LiquidPiston says that their X-mini is still in a testing phase, and they hope to get the weight down to 3 pounds and the power up to 5 hp. (The 40-pound piston engine produces about 6.5 hp.) In the meantime, their little engine already packs a punch, as you can see in the video below.
LiquidPiston says the X-mini can run on Jet Propellant 8, the military's fuel of choice, making it an ideal candidate for all sorts of military applications. For instance, the X-mini is small and light enough to power a UAV, it can be part of a generator that can be carried in a backpack, or it can even be used to power military robotics. According to Alec Shkolnik, LiquidPiston's co-founder and president, "[DARPA] is kinda agnostic as to the actual application … they have so many different applications that need power."
Shkolnik said that the X-mini is still early in testing, and they have only just built their first working prototype, but he's hopeful that the engine could see a commercial release sometime in the next few years. When that happens, the X-mini could find its way into lawnmowers, emergency generators, and even small vehicles like mopeds.