Most research investigating diesel-fueled vehicles as important sources of air pollution that can increase the risk of asthma, bronchitis, and other health problems has focused on diesel soot, rather than emissions produced by lubrication oil.
However, lubrication oil appears to be an important yet little-recognized source of toxic particle emissions from internal combustion engines, according to a joint study by government and academic researchers in Washington State and Minnesota. Their study is published in the 1 Oct. issue of the journal Environmental Science & Technology.
Under normal diesel engine operation, trace metals are vaporized and adsorbed or condensed onto the surface of soot particles. The origin of the metals may be from trace metals in the fuel or from the metallic fuel additives in the fuel used for diesel particulate filter regeneration. However, more typically, the metals in exhaust particles originate from lubrication oil that is spread onto the cylinder walls by the piston rings or that enters the combustion chamber via reverse blow-by of the rings. This is evidenced by data showing measurable levels of metallic lube oil additives in bulk diesel particulate matter (DPM) samples.
Other recent data demonstrate a correlation between the mass of calcium and organic carbon (OC) in bulk particulate samples because of their common origin in the lubrication oil. The latter is consistent with previous diesel particulate data showing that OC is derived from hydrocarbons associated with lubrication oil. In diesel engines, the combustion of lubrication oil contributes to particle formation by increasing the amount of semivolatile hydrocarbon species available for nucleation upon exiting the tailpipe. In addition, the (metallic) ash residues combine with soot particles, and in some cases where the metal-to-carbon ratio is high, metal vapors self-nucleate inside of the engine to form a population of metal-rich nanoparticles.
The primary goal of this study was to investigate the formation of combustion-generated particles derived from lubrication oil in the absence of fuel-produced soot. Arthur L. Miller and colleagues modified a truck diesel engine to run on hydrogen instead of diesel fuel, allowing the researchers to focus solely on particle emissions from lubrication oil.
They found that the hydrogen-powered engine emitted higher levels of metal-rich particles than the diesel-fueled engine. Lubrication oil was the primary source of these increased emissions. Emission particles identified include calcium, phosphorous, zinc, magnesium, and iron nanoparticles, all of which have the potential to cause lung damage when inhaled over long periods, they say.
Results of this work imply that new engine technologies that reduce soot levels in the engine or include trace metals in the fuel, may increase both the metal-to-carbon ratio of emitted particles and the generation of metal-rich nanoparticles via self-nucleation. The source of metals for such particle generation could be from fuel impurities, lubrication oil consumption, metals derived from engine wear, or metal-containing fuel additives. Because of their small size and thus efficient lung penetration, the generation of such metallic nanoparticles may introduce health concerns.
Miller, A.L., Stipe, C.B., Habjan, M.C., and Ahlstrand, G.G. “Role of Lubrication Oil in Particulate Emissions from a Hydrogen-Powered Internal Combustion Engine” Environ. Sci. Technol., 41, 19, 6828 - 6835, 2007, 10.1021/es070999r