3 March 2011

The EU Joint Research Centre published a final report on inter-laboratory correlation of PMP particle mass (PM) and solid particle number (PN) emission testing from heavy-duty engines. All measurements were conducted at ultra-low particle concentrations, using a diesel engine equipped with a particulate filter. The report is titled “Particle Measurement Programme (PMP) Heavy-duty Inter-laboratory Correlation Exercise (ILCE_HD)”.

The inter-laboratory correlation exercise conducted testing at 5 laboratories in Europe in order to demonstrate the practicality, robustness, repeatability and reproducibility of the PM/PN measurement methods proposed by the PMP program. The exercise involved testing a Golden Engine—a Euro III Iveco Cursor 8, equipped with a wall-flow diesel particulate filter (DPF)—at all participating laboratories. Each laboratory tested the engine over the heavy-duty World Harmonized Transient Cycle (WHTC), World Harmonized Steady state Cycle (WHSC), as well as current EU regulatory cycles: the European Transient Cycle (ETC) and European Steady state Cycle (ESC). Particle emissions measurements were taken from both full flow (CVS) and partial flow (PFDS) dilution systems at each laboratory. Two ‘Golden’ PN measurement systems were circulated between the test laboratories, one for use in CVS measurements one for use in PFDS measurements. In addition laboratories made particle number measurements using several alternative, PMP type systems to compare the performance of different measurement systems.

The Golden Measurement systems performed reliably at all laboratories and agreed with one another to within 5% when making measurements in parallel, said the report. PM emission levels from the Golden Engine using CVS sampling were below 6 mg/kWh across all test cycles after exclusion of outlying test results. PFDS systems returned slightly lower PM results, below 4 mg/kWh. In this exercise tunnel background PM measurements were generally found to be similar to engine measurements. PM measurement repeatability from PFDS was 20-30% for all test cycles and rather higher, 35-56%, for CVS measurements after exclusion of outliers. Reproducibility between laboratories was 35-45% for PFDS measurements and 35-55% for CVS measurements.

The following are selected findings of the report on PN measurements:

  • PN emission levels from the Golden Engine varied significantly from cycle to cycle. The cold start WHTC gave the highest PN levels, approximately 4×1011/kWh from both CVS and PFDS dilution systems. At these levels tunnel background PN concentrations did not significantly influence measurements. Hot start WHTC and ETC cycles gave PN levels around 5-9×109/kWh, while steady state cycles gave higher results (2-3×1010/kWh on the WHSC and 6-8×1010/kWh on the ESC), possibly due to higher exhaust temperatures resulting in some passive DPF regeneration that caused a reduction in filtration efficiency. On these test cycles tunnel background levels were found to have a significant impact in the case of some laboratories’ CVS systems. PFDS tunnel background levels, however, were significantly lower and did not influence PN results. Where tunnel background concentrations were low, correlation between CVS and PFDS measurements was excellent.
  • PN repeatability levels across the different test cycles ranged from 20-60% for CVS sampling, with best repeatability being on the cold WHTC (where PN levels were highest), and worst on the WHSC, where partial passive regeneration of the DPF may have occurred. PFDS repeatability ranged from 20-70%, with best and worst results again on cold WHTC and WHSC respectively. PN reproducibility between laboratories was generally similar to repeatability. Across the different test cycles PN reproducibility ranged from 30-80% for CVS sampling and 50-86% for PFDS sampling.
  • Results from alternative PN measurement systems conforming to PMP principles generally correlated to the Golden System measurements within 15% although some systems showed greater offsets.
  • Additional experiments conducted during the exercise for investigative purposes showed that reported concentrations of solid particles of less than 23 nm diameter were low relative to those of larger than 23 nm particles, thus confirming the suitability of the 23 nm lower size cut-off for the particle number counter. This finding was contrary to results from some US research, noted the report. (The 23 nm size cut-off adopted by the PMP has been questioned by some as arbitrary.)

The results demonstrate that the PM measurement method is suitable to confirm that engine emissions are below 10 mg/kWh, concludes the report. In this exercise, PM measurements could not generally be discriminated from tunnel background PM measurements. The PN emissions measurement method was able to discriminate between the emissions levels on different test cycles of an engine equipped with an efficient wall-flow DPF. PN was also able to discriminate engine emissions from tunnel background levels, except in the case of high tunnel background sampling systems during testing on cycles with lower emissions levels.

The EU PMP program has developed new testing methods for diesel particulate emissions. While the particle mass methods are generally similar to the US EPA 2007-2010 procedures, the new particle number methods will be used in the first-ever regulatory PN emission testing at the Euro 5b/6 stages for light-duty vehicles and Euro VI for heavy-duty engines. Solid particle number measurements based on the PMP methods are also being incorporated into California LEV III emission standards.

Source: JRC: ILCE_HD Final Report