Sunday, February 25, 2007

Q&A: Biodiesel at low temperatures

Rebeccalin writes:
Hi, I was wondering why viscosity is a concern when using biodiesel. Is it because it will freeze in lower temperatures and the car would not run?
Also, what can be added to biodiesel to reduce the problem of viscosity?

Hi Rebeccalin,

That's a great question!

Before we begin: viscosity is a measure of how fast a liquid flows. Something with a low viscosity (like water) flows very quickly. Something with a high viscosity (like honey or pancake syrup) flows very slowly. Also keep in mind: viscosity can change a lot with temperature. for example: cold honey barely moves at all, but if you warm it up, it flows very easily.

Viscosity of biodiesel fuel is actually pretty similar to petrodiesel fuel.

Here's how they compare at 40C (I know, it rarely reaches 40C during the winter, but bear with me, that was the only data I could find) - I threw in the values for canola oil and water for comparison, so you can see how close biodiesel really is to petrodiesel.

The reasons the petrodiesels have a range of values here because the viscosity varies depending on the different blend of petrodiesel used - (I'll discuss that in a second). Biodiesel also has varying viscosities, depending on the source of the grease used to make biodiesel. Pretty much, if you use very viscous grease, you get very viscous biodiesel.

Really, as long as biodiesel stays liquid, it should have no problem going through your diesel engine. Viscosity isn't the problem.

The real problem with using any diesel engine fuel at low temperatures isn't the viscosity - it's what is called the cloud point. The cloud point is the temperature below which little crystals form in the fuel.

If the temperature drops below the cloud point, those little crystals grow into bigger crystals and your tank and fuel lines are filled with a solid "gel" of fuel. That sounds bad, and it is bad. Gelled fuel will prevent your car from starting, and will probably damage your fuel filters. Incidentally, the viscosity of gelled (or even clouded) fuel is way off the top of the viscosity chart above.

The cloudpoint of commercially available petrodiesel is controlled by blending - it can range from -34F to +41F.

Depending on where you are and what time of year it is, the petrodiesel available commerically will be blended differently to have a cloudpoint appropriate for the weather. This is done to save money for the oil companies. The higher (or warmer) the cloudpoint is, the less expensive the fuel is.

For example, in Minnesota in January, the cloudpoint of diesel at the pump is -34F. In Louisiana during the same month, the cloudpoint will be +25F. In June, it may be as high as +41F.

100% biodiesel has a cloudpoint of +31F to +51F, depending on what kind of grease the biodiesel was made from.

Now we're beginning to see the problem with biodiesel in the winter. Unless you live in Louisiana, you might have problems using 100% biodiesel in the winter.

So, it's very cold outside, but you want to drive greeen - what can you do?
The easiest solution is to blend your biodiesel with petrodiesel. I'd suggest a B20 blend (20% biodiesel/80% petrodiesel).

With regard to winter use, the National Biodiesel board says that over seven years of testing:
"The cold flow properties of biodiesel and biodiesel blends have been thoroughly tested with a variety of diesel fuels, both with and without cold flow enhancing additives. Biodiesel blends (primarily B20) have also been used in a variety of climates—including some of the coldest weather on record—without cold flow problems."

"High concentrations of biodiesel (i.e. blends over 20%) may not be appropriate for use in cold climates without blending...with proven cold flow improvers specific to conventional diesel fuels."
There are some people who have had success using antigel additives with higher blends of biodiesel (and even 100% biodiesel). The best source of info would be your biodiesel supplier or people with lots of experience using biodiesel in cold weather.

I hope this helps!

Sources: ASTM, National Biodiesel board, Wikipedia

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