...It could run for ten or more hours on a container of diesel fuel slightly larger than a D battery; when the fuel cartridge ran out, a new one could be easily swapped in. Each disposable cartridge would pack as much energy as a few heavy handfuls of lithium-ion batteries. As a result, a small pack of the cheap and light cartridges could power a PDA or cell phone through several days of heavy usage, no wall-outlet recharging required—a highly attractive feature for soldiers in remote locations or travelers. What’s more, the miniature turbine takes up about a quarter of the volume of a typical cell-phone battery.

Not that a micro engine is without drawbacks. It would shoot a tiny stream of hot exhaust gas, for one thing, making it more suitable for devices clipped to belts or carried in briefcases than for those stuffed in pockets. The engine itself would get hot, though an exhaust suppressor would easily keep devices from getting much warmer than they do today. But for many energy-hungry applications, says Epstein, a tiny turbine’s remarkable power output would far outweigh any disadvantages. Suggests Epstein, “You don’t need a very good jet engine to do better than batteries.”