A compressed natural gas (CNG) refueling system has banks of cylinders containing CNG, a hydraulic fluid reservoir containing a hydraulic fluid which does not readily mix with CNG, and reversible flow valves. Each cylinder has a fitting installed in an opening at one end. The fitting contains a hydraulic fluid port and a gas port. The other end of each cylinder is closed. Hydraulic fluid is pumped from the reservoir into each cylinder through the hydraulic fluid port. Inside each cylinder, the hydraulic fluid directly contacts the CNG, forcing the CNG out through the gas port. When a sensor detects that the cylinders are substantially drained of CNG, the reversible flow valves will reverse orientation, allowing the hydraulic fluid to flow back into the reservoir.