Model allows engineers to test fuel systems on computers

The fuels model will increase the pace of injector design for greater efficiency, better gas mileage and more horsepower in cars and trucks. But the beauty of this approach is that it works for all combustion processes and fuels, from mopeds to missiles and from gasoline, ethanol and diesel fuel to decane/hexadecane.

Instead of costly real-world modeling, which requires the design, machining and production of parts before they can be bench tested and performance modeled, the mathematical model lets designers test their ideas on computers first. The model also brings research into alternative fuels into the computer before it needs to be prototyped.

"That's the reason we are so excited about this research, is that it cuts down on the expense of the calculations to model fuel efficiency," said Dr. Chien-Pin Chen, chair of UAHuntsville's chemical engineering department, who along with graduate student Omid Samimi Abianeh wrote a research paper on the fuels model ("A discrete multicomponent fuel evaporation model with liquid turbulence effects," International Journal of Heat and Mass Transfer, Vol. 55, Issues 23-24, November 2012, Pages 6897-6907). Chemical engineering professor, Dr. Ramon Cerro (see also: "Batch disillation: The forward and Inverse Problems," Ind. Eng. Chem. Res., Vol. 51, 12435-12488, 2012) and Dr. Shankar Mahalingam, dean of the College of Engineering, are also involved in the research.

"If somebody wants to do a numerical diagram of an internal combustion engine -- and I'm a numbers guy¬ -- the first thing they need to study is the fuel," Dr. Chen said. But because fuel is a highly complex substance, a researcher would need a supercomputer to do that. Gasoline, for example, contains hundreds of substances with different evaporation rates and ignition points.

"So we designed a surrogate fuel with three components instead of hundreds," Dr. Chen said. "It performs the same but it is not as complex to study." While it can be created as a physical substance, in the model the fuel is represented mathematically. "That model is our contribution," he said, and it works across all fuels, from rocket fuel to common ethanol/gasoline mixtures and the new E85 ethanol fuels. Their research has been funded by NASA and Gulf of Mexico Research Initiative grants.

In modern engines, injectors spray fuel into the combustion chamber at precisely timed intervals for combustion. The size, composition, behavior, temperature and pressure of those droplets all determine how efficiently the fuel will perform, Dr. Chen said. The model can demonstrate how fuel droplets from different injector designs will behave as far as their evaporation characteristics and combustion efficiency in the combustion chamber. All fuel types are certified by the National Institute of Standards and Technology, and that was the database used to validate the research results, Dr. Chen said.

"We are already changing the injector designs," Dr. Chen said, adding that the fuels model allows engineers to better answer the question, "What is the best injector design to give you the best flame propagation?

The new model has led to additional research in fuel turbulence, the rich to lean swirl of fuel in a combustion chamber that provides for even flame propagation.

In car and truck engines, it is important that fuel burns and expands in a controlled fashion rather than exploding. Explosions cause detonation, that pinging or clunking sound drivers sometimes hear that leads to premature engine wear and failure.

To accomplish even propagation, modern gasoline engines are designed to layer the fuel so that it has a higher density in relation to the available air (a rich mixture) near the spark plug and swirls to a lower density (a lean mixture) near the top of the piston. The plug's spark can more easily start combustion in the rich fuel, and the leaner mix underneath is more efficiently burned. Injector nozzle design and placement in the chamber are both important to this process. This summer at a Korean conference, Dr. Chen will present a paper and discuss the research done at UAHuntsville on how the turbulent swirling process affects the fuel droplet evaporation process.

The UAHuntsville researchers are also working to develop a combustion flame propagation model that could bring that process, too, inside the computer first before real-world testing is undertaken and result in gains in efficiency. "We are studying the flame front and how they wrinkle as the fuel burns," said Dr. Chen, who plans to submit a proposal to the U.S. Dept. of Energy to further that study. The research could increase the efficiency of future combustion chamber designs.

"The long-term goal," Dr. Chen said, "is to find a way to burn fuel more efficiently for more power and cleaner combustion."