel will be evaporated during the engine warm operation hence fuel content in the oil is reduced. Due to the shorter combustion engine running times this effect has been reduced and in combi-nation the increased oil dilution will lead to critical conditions for the engine oil. As the


wall wetting will be the main source for PN emission and oil dilution, the above ex-plained strategies reduce the oil-dilution and the same has been observed in the CFD simulations.

As an additional option for further improvements the hydraulic system capabilities can be enlarged to fuel pressures up to 500 bar. This gives additional benefits for the en-gine operation without a negative impact on the injection performance regarding shot to shot and low injection amount capability.

One benefit of the higher fuel pressure will have a better fuel preparation which coun-ters the reduced time for evaporation of the compression injections. At -7 °C the cold tests with optimized multiple-injection strategies show a reduction of approximately 30 % of PN raw emissions (23 and 10 nm) for a pressure increase of 100 bar, which can be seen in figure 8.

Fig. 8: Particle number reduction by injection system

In warm full load operation, the combination of pure compression injections enables better robustness for low octane fuels as shown in figure 9. This can be used for im-proved knock resistance at a higher compression ratio and hence enable a further CO2 reduction of the engine.

Fuel injection after intake valve closing maximizes the internal cooling due to the fuel evaporation enthalpy. The higher PN emissions at 350 bar can be compensated by the fuel pressure increase. Due to the extreme strategy the 500 bar fuel pressure is man-datory to have reasonable fuel preparation in combination with acceptable PN emis-sions.


Fig. 9: Knock control by injection

2.4 Active Purge Pump

In [2] the Active Canister Purge System (APS) for electrified powertrains has been presented. This APS contains a cost effective and robust determination of HC concen-tration by means of pressure sensors before and after the purge pump. This system determines the HC concentration of the purge gases before introducing into the intake manifold. The use of the APS can guarantee that the required purge rate of the char-coal canister to assure compliance to evaporative emission regulatory requirements for PHEV's with increased engine off times due to electric driving.

The active purge pump allows not only the controlled purging of the canister, but also the possibility to add either air and thus oxygen or an air fuel mixture to the exhaust, which is of advantage for the Gasoline Particle Filter (GPF) regeneration. With the help of air injection catalyst pre-heating measures can be supported. The injection of air can create exothermal energy in the catalyst to heating-up the catalyst. The system configuration is shown in figure 2.

3 Exhaust Aftertreatment

To fulfill the above mentioned future requirements and to achieve robust emission re-duction even at short driving cycles, new challenges are given to the aftertreatment system. In addition, the electrification of the powertrain reduces the temperatures in the exhaust system (figure 4), which leads to the need of active heating measures.


Beside increasing the catalyst temperature, the goal of the active heating measures is the chance to have the freedom for a CO2 optimized calibration of the engine, for ex-ample by replacing the heating up strategy by the engine using the electrical heated catalyst.

The idea of electrical heating of catalytic converters in exhaust systems of combustion engines is quite old. In the 90s this solution could be realized in series production the first time see reference [7].

The electrically heated catalyst in figure 10 is a proven component and is also used today in production for 12 V diesel applications. It consists mainly of three parts: a mantle with an electrical heated disk and support catalyst which provides mechanical stability to the pan cake shaped heating disk. To contact the heating disk electrically variants with one or two electrical ducts are available. Usually one duct is used and the second pole is the mantle connected with the vehicle mass. In the past it was proven that the EHC with a 12 V power consumption already improved the emissions signifi-cantly and also could have a positive impact of the CO2 emissions [8].

Fig. 10: Design of the electrically heated catalyst EMICAT®

The 48V battery system of the electrified gasoline powertrain gives the possibility for increased heating power. By that the power can be increased from 1-2 kW to 3-4 kW. As a result, an even faster heating-up of the catalyst can be achieved and the heating-up time gets shorter and thus the total energy consumption is even lower. In this paper the version with an electrical power of 4 kW and a voltage of 48 V was used.

3.1 Combination of Active Purge Pump and EMICAT®

The electrically heated part of the EHC is defined to be small to react very quick in case the catalyst temperature has to be increased during cold start and/or low load operation. Because the heated disk is also coated it reacts like the head of a match,


starting to convert the emissions and by that creating exothermal energy which sup-ports the heating of the complete exhaust system.

If preheating before engine cranking is an option, it would make sense to heat up a larger volume. These two requirements will lead to 2 different designs of which both might be needed for real driving with an electrified vehicle.

Looking at the EMICAT®, the heated slice is mounted in direct neighborhood to the supporting catalyst, by that the heat transfer happens mainly via radiation and thus is limited. Increasing the heat transfer by convection would help to heat-up a larger cat-alyst volume during preheating.

Using the already described P0 application in figure 4 the combination of EHC with the Active Purge Pump has been used to evaluate the impact of active catalyst heating incl. pre heating on emissions and energy consumption. The active purge pump has been used to introduce air in front of the EMICAT®. The air flow enables temperature control of the EMICAT® and increases the heated volume of the TWC which is above light-off before engine cranking. Pre-heating of the EMICAT® can start with opening the door by remote control or the door handle. Measurements of the time between opening of the door and start of the ICE have shown below in figure 11, that most of the people need 15 seconds or longer until start of the ICE. Therefore 15 seconds was chosen as a standard preheating time. Longer preheating time would give even better emission results. 15 seconds preheating needs 60 kJ or 17 Wh battery capacity. That means preheating could be repeated without a high influence on the SOC of the bat-tery, even if the driver is opening the door several times without starting the engine.

Fig. 11: Influence of EMICAT® during the first seconds of an FTP test cycle

Figure 11 shows the comparison of a hybrid vehicle in the first seconds of an FTP test cycle with different EMICAT® heating strategies with the same thermal energy in front of the catalyst system. Without heating the EMICAT® the light off temperature inside the TWC is reached only after vehicle take off. With 4 kW heating of the EMICAT® the


light off temperature is reached already before vehicle take off. 15 seconds preheating the EMICAT® enables light off few seconds after ICE start and a very fast temperature increase.

The emission improvement and the energetic energy balance until catalyst light off are shown in figure 12.

Fig. 12: HC+NOx-Emissions and electrical heated energetic balance until catalyst light off

If the EHC has been heated with engine cranking, the light off temperature at the outlet of the TWC is 22.5 seconds earlier with 53 % of HC+NOx reduction. 15 seconds pre-heating of the EMICAT® enables 17 % of HC+NOx emissions and the light off at the gas-outlet of the TWC is reached 29 seconds after ICE start. 95 % of HC+NOx conver-sion has been achieved with preheating in the idle phase of 14.5 seconds after ICE start. Without preheating, the 95 % of HC+NOx conversion points have been reached seconds later when compared to with preheating. Also the particulate emissions are considerably (80 %) lower by using the electrically catalyst heating instead of engine based catalyst heating.

Half of the thermal exhaust energy and CO2 emissions are required to bring the catalyst above light off with preheating of the EMICAT®. The electric energy consumption of the EHC and the BSG until catalyst light off was calculated as CO2 equivalent with an efficiency of 70 % and was added in figure 12 to the CO2 from the ICE.

The faster the catalytic converter is heated above light off, the less thermal and elec-trical energy is required!

With this concept in place the catalyst heating by the engine becomes less important and providing additional torque by the electric motor can be used for the first acceler-ation. This will lead to reduced CO2 emissions. Without preheating this strategy will lead to slower temperature increase of the exhaust system as shown in figure 3.


3.2 Particle Filter Regeneration

For P2 hybrids and plug-in hybrids, the oxygen content in the exhaust system has been reduced for GPF regeneration. This leads to an increase of the GPF soot load during driving, refer reference [5]. Active particle filter regeneration with additional introduction of oxygen at high temperature upstream of the GPF will be necessary to reduce the back pressure in the exhaust system.

Fig. 13: GPF regeneration strategies

In reference [2], a system for the regeneration of a coated underbody particulate filter has been presented to ensure regeneration in all driving conditions. With this system there has been a possibility to regenerate the particulate filter even at very low driving speeds without sacrificing the drivability. The ICE has been operated with rich air/fuel ratio and secondary air has been introduced at upstream of the coated particulate filter to heat up the particulate filter by oxidation of the unburned CO and HC and excess oxygen required for soot oxidation also provided. During the rich operation of the en-gine NH3 has been formed in the Pt/Rh closed coupled catalyst which has been oxi-dized to NOx with an excess oxygen in the Pt/Rh coated GPF. During the regeneration of the particulate filter, slightly increased NOx emissions above NOx emission limits are emitted, which must be averaged over the distance travelled to remain below the NOx limit value, refer figure 13. In order to keep the NOx emissions below the emission limits during the GPF regeneration, the system was extended with a 3rd lambda sensor downstream of the particulate filter. The Oxygen Storage Capacity (OSC) of the coated particulate filter has been used for control. To assure oxidation of the soot in the GPF, the secondary air flow has been adjusted so that the excess air ratio after the GPF at stoichiometric. The secondary air flow rate of the APP has been changed in such a way that the secondary air flow rate has been slightly increased in the case of a rich breakthrough and slightly reduced in the case of a lean breakthrough after the coated GPF (cGPF). With the OSC regeneration strategy of the cGPF the NOx emissions are


below the NOx limits even during the regeneration cycle and slightly increased to nor-mal control lambda equal to one. Figure 14 shows that the cGPF heating and regen-eration phases during the first 800 seconds with different regeneration strategies of a pre-conditioned WLTP.

Fig. 14: Benefit of active GPF regeneration system

Because of the energy required to heat the GPF through rich operation, there has been a 10 % fuel consumption penalty during regeneration with secondary air. The fuel con-sumption increase during the GPF regeneration has been at 3.4 % relative to the entire WLTP cycle. As only the GPF is heated using APS regeneration and the thermal mass of turbocharger and the close coupled catalyst don’t play a major role, the fuel con-sumption increase has been minimized. With secondary air injection by the APS, the GPF regeneration is possible throughout the full range of vehicle speeds. If the injec-tion of secondary air by the APS takes place when catalyst temperatures are high and space velocities in the exhaust system are low there is no fuel consumption penalty and the NOx increase due to inactive NOx catalyst volume in the cGPF is moderate.

3.3 Combination of HC-Trap and Electrically Heated Catalyst

The use of an electrically heated catalyst in combination with preheating improves the tailpipe emission has been demonstrated. But, depending on the real driving conditions after cold start further improvements to achieve a zero impact vehicle will be needed.

In the past HC adsorbers were discussed and already in production as a solution for this problem. Not finally solved was the gap between desorption temperature of the adsorber and the light off temperature of the catalytic converter. By that with aged catalyst systems a reasonable portion of the previously stored HCs slipped through the 3-Way catalyst. Also coated HC-traps could not solve this problem in a sufficient way.


Even influencing the gas inlet temperature by engine measures, for example by rapid heating could not eliminated the problem, because always the adsorber has been heated up above desorption temperature before the catalytic converter got active.

One solution, already used in the past, see reference [6], could be the EMICAT® posi-tioned behind the adsorber. The supporting catalyst has been working as TWC located downstream of the adsorber and the heating slice. After engine cranking the adsorber stores the HC emissions during the first seconds while the EHC heats up the catalyst behind. Of course, that also works in combination with preheating which would lead in theory to an extremely low cold start emissions. Taking the experience from the past a test program was started.

Fig. 15: Gas inlet temperature at 2 different trap positions and amount of HC trapped

In the first step, different gas inlet temperature conditions have been studied and demonstrated with close-coupled catalyst and underfloor positions. Figure 15 shows the two different gas inlet temperatures and the HC amount trapped. It has been ob-served, that the total HC trapping capacity has been lower at the close coupled catalyst position and HC trapping efficiency has been already reduced after 7 seconds. Since the temperature of close-coupled position catalyst at the gas-inlet side of the trap al-ready starts desorbing while the gas-outlet side temperature has been lower enough for adsorbing. Due to that the trap volume has been indirectly reduced.

An underfloor position seems to be of an advantage. In the second step the combina-tion of a close coupled TWC with an underfloor adsorber was analysed in comparison

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to an underfloor only system. Pretests already showed that the close-coupled TWC volume should be relatively small in order of not delaying the desorption, which would lead to long active heating times. By that a catalyst volume of 0.49l was chosen. Figure 16 shows the 2 system configurations.

Fig. 16: Possible System Configuration of HC-Trap Exhaust Aftertreatment Systems

Fig. 17: Exhaust gas temperature, tailpipe HC and heating time of 2 different HC-trap exhaust systems

In case of the close-coupled TWC combined with the underfloor HC-Trap system it can be seen in figure 17, that the TWC starts working after 8 seconds. During that time the

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underfloor HC trap has been working and storing the HC. In addition, the heated cata-lyst as TWC starts working after 10 seconds reducing the emissions at the downstream of the adsorber which gives the lowest tailpipe emissions. After 60 seconds the HC trap will be almost empty and would be prepared for the next cold start. In case of the underfloor system only, the HC trap has been able to trap the HC during the first 60 seconds. The electrically heated TWC has been active and starts converting the de-sorbed HC and the engine out emissions after 10 seconds.

In the next step the HC-trap volume has been optimized and also the possibility of using 2 EHCs in order to heat up more catalyst volume behind the adsorber. Figure 18 shows that the impact of trap volume with heated TWC behind. As can be seen the best result could be achieved by having 1,25 to 1,67 l of trap volume. Analysing the temperatures the 1,25 l gives an advantage in desorption time, and also in sense of total system cost.

Fig. 18: Tailpipe emissions as a function of HC-Trap volume.

In the next step 2 EHCs were installed to increase the heated catalyst volume behind the HC-trap. Figure 19 shows the impact of using 2 EHC (which would be similar to a preheating function). The result shows a reduction of the tailpipe emissions by 45%. As already mentioned that is due to a higher TWC volume being on catalytic active temperature by using 2 heating zones.

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Fig. 19: Influence of using 2 EHC behind the HC-trap

Fig. 20: Active desorption of the HC-trap by using a heated part before the adsorber

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Also the use of heated part in front of the HC trap will make sense only if trap desorption has been actively controlled. Figure 20 shows that the desorption with active heating has been realized in very short period of time. The adsorber has been almost empty after 25 seconds in case of the active desorption instead of 60 seconds (passive). By that the need for active heating has been reduced and also the lambda control which has to be done behind the HC-trap during desorption can be optimized.

Based on the results shown above it also can be stated that depending on the appli-cation it`s exhaust gas mass flow and temperature of the HC-trap volume has to be adapted to the application. Also a “right sized” close coupled TWC supports very low emission levels.

Tests on a dynamic engine test bench will be carried out with different load cycles and lambda controls to get the optimum system for zero impact emissions levels.

4 Conclusion

The combination of the high power 48 V electric machine, optimized injection system, active purge pump and the exhaust aftertreatment with the EMICAT® gives the possi-bility to cover future legislations and public acceptance. The mild hybrid P2 concept will be needed to support the conventional combustion engine and improve the total performance with regards to fuel consumption. This generates additional requirements due to electric driving capability with delayed activation of the combustion engine at high torque demand.

The standard system for compliance with emission limits at reduced driving distances consists of a further improved 350 bar injection system with multiple injection capabil-ities during compression stroke, coated gasoline particle filter, electrically heated cat-alytic converter and active purge pump to cope with evaporative emission legislation.

Figure 21 shows that the emission reduction of the gaseous exhaust components by electric catalyst heating will be much higher than a load point shift by the electric motor. The active purge pump brings part of the catalyst volume above the light off tempera-ture during the electric drive and cold start. This reduces HC + NOx emissions by more than 80 % which is mandatory for future RDE compliance with short distance driving. The additional application of the active purge pump allows the GPF regeneration with low emission increase.

The option to increase the fuel pressure to 500 bar reduces the particle raw emissions by 30 % in cold conditions and enables knock robustness at high compression ratio with therefore improved CO2 emissions.

The option to apply HC trapping technologies reduces HC during start of the internal combustion engine. The downstream electrically heated catalyst has been mandatory to heat up the 3-way catalyst volume after the HC trap to overall reduce HC emissions under RDE conditions.


Fig. 21: Influence of torque assist and electric catalyst heating on gaseous emis-sions

On the road to unlimited clean mobility the presented technologies of raw emission reduction and fast light-off aftertreatment allow the low emission application of high power 48 V electrified vehicles under RDE driving conditions.

5 References

[1] BRÜCK, Rolf, HIRTH, Peter, KONIECZNY, Roman, MAUS, Wolfgang: The Fu-ture of Exhaust Aftertreatment Design for Electrified Drive Trains. 30th Vienna Motor Symposium 2011.

[2] ACHLEITNER, Erwin, FRENZEL, Holger, GRIMM, Jürgen, MAIWALD, Oliver, RÖSEL, Gerd, SENFT, Peter, ZHANG, Hong: System Approach for a Vehicle with Gasoline Direct Injection and Particulate Filter for RDE. Vienna Motor Sym-posium 2018.

[3] KNORR, Thomas, ELLMER Dietmar, BAENSCH Simon, SCHATZ Axel: “Opti-mization of the 48 V Hybrid Technology to Minimize Local Emissions in the RDE” in 27th Aachen Colloquium Automobile and Engine Technology, Aachen 2018.

[4] AVOLIO, Giovanni, BRÜCK, Rolf, GRIMM, Jürgen, MAIWALD, Oliver, RÖESEL, Gerd, ZHANG, Hong: “Super Clean Electrified Diesel: Towards Real NOx Emissions below 35 mg/km” in 27th Aachen Colloquium Automobile and Engine Technology, Aachen 2018.


[5] ROSE, Dominik, NICOLIN, Per, COULET, Bertrand, CHIIJIIWA, Ryoko and BOGER, Thorsten: Germany Gasoline Particulate Filter Application and Soot Management – Concepts for Conventional and Hybrid Powertrains. The 18th Hyundai-Kia International Powertrain Conference 2018.

[6] KIM, Chang Hwan, LEE, Hyokyoung, KANG, Chun Yong, CHUNG, Jin Woo: For a New Paradigm in Aftertreatment: The Almost Zero Concept for Gasoline NOx and Hydrocarbon Emissions. 27th Aachen Colloquium Automobile and En-gine Technology, Aachen 2018.

[7] KIEFER, Wolf, PLODEK, Bernd, EHMANN, Peter, FELDWISCH-DRENTRUP, Rupert, DIRINGER, Joachim: BMW 750i mit elektrisch beheiztem Katalysator. MTZ Motorentechnische Zeitschrift, Bd. 11, Nr. 59, pp. 3-11, 1998.

[8] BRÜCK, Rolf and KONIECZNY, Roman, Thermal Management for Low Emis-sion Concepts of modern Engines; The electrically heated Catalyst, in 19th Aa-chen Colloquium Aachen 2010

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