An international team of researchers has discovered a new type of cool-burning flames that could lead to cleaner, more efficient engines for cars. The discovery was made during a series of experiments on the International Space Station by a team led by Forman Williams, professor of mechanical and aerospace engineering at UC San Diego. During the experiments, researchers ignited large droplets of heptane fuel. Initially, it looked like the flames had extinguished themselves, just as they would have on Earth. But sensors showed that the heptane was still burning - the resulting cool flames invisible to the naked eye. A better understanding of these flames’ chemistry could help improve internal combustion engines in cars, for example by developing homogenous-charge compression ignition - a technology not currently available in cars. It could potentially lead to engines that burn fuel at cooler temperatures, emitting fewer pollutants such as soot and nitric oxide and nitrogen dioxide, while still being efficient.