Heavy duty diesel engine exhaust characteristics were studied with direct tailpipe sampling on an engine dynamometer. The exhaust particle size distributions, total particle mass, and gaseous emissions were measured with different load conditions without after-treatment. The measured particle size distributions were bimodal; distinctive accumulation and nucleation modes were detected for both volatile and dry particle samples. The condensing volatile compounds changed the characteristics of the nonvolatile nucleation mode while the soot/accumulation mode characteristics (concentration and diameter) were unchanged. A clear dependence between the soot and the nonvolatile nucleation mode number concentrations was detected. While the concentration of the soot mode decreased, the nonvolatile nucleation mode concentration increased. The soot mode number concentration decrease was related to soot-NOx trade-off; the decrease of the exhaust gas recirculation rate decreased soot emission and increased NOx emission. Simultaneously detected increase of the nonvolatile nucleation mode concentration may be caused by the decrease of the soot mode sink or by changed combustion characteristics. However, the total particle number concentration increased with decreasing soot mode number concentration. The proportion of the particle number concentration between the nonvolatile nucleation and soot mode followed the NO2:NO ratio linearly. While ratio NO2:NO increased the proportion of soot mode number concentration in total number concentration increased. Regardless of the mechanism that causes the balance between the soot mode and the nonvolatile nucleation mode emissions, the changes in the particle number size distribution should be taken into account while the particle mass emissions are controlled with combustion optimization.

The effects of the fuel injection pressure on a heavy-duty diesel engine exhaust particle emissions were studied. Nonvolatile particle size distributions and gaseous emissions were measured at steady-state engine conditions while the fuel injection pressure was changed. An increase in the injection pressure resulted in an increase in the nonvolatile nucleation mode (core) emission at medium and at high loads. At low loads, the core was not detected. Simultaneously, a decrease in soot mode number concentration and size and an increase in the soot mode distribution width were detected at all loads. Interestingly, the emission of the core was independent of the soot mode concentration at load conditions below 50%. Depending on engine load conditions, growth of the geometric mean diameter of the core mode was also detected with increasing injection pressure. The core mode emission and also the size of the mode increased with increasing NOx emission while the soot mode size and emission decreased simultaneously.

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Exhaust gas particle and ion size distributions were measured from an off-road diesel engine complying with the European Stage IIIB emission standard. The measurements were performed at idling and low load conditions on an engine dynamometer. Nucleation-mode particles dominated the diesel exhaust particle number emissions at idle load. The nonvolatile nucleation-mode geometric mean diameter was detected at 10 nm or below. The nonvolatile nucleation-mode charge state implied that it has evolved through a highly ionizing environment before emission from the engine. The determined charging probabilities were 10.0 AE 2.2% for negative and 8.0 AE 2.0% for positive polarity particles. The nonvolatile nucleation particle concentration and size was also shown to be dependent on the lubricant oil composition. The particle emissions were efficiently controlled with a partial filter or with partial filter þ selective catalytic reduction (SCR) combination. The particle number removal efficiencies of the aftertreatment systems were over 95% for wet total particle number (>3nm) and over 85% for dry particle total number. Nevertheless, the aftertreatment systems' efficiencies were around 50% for the soot-mode particles. The low-load nonvolatile nucleation particle emissions were also dependent on the engine load, speed, and fuel injection pressure. The low load particle number emissions followed the soot-core trade-off, similar to medium or high operating loads.Implications: Idling and low-load diesel engine exhaust emissions affect harmfully the ambient air quality. The low-load particle number emissions are here shown to peak in the 10-nm size range for a modern off-road engine. The particles are electrically charged and nonvolatile and they originate from the combustion process. Tailpipe particle control by open channel filter can remove more than 85% of the nonvolatile 10-nm particles and about 50% of the soot-mode particles, while the fuel injection pressure increase can lead to particle number increase. The study provides a new viewpoint for low-load particle emissions and control.