4 August 2009

Gasoline-diesel combo powers clean engine

Diesel and gasoline fuel sources bring different assets and liabilities to powering internal combustion engines.

There is now a movement afoot to harvest the best properties of both fuel sources at once, on the fly, by blending the fuels within the combustion chamber.

That could lead to a diesel engine that produces significantly lower pollutant emissions than conventional engines, with an average of 20% greater fuel efficiency as well. These dramatic results came from a technique University of Wisconsin-Madison Distinguished Professor of Mechanical Engineering Rolf Reitz would call “fast-response fuel blending,” in which an engine’s fuel injection is able 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% gasoline to 15% diesel; under lighter loads, the percentage of diesel would increase to a roughly 50-50 mix. Normally this type of blend would not 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,” Reitz said. “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.”

Reitz estimated if all cars and trucks were to achieve the efficiency levels demonstrated in the project, it could lead to a reduction in transportation-based U.S. oil consumption by one-third.

“That’s roughly the amount that we import from the Persian Gulf,” Reitz said.

Two things happen in the gasoline-diesel mix, Reitz said. First, the engine operates at much lower combustion temperatures because of the improved control—as much as 40% 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 lost in the exhaust, and also fewer pollutant emissions produced by the combustion process. In addition, the system can use relatively inexpensive low-pressure fuel injection (commonly used in gasoline engines), instead of the high-pressure injection required by conventional diesel engines.

Development of the blending strategy came from advanced computer simulation models. These computer predictions then went through a real-life application using a Caterpillar heavy-duty diesel engine at the UW-Madison Engine Research Center. The results were “really exciting,” said Reitz, confirming the predicted benefits of blended fuel combustion. The best results achieved 53% 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% thermal efficiency.

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

For related information, go to www.isa.org/manufacturing_automation.