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. The fact is, 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. The rumors about incompatibility, however, stem from concerns about the possibility of increased engine oil dilution that can come into play with the use of higher biodiesel blends (B10 and higher) 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 to meet stringent new emissions standards for particulate matter. This is predominantly limited to the light duty diesel product offerings from Volkswagen, Audi, Mercedes and BMW, which are a small portion of the U.S. market. The other light, medium and heavy duty diesel engine manufacturers 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.

While recent research studies by Volkswagen in Germany as well as the National Renewable Energy Lab did conclude that use of biodiesel blends above B10 could lead to slightly higher engine oil dilution levels in diesels using in-cylinder post-injection systems, both studies also found that 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. 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 instead of 10,000 miles). VW and NREL are also planning further studies on this issue.

GM’s next-generation heavy-duty Duramax diesel V-8 won’t just burn cleaner to meet tough new emissions standards for 2010; it will also burn greener — fuel that is. The so-called LML Duramax will be certified to run on biodiesel blends of up to B20, which is 80 percent ultra-low-sulfur diesel and 20 percent biodiesel, in GM’s 2011 model year 2500 and 3500 Silverado and Sierra pickups.

The move finally matches the B20 capability of the current 2007-09 Cummins 6.7-liter inline-six that powers the Dodge Ram HD lineup. The 2007 Dodge Ram 3500 was the only pickup in our last Heavy Duty Shootout that was able to burn B20. B20 is available at many truck stops today, but the current 2007-10 LMM Duramax and Ford’s 2008-10 6.4-liter Power Stroke V-8 are only approved for B5 biodiesel.