India is closing the gap to regions with the most stringent exhaust emission regulations by introducing Bharat Stage VI emissions legislation in advance. For passenger car diesel engines, this results in the need of exhaust aftertreatment regarding particulates and nitrogen oxides, which is intended to contribute to improved air quality in urban areas. Other than in Europe, the traffic situation and system costs favour systems based on a lean NOx trap for this purpose, as Graz University of Technology and AVL show in this article.
India is an emerging market for passenger cars and the annual growth rate for new registrations accounts for 9 %. In 2017, 3.2 mn passenger cars were registered, making India the third largest Asian key market, only outranked by China and Japan [1, 2]. In the last decade, India has been among the regions with the highest diesel shares. The diesel peak, which was based on the favourable fuel pricing, was reached in 2012/13, when more than 50 % of the cars sold were powered by diesel engines. Since then, the gasoline and diesel prices have gradually converged and, as a result, the demand for diesel cars in the compact and medium class has dropped to 23 % by 2018 .
In the same time span, the diesel share in the pick-up and SUV segment changed from 97 to 84 %, leading to an overall diesel portion of 41 % . In addition to the pricing and fiscal policy, further reasons for this decline can be found in former diesel prohibition and the environment levy as its substitute, which applies for diesel engines with an engine displacement above 2 l. Air pollution of many cities can, however, be considered as the actual flash point of the diesel debate. With regard to particulates, Indian metropolises are among the most polluted on a global scale [5, 6] and diesel vehicles without DPF make a contribution here.
The Indian government wants to address the problem of high CO2 emissions partly caused by the coal-dominated energy sector by reinforcing solar energy. Concerning transport, electric mobility is propagated. Nevertheless, the rising demand for vehicles will hardly be satisfied without the use of combustion engines in the upcoming years. The diesel propulsion can still benefit from a CO2 advantage over short-term available and affordable alternatives here. The necessity to reduce engine exhaust pollutants from vehicles will be accommodated by the implementation of Bharat Stage VI (BS VI). However, for the small and medium car segments, the exhaust aftertreatment systems, which are required for its fulfilment, need to be reasonably priced in order to retain the attractiveness of the diesel propulsion for the Indian customers.
BHARAT STAGE VI & RDE
From 2020 onwards, passenger cars in India have to comply with the emission standard BS VI. As depicted in (1), it will come into force nationwide, unlike its predecessor BS IV, which was introduced gradually throughout the country until 2017. The implementation is comparable with a transition from Euro 4 to Euro 6 and therefore requires a significant reduction of particulates and nitrogen oxides, (2). Therefore, in addition to a particulate filter, nitrogen oxide aftertreatment will also be required in the future.
According to the draft of BS VI (AIS-137) , the type-1 test cycle will still be represented by the Modified Indian Driving Cycle (MIDC). It corresponds to the NEDC (New European Driving Cycle); however, it features a speed limitation of 90 km/h. In addition, the start of Real Driving Emissions (RDE) monitoring is planned for 2020, which will serve as a basis for the introduction of RDE Conformity Factors (CF) in April 2023. RDE India is also deduced from the European RDE packages, although some points are still to be defined.
As a reason, the differences between the Indian and European overall conditions can be stated, which can mainly be found in traffic flow and cruising speed beside ambient temperature and geodetic situation. Traffic in India is generally characterised by a lower speed level, (3). This is due to congestion in the metropolises and a very coarse network of high-speed roads (expressways) with a speed limitation of 120 km/h and mostly charged access. The busy highways are usually limited at 80 km/h. In the problem zones concerning air quality, the urban areas, heavy and nose-to-tail traffic dominates, which exceeds European scenarios.
The typical operation at low speed is considered in the MIDC and moreover, in the planned RDE trip requirements. (3) shows two provisional variants of obligatory trip parameters for the vehicle category M, which were announced in the AIS-137 draft. Variant A demands a lower speed level as B, which is closer to the European requisites. Both alternatives should, however, comply with a maximum speed of 110 km/h, which is distinctively below the European speed limit of 160 km/h.
DIESEL EXHAUST AFTERTREATMENT CONCEPTS FOR INDIA
It can be assumed that in addition to legislation, exhaust aftertreatment concepts will also be based on Europe. Nevertheless, the above stated differences imply special challenges. In India, these are the costs, the special traffic characteristics and the comprehensive supply of an appropriate fuel quality. The latter is connected with the introduction of BS VI fuel containing less than 10 mg/kg of sulphur. The Ministry of Petroleum and Natural Gas has announced national availability until the launch of BS VI in April of 2020. The disposal already started in April 2018 in Delhi. Accordingly, Lean NOx Trap (LNT) and Selective Catalytic Reduction (SCR) systems are both conceivable for nitrogen oxide reduction.
Based on the partly undetermined framework, adequate emission concepts for the legal compliance and the reduction of real emissions have to be identified. Due to the operating conditions, the heat- up and the temperature maintaining of the aftertreatment system will be crucial. Hence, a close-coupled position of the NOx reduction catalyst is taken for granted. (4) shows systems which fulfil this prerequisite; for example, by integrating SCR on the DPF. This leads to the question if LNT, SCR or a combination can best meet the Indian requirements. Subsequently, a combination of LNT-SCR without active AdBlue dosage is investigated, as it offers cost and conceptual advantages.
LNT & PASSIVE SDPF
The benefit of LNT-based systems is the possibility to place them in a hot-end position in the exhaust path. The fast heat-up behaviour and the storage capacity at low temperatures allow NOx to be reduced without excessive heating measures. Therefore, most of the trips in India can be covered by the LNT itself. Active SCR systems would need to ensure sufficient temperature for dosing release and NOx reduction, most often with fuel consuming thermal management strategies in the urban areas of India.
The application of a passively fed SCR-coated Diesel Particulate Filter (SDPF) extends the operating range of nitrogen oxide reduction in areas where NOx slips over the LNT at conditions with high space velocity or temperature. The functionality of passive SCR has been content of several studies and basically relies on the formation of NH3 during the regeneration phases of the LNT. An adjacent SCR catalyst can store the ammonia and use it for further NOx reduction.
In a reduced manner, the process is schematically illustrated in (4) and can be found in more detail for example in . For Europe, passive SCR on an under floor converter was investigated in  and an SDPF was used in . However, a lack of robustness at high-speed driving combined with road incline could be identified. This operating mode is not typical for India though. The RTS-95 cycle was chosen to demonstrate the advantage of passive SCR in highly dynamic operation, (5).
The analysis was conducted on an engine test bed and a 1.6 l diesel engine as test carrier. It was equipped with high and low pressure exhaust gas recirculation, a 1,600 bar common rail system and a VTG turbocharger. The underlying vehicle weighs 1,500 kg. The baseline tests were executed with an aged LNT and six purging events. The lengths of the LNT regenerations were conventionally controlled by lambda probes. The LNT-only operation is depicted leftmost as reference with 0 % fuel penalty. By tolerating a fuel penalty, the amount of formed NH3 was increased by changing rich calibration and length of the events . As a consequence, the contribution of passive SCR increased and an NOx CF below 2.1 could be reached, without a considerable raise of other pollutant components.
The benefit of the SDPF depends on the prevailing temperature level, NOx slip, regeneration frequency and NH3 formation. (6) therefore shows the operation for a series of three WLTC Low parts, representing dynamic city operation, and the RTS-95, as an aggressive city-interurban-highway cycle. The results are depicted with a cold and a hot start, where for the cold start cases the use of an exhaust heating measure was omitted. It can be seen that the contribution of passive SCR is lower during the cold-started tests. Here, NOx reduction is based almost entirely on LNT as NH3 can be formed and stored in the SDPF, but the SCR light-off is reached late. With sufficiently warm catalytic substrates, the stored ammonia can be successfully used for SCR, so that in the warm RTS-95 almost 50 mg/km of NOx could be converted.
Due to the advantages in low-load operation and the costs, it can be assumed that from 2020 LNT-based systems, similar to many Euro 6b applications, will be introduced for small and medium diesel cars in India. The extension of the LNT by a passive SDPF allows a further significant reduction of NOx emissions in highly dynamic operation. Nevertheless, the NH3 formation is linked to the regeneration capability of the LNT. If operating conditions occur that prevent rich operation for a long time, the SDPF function for NOx reduction will decay. These circumstances can occur especially on European highways with significantly higher average speed, but hardly in India.
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DIPL.-ING. REINHARD RATZBERGER was Research Assistant at the Institute of Internal Combustion Engines and Thermodynamics at the Graz University of Technology (Austria).
UNIV.-PROF. DR. HELMUT EICHLSEDER is Head of the Institute of Internal Combustion Engines and Thermodynamics at the Graz University of Technology (Austria).
DIPL.-ING. MARTIN WIESER is Lead Engineer Exhaust Aftertreatment Passenger Car Diesel at AVL List GmbH in Graz (Austria).