Fuel, Driving Habits Affect MPG

Want to put a tiger in your tank? Don’t start with a corn cob, say NC State researchers, who have compared the fuel economy and emissions of various biofuels with regular gasoline and diesel.

Dr. Chris Frey, a professor of civil, construction and environmental engineering, test drives various cars, minivans, and SUVs to obtain second-by-second data on engine performance and tailpipe emissions for traditional versus alternative fuels. Likewise, he tests biodiesel in tractor-trailers, dump trucks, and construction vehicles. “Ethanol is an alternative to oil but is not yet as good from an efficiency and emissions standpoint,” Frey says. “Biodiesel provides a more favorable comparison.”

The energy density of ethanol is lower than gasoline, Frey says, meaning vehicles running on ethanol get fewer miles per gallon. Ethanol does cut carbon monoxide emissions in the “field-to-tank” tests, he says. But biodiesel does even better, cutting greenhouse gas emissions and particulate pollution without sacrificing much energy when compared with petroleum-based diesel. Frey also is working with
NC State’s Institute for Transportation Research and Education (ITRE) to study the fuel economy and emissions of plug-in hybrid and fuel cell vehicles. Hybrids will reduce energy consumption by up to 20 percent, he says, but it will take years for people to swap out a significant percentage
of older vehicles for hybrids.

“How cars are driven is a major contributing factor to energy use and emissions.”

Miles per gallon is only part of the equation for more energy-efficient vehicles, according to ITRE Director Nagui Rouphail. “How cars are driven is a major contributing factor to energy use and emissions,” he says. Data that Frey has collected show aggressive driving—jack-rabbit starts and stops, and speeding—uses far more fuel than trying to maintain a constant speed as much as possible. Proper timing of traffic lights to reduce idling was also found to cut fuel consumption and emissions. Research at ITRE using traffic predictions for the Raleigh-Durham region shows that an expansion of the current sprawling growth pattern over the next 20 years will produce increased fuel use and emissions even when newer, more efficient cars take to the highways.

Ethanol cuts carbon monoxide emissions in the “field-to-tank” tests, but biodiesel does even better.

Rouphail says ITRE is also working with regional planning researchers at UNC-Chapel Hill on a computer model to study the impact of a “smart growth” land-use plan for Charlotte that would reduce vehicle miles traveled and slash fuel consumption. “A combination of factors needs to be looked at to decrease energy use,” he says. “It’s really a complex picture for which there is no single silver bullet.”


Dr. Chris Frey (top) and graduate research assistant Hyung-Wook Choi (bottom, at right) monitor the emissions produced by different vehicles and different fuels.

ITRE Director Nagui Rouphail says properly timed traffic lights can reduce fuel consumption and vehicle emissions.