Particulate matter formation in spark-ignition engines

Abstract

Recent health concerns over airborne particulate matter (PM) have prompted examination of the mechanisms by which PM is formed in spark ignition (SI) internal combustion engines. A study was undertaken in order to understand the effects of dilution on measured PM, to examine and model the effect of steady state engine operating conditions on engine-out PM, and to characterize the effect of transient engine conditions on particle growth and dynamics. Particle dynamics in diluted SI and compression ignition (Cl) engine exhaust are examined and discussed in the context of SI exhaust dilution. Temperature measurements in the exhaust pipe and dilution tunnel reveal the degree of mixing between exhaust and dilution air, the effect of flowrate on heat transfer from undiluted and diluted exhaust to the environment, and the minimum permissible dilution ratio for a maximum sample temperature of 52°C. Measurements of PM concentrations as a function of dilution ratio, using a Scanning Mobility Particle Sizer (SMPS), show the competing effects of temperature and particle/vapor concentrations on particle growth dynamics, which result in a range of dilution ratios - from 13 to 18 - where the effect of dilution ratio, independent of flowrate, is kept to a minimum and is therefore optimal in order to achieve repeatable PM concentration measurements. Particle dynamics in transit through the dilution tunnel are measured and compared to previous research. PM emissions are strongly affected by steady state engine parameters that affect global and local air/fuel ratios, the concentration of liquid fuel in the cylinder, and the availability of soot precursors. PM emissions vary by up to six orders of magnitude between the fuels tested, when at the same fuel/air equivalence ratio. Minimum PM concentrations are emitted at a global fuel/air ratio within 10% of stoichiometric, with the exact value depending on the particular fuel, and concentrations can increase by more than three orders of magnitude when the fuel/air ratio is either increased or decreased 30% from stoichiometric. Burning liquid fuel is a significant source of PM, as evidenced by the fact that open valve fuel injection increases PM emissions by up to three orders of magnitude relative to closed valve injection. Coolant and oil temperatures, spark timing, 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. Particles derived from oil consumption were found to be between zero and 40% of the total PM concentration for the oils used in the present experiments. Differences in PM emissions with and without the catalytic converter are not statistically significant. Particulate number and mass concentrations plus particle sizes are addressed in the present paper, as is the correlation between PM and emissions of gaseous pollutants - hydrocarbons (HCs), oxides of nitrogen (NOx), oxides of carbon (CO and CO2) - as well as oxygen and characteristic temperatures and pressures during the engine cycle. A model of PM formation via homogeneous- and heterogeneous-phase reactions, growth via condensation and adsorption/absorption of vapors, and diminution via oxidation explains the observed behavior of PM emissions with respect to each of the engine, fuel, and dilution parameters above. PM emissions during transient engine operation are generally a first-order time response with characteristic times similar to those involved in the fuel evaporation process, suggesting that PM emissions respond to instantaneous engine conditions and may be modeled using a quasi-steady state application of the model.