Different conditions in the U.S. and Europe led Daimler Trucks to offer two different emission-reduction solutions for the two markets. Markus Kemmner, who is responsible for Exhaust Treatment at Mercedes-Benz Commercial Vehicle Development, believes that the European solution using an SCR catalytic converter and AdBlue is clearly more economical than the U.S. solution. The technology chosen in the United States uses intensive exhaust gas recirculation to reduce nitrogen oxide emissions, whereby the resulting increase in particulates is kept below the limits for that substance through the use of a filter. Nevertheless, the engineer refuses to divide the procedures into rigid categories of “good” and “bad.” Instead, in view of future emission limits that will likely be even more restrictive than today’s, he predicts that “both technological approaches will probably merge together because that will be the only way to meet the standards then required.” In other words, both an SCR catalytic converter and a closed particulate filter will be needed in the exhaust gas line to ensure compliance with the respective limits in the future.

The special EEV version of the Citaro urban bus offers a look at what form such a combined system might take. The model comes in three engine variants whose emissions have more or less been reduced to the greatest extent possible, using today’s engine and exhaust treatment technology. Emissions from the EEV Citaro are not only much lower than the level stipulated by EURO 5; in some cases, they are also well below the EEV limits, which themselves are between 25 and 30 percent lower than EURO 5 when it comes to emissions of carbon monoxide, hydrocarbons and, above all, particulates. At the same time, the fuel economy of the EEV Citaro measures up to the extremely efficient BLUETEC 5 vehicles.

This outstanding result was achieved through an extremely compact system that consists of a box containing a closed particulate filter and an SCR catalytic converter that is much more efficient than the one used in BLUETEC 5 vehicles. Of course, it’s not enough to simply install two catalytic systems side by side: urban buses generally travel on short routes, and they also stop repeatedly at traffic lights and bus stops. This leads to constantly changing engine loads, so the real challenge here is to be able to regenerate the particulate filter on a regular basis — to continually burn off the soot, in other words. “And you can only do that if the exhaust gas temperature doesn’t go below 250°C,” Kemmner explains. The Citaro engineers accomplished this by controlling a throttle valve in such a manner as to ensure it will not allow exhaust gas temperature to fall too much, even under tough operating conditions. They also installed a soot charge model in the control unit that prevents the particulate filter from reaching a critical state.

In view of these developments, Kemmner’s goals for the future are clear: “This is not about a particular technology. Our objective is to optimize the complete exhaust gas chain — from the engine to the closed diesel particulate filter and the denoxification process in the SCR catalytic converter with AdBlue. The end result will be a system that keeps operating costs at an acceptable level, even under more demanding conditions, and also offers customers maximum utility.”