Effect of Speed and Speed-Transition on the Formation of Nucleation Mode Particles from a Light Duty Diesel Vehicle

This work studies the formation of nucleation mode (NM) particles from a Euro 3 passenger car operating on 280 ppm wt. sulfur fuel, during on-road plume chasing and in the laboratory. The vehicle produced a distinct NM when its speed exceeded 100 km/h in both sampling environments. A higher particle number (up to 8 times) after 4 min at constant speed was measured when this speed was approached from a lower than from a higher speed. The variability in the measurement of NM particles was explained using literature information on sulfur-to-sulfate conversion over a catalyst and, in particular, on the extent and rate of sulfate storage and release mechanisms. All evidence led to the conclusion that storage and release processes take several minutes to conclude after a step-wise change in speed and have significant implications in the total particle number measurements during steady-speed testing. However, their effect was much less important during transients due to the averaging effect of transient operation on aftertreatment condition. Appropriate preconditioning is therefore necessary in each case to reduce measurement variability.

Over speed transitions, the solid particle number concentration generally followed the inverse of the air-to-fuel (A/F) ratio. The total particle number concentration also followed the inverse A/F ratio at low speeds (i.e. <100 km/h), when no NM was monitored at either the initial or the terminal speed. At higher speeds though, the total particle number increased both during acceleration and during deceleration. The increase during acceleration was explained due to the increase in the exhaust gas temperature which increased the sulfur-to-sulfate conversion. Over deceleration, the total particle number increase could only be explained by the reduction in the available surface area of solid particles as the A/F increased, which promoted the homogeneous nucleation of sulfate.