A combined experimental and modeling effort was performed in order to understand how particulate matter (PM) is formed in spark-ignition (SI) internal combustion engines. Parameters that affect global and local air/fuel ratios strongly affect PM.Minimum PM number and mass concentrations are emitted at a global air/fuel ratio within 10 of stoichiometric, and concentrations increase by as many as 3 orders of magnitude when the air/fuel ratio is either increased or decreased 30 fromstoichiometric. Burning liquid fuel is a significant source of PM, as evidenced by the fact that open valve fuel injection increases PM concentrations by up to 3 orders of magnitude relative to closed valve injection. Coolant and oil temperatures, sparktiming, and exhaust gas recirculation (EGR) affect PM through their effect on intake port and cylinder temperatures as well as through the effect on the availability of liquid fuel in the cylinder. Particle sizes as a function of engine operatingconditions are discussed.
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