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Portable Nano-Particle Emission Measurement System

Periodic Reporting for period 2 - PEMs4Nano (Portable Nano-Particle Emission Measurement System)

Reporting period: 2018-04-01 to 2019-12-31

The ultrafine particles (UFP) emitted by combustion engines include some nanoparticles that are smaller than current exhaust testing systems can reliably detect. Current systems are certified to detect particles down to 23 nm. The reason for the limit at 23 nm is because a large proportion of particles below this size are composed of semi-volatile material. The number of these semi-volatile particles changes with temperature and can strongly influence the counting of the solid particles; the particle count at 10 nm may vary by an order of magnitude. In addition, a significant proportion of sub-23nm particles can be lost to the walls of the measurement equipment before the particles are counted. The current procedures (that define the conditions under which emissions are measured) have tolerances on particle losses that are not meaningful for particles smaller than 23 nm.

The goal of PEMs4Nano is to develop the following: (1) A fundamental understanding of sub-23 nm particle formation, composition, number concentration, size distribution and transport, and the corresponding impact on the measurement procedure. (2) Develop sub-23 nm measurement systems: a device for stationary measurements and two portable emissions measurement (PEMS) devices for mobile applications that are suitable for the reliable and reproducible measurement of particles down to 10 nm. (3) Robust procedures: measurement conditions (i.e. measurement procedures) for which the losses, detection limits and tolerances are acceptable for reliably detecting solid 10 nm particles. (4) A model-guided application (MGA) that provides an understanding of the full trajectory of the particle emissions, from their formation inside the engine to the effect of aftertreatment devices in the tailpipe. These goals help to support potential future international standards and regulations on nanoparticle emissions, and in particular to make the future EU certification tests robust and reliable.
Detailed multi-technique analyses were performed on particles generated from a single cylinder engine. For the first time it was possible to identify key molecular markers and discriminate particles by source, size and engine operation regime. Particles were identified according to gasoline-specific bonds, lubricant- specific bonds and engine-specific bonds. The measurements clearly and unambiguously showed the effectiveness of the catalytic stripper used to remove the semi-volatile particles. A large amount of data was collected and used to support the development of MGA.

MGA combines physico-chemical simulations with statistical algorithms to simulate the formation of particulate emissions. During the project, the MGA was applied to a single cylinder engine, a multi-cylinder test engine with exhaust aftertreatment, and to simulate the emissions from vehicles in various drive cycles, including real driving emissions (RDE). The physico-chemical model was extended to include not only particles from incomplete combustion, but also from other important sources (e.g. fuel impingement on cylinder walls in the engine, ash formation from lubrication oil and condensation of semi-volatiles in the exhaust). The MGA results matched the physical characteristics from the single cylinder engine studies.

Three prototype measurement devices were developed and they were optimized using a two-step approach. In a first step, the condensation particle counters were optimized and re-calibrated, including a temperature adjustment for the saturator and condenser. In the second step, the configuration was optimized to reduce particle losses. The highest losses occur in the catalytic stripper, where a trade-off is made between reducing losses and removing semi-volatiles. The optimized catalytic stripper achieved a penetration efficiency of over 60% (i.e. losses less than 40%) for 8 nm particles, and a semi-volatile removal efficiency of over 99% for particles composed of tetracontane.

An extensive testing campaign was used to validate the performance of the prototype measurement devices. Three test configurations were used: chassis dyno tests with the measurement equipment located at the tailpipe, chassis dyno tests with the measurement equipment located after the CVS tunnel, and real road tests. The results show that sub-23 nm particles are mainly generated at the engine start and during acceleration phases. There was a high consistency between different real driving emission (RDE) tests and good repeatability: the differences are all between 15% and 35%, so well below 50% (the current legislative limit).

The PEMs4Nano consortium participated in 47 conferences, including the major international aerosol meetings and invited presentations at the TRA2018 in Vienna and TRA2020 in Helsinki. Seven peer-reviewed articles have been published, and five more are under review. The project was highly visible, with two articles making it to the journal cover. It has won 4 prizes, including the Trojan Horse award at the ETH Particle Meeting 2019, a paper selected as one of the best papers at the SAE 2019 conference, and the Prix Jean Bricard 2020 prize. Finally, a patent on a semi-volatile mass detector was filed.
For the first time, extensive chemical analysis allowed the identification of key molecular markers to discriminate particles by source, size and engine regime. The results clearly show that the catalytic stripper removes the organic fraction from both the particles and the gas phase, which has the highest impact on the smallest particles. Key chemical classes were measured as input for the model-guided application (MGA). The MGA introduced a physico-chemical model that, for the first time, enables the simulation of particulate emissions composed of a mixture of solid particles, semi-volatile material and ash arising from different processes within the vehicle. The MGA can be applied to a wide variety of vehicles, thus enabling the results of PEMs4Nano to be scaled to other applications. The state of the art of the resulting measurement technology has been extended to include an optimized catalytic stripper. The outcomes of the project are new lab and on-board systems for the measurement of sub-23nm particles. The new systems are used in the same way as the established PN23 measurement systems, and are therefore suitable for being widely adopted. For the vehicles used in this study, the 10 nm measurement was demonstrated to be robust and reliable under current certification conditions.

The impact of the project is that (1) the PEMs4Nano experimental procedures for collecting and analyzing nanoparticles are available to be adopted internationally, (2) the prototypes are ready to be evaluated by experts and (3) the model-guided application is ready to be used to support the development of future robust emissions regulations and future engine technologies and aftertreatment systems.

In a broader perspective, these innovations will significantly contribute to reducing particle emissions from the transport sector. Reduced particle emissions will contribute to human health; improving the air quality is especially important in urban environments and for the millions of people living near major roads. Finally, PEMs4Nano contributes to employment in the emission measurement technology sector and to improved working conditions for outdoor workers, especially in cities and in the road infrastructure maintenance sector.
PEMs4Nano concept figure