Researchers at Purdue University have developed a new advanced closed-loop control system which prevents biodiesel fuel blends from producing more nitrogen oxides that conventional diesel fuels. Nitrogen oxides are the component of burnt diesel fuel that contributes to smog. According to the researchers, vehicles burning biodiesel can produce as much as 40% more nitrogen oxides at some operating conditions and fuel economy declines by as much as 20% compared to conventional diesel fuel.

The reason biodiesel produces more nitrogen oxides during the combustion process is because the fuel has oxygen in it whereas conventional diesel fuel doesn't. One key innovation that the new engine control system uses to help reduce nitrogen oxides produced when burning biodiesel is exhaust gas recirculation. The technique reroutes a portion of the exhaust gases back into the cylinders of the engine to reduce the amount of emissions that are released. Using the exhaust gas recycling method the researchers were able to reduce the amount of nitrogen oxides produced and increase fuel economy.

"We were able to improve the fuel economy with a biodiesel blend while reducing nitrogen oxides to where they were with conventional diesel," Gregory Shaver, assistant professor of mechanical engineering said. "At the same time, we were able to maintain the customary biodiesel reductions in particulate matter emissions compared to ordinary diesel fuel while not increasing noise emissions."

Using the closed-loop control system and exhaust gas recirculation the team was able to reduce the nitrogen oxide emissions produced by the vehicles burning biodiesel to the same level produced by vehicle burning conventional diesel. However, while the new techniques did improve the fuel economy of biodiesel vehicles, the fuel economy is still not as great as that of traditional diesel fuel.

"This means you get lower mileage for biodiesel compared to ordinary diesel fuel," Shaver said. "We improved the combustion efficiency and were able to get better mileage than before, but still not as good as conventional diesel fuel."

The reason that fuel economy with biodiesel is lower than that of conventional diesel is because biodiesel has 10 to 12% lower energy density than conventional diesel when burned during the combustion process. The closed-loop control technique uses advanced models to adjust the engine automatically as it senses what blend of fuel I being used in the engine. The software in the control system adjusts fuel injection timing and the air-to-fuel ratio along with the amount of exhaust gasses returned to the cylinders depending on the type of fuel being used in the vehicle.

"You need to be able to estimate what the blend ratio is so you know what's going on in the engine," Shaver said. "Is it 20 percent biodiesel fuel mixed with 80 percent regular fuel? Then we can do something to reduce the nitrogen oxides to levels consistent with a conventional fuel that didn't have oxygen in it."

The technology needed to implement the new control system into existing diesel vehicles is already here. Shaver says that it only adds an "extra wrinkle or two" to what is already used inside a vehicles control module. The future of biodiesel production itself is in question as companies producing the fuel have gone out of business after tax credits supporting the industry were ended.