Fluid Power Research Update: Free Piston Engine Hydraulic Pump

We’ve written before about fluid power research at the Center for Compact and Efficient Fluid Power (CCEFP)—the network of fluid power research laboratories, academic faculty, graduate and undergraduate students at seven universities—that is making a difference when it comes to preparing a better-educated workforce for the fluid power industry. The CCEFP has created a 500% increase in the number of fluid power-focused advanced degrees awarded in the United States, with almost half of its graduates going on to work in the fluid power industry.


 In addition to increasing interest in fluid power, the CCEFP has also been home to a number of research projects in fluid power. One such project involves investigating the free piston engine (FPE) hydraulic pump.

Free Piston Engine (FPE) Hydraulic Pump

Faculty Investigator: Prof. Zongxuan Sun and graduate student, Chen Zhang, PhD Candidate, University of Minnesota.

The free piston engine (FPE) hydraulic pump is a compact fluid power supply that combines the internal combustion engine and the hydraulic pump into one device. As shown in Figure 1, combustion in the right cylinder will push the inner piston to the left and outer piston to the right, which will compress the gas in the left cylinder and generate high pressure fluid in the center hydraulic chamber. Similarly, combustion in the left cylinder will return the inner piston to the right and outer piston to the left. Due to the absence of the mechanical crankshaft, the FPE has the ultimate freedom on its piston motion and offers us an additional control means, i.e. compression ratio and piston trajectory, to enhance the engine efficiency and performance. A major technical barrier for the wide spread of the FPE is the lack of robust and precise control of the piston motion. Previously, an active piston motion controller was developed to act as a “virtual crankshaft”, which regulates the piston motion to follow any given reference trajectory. Attributed to the “virtual crankshaft”, continuous combustion performance in the FPE has been achieved at the University of Minnesota.

fluid power research

Figure 1. The picture and the schematic of the free piston engine hydraulic pump

In addition, the trajectory-based combustion control has also been developed. By changing the piston trajectory in real time, we are able to vary the combustion chamber volume, affect the thermal dynamics of the in-cylinder gases and therefore tailor the combustion process to maximize the engine thermal efficiency and minimize emissions. Furthermore, the trajectory-based combustion control also offers the FPE ultimate fuel flexibility since a variety of liquid fuels can be utilized to power the FPE by changing the compression ratios accordingly.

For fluid power applications, such as construction and agriculture machines, the free piston engine hydraulic pump (FPEP) offers extra advantages beyond the engine efficiency improvement. Given the modular nature of the FPEP, different power modules can be used for different functions such as driving or working. The various power modules can operate at different pressures and flow rates, and they can be turned on and off in real time. Even better they can also be located at various locations on the machine. This modular arrangement will significantly reduce throttling losses, improve part load efficiency, and offer extra degree of freedom for machine design and packaging.

Prof. Zongxuan “Sunny” Sun may be reached at zsun@umn.edu and graduate student Chen Zhang may be reached at zhan2314@umn.edu.

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