Review on engine exhaust sub-23 nm solid particles
The Particle Number (PN) standard was introduced for Diesel Light Duty vehicles in September 2011 (Euro 5b) and is limiting the number of non-volatile particles emitted over the NEDC to 6x1011 p/km. The new standard effectively necessitated the installation of high-efficient wall-flow DPFs in all diesel vehicles. The same standard will be introduced to gasoline vehicles utilizing direct injection (G-DIs) at Euro 6 stage (September 2014) initially with a limit of 6x1012 p/km and from September 2017 with 6x1011 p/km. This 3 year delay was decided in order to allow the manufacturers sufficient... time to investigate the possibility of compliance through improvements in the combustion process without the need of Gasoline Particulate Filer (GPF). The PN standard will also be introduced for heavy-duty engines in 2014 (Euro VI). The PN standard was based on investigations of diesel vehicles and concerns were raised regarding its suitability for gasoline engines. In the current legislation particle number (PN) limits for solid particles >23 nm are prescribed. However it is feared that gasoline engines have smaller particles and thus the current standard cannot capture them. Target of this report was to investigate whether it is necessary and possible to measure <23 nm particles. The main conclusions based on a literature survey and experimental investigations at JRC are: 1) Engines emit solid sub-23 nm particles. The average percentage over a cycle (WLTP) is similar or higher for G-DIs (40%) compared to diesel engines (20%). These percentages could be higher if the losses in the PN measurement systems would be taken into account. These percentages are relatively low considering the emission limit levels (6x1011 p/km) and the repeatability (10-20%) and reproducibility of the method (50%). These percentages are close to the percentages expected theoretically not to be counted due to the 23 nm cut-off size (5-15%). High emissions can be found when additives are added in the fuel or lubricant. 2) It is technically feasible to decrease the lower size approximately to 7-10 nm without major changes of the PN measurement systems. 3) The volatiles are not always removed efficiently in the PN measurement systems. The major issue is re-nucleation of sulfuric acid downstream of the evaporation tube. These particles typically do not grow at sizes above 23 nm. 4) There are indications of formation of 10 nm solid particles from hydrocarbons and sulfuric acid in the PN systems, but this needs further investigation. 5) The >10 nm measurement uncertainty due to differences between commercial systems will remain at the same levels when no separate solid nucleation mode exists.