Due to the introduction of ever larger low emission zones and the raising of air-quality targets, the EU is tightening its rules for the public transport buses that plough the roads of Europe's towns and cities. And although buses have long been kitted out with SCR catalytic converters to reduce their pollutant emissions, new challenges lie ahead. This is because buses that are primarily deployed in inner-city traffic often get caught up in traffic jams and stand at bus stops with their engine running. Generally speaking, then, this means that they do not reach the exhaust-gas temperatures required for the SCR catalyst system to function optimally. These "cold phases" limit the effectiveness of the system and prevent it from optimally neutralising all of the pollutants emitted by the engine. Relief is provided by the "Thermo Management Technology (TMT)" solution invented by HJS Emission Technology.
Thanks to the innovative TMT system, which raises the exhaust-gas temperature, the efficiency of the SCR system installed as standard is increased significantly. The very name of the system, "Thermo Management Technology", indicates the approach followed by HJS during its development: if the low engine load means that the exhaust gases do not automatically reach the temperature levels required, a system needs to be installed that monitors the exhaust temperature and raises it as and when necessary. Two development goals were of particular importance to HJS: the system's design and operating principle needed to keep costs and therefore the retail price as low as possible and also enable the system to be easily installed as a retrofit kit. And HJS succeeded in achieving precisely these two goals with its SCR upgrade – TMT. The system is extremely efficient, inexpensive and can also be installed in the vehicle without any complication and any modification to a bus' engine management system. The main components of the upgrade kit are a control unit, an exhaust flap, a coolant heater and insulation material for the standard SCR catalyst.
The heart of the TMT system is the control unit, which measures the temperature and pressure of the exhausts upstream of the SCR catalyst and the temperature downstream. This data is picked up by temperature sensors. If, on the basis of the measured data, the control unit determines that the exhaust temperature is too low, it actuates an exhaust flap. By changing the position of this flap, the backpressure of the exhausts is increased and this in turn causes the temperature of these gases to increase significantly. This effect is further aided by additional insulation material that is wrapped around the SCR catalyst. If the exhaust temperature needs to be increased even more, the control unit activates the coolant heater. Overall, the TMT system is able to increase the exhaust-gas temperature by up to 50°C.
In tests conducted with TMT in actual inner-city operation, the average temperature of vehicle exhausts was able to be raised from 185°C to over 220°C. This resulted in the share of nitrogen oxides removed from the exhaust gases increasing from 37% to 72% on average.
The system developed and manufactured by HJS is a retrofit system that supplements the SCR exhaust treatment systems installed as standard and makes them more efficient. It's an easy-to-install and inexpensive solution for Euro IV, Euro V and EEV buses that any specialist workshop is more than capable of fitting.