As the owner of a newer model Volkswagen Jetta TDI and a biodiesel fan, I took particular interest in concerns related to engine oil dilution in diesels using in-cylinder post-injection technology. Alarming rumors about catastrophic results from use of biodiesel blends higher than B5 have trickled through discussion forums and industry gossip.

Like many biodiesel myths, rumors about the incompatibility of biodiesel blends with new light-duty diesel vehicles' emission control technologies are, for the most part, inaccurate. Of course, B5 blends are virtually indistinguishable from diesel fuel in terms of engine performance and compatibility with emission control systems, making all diesel engines easily compatible with blends up to at least B5.

Reports about incompatibility stem from concerns about the possibility of increased engine oil dilution that can come into play with the use of biodiesel blends higher than B10 in some of the new light-duty diesel vehicles that utilize an emissions control system with in-cylinder post-injection. This system utilizes a late in-cylinder injection of raw fuel to burn off the material collected on particulate traps required by U.S. EPA to meet stringent, new emissions standards for particulate matter. The design is predominantly limited to the light-duty diesel product offerings from Volkswagen, Audi, Mercedes and BMW. While they are a small portion of the total U.S. diesel market, they are among the best known light-duty diesel passenger options in the states-my diesel Jetta included.

So I was pleased to learn that avoiding operation concerns with in-cylinder post-injection technology using higher biodiesel blends could be as easy as implementing more frequent oil changes. The fact is, some amount of oil dilution in diesel engines will occur over time with normal operation even with regular diesel fuel, leading to oil degradation and the need for oil replacement. But with use of in-cylinder post-injection, the oil dilution impacts can increase whether fueled with a biodiesel blend or petroleum diesel alone. The late injection events increase the opportunity for unburned fuel to reach the cylinder walls and, in turn, enter the lubricating oil. With conventional diesel fuel, some of it can boil out of the lube oil, lessening the long-term dilution effect. But it still occurs. This effect, however, is accentuated with biodiesel because of its higher boiling point relative to petroleum diesel, which can lead to more fuel being retained in the lube oil. The increase in oil dilution can be easily addressed with an oil change and can be further mitigated through a proactive engine oil change service interval (e.g., consider an oil change at 7,500 miles instead of 10,000 miles).

Volkswagen's concern lies in the fact that their normal 10,000-mile service interval, which is longer than the industry standard, may allow more than the desired amount of engine oil dilution to occur. However, recent research studies by Volkswagen as well as the National Renewable Energy Lab both concluded that, despite the slightly higher dilution levels with biodiesel blends above B10, the absolute level of viscosities still remained in an uncritical range for the applied oil quality, and there were no negative impacts on vehicle emissions, engine performance, or parts wear as a result of the biodiesel use. Volkswagen and NREL are also planning further studies on this issue. Other light-, medium- and heavy-duty diesel engine manufactures do not generally use late in-cylinder injection of raw fuel and have not reported problems with B20 compatibility, or excessive engine oil dilution in their new diesel models. They have opted for systems that utilize an exhaust-stream injection of fuel to regenerate the particulate traps, therefore mitigating the risk of engine oil dilution.

Volkswagen's warranty position on biodiesel remains in support of blends up to B5, although it is currently conducting further research on higher biodiesel blends. However, the bottom line is that with more frequent oil changes, which would still be a longer mileage span than any gas guzzler, and the responsible use of high-quality biodiesel blends meeting ASTM specifications, users can still fuel up confidently with blends up to B20 without concern for engine performance.

Look for a white paper, on "2010 Diesel Technology and Biodiesel Compatibility," coming soon and available for NBB members a www.biodiesel.org.

Jennifer Weaver with ASG Renaissance is the OEM Outreach & Education Program Manager for NBB