PALO ALTO, Calif.--This is sort of like the "two, two, two mints in one" solution for the clean-tech set.

Australia's Terra Fuel Technologies has come up with a device that, when added to a diesel car, bus or truck, lets the vehicle also run partly on ethanol. You need to add a second gas tank on the vehicle to hold ethanol, but in the end, it's an ethanol-diesel car. The device, a black box, controls the flow of ethanol into the engine.

The company has tested it and will start selling it in the United States.

Who in their right mind would want this? Adding ethanol to a diesel engine actually improves performance of the vehicle by about 10 percent while reducing emissions, said Alexander Daniel, vice president of Business Strategies International, which is trying to help Terra Fuel get traction in the States.

Running a diesel engine on vegetable oil, a clean alternative, can degrade performance a little, even according to biodiesel fans.

Diesel drivers, of course, can reduce emissions by running their cars on a mix of regular diesel and biodiesel. But biodiesel is made from oil, Alexander and others at BSI noted, and a lot of countries don't have a lot of spare vegetable oil. Australia, for instance, grows sugarcane, which can be turned into ethanol.