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Study: Lube Oil is an Important Source of Toxic Particle Emissions, Even in Cleaner Burning Engines

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.

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Comments

Richard C Burton

Having been at motorcycle races, where vintage suped up 2 strokes were fouling the air with the strong smell of oil, gets me wondering; just how much, and what type of pollution they are (still)putting out...

Aussie

2-stroke fuel can be made by mixing a small amount of biodiesel (maybe 5%) or perhaps ULSD with petrol. It seems to give the engine a smoother idle perhaps since it is both a lubricant and a fuel. Tests would be needed to find the optimum ratio.

Rafael Seidl

Oil consumption can be minimized by using an all-aluminium block with silicate inlay casting instead of a cast iron piston liner. VW and Audi do this on a number of their engines. Using the same material reduces the clearance between piston/piston rings and liner when the block is still cold.

In addition, the profile of the piston rings should be optimized for rapid initial wear so the engine is broken in faster. The third requirement for minimal oil consumption is a contoured head gasket that spreads the forces from the cylinder bolts as evenly as possible, to minimize clover-leaf distortion of the liner cross-section. Finally, the crankcase ventilation system must feature an effective cyclone to remove as much of the oil mist as possible.

Diesel vehicles with DPFs must use special oil grades without any ash-forming addditives, which would permanently clog up the filter.

coal_burner

I think VW may have pretty much solved this problem With their newer TDIs. My '04 Golf has 47,000 miles on it, my last oil change was 13,000 miles ago, and there is absolutely no noticeable difference in the oil level from the last time I changed the oil.

Harvey D

Another good reason to put an end to ICE vehicules-machines including diesels. Electric motors do not have this problem.

Luke

Another good reason to put an end to ICE vehicules-machines including diesels. Electric motors do not have this problem.

Harvey D - good luck with that.

Batteries have the potential to be an environmental disaster on their own and, even with the advanced development that is taking place for the next round of hybrids, the energy density is nowhere near that of a conventional fuel. Building lots of electrical plants has obvious environmental consequences, even if done well. Buring fuel in vehicles using fuel cells still basically burns the fuel, even if it's done in a fuel cell -- there is a lot of heat, there are emissions, and I can't afford a fuel cell. Also, since there are no hydrogen mines on earth, hydrogen fuel cell systems are basically an expensive battery technology. Good luck with that.

If you can solve any of these problems, engineers, citizens, and even politicians the world over will deem you a hero - I know I would. But the proof is in the labratory test-rig and in the production unit, and a lot of really smart people (many of whom are educated by the university I work for) are working on this problem right now.

Roger Pham

Coalburner,
My observation is similar to your. After 5000 miles on my Prius, there was no noticeable change in oil level.

On older cars, oil consumption can be much alleviated by the use of thicker-grade oil and use oil designed for high-mileage vehicles.

George

It seems like gasoline engines should have the same problem with lubricant derived particulates. Anyone know if that's the case? I could imagine that sealing a diesel piston might be harder because it's under more stress than a gas piston. I had the same thought as Harvey- ICEs are tough to make clean over the lifetime of the engine. And Luke, duh, we know EVs are not quite there yet. I don't think batteries are going to be an "environmental disaster". They can be sanely recycled, and pollution due to their manufacture is wildly overblown. These environmental issues can be managed. ICEs are fundamentally unclean. In a couple decades I suspect that LDVs will be largely electric drive, our air will be cleaner and people will be healthier.

Harvey D

Luke:

Most batteries can be recycled over and over again.

Extra electrical energy required could be produced from clean sources such as Wind, Sun, Geothermal, Hydro, Waves, Nuclear etc.

Eventually, we may not have much of a choice to switch to electric power for ground transport vehicles. It is by far the cleanest solution. Cost will come down when batteries + motors + controls are mass produced.

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