In the version designed to replace conventional gasoline engines, the diesel fuel is replaced with gasoline that’s mixed with an additive to make it more reactive, improving ignition of the fuel. Instead of having two fuel tanks, the car needs only one gasoline tank and a small reservoir the size of a window-washing-fluid bottle to hold the additive. Ordinary gas is injected by the port injector, and gas mixed with the additive is injected directly into the chamber. The result is an engine that’s 45 percent efficient, compared to about 30 percent efficient for conventional gasoline engines.
In both systems, the approach reduces engine pressure and temperature, which reduces the formation of smog-forming and other dangerous pollutants. The lower temperatures also reduce the amount of energy that’s lost to heat, making it available to drive the piston. “We extend the combustion event over a controlled period of time to get a gentle heat release that doesn’t lead to violent pressure rise and high temperatures in the combustion chamber,” Reitz says.
Robert Dibble, a professor of mechanical engineering at the University of California at Berkeley, says the new design “is a clever idea.” He says the two-fuel design could be difficult to get automakers and consumers to adopt, but he notes that current diesel after-treatment systems already require drivers to add a separate exhaust treatment fluid when refueling, so this barrier may be lower now. The version of the new design that uses a gasoline additive would only require refilling the additive every oil change, Reitz says, reducing the inconvenience.
Dibble says that another researcher is taking a similar approach to improving engine efficiency, but his approach uses only one fuel. Bengt Johansson, head of the division of combustion engines at Lund University in Sweden, has shown high efficiencies and low emissions by controlling the timing and duration of combustion with multiple fuel injections. But unlike the UW-Madison method, which uses fuels with different combustion properties, Johansson’s approach controls combustion by creating regions within the combustion chamber with varying concentrations of fuel and air. One potential disadvantage is that, because of the high compression levels used, it requires a more expensive engine than those used in conventional gas-powered cars.