Professor Kittelson gave a presentation on solid particle measurement from a number of studies, in particular recent research with the University of California Riverside.

He noted that most penetrating particle size for DPFs is around 300nm, so limits controlling >23nm particles are expected to be effective in controlling emissions of all particles. However, for non-DPF equipped diesel engines significant concentrations of sub 10nm ash particles are produced at idle conditions. For petrol engines, significant sub 23nm particle concentrations were observed on PFI engines using fuels with metallic additives, and measurements at SWRI showed around 20% of solid particle concentrations from GDI engines to be in the 10-23nm size range. Solid particle emissions for HCCI operation were seen to be entirely in the sub 23nm size range.

Measurements with University of California Riverside’s on-road mobile lab had shown unexpectedly high concentrations of sub 23nm particles in post DPF exhaust at high loads. Post VPR measurements at high load with using 23nm, 10nm and 2.5nm cut-size PNCs suggested no particles were present in the 10-23nm size range, but there were substantial post VPR concentrations in the 2.5-10nm size range. Measurements using a Catalytic Stripper also initially showed no particles in the 10-23nm size range and initially no particles in the 2.5-10nm size range. However over prolonged sampling particles in the 2.5-10nm began to appear downstream of the Catalytic Stripper also. At lower load much lower particle concentrations were observed. Post VPR measurement showed no particles in the 10-23nm size range, but presence of particles in the 2.5-10nm range. However, measurements downstream of the Catalytic Stripper showed little evidence of particles in the 2.5-10nm size range. CVS measurement with no VPR showed lower concentrations suggesting formation of particles within the VPR.

Laboratory experiments with Thermodenuder and Catalytic Stripper showed the Catalytic Stripper to be significantly more effective at removing heavy hydrocarbon and sulphuric acid particles. They also suggested some evidence of formation of particles by the Thermodenuder.

In conclusion Professor Kittelson noted that for engines equipped with DPFs regulating for >23nm particles effectively controls all solid particle emissions. However for non-filter equipped engines there can be substantial concentrations of sub 23nm solid particles. Extending the regulatory particle number measurement technique down to a 10nm cut-off size would be problematic with the current VPR due to nucleation of semi-volatile (sulphuric acid) particles, but a catalytic stripper
would be an efficient means of conditioning the sample to eliminate formation of these particles. Extending the measurement below 10nm would be extremely problematic as nucleation of sub 10nm particles was seen downstream of both VPRs and Catalytic Strippers and there was some evidence of VPR and thermodenuders creating solid particles in this size range.

Dr Mayer gave a presentation on TTM’s work on nanosize metal oxide particle emissions. He noted that peak penetration of particles into the alveoli occurred at 20nm size and that peak penetration of the bronchial tract was at sub 10nm size and commented that it was unclear whether particles deposited in the bronchial tract were captured and released or may travel via the olfactory nerve to the brain.

Data on toxicity for different compounds showed Copper, Zinc and Iron oxides exhibiting higher toxicity than Carbon. Engine wear, lubricating oil, additives and catalyst coatings were all potential sources of metallic compound particles. TTM had conducted mass spectrometry measurements on particles collected on ELPI impactor stages. For a non-DPF diesel, size distribution measurements showed a peak particle concentrations at around 20nm under engine idling conditions, where significant lubricant burn can occur. Spectrometry of the material collected on the ELPI (<30nm) back-up stage at idle showed significant quantities of Calcium, Iron and Zinc. With an Iron fuel-borne catalyst, mass of iron in the engine-out particulate increased 30 fold, although a DPF proved almost 99% effective at capturing this material. Further VERT measurements confirmed that DPF filtration efficiency is excellent even for sub 30nm particles.

Measurements on a range of petrol engined vehicles showed an old car and a motorcycle to have significant particle concentrations in the sub 23nm size range. A more recent port fuel injection (PFI) car exhibited much lower concentrations, as did a recent scooter at idle although at 50km/h it exhibited similarly high concentrations to the old car and motorcycle. Total particulate emissions included significant quantities of Calcium and Zinc in particular.

Dr Mayer concluded that engine wear and lubricant oil consumption produce significant quantities of metallic particles of around 20nm size in the case of both petrol and diesel engines. These particles are likely to have higher toxicity than soot, but are efficiently removed by DPFs if these are fitted.

JRC presented the results of a recent measurement programme on sub 23nm particles. Their work involved using the current VPR specified in Regulation 83 with a range of PNCs with 4.5, 10 and 23nm D₅₀ cut sizes. They had tested 3 GDI, 2 PFI and 2 DPF vehicles (including measurements on a Flex Fuel GDI running on E85 and a bi-fuel PFI running on CNG) over cold start NEDC and hot start CADC cycles.

NEDC results suggested 10-20% of post VPR particles were in the 10-23nm size range for GDI vehicles, 35-56% for PFI and (surprisingly) 26-45% for DPF vehicles. Measurements using the 4.5nm cut size PNC were higher for all technologies, but not significantly so. Results on the CADC Motorway cycle gave slightly higher proportions of post VPR particles in the 10-23nm size range, but much higher for
the 4.5nm cut size PNC in some cases. However 4.5nm measurements were found to be sensitive to PCRF setting (higher settings giving much lower particle concentration measurements) suggesting volatile material in this size range was penetrating the VPR.

Active DPF regeneration measurements on the CADC cycle showed an enormous increase in volatile particle concentrations, but post VPR concentrations (with all PNCs) were also increased by more than 2 orders of magnitude. Again 4.5nm cut size PNC results were sensitive to PCRF setting, suggesting volatile particles in this size range are penetrating the VPR.

JRC confirmed that the DPF vehicles tested were fitted with catalysed DPFs. Professor Kittelson commented that the post DPF sub 23nm particles were probably volatile particles (e.g. condensed sulphuric acid). VW asked if JRC had made any measurements with a Catalytic Stripper (which would be more effective at removing such volatile particles) instead of the VPR. JRC confirmed that they had not, but hoped to do so during the WLTP Validation 2 exercise as part of their DPF regeneration investigative measurements.

The chairman indicated that he would report back to GRPE on the discussions on sub 23nm particle measurement and seek GRPE’s views on whether PMP should discuss and investigate this subject further.