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The reductions in N[O.sub.x] emissions for the Cummins ISX-15 over the HHDDT cycle and the Cummins ISB6.7 over the UDDS cycle are consistent with the findings of previous studies, which have generally shown reductions in N[O.sub.x] with HVO relative to petroleum-based fuels [22-23, 32-33, 38].
Further analyses showed that a relatively large portion of the N[O.sub.x] emissions were produced at the start of the UDDS and HHDDT cycles when the SCR system temperature was <250[degrees]C, as illustrated in Figure 5 (a-c), which shows profiles of real-time N[O.sub.x] and N[H.sub.3] emissions of the CARB ULSD, HVO100, and WCO20 fuels for the Cummins ISX-15 vehicle over the UDDS cycle.
For the HHDDT cycle for the Cummins ISX-15 vehicle, all biofuels led to reductions in C[O.sub.2] emissions compared to CARB ULSD.
The same picture was seen for the HHDDT cycle, with the HVO fuels showing lower C[O.sub.2] emissions and the biodiesel blend showing higher C[O.sub.2] emissions than CARB ULSD.
Overall, the results reported here for the Cummins ISX-15 vehicle did not show strong differences in carbon balance fuel economy between the test fuels for either the UDDS or the HHDDT cycles.
For the HHDDT cycle, HVO100 showed a marginally statistically significant reduction in fuel economy of 3% compared to CARB ULSD, whereas the biodiesel blend showed a 3% reduction in fuel economy compared to CARB ULSD, at a statistically significant level.
PM mass was collected cumulatively for the entire triplicate UDDS and HHDDT cycles for all vehicle/fuel combinations due to the expected low PM levels.
For the HHDDT cycle for the Cummins ISX-15 vehicle, solid particle number emissions did not show strong fuel trends.
For both the UDDS and HHDDT cycles for the Cummins ISB6.7 vehicle, the biofuel blends showed strong reductions in volatile particle number emissions compared to CARB ULSD, with these differences being statistically significant in most cases.
Figure 12 and Figure 13 illustrate the particle size distribution profiles for the Cummins ISX-15 vehicle over the UDDS and HHDDT cycles, respectively.
Figure 14 and Figure 15 present the average particle size distributions for the Cummins ISB6.7 vehicle over the UDDS and HHDDT cycles, respectively.
Testing was performed over the Heavy-Duty Urban Dynamometer Driving Schedule (UDDS) and Heavy Heavy-Duty Diesel Truck (HHDDT) Transient Cycle.
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