Off-highway mining and construction equipment typically converts all the power output of the engine to hydraulic power, with this power then used to perform the earth-moving operations, and also to propel the vehicle. This equipment presents significant opportunities for a new type of powerplant designed to deliver hydraulic power directly. An alternative to the conventional engine driven pump is a free-piston engine-pump (FPEP). The FPEP incorporates the functions of both an internal combustion engine and a hydraulic pump into a single, less-complex unit. The design presented in this paper utilizes two double-ended, reciprocating, opposed pistons, with combustion at one end of each piston and pumping at the opposite end. The opposed piston layout provides balance and also facilitates uniflow scavenging through intake and exhaust ports in the combustion section of the engine. An important feature of this FPEP design is the rebound accumulator circuit. The purpose of this circuit is to provide for control of the piston return to Top Dead Center. This permits the effective engine speed to be varied and also provides for synchronization of the opposed pistons. The energy stored in the rebound accumulator is also to be used for starting. The design of the FPEP should provide efficient delivery of hydraulic fluid over a wide range of pressure by controlling the amount of fuel injected, by varying the supercharge pressure, and by providing for variable delivery.