Making a clean fuel all the cleaner

2008-09-12 09:50:41

The appeal of bio-fuel is further enhanced when its advantages as a sulphur-free, renewable energy source can be combined with the fuel efficiency and low emissions made possible by the latest generation of high pressure diesel engines. As their design continues to evolve, Dr. Nasser Ashgriz is working to ensure that the prospects for biodiesel will continue to be bright.

The University of Toronto engineering professor leads the AUTO21 Ultra-Clean Biodiesel Engines project, which is examining the various technical challenges that must be overcome for this fuel to start showing up in gas tanks across North America. Among the foremost of those challenges is the issue of atomization of bio-fuels as they enter the combustion chamber.

"Control of the droplet size distribution, and its mixing with the air inside the engine is the crucial thing in terms of combustion characteristics, engine performance, and emissions," he says, noting that diesel technology has increased the internal operating pressure by several orders of magnitude, reducing the diameter of droplets by as much as two-thirds. "As the droplet size goes down, that allows for quicker evaporation and mixing between the fuel and air, and therefore emissions such as particulates reduces dramatically."

Such progress can be offset by the specific characteristics of biodiesel, Dr. Ashgriz points out. "Viscosity of bio-oils is higher than petroleum diesel, and that viscosity causes the droplet sizes to go up," he says.

The researchers are assembling a detailed numerical model describing this mixing behaviour, which should point the way toward optimal settings and sizes of the nozzle that sprays fuel for combustion. In fact, Dr. Ashgriz explains, the use of pure biodiesel actually increases of some emissions, as well as cold starting problems.

These shortcomings, combined with the likelihood that industrial production of biodiesel will remain limited for the foreseeable future, mean this fuel will likely wind up being blended with its conventional counterpart. As part of their studies, the AUTO21 researchers are examining what blend of these fuels would be best, and what kind of engine configuration would best suit that blend.

 

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