Can a gasoline-diesel blend deliver cleaner, more efficient engines?
Tests using a combination of diesel and gasoline fuels to power internal combustion engines achieve extremely high thermal efficiency, 20% greater fuel efficiency, and could significantly reduce U.S. dependence on imported oil.
Researchers at the University of Wisconsin-Madison are asking: What if an engine could be programmed to harvest the best properties of both gasoline and diesel fuel sources at once by blending the fuels within the combustion chamber itself?
The answer, based on tests by the University's engine research group (headed by Rolf Reitz), would be 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 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's 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 wouldn't ignite in a diesel engine, because gasoline is less reactive than diesel and burns less easily. But in Reitz's strategy, just the right amount of diesel fuel injections provides the kick-start for ignition.
Reitz describes his strategy as "[changing] the fuel properties [of each] 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."
"Two remarkable things happen in the gasoline-diesel mix," Reitz says:
- 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.
- The customized fuel preparation controls the chemistry for optimal combustion. That translates into less unburned fuel energy lost in the exhaust, and fewer pollutant emissions being 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 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 University's Engine Research Center.
The best results achieved 53% thermal efficiency in the experimental test engine. According to Reitz, 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 says. "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."
Thermal efficiency is defined by the percentage of fuel that is actually devoted to powering the engine, rather than being lost in heat transfer, exhaust or other variables.
The big-picture implications for reduced oil consumption are even more compelling, Reitz says. The United States consumes about 21 million barrels of oil per day, about 65% (13.5 million barrels) of which is used in transportation. If this new blended fuel process could convert both diesel and gasoline engines to 53% thermal efficiency from current levels, the nation could reduce oil consumption by 4 million barrels per day, or one-third of all oil destined for transportation.
"That's roughly the amount that we import from the Persian Gulf," says Reitz.
The work is funded by DOE and the College of Engineering Diesel Emissions Reduction Consortium at the University, which includes 24 industry partners.
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