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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%).

Gas-operated vehicles powered by natural gas (NG) or other gaseous fuels such as liquefied petroleum gas (LPG), are seen as a possible option for curbing CO₂ emissions, fuel consumption and operating costs of goods and passenger transport. Initiatives have been adopted by various public organizations in Europe and abroad in order to introduce gas-fueled vehicles in their fleets or use retrofit fueling systems in existing ones. In this study a retrofit dual fuel (diesel-gas) fuelling system was investigated as a potential candidate technology for city bus fleets. The system is marketed under the commercial name d-gid. It is a platform developed by the company Ecomotive Solutions for the control and management of a diesel engine fuelled with a mixture of gaseous fuels. In order to assess its environmental and cost effectiveness the system was tested on a Volvo city bus. The tests were performed on an HDV chassis dyno under various driving conditions.