A fluid becomes supercritical when its temperature and pressure exceed a critical boundary point, causing it to take on novel properties between those of a liquid and a gas. George Anitescu, a research associate at the Department of Biomedical and Chemical Engineering at Syracuse University in New York state, who developed the new engine design, says that supercritical diesel can be burned more efficiently and cleanly.
By raising diesel to a supercritical state before injecting it into an engine's combustion chamber, viscosity becomes less of a problem, says Anitescu. Additionally, the high molecular diffusion of supercritical fluids means that the fuel and air mix together almost instantaneously. So instead of trying to burn relatively large droplets of fuel surrounded by air, the vaporized fuel mixes more evenly with air, which makes it burn more quickly, cleanly, and completely. In a sense, it is like an intermediate between diesel and gasoline, but with the benefits of both, says Anitescu, who presented his work last week at Directions in Engine-Efficiency and Emissions Research, a conference held in Dearborn, MI.
In order for the diesel to reach a supercritical state, Anitescu's fuel system has first to heat it to around 450 degrees Celsius at a pressure of about 60,000,000 Pascal. Achieving the pressure is not a problem, Anitescu says, but increasing the temperature is more demanding.
Because fuel systems usually operate at temperatures below 80 degree Celsius, Anitescu and his colleagues used the heat from the engine's exhaust to raise the fuel's temperature. This causes further complications. "You need to prevent it from coking," he says. Coking occurs when hydrocarbons in the fuel react, producing sticky deposits that can lead to fuel-system failures. The phenomenon can be avoided by diluting the fuel with an additive, such as carbon dioxide or water. In the Syracuse engine, a small amount of exhaust gas is introduced to act as an anticoking agent, a technique known as exhaust-gas recirculation.