Engine Exhaust Solid Sub-23 nm Particles: I. Literature Survey

In the current diesel vehicle exhaust emissions legislation Particle Number (PN) limits for solid particles >23 nm are prescribed. The legislation was extended to include Gasoline Direct Injection (G-DI) vehicles since September 2014. Target of this paper was to investigate whether smaller than 23 nm solid particles are emitted from engines in considerable concentration focusing on G-DI engines. The literature survey and the experimental investigation of >15 vehicles showed that engines emit solid sub-23 nm particles. The average percentage over a test cycle for G-DIs (30-40%) is similar to diesel engines. These percentages are relatively low considering the emission limit levels (6×1011 p/km) and the repeatability (10-20%) of the particle number method. These percentages are slightly higher compared to the percentages expected theoretically not to be counted due to the 23 nm cut-off size (5-15%). Higher fraction can be measured with high ethanol content fuels, at sub-zero ambient temperatures and when additives are added in the fuel or lubricant. Based on the information today, the PN legislation should remain the same. However measurements of particles <23 nm should go on in order to monitor the sub-23 nm fraction of current and future engine technologies.