Diesel Particle Exhaust Emissions from Light Duty Vehicles and Heavy Duty Engines

Diesel engines are widespread in both passenger car and heavy duty truck applications. However, despite that the combustion concepts are similar in the two cases, the engine calibration required for compliance with the different emission standards leads to distinct particle emission behavior from the two categories. This paper compares the exhaust particle emissions from heavy duty engines with typical diesel passenger cars of similar emission standard and/or emission control technology. Measurements were conducted with the same sampling system and sampling protocol to avoid interferences induced by the sampling methodology. A range of particle properties were studied, including mass, number of solid and total particles and total particle surface. For comparability, the results are expressed per unit of exhaust volume, per unit of fuel consumed and per unit of distance driven. The results show that while distancespecific emission levels (in particular of solid particles) are proportional to engine size in urban driving, they become similar in highway conditions. When expressed per unit of fuel consumed, heavy duty engines may emit up to one order of magnitude lower soot particle numbers than passenger cars. This is a combination of factors, including the shorter time available for combustion, the higher EGR rate and the higher PM emission limits of light duty vs. heavy duty vehicles. As a result of the lower soot emission though, a distinct nucleation mode is more common in the exhaust of heavy duty vehicles, especially at low loads and when no oxidation-type of aftertreatment is present. On the contrary, light duty vehicles commonly equipped with oxidation catalysts mainly produce a nucleation mode at higher speeds, when sulfate production increases. The results of this study show that the contribution of diesel vehicles to ambient PM pollution is not necessarily proportional to their size and that the effectiveness of potential aftertreatment options depends on the engine type.