Exhaust emissions below the EU4 limits, optional diesel particulate filter

The diesel models of the new A-Class have also made great strides where exhaust emissions are concerned. The newly developed diesel engines are so clean that they are able to meet the stringent exhaust limits of the EU4 guideline without a particulate filter system. Compared to the previous CDI units, gaseous emissions have been reduced by up to 56 percent while particulate emissions are now down to 0.025 grams per kilometre.

Sophisticated engine improvements and use of the latest common-rail technology are in large measure responsible for this. The new A-Class is also equipped with two oxidation-type catalytic converters, whose task is to reduce gaseous pollutants by combining them with oxygen (oxidation). In this way unburned hydrocarbons (HC) and carbon monoxide (CO) are converted to water (H2O) and carbon dioxide (CO2). One of these catalytic converters (volume 2.1 litres) is located immediately downstream of the turbocharger turbine, while the second with a volume of two litres is positioned in the underbody area. An oxygen sensor upstream of the catalytic converter close to the engine monitors the exhaust gas constituents.

Mercedes-Benz also offers an optional, maintenance-free particulate filter system, which reduces particulate emissions by a further 99 percent. The particulate filter (volume 2.5 litres) replaces the underbody catalytic converter and is connected to the upstream converter close to the engine by an air-gap-insulated exhaust duct. The particles retained on the walls of the filter ducts are burned off at an exhaust temperature above 600 degrees Celsius.

The particulate filter system developed by Mercedes-Benz regenerates itself without the use of additives and remains effective over a very high mileage, which distinguishes it from previously available processes of this kind. Dispensing with fuel additives, whose purpose is to help burn the soot particles trapped in the filter at low temperatures, has a positive effect on fuel consumption, durability and engine output. Studies have shown that fuel additives used to clean the filter remain in the filter ducts as non-regenerable ash, and can block these after a high mileage. This increases the exhaust counterpressure and therefore the fuel consumption, while the output of the diesel engines correspondingly reduced.

With this filter system, high mileages can be achieved by the diesel models in the new A-Class without any additional servicing. The exhaust temperature required for particulate regeneration is reached either at high engine speeds or by appropriate adaptation of various engine functions. These are controlled as a function of the exhaust pressure and temperature at the particulate filter. Variable, second-generation common-rail technology makes a major contribution in this respect, as it allows a brief post-injection of fuel depending on the operating status and filter condition. This specifically increases the exhaust temperature, burning off the particles trapped in the filter in a controlled manner.