For those interested in the future of biodiesel, particularly higher percentage blends in newer DPF equipped diesel engines, a recent Popular Mechanics article offers some interesting information. For now the best advice for new 'clean diesel' TDIs disappointingly seems to be to stick with Volkswagen's B5 recommendation (although I know owners comfortably running B10 & B20 without problem to date).

Until two years ago, all diesel engines were B100-compatible (biodiesel cannot run in gasoline engines because it needs an engine that ignites by compression). Then standards set by both the Environment Protection Agency and California Air Resources Board, phased in for 2007, required all passenger vehicles to meet the same, stricter emissions. That meant diesel manufacturers had to reduce emissions of NO_X and particulate matter to meet those of gas-powered cars. These standards were created with good intentions—to look out for our health by improving the air that we breath. (After all, particulate matter is a known carcinogen.) But the way most manufacturers did this created a setback for those of us trying to use biofuels.

To get rid of particulate matter, the diesel manufacturers came up with what's called a DPF (diesel particulate filter). But this catalytic filter becomes poisoned if sulfur dioxide is pumped through it. So as of 2007, the standard for diesel fuel was revamped as well, and fuel refineries had to reduce sulfur content to no more than 15 parts per million (now known as ultralow-sulfur diesel). The DPF is placed in the exhaust system in front of the muffler and looks like a catalytic converter used on gasoline engines. It captures particulate matter in its inner core. Periodically, the DPF has to be taken up to high temperatures to burn off the matter it has collected. This is called regeneration or postinjection regeneration. The idea is to inject fuel into the exhaust that has been vaporized, and when the fuel comes into contact with the DPF, an exothermic reaction heats it up and incinerates the plug of soot.

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