Researchers in New York have demonstrated a supercritical diesel fuel-injection system that can reduce engine emissions by 80 percent and increase overall efficiency by 10 percent.

Diesel engines tend to be more efficient than gasoline, but the trade-off is that they are usually more polluting. Because diesel is heavy, viscose, and less volatile than gasoline, not all the fuel is burned during combustion, resulting in carbon compounds being released as harmful particulate soot. The higher combustion temperatures required to burn diesel also lead to increased nitrogen oxides emissions.

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 the past, another related approach, called homogeneous charge compression ignition, has been used to improve the performance of diesel. This involves premixing diesel and air before injecting it as a vapour into a combustion chamber under high pressure. But while this mixture burns more efficiently, it also makes combustion more difficult to control, which can lead to engine knocking: shockwaves within the engine’s cylinders caused by pockets of unburned fuel and air. In contrast, supercritical diesel injection produces very small vapour-like droplets, but with fuel densities equivalent to a liquid, says Anitescu.

Andreas Birgel, a researcher with the Internal Combustion Engines and Fuel Systems Research Group at University College London, UK, says there is plenty of interest in producing diesel that vaporizes more easily, for example, by using corn or rapeseed oil to make biodiesel, which has a relatively low viscosity. Another approach is to treat conventional diesel with additives, he says.

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.

The system has only been tested in a laboratory setup, but a prototype could be ready for testing by the end of the year, says Anitescu. The fuel system is designed to use conventional fuel injectors, even though these are designed to work with regular fluids. Anitescu says it may be possible to improve the performance by switching to a fluid state just below supercritical. This may allow vaporization to occur while getting better performance out of the injectors. “We have many options here,” he says.

At the same conference, Transonic Combustion, a company based in of Camarillo, CA, presented details of an alternative way to use supercritical fuels that involves a novel fuel injector and redesigning the engine’s entire valve system and combustion chamber.

But with either approach, going supercritical does not come without a cost, says Birgel. “You still need the viscosity because most diesel fuel systems depend upon the fuel for lubrication,” he says.

“This is an issue which has yet to be addressed,” admits Anitescu. He says it may be possible to introduce lubricants, but this would only be necessary in the final stage of the fuel system, where the fluid is at its hottest. For subcritical fuels, it may not be an issue, he says.