Nucleation mode particle evolution in a gasoline direct injection engine with/without a three-way catalyst converter

For gasoline direct injection (GDI) engines, as the exhaust gas temperature decreases, the hydrocarbon-to-particle conversion may occur, generating nucleation mode particles. A three-way catalyst converter (TWC) may have effects on the particle evolution as exhaust temperature decreases. In the present study, the effects of TWCs on particulate matter emissions and hydrocarbon-to-particle conversion process under different exhaust gas temperatures are researched. The particle size distributions are compared between the pre-TWC (upstream of the TWC) and post-TWC (downstream of the TWC) positions. Results show that the reduction of total particle number concentrations is 40-80% after exhaust gas passes the TWC, which is higher at higher load. The TWC has higher reduction efficiencies for particles in smaller size ranges. In the size range of 4-8 nm, the reduction efficiencies for particles are more than 96%. For particles larger than 50 nm, the TWC doesn't show remarkable effects on particle number (PN) concentrations. The particle evolution and the hydrocarbon-to-particle conversion with regard to the exhaust gas temperatures are also investigated. At the post-TWC position, the PN concentrations of nucleation mode are much lower compared with those of accumulation mode, and not sensitive to exhaust temperature variations. The PN concentrations of nucleation mode at the pre-TWC position with the sampling temperature of 170 degrees C are slightly higher than those at the post-TWC position. PN concentrations of nucleation mode at the pre-TWC position increase sharply when the sampling temperature decreases from 110 degrees C to 50 degrees C. It is suggested that TWCs don't convert the nucleation mode particles directly, but remove those semi-volatile hydrocarbon components as precursors of nucleation mode particles forming in low exhaust temperatures.