Comparing emissions

Testing showed that emissions from the bio-fuels 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 (ppm) and B100 put out nine parts. 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 both B100 and cooking oil produced less than the B5 and standard diesel.

NOx

Cooking oil had the lowest NOx 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. However, advocates claim that the CO2 from burning biofuels is absorbed by growing the plants the fuel is made from. Critics, however, dispute this claim.