Independent Pressure and Flow Rate Control Enabled by Hydraulic Free Piston Engine

Keyan Liu, Chen Zhang, Zongxuan Sun

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


Fluid power consumes a significant portion of energy annually with relatively low efficiency, especially for mobile applications. This is mainly due to the current architecture where an internal combustion engine (ICE) driven hydraulic pump is used to supply fluid to various driving and working circuits. This architecture as well as the response time limitation of the ICE and the hydraulic pump cause significant fluid throttling loss and low-efficiency engine operation at part load conditions. As a novel alternative, the hydraulic free piston engine (HFPE) eliminates the mechanical crankshaft and integrates the ICE and the hydraulic pump into one unit. Due to its ultimate freedom on the piston motion and reduced inertia, the HFPE has the potential to produce variable fluid power in an efficient manner with fast response time. In this paper, such potential is investigated. The working principle of a prototype HFPE is described and a comprehensive HFPE model is developed. A novel control method, namely the independent pressure and flow rate control (IPFC), is proposed to regulate the HFPE's output flow rate at any given load pressure and realize throttleless fluid control. The effectiveness of the method is demonstrated through simulations, which show the capability of the control method in providing different output flow rates at variable load pressures within tens of milliseconds. Additionally, a detailed analysis of the HFPE efficiency, in terms of thermal efficiency of the engine and mechanical efficiency of the pump, has been conducted under different working conditions. The results show that with the IPFC, the HFPE is an efficient and flexible mobile fluid power source.

Original languageEnglish (US)
Article number8675501
Pages (from-to)1282-1293
Number of pages12
JournalIEEE/ASME Transactions on Mechatronics
Issue number3
StatePublished - Jun 2019

Bibliographical note

Funding Information:
Manuscript received October 26, 2017; revised February 5, 2018, June 18, 2018, September 21, 2018, and January 9, 2019; accepted March 16, 2019. Date of publication March 27, 2019; date of current version June 14, 2019. Recommended by Technical Editor J. Mattila. This work was supported by the NSF Center for Compact and Efficient Fluid Power under Grant EEC-0540834. (Corresponding author: Zongxuan Sun.) The authors are with the Department of Mechanical Engineering, University of Minnesota, Twin Cities, Minneapolis, MN 55455 USA (e-mail:,;;

Publisher Copyright:
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  • Fluid power source
  • free piston engine
  • mobile hydraulic application
  • throttleless fluid control


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