The sources of polycyclic aromatic compounds in diesel engine emissions

The application of a radiotracer technique to investigate the sources of polycyclic aromatic hydrocarbons (PAH) in diesel exhaust emissions is described. In separate experiments, ¹⁴C radiolabelled naphthalene, fluorene, fluoranthene and pyrene were each added to diesel fuel which was combusted in a 2L direct injection Perkins Prima diesel engine run at steady state under a series of speed (1000-3200 rpm) and load (20-80 N m) conditions. Each PAH was recovered in the exhaust emissions, using a novel sampling system, and its specific activity used to reveal the extent of its survival and pyrosynthesis of the PAH from other fuel fragments. Survival of the PAH varied from below detection limit for fluoranthene at low speed and load to 1.25% for fluorene at high speed and low load. For mid-speed and mid-load conditions, a linear relationship was observed between the extent to which individual PAH survived combustion and the energy level of the lowest unoccupied molecular orbital (LUMO) of the molecule calculated from Hückel molecular orbital theory. LUMO is an indirect parameter related to gas phase oxidation rates of PAH. Linearity was not so apparent at other speeds and loads. Resynthesis of the PAH varied from less than 0.1% for pyrene at mid-speed and mid-load to 4.93% for naphthalene at high-speed and high-load. Probable mechanisms for the production of some of the pyrosynthetic PAH were discussed.