LiquidPiston has built functional proof-of-principle 70 and 40 HP diesel engine prototypes (X1, X2, and X4), which have demonstrated initial operating capabilities of the engine architecture.

These diesel engine prototypes, are capable of running for a short time at light load, and demonstrate compression ignition of diesel or JP-8 fuel, and net indicated efficiency of 33% (at light-load), which compares to ~30-40% efficiency in this size range of diesel engines at part-load. Intended as proof-of-principle demonstrators to show the engine is capable of running, these initial prototypes were designed without cooling systems. X4 builds improves upon these designs by implementing a water-cooled engine block.