Effects of Piston Wetting on Size and Mass of Particulate Matter Emissions in a DISI Engine 2002-01-1140
We have examined the influence of piston wetting on the size distribution and mass of particulate matter (PM) emissions in a SI engine using several different fuels. Piston wetting was isolated as a source of PM emissions by injecting known amounts of liquid fuel onto the piston top using an injector probe. The engine was run predominantly on propane with approximately 10% of the fuel injected as liquid onto the piston. The liquid fuels were chosen to examine the effects of fuel volatility and molecular structure on the PM emissions.
A nephelometer was used to characterize the PM emissions. Mass measurements from the nephelometer were compared with gravimetric filter measurements, and particulate size measurements were compared with scanning electron microscope (SEM) photos of particulates captured on filters. The engine was run at 1500 rpm at the Ford world-wide mapping point with an overall equivalence ratio of 0.9. The liquid fuels examined were California Phase II gasoline, n-pentane, iso-octane, toluene, and n-undecane.
With the injection of liquid fuel onto the piston particulate matter mass emissions increased greatly over operation on propane alone and over operation where a similar fraction of the liquid fuel was port injected. Toluene and n-pentane had the highest mass emissions reflecting the effects of structure and volatility. They also had the largest mean particulate size. The particulates had mean scattering diameters in the range of 500 to 1000 nm. Time-resolved measurements taken at 2 sec. intervals showed the temporal variation of PM characteristics.
Particulate mass emissions correlated well with prior measurements of unburned hydrocarbon emissions taken under the same conditions (e.g., due to piston wetting).