Diesel and gasoline fuel sources both bring unique assets and liabilities to powering internal combustion engines. But what if an engine could be programmed to harvest the best properties of both fuel sources at once, on the fly, by blending the fuels within the combustion chamber?

Such an engine just might be possible thanks to the University of Wisconsin-Madison engine research group headed by Rolf Reitz. The research team is developing a diesel engine that produces significantly lower pollutant emissions than conventional engines, with an average of 20 percent greater fuel efficiency as well. 

These dramatic results came from a novel technique Reitz describes as "fast-response fuel blending," in which an engine's fuel injection is programmed to produce the optimal gasoline-diesel mix based on real-time operating conditions.

Under heavy-load operating conditions for a diesel truck, the fuel mix in Reitz' fueling strategy might be as high as 85 percent gasoline to 15 percent diesel; under lighter loads, the percentage of diesel would increase to a roughly 50-50 mix. Normally this type of blend wouldn't
ignite in a diesel engine, because gasoline is less reactive than diesel and burns less easily. But in Reitz' strategy, just the right amount of diesel fuel injections provide the kick-start for ignition.

"You can think of the diesel spray as a collection of liquid spark plugs, essentially, that ignite the gasoline," says Reitz, the Wisconsin Distinguished Professor of Mechanical Engineering. "The new strategy changes the fuel properties by blending the two fuels within the combustion chamber to precisely control the combustion process, based on when and how much diesel fuel is injected."

If the team's efforts are successful, U.S. oil consumption could be reduced by as much as one-third. "That's roughly the amount that we import from the Persian Gulf," says Reitz.

Two remarkable things happen in the gasoline-diesel mix, Reitz says. First, the engine operates at much lower combustion temperatures because of the improved control — as much as 40 percent lower than conventional engines — which leads to far less energy loss from the engine through heat transfer. Second, the customized fuel preparation controls the chemistry for optimal combustion. That translates into less unburned fuel energy.

Development of the blending strategy was guided by advanced computer simulation models. These computer predictions were then put to the test using a Caterpillar heavy-duty diesel engine at the UW-Madison Engine Research Center. The results were "really exciting," says Reitz, confirming the predicted benefits of blended fuel combustion.

The best results achieved 53 percent thermal efficiency in the experimental test engine. This efficiency exceeds even the most efficient diesel engine currently in the world — a massive turbocharged two-stroke used in the maritime shipping industry, which has 50 percent thermal efficiency.

"For a small engine to even approach these massive engine efficiencies is remarkable," Reitz says. "Even more striking, the blending strategy could also be applied to automotive gasoline engines, which usually average a much lower 25 percent thermal efficiency. Here, the potential for fuel economy improvement would even be larger than in diesel truck engines."

Perhaps, however, the greatest benefit of these futuristic engines would be the reduction in pollution emitted each year. The EPA has mandated incredibly stringent standards which are set to take affect in 2010. The regulations are designed to cut about 90 percent of all particulate matter (soot) and 80 percent of all nitrogen oxides (NOx) out of diesel emissions.

"What's more important than fuel efficiency, especially for the trucking industry, is that we are meeting the EPA's 2010 emissions regulations quite easily," Reitz says. 

Research presented at the 15th U.S. Department of Energy Diesel Engine-Efficiency and Emissions Research Conference in Detroit.