Mechanism of particulate matter formation in spark-ignition engines. 3. Model of PM formation

A combined experimental and modeling effort was; performed in order to unde rstand how particulate matter (PM) is formed in spark-ignition (SI) interna l combustion engines. Application of the model allows quantification of the amount of PM nucleated at sites of burning liquid fuel (either droplets or pools) and in homogeneous gasphase chemical reactions. Moreover, it quanti fies PM growth by condensation and absorption/adsorption of hydrocarbon (HC ) vapors as well as PM reduction by oxidation both in the cylinder and in t he exhaust pipe. Model parameters, fit: by comparison to experimental data, show the strong dependence of PM formation on the presence of liquid fuel in the cylinder and HCs in the exhaust, temperatures in the intake port and cylinder, air/fuel ratio, and propensity of the fuel molecule to break dow n into soot precursors. PM emissions calculated by the 13-parameter model c ompare to 84 experimental data sets with an R-2 correlation coefficient of 0.81, demonstrating goad correlation. The effects of individual engine, fue l, and dilution parameters an modeled PM emissions are discussed, elucidati ng the mechanisms causing experimentally measured trends in PM emissions as a function of the respective parameters.