Mechanical engineers at Iowa State University in Ames, US, have demonstrated in a new study how to boost the power output of biodiesel simply by adding waste plastic to the fuel.
According to a report in New Scientist, the study, funded in part by the Department of Defense, was conducted to find a way to dispose of trash and generate power under battlefield conditions.
"One can recycle any kind of plastic, but if you are camped in a remote area, recycling is not an option," said Song-Charng Kong, a co-author of the study. "Turning plastic into fuel is a way to get rid of garbage and generate electricity," he added.
Kong and colleagues dissolved polystyrene - a polymer used to make disposable foam plates and cups - into biodiesel at concentrations ranging from 2 to 20 percent polystyrene by weight.
"A polystyrene cup will dissolve almost instantly in biodiesel, like a snowflake in water," Kong said, although the plastic doesn't break down as well in petroleum-based diesel and other liquid fuels.
Tests of the mixed fuel in a tractor engine used for electricity generation showed that as polystyrene concentrations increased to 5 percent, power output increased at roughly the same rate.
However, there was a drop off in output for plastic concentrations above 5 percent.
According to Kong, the change is due to the fuel's increasing viscosity as more and more polystyrene is added.
"Initially, the thicker fluid creates greater pressure inside the generator's fuel injector causing earlier injection of fuel into the engine and increasing its output," he said.
But eventually, the fluid gets so viscous that it doesn't completely combust in the engine and power output decreases.
At 15 percent polystyrene, the fuel is so thick the fuel injection pump overheats.
The new fuel mix is not without its problems, however - as the concentration of polystyrene increases, so do emissions of carbon monoxide, soot, and nitrous oxides.
"You are putting large polymer compounds in, it's hard to burn them completely," Kong said, adding that he hopes to now work on refining the engine's fuel injection system to yield a more complete burn with fewer emissions.