Modular HJS heavy-duty exhaust gas aftertreatment system with independent thermal management for high NO_X conversion especially in urban operation

Zusammenfassung

With the European Directive 2008/50 EC on Ambient Air Quality [1], the European Commission is pursuing the goal of ensuring that the annual average ambient air quality limit of 40 μg/m3 for NO2, which has been in force since 2010, is ensured. Compliance with this directive is a major challenge for many municipalities and federal states.

Especially in urban areas, NO2 limit values are often exceeded. Transient low-load operation with low exhaust temperatures and sometimes high NOX raw emissions during the acceleration phases also pose special challenges for the complex exhaust aftertreatment systems of buses and commercial vehicles. With regards to the problem of Real Driving Emissions (RDE) in the urban environment and in light of the expected future exhaust emission legislation, HJS has further developed the well-known SCRT technology and added important technology components.

In addition to highly efficient catalytic converters and optimized NH3 mixture preparation, the focus was placed on autonomous thermal management technologies and corresponding control algorithms in order to achieve very high NOX conversion rates even in demanding urban operation. The first application of this modular HJS heavy duty exhaust aftertreatment platform is in city buses.

Diesel particulate filters are well established to reduce particulate emissions from diesel engines and are being used in most new applications already.

The guidelines for retrofitting diesel buses as part of the “Sofortprogramm Saubere Luft 2017 – 2020” have defined emission limits and testing boundary conditions which even go beyond the IUC regulations for Euro VI buses, especially in inner-city operation.

In order to meet the emission requirements of this directive in real city bus operation, a thermal management is required in addition to an optimised EURO VI exhaust gas aftertreatment with DOC, DPF and SCR in order to operate the SCR system in an efficiency-optimised temperature window even at the lowest speed profiles.

The modular heavy duty exhaust aftertreatment system with independent thermal management presented in this paper aims to significantly increase the efficiency of the SCR system in transient inner-city operation. The exhaust gas temperature in the low load range is raised by up to 50 °C for this purpose. Central elements for this are a quick, electrically controlled and operated exhaust flap along with an electrically heated diesel oxidation catalyst.

With the aid of the exhaust flap, the exhaust back pressure is increased in certain driving situations. The electrically heated catalytic converter arranged in front of the particulate filter increases the exhaust gas temperature and increases the engine load via the additional load on the alternator.

The HJS ACU control unit processes the measured values such as NOx flux, temperatures, pressures, exhaust gas mass flow, status of the electrical system, etc. and controls the actuators in such a way that the best possible NOX reduction is achieved with minimum fuel consumption penalty. The system is also fully diagnosable.

The HJS system contributes immediately to the reduction of NOx emissions and thus to improved air quality in urban areas. The modular concept and the smart CAN bus and sensor-based control also qualifies the HJS system for other commercial vehicle applications beyond city bus applications and EURO VI legislation. For the first time, engine heating measures can be enhanced by engine operating point independent thermal management measures to help reliably meet future RDE-focused legislation in critical city operation.

In the following, the performance of the system will be demonstrated as an example for retrofitting a city bus application.