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Diesel vs. biodiesel vs. vegetable oil

We test homegrown fuel alternatives to gasoline

Last updated: April 2014

With gasoline prices volatile and the Obama administration committed to easing the United States' addiction to oil, Americans seem to be taking more interest in alternative fuels, including those derived from farm crops and other renewable organic sources. Among the most widely available are biodiesel and vegetable oil, both of which can be used to power a diesel engine.

Developed from vegetable or animal fats, biodiesel is functionally identical to petroleum diesel. Adherents claim it pollutes much less than regular diesel.

Biodiesel is most commonly sold in blends with normal diesel; B5, which is 5 percent biodiesel and 95-percent petroleum diesel, and B20, or 20 percent bio diesel. B20 sells for about 20 cents a gallon more than petroleum diesel according to the U.S. Department of Energy. Pure biodiesel (B100) sells for aboiut 85 cents more per gallon than regular diesel.  

A relative of biodiesel is plain, edible cooking oil. But because it's not financially practical to fuel a car with cooking oil from grocery store shelves— a gallon costs about $8—some people are modifying diesel engines to run on the used deep-fryer oil that restaurants often throw away. Discarded oil is sometimes available free, though more restaurants are now charging for it.

To see how biodiesel—B5 and B100—and fryer grease compare with conventional petroleum diesel fuel, we converted a diesel-powered 2002 Volkswagen Jetta TDI so that it could operate on all three. We found that they all allowed the car to perform well but differed in price and convenience.

We experienced the best overall results using B5, the 5 percent biodiesel blend. It provided the best balance of performance, emissions, fuel economy, and convenience. B5 will run in any diesel engine without requiring vehicle modifications, and it is pumped into the tank just like any standard fuel. But because it is 95 percent petroleum diesel, it does little to wean drivers from fossil fuels.

Our Jetta ran well on the used cooking oil, but the inconvenience of finding fuel sources and preparing the oil for use in the engine limits its appeal and offsets its low price.

Automakers are now beginning to warranty new diesel cars on biodiesel blends of up to 20 percent. At concentrations higher than that, or on cooking oil, engineers say they find too many impurities and inconsistencies in the fuel to be comfortable providing warranty coverage.

What we found  

As the chart below shows, fuel economy and acceleration were similar with all four fuels. The B5 blend turned in the best fuel-economy results. On the highway, it yielded almost 49 mpg while the regular diesel and B100 achieved about 45 and 44 mpg, respectively. The cooking oil recorded just 42 mpg.

The difference in fuel consumption over 15,000 miles is only 47 gallons. But even though the cooking oil was free, it required a big time investment to collect and filter.

Using regular diesel, our 2002 Jetta TDI clocked 15 seconds to reach 60 mph from a standing start. Running pure cooking oil returned near-identical times, but using B5 shaved off 0.8 seconds for a time of 14.2. Even that difference probably wouldn't be noticeable for a normal driver, but compared to other conventional cars, that's still slow.


Comparing emissions

Our testing showed that emissions from the biofuels were the same or better than from regular diesel by most measures. None of the four fuels generated significant amounts of carbon monoxide. Cooking oil produced less smog-causing NOx than regular diesel, while our B100 produced a little more.

Hydrocarbons are related to smog formation. Cooking oil and B100 turned out to produce slightly more hydrocarbons than either regular diesel or B5. HC emissions reflecting unburned fuel and cooking oil racked up 14 parts per million and B100 put out nine ppm. B5 and regular diesel produced only 3 ppm each. Since the Connecticut limit on HC is 150 ppm, all far exceeded the requirements.

Particulates. Particulates are a concern with diesel engines. In Connecticut and some other states, particulate content (aka soot) is measured by the opacity (cloudiness) of the exhaust smoke. It's expressed as a percentage, and the Connecticut state limit is 20 percent. All four fuels had a much lower percentage, though B100 and cooking oil produced less than the B5 and standard diesel.

NOx. Cooking oil had the lowest oxides of nitrogen emissions, while B100 had the highest.

CO2. Carbon dioxide is considered a major contributor to global warming, and CO2 emissions are the same no matter what liquid fuel an engine burns: about 19 pounds of CO2 for every gallon of fuel. But advocates claim that the CO2 from burning biofuels is offset by the plants absorbing carbon as they grow. Critics dispute that claim.

  Diesel Cooking oil B100 B5
Unburned hydrocarbons, ppm 3 14 9 3
Carbon monoxide, % 0 0.01 0 0
NOx, ppm 237 214 248 240
Particulates (opacity %) 2.9 1.6 1.1 2.4
City, mpg 20.1 20.8 22.3 22.6
Highway, mpg 44.9 42.1 44.2 48.5
0-30 mph, sec. 4.5 4.5 4.5 4.5
0-60 mph, sec. 15 14.9 14.5 14.2
1/4-mile, mph 69.9 70.3 71.3 72.2

CR's take

The Jetta ran well on vegetable oil. But you have to switch to diesel before you shut the engine down every time to purge impurities from the fuel system, which is a nuisance. And if you forget and leave the switch on after shutdown, the cooking oil tank will fill with diesel fuel.

When running on cooking oil, the tailpipe exuded a familiar fried-food scent.

B100 is a fully renewable fuel and provided similar fuel economy and performance to petroleum diesel, but it typically carries a hefty price premium. For now, there is no financial incentive to use B100. Also, it is unlikely that manufactures will warranty B100 in their engines, though most will cover up to B20.

If you have access to biodiesel, running a moderate biodiesel blend in your diesel engine may provide slightly better fuel economy and acceleration than conventional diesel, while costing about the same.

Used cooking oil is another fully renewable fuel, but it's a viable alternative only for those up to the challenge. Operating a grease car requires sacrificing trunk space to an auxiliary tank, arranging your own fuel sources, picking up, filtering, and storing the fuel, starting the fuel system with regular diesel after every time you drive, and cleansing the system before shutting down.

How we tested

To modify the Jetta to run on used cooking oil, we bought a conversion kit from Greasecar Vegetable Fuel Systems ( It included a 13-gallon auxiliary fuel tank for the cooking oil, hoses and fittings for rerouting the engine coolant to warm the veggie-oil tank, extra fuel lines, a special inline filter, electrical components, and switches. We supplied some missing wire needed to correctly complete the installation. The latest kits from Greasecar cost $1,550.

The involving installation is best suited for proficient do-it-yourselfers and professionals. Because the auxiliary fuel tank is mounted in the spare-tire well, displacing the spare, we had to drill holes in the trunk. Properly routing and connecting the new fuel lines, heater hoses, undercar wiring, and dashboard switches required hands-on skill. One of our certified mechanics took about 10 hours to complete the job, using our well-equipped workshop.

We also installed two more 5-gallon auxiliary tanks so we could test multiple fuels. One of the extra tanks held B100 while the other contained B5. Including the petroleum diesel in the main tank, we had four different fuels on board for comparison tests.

We made arrangements to pick up used fryer oil from a couple of local restaurants free. Collecting, filtering, storing, and fueling became a much greater chore than performing a simple gas-station fill-up. We collected it in covered pails and filtered it through a large paper paint filter to remove any food residue, which was a messy chore. We then had to use a funnel to pour the oil into in the tank in the car's trunk.

Our instrumented tests measured performance and pollution. We conducted our standard tests for real-world fuel economy and acceleration, measuring 0-30 mph, 0-60 mph, and quarter-mile times. To measure tailpipe emissions, we contracted with a Connecticut emissions test station. That center measured the output of Hydrocarbon, carbon monoxide, oxides of nitrogen, and particulates.

Backyard biodiesel

Vegetable oil is often confused with biodiesel because some people manufacture biodiesel at home. And like home winemaking, to make an acceptable product takes care, skill, and some specialized equipment.

To "crack" the cooking-oil triglycerides into an acceptable diesel fuel, the most common home process uses methanol as a solvent and lye as a catalyst. But lye and methanol are both highly toxic and should be used with great care. The main byproduct of the refining process is glycerin, which you can compost or use to make your own soap.

One of the concerns with homemade biodiesel, and indeed with any biodiesel, is that if it can still contain methanol traces. Methanol is a solvent that attacks rubber aggressively, so it can eat its way through any rubber hoses or seals in a car's fuel system. Fortunately, most modern diesels already use nonrubber synthetic fuel-system parts. Otherwise, those parts can be replaced with readily available synthetic materials that are immune to methanol's predations.

The proportions of catalyst and solvent needed to make biodiesel will vary depending on the source of the feedstock used, so the recipe will differ with every batch. The whole process of mixing, pouring, settling, and then separating the liquids and solids produced can take anywhere from a day up to a week. For that reason we conclude that moonshine biodiesel is best left to a dedicated hobbyist with experience and a focus on safety.

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