F2008-09-052
Direct NO2 Emissions from Urban Buses and Heavy-Duty Trucks - A Comparison of Engine and Emission Control Technologies
Emissions of nitrogen oxides from motor vehicles are an important source of air pollution that affects not only acidification of the natural environment, but also has a severe impact on air quality in urban surroundings all over the world. Therefore, since late 1980´s, legislative efforts has been targeted towards bringing down the levels of these substances that are mainly originating from the reactions of the nitrogen in ambient air and oxygen during the high-temperature combustion process in internal combustion engines. Indeed, continuous amendments in regulations concerning both heavy and light-duty motor vehicles have succeeded in bringing down the certification level of nitrogen oxide emissions. For example after 2008, a EURO5 compliant engine for buses and trucks is allowed to emit only 2.0 g/kWh of NOx. This is only some 14 % of the original EURO0 level implemented in 1990.
However, in recent years the ambient air quality monitoring in many European cities has shown that the recorded levels of NO2 in urban environments with exposure to exhaust gases from motor vehicles have not much improved, if changed at all. This has happened, despite the fact that more stringent NOx emission control has been implemented in all new motor vehicles and many types of quite effective emission reduction systems are being employed.
As many studies have shown that exposure to NO2 has more adverse health effects than those of NO, it is of primary concern, even if after a transformation in air, all NO will eventually be turned into NO2. Because this transformation usually takes place within minutes or few hours, depending on many atmospheric parameters (temperature, humidity, local ozone concentration etc.), emissions are usually diluted and transported away from the immediate release and exposure areas thus alleviating the ill-effects. However, if more NO2 is directly emitted in urban air, elevated concentrations are an inevitable result.
Usually, as legislative standards only regulate NOx, the sum of NO and NO2, there have not been many studies presenting NO2 emission rates, nor reporting of NO/NO2 ratio in the NOx emissions. Only in recent years some studies have been focusing on this issue, and reported that especially the latest emission control technology employed in diesel-powered vehicles is susceptible for high direct NO2 emissions. During 2006 VTT also initiated an effort to chart the typical levels of NO2 associated with different types of motor vehicles. For this purpose, NO and NO2 were measured separately on a full vehicle chassis dyno using real-world duty-cycles from a number of heavy duty vehicles. The emphasis has been on late-model urban buses, as in many urban street canyons buses are a major source of NOx emissions.
The work is still in progress, and the paper will highlight the results and summarize the eventual conclusions of this study due Q1 of 2008.
Session: Exhaust Gas Aftertreatment