I ran across an interesting article in Popular Mechanics about some issues that pure biodiesel might have in some new diesel engines.

Dave Hubbard, who follows developments in biodiesel and even makes the green fuel himself, writes that changes in environmental laws have actually made formerly B100-compatible engines unable to burn the pure form of the biofuel:

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 NOX 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. (Squirting fuel down the exhaust? Gee, I wonder why the newer models have poorer fuel mileage.)

And here is where the pitfall lies for biodiesel users like myself. Most of the manufactures decided to inject fuel into the cylinders just after the cylinder fires and the exhaust valve opens. At this point, the fuel vaporizes and the vapors move down the exhaust to the DPF and clean it. Because biodiesel is denser than conventional diesel fuel (it has a longer hydrocarbon chain) and has a higher distillation temperature and boiling point, it does not vaporize as easily. Some of the fuel ends up adhering to the cylinder wall and runs past the rings, diluting engine oil.

Hubbard goes on to say that there’s been lots of debate about why the car makers moved to this type of injection, and of course, now, it would cost too much to redesign to make it more B100 friendly. But he does point out that there are still some Caterpillar and Cummins engines that use technology that make them biodiesel-compatible.

Just thought you’d want to know in case you were buying a new car with the intent of running pure biodiesel. Maybe that 1980’s VW Rabbit diesel is looking better every day!