C O V E R S T O R Y FUTURE MOBILIT Y
REAL DRIVING EMISSIONS — A SHIFT IN VEHICLE APPLICATION The measurement of emissions in real-world driving situations will play an increasingly important role in vehicle and engine development in the future. AVL has developed a method that optimises the interaction between emission prevention inside the engine and emission reduction using exhaust gas aftertreatment systems. At its core is a new, mobile system for measuring emissions in transient and dynamic driving operation.
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AUTHORS
DIPL.-ING. (FH) STEFAN STRIOK is Lead Engineer Generation 1 Development Passenger Cars, Engineering and Technology Powertrain Systems, at the AVL List GmbH in Graz (Austria).
DIPL.-ING. HANNES WANCURA is Development Engineer Aftertreatment Systems, Engineering and Technology Powertrain Systems, at the AVL List GmbH in Graz (Austria).
DIPL.-ING. (FH) STEFAN PLATNER is Lead Engineer Performance and Emission Calibration Passenger Cars, Engineering and Technology Powertrain Systems, at the AVL List GmbH in Graz (Austria).
CHALLENGE EMISSIONS LEGISLATION
The current discussion about future emissions limits for passenger cars creates several scenarios for the years after 2017. Most probably, from today’s perspective, the proof of emissions limit compliance in vehicles will be performed under real driving conditions on the road. The consequence of the introduction of these socalled Real Driving emission (RDE) for vehicle development in Europe is that emissions development and verification will no longer be done exclusively under standard conditions on a chassis dynamometer. To take this scenario into account, AVL has launched an extensive program to develop strategies to meet the new challenges. This article describes the topics of combustion development and exhaust aftertreatment of a diesel engine with 1.6-l displacement for a passenger car with a vehicle mass of 1500 kg. The currently valid emission limits for passenger cars are measured in the NEDC. This cycle does not cover the entire speed and load range of the engine, ➊. In order to meet future RDE, the hardware and the calibration of the engine emissions must be adjusted so that the exhaust emission limits are met in a significantly expanded operating range. These limits have not yet been fixed by law. The goal is to be prepared for such future emission legislation by a range of engine development meas-
ures and by choosing a suitable exhaust after treatment system.
PROCEDURE
For the investigations, a model-based development methodology was used in addition to the engine development work [1]. This included a semi-physical model of the engine and the exhaust after treatment system, a software ECU with sensors and actuators. The individual systems employed are shown in ①. The simulation results provided the input parameters for the emission development on the steady state engine test bed and the chassis dynamometer. In addition to the legislative NEDC, the higher load cycles WLTC (Worldwide Harmonized Light Duty Driving Test Cycle) and Artemis were investigated. The verification of the exhaust emission was carried out on public roads using a mobile measuring system called AVL M.O.V.E. The selection of the appropriate exhaust after treatment concept was done by means of simulation-based development methodology.
ENGINE-OUT EMISSIONS
The target on the steady state engine test bed was the reduction of nitrogen oxide (NOx) emissions in the higher load operating range of the engine without detrimen-
ING. MARCO SCHÖGGL is Business Development Manager In-Vehicle Measurement Systems, Instrumentation and Test Systems, at the AVL List GmbH in Graz (Austria).
❶ Development tasks, test cycles autotechreview
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C O V E R S T O R Y FUTURE MOBILIT Y
❷ Exhaust emissions on the chassis dynamometer (engine-out)
tal effects to other legally limited emission components (HC, CO, PM) and fuel consumption. The basis of the development was a Euro 5 series calibration. A previously performed simulation of the drivetrain for the selected vehicle/engine combination provided the necessary input data of to the WLTC and the Artemis cycle for test bed development. The main focus of the steady state test bed work lay in the extension of the EGR range to higher loads of the engine. At the same time the application of further relevant parameters for the combustion in the engine control unit was done. In following the emissions were measured on the chassis dynamometer in the different driving cycles. The exhaust after treatment system of the vehicle for these tests consisted of a diesel oxidation catalyst (DOC) and a particulate filter (DPF). ➋ shows the differences in the engine-out emissions between Euro 5 series calibration and RDE calibration status. A significant reduction in engine-out NOx emissions can be achieved without any deterioration in fuel consumption. In the WLTC the NOx emissions were reduced by 58 % compared to the baseline. In the Artemis cycle, a reduction of NOx by 78 % was achieved. The particulate emissions upstream of the DPF were in a low range; so long DPF regeneration intervals are guaranteed. The comparison between the real measured values and the results applied from the drive train simulation in ② indicates a good correlation.
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To verify the results of the chassis dynamometer the vehicle emissions were measured on a public road with the portable measurement system M.O.V.E. A route with a one-third each time-based percentage of urban, suburban and highway driving, with a total duration of approximately 1 h driving time was selected as the reference cycle. This cycle was run on the chassis dynamometer to
obtain consistent identical environmental conditions during the development activities. In parallel, the measured data from the reference cycle (time-resolved emissions and values from the engine control unit) were used as input data for a semi physical engine model. Based on this data, the existing engine model could be refined for transient applications. Thus for example, the influence of different
❸ Exhaust emissions in the reference cycle (engine-out) www.autotechreview.com
environmental conditions on the exhaust emissions could be determined by simulation. In ❸, the emission results in realworld driving on public roads were compared with those driven on the chassis dynamometer. The comparison to the WLTC and Artemis emission results in ② show that these two cycles reflect a good correlation with “real driving”.
EXHAUST AFTERTREATMENT
❹ Exhaust aftertreatment concepts, tail pipe emissions
The selection of an appropriate exhaust after treatment concept was carried out by means of simulation based development methodology [2]. As already mentioned, the emission limit values for RDE have not yet been fixed by legislation. Therefore, the emission limits for the simulation were taken from the existing Euro 6 legislation. The simulation was performed for two different concepts of exhaust after treatment systems for the WLTC, the Artemis- and the AVL reference test cycle. The investigated exhaust after treatment systems are shown in ➍. The first system was a combination of a NOx storage catalyst (NSC) and a DPF. The second system consisted of an arrangement of DOC close to the engine, a DPF and a SCR catalytic converter in an underfloor position.
WORLDWIDE HARMONISED LIGHT DUTY DRIVING TEST CYCLE ④ shows the tail pipe (TP) emissions for
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both after treatment concepts. The nitrogen oxides (NOx) of system No. 1 with 89 mg are almost identical compared to 86 mg of system No. 2 and slightly above the Euro 6 limit. Due to the necessary multiple NSC purge (system No. 1), the tail pipe emissions of CO and THC emission components increases significantly compared to system No. 2 (SCR). An engine operation mode with a lambda value below one must be carried out in the engine control unit for a proper use of the NSC system. In addition, the fuel consumption and CO and THC emissions in this operation mode have to be observed. For the effective use of the SCR system at low loads (< 200 °C upstream SCR catalyst), appropriate measures must be taken for heating the SCR catalytic converter. This measure results in increased fuel
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consumption. ➎ shows the exhaust gas temperatures and the accumulated NOx emissions in the individual test cycles.
sary for the reduction of nitrogen oxides. During the specification of the DOC, the ratio of NO2/NOx has to be taken into account, since this is an important parameter for the NOx conversion rate.
ARTEMIS
During the warm-up phase of the test cycle, the system with the NSC offers a distinct advantage compared to the system with SCR catalytic converter. The efficient use of NSC system requires certain prerequisites: temperature window, engine load (BMEP, exhaust mass flow) and NOx storage condition. On the other hand achieves the SCR compared to the NSC system a higher NOx conversion rate after reaching a sufficiently high temperature (> 250 °C) upstream the SCR catalyst.
AVL REFERENCE CYCLE
In the AVL reference cycle, the NSC system offers advantages compared to the system with SCR catalytic converter during the warm-up phase. An increase of the exhaust gas temperature in the low load range of the engine is preferred. This will ensure that appropriate conditions for the catalyst purging are given. The system with pre-charged SCR is able to meet the NOx limit value of 80 mg/km. To use the full potential of the NSC catalyst, defined measures were taken to avoid excessive storage mode. Both systems have advantages and disadvantages depending on the selected test cycle.
PARTICULATE FILTER WITH SCR COATING
An alternative to the conventional SCR catalyst is a particulate filter with SCR coating (sDPF). The main advantage of these components is their close position to the engine. Improved efficiency in the NOx conversion rate during the warm-up phase of the engine, in particular, can be achieved due to the increased temperature level of the upstream catalyst. However, an SCR coating on the DPF also offers some challenges. Due to the functionality, no precious metal coating can be applied from a current perspective, as this would oxidise the existing NH3, which is neces-
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VEHICLE MEASURING SYSTEM
Engine emissions are measured on a public road with the M.O.V.E system. This is a mobile development platform for the vehicle tests in “real-life”, which provides an indication system, a fuel consumption measuring and a particulate and gaseous emission measuring system. According to the specific requirements of the legislation, the socalled M.O.V.E GAS PEMS (Portable Emission Measurement System) was developed for RDE LDV. This meets the legal accuracy requirements of R49/R83, has a very good correlation with the test bed measurement technology and is specially tailored to ensure the safety of the driver (outside installation on the vehicle): The measuring system can be adapted in a few minutes without any modifications on the vehicle and the exhaust sampling and can be easily mounted on a standard bicycle carrier. Height and weight of the measurement system are optimised and, being equipped with special battery systems, autonomous measurement during several hours of operation is ensured. The overall M.O.V.E system provides full integration into the development process via a user-controlled interface, automated data analysis and the return of the data to the simulation and test environment. A very good correlation of the mobile instruments with the emission measurement on chassis dyno test bed is important. Emission calibration work packages can be shifted from the chassis dynamometer to the road or test track. Stationary, transient and high dynamic calibration tasks can be easily performed with the aid of a controlled trailer brake. Especially during vehicle climate test trips, the combustion parameters are modified according to the ambient conditions. When confirming the robustness of developments, endurance test vehicles are equipped with the M.O.V.E system, which sup-
ports any error cause analysis.
SUMMARY
The future verification of the RDE by the legislation represents a new challenge to the development of the passenger car diesel engine. A well-defined combination of engine-out emissions and an efficient exhaust after treatment system is of great importance. Compliance of the emissions in an extended operating range of the engine requires more calibration effort. By means of the mobile emission measurement M.O.V.E., exhaust emissions on public roads can be verified under different environmental conditions and further calibration steps can be taken. Thus the continuity in the development process using the whole powertrain simulation, the engine and vehicle test beds and the vehicle measurement in real life on the road is guaranteed. REFERENCES
[1] Platner, S.; Kordon, M.; Fakiolas, E.; Atzler, H.: Modellbasierte Serienkalibrierung – der effiziente Weg für Variantenentwicklung. In: MTZ 74 (2013), No. 10 [2] Beichtbuchner, A.; Wancura, H.; Weissbäck, M.; Hadl, K.: Konzepte zur Diesel-Abgasnachbehandlung für die Richtlinie LEV 3. In: MTZ 74 (2013), No. 7/8 [3] AVL Emissionsreport. www.avl.com/legislationservices
THANKS The authors thank the project team, here is representative DI Johann Kleinberger, AVL List GmbH, mentioned.
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