Free Piston Engines for Mobile Fluid Power Equipment

Technology No. 20160384
IP Status: Pending US Patent; Application #: 15/607,937

Improved Efficiency of Fluid Power Sources

A systematic control method can significantly improve the efficiency of fluid power sources. The algorithm calculates piston trajectory for hydraulic free piston engine (FPE) operation based on the associated hydraulic force and combustion force. This desired piston trajectory is then used as a reference signal for piston motion tracking control so that the load pressure and output flow rate of the FPE can be independently controlled, in real-time, according to required loads. In addition, the piston trajectory reference can be varied digitally in each stroke, so the response time of the FPE for variable loads occurs within just milliseconds.

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Independent Control of FPE Output Flow Rate and Pressure

Fluid power sources are currently sized for a maximum load, maximum pressure and flow rate, but most duty cycles only require a partial load. Consequently, using relief valves and throttling valves to reduce output flow rate and adjust pressure compromises system efficiency due to throttling loss. This technology offers a way to significantly improve such fluid power efficiency by independently controlling the output flow rate and pressure of the FPE by implementing the desired piston trajectory reference. Using this algorithm seamlessly matches the output power to the required load, reducing throttling loss and improving system efficiency.


  • Improved efficiency
  • Ability to adjust power output in real-time, within milliseconds
  • Can compensate for load changes
  • Novel approach not yet disclosed by others


  • Any hydraulic free piston engine or fluid power source
  • Mobile applications (hybrid on-highway vehicles, off-road vehicles (agriculture and heavy-duty construction machinery such as excavators, wheel loaders), distributed power generation units

Phase of Development - Modeled design. Validated with numerical simulations. The algorithm has been tested as a computer simulation.

Zongxuan Sun, PhD
Professor, Mechanical Engineering
External Link (

Free Piston Engine Based Mobile Fluid Power Source, 2016 ASME Proceedings

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