Regulating particle number measurements from the tailpipe of light-duty vehicles: The next step?

Abstract

Light-duty vehicle emission regulation in the European Union requires the dilution of the whole exhaust in a dilution tunnel with constant volume sampling prior to emission measurements. This methodology avoids measurement uncertainties associated with direct raw exhaust emission measurements from the tailpipe, such as exhaust flow determination, exhaust flow pressure pulsations, differences in the response time between exhaust flow and instrument signals, or their misalignment. Transfer tubes connecting the tailpipe to the dilution tunnel of different lengths, and mixing of the exhaust gas with the dilution air in the dilution tunnel may increase differences in measurements performed at different facilities. Recently, the light-duty vehicle regulation was complemented by on-road measurements with Portable Emissions Measurement Systems (PEMS). PEMS measurements are conducted from the vehicle tailpipe. Differences between tailpipe and full dilution tunnel measurements have not been adequately addressed so far. In this study we compare particle number emissions measured at the full dilution tunnel or directly at the tailpipe. The measurements covered solid particles with diameter larger than 23 nm, as required by the current regulation, but also solid particles larger than 10 nm, as recommended for future regulations. The studied vehicle technologies were diesel, gasoline, and compressed natural gas. The differences between tailpipe and dilution tunnel particle number emissions were found to be small (<15%) for both size ranges, with the exception of engine cold start (up to 35% in some cases). Theoretical estimates showed that agglomeration in the transfer line from the vehicle to the dilution tunnel might reduce particle concentrations by up to 17%. Exhaust flow rate determination and time misalignment of exhaust flow and particle concentration signals can introduce uncertainties of ±10% and ±5%, respectively, to the tailpipe measurements. The results suggest that tailpipe sampling is not only possible, but it can additionally give more representative (“real”) emissions of the vehicle and should be considered in post Euro 6 regulations.

Particle Number Measurements Directly from the Tailpipe for Type Approval of Heavy-Duty Engines

“…The SPN emissions measured with the systems connected to the proportional partial flow dilution systems were calculated according to the regulation. For the systems at the tailpipe the equation given in the RDE (real-driving emissions) regulation of light-duty vehicles was used replacing distance driven with engine work [27]. This calculation needs correct time alignment of the particles signal with the exhaust flow rate.…”

“…The losses should be similar for all systems examined, thus the conclusions should be the same even correcting separately for the losses below 30 nm. If losses would be taken into account, the results of the 10-23 nm levels would need a correction of approximately 1.7, as explained elsewhere [16,27]. Figure 2 summarises the results of the tests where the length of the heated line to the "Golden" system TP(CS) varied between 0.5 and 4 m. The tests were conducted with three engines.…”

A Sampling and Conditioning Particle System for Solid Particle Measurements Down to 10 nm

“…time in the VPR [14]. The inclusion of sub-23 nm particles in the legislation may result in stricter limits for the tetracontane removal [7].…”

“…Measurements with polydisperse particles are not as precise as with monodisperse particles may give an approximate penetration fraction for more sizes. In setup A, the particle size selection was done with a long-DMA (TSI, 3081) for sizes Dm=[100, 50, 30] nm and a nano-DMA (TSI, 3085) for sizes Dm= [15,10,7.5] nm. The sheath to aerosol was 1.5:15.…”

“…CVS and tailpipe measurements, in general, correlate well with small differences (<15%). However, particle agglomeration, diffusion and thermophoresis in the transfer line between the exhaust and the CVS tunnel can result in significant particle losses (estimated theoretically to be circa 17%) [7]. On top of that, there is much uncertainty resulting from errors in exhaust flow determination and discrepancies in the time alignment among the different signals from the measurement analyzers and the exhaust flow sensors due to the large tunnels needed.…”