Contributed by Ke Li, PhD student, University of Minnesota (Advisor: Professor Zongxuan Sun)
A conventional mobile fluid power generation system consists of a internal combustion engine (ICE) and a rotational hydraulic pump. An alternative is the use of a free piston engine (FPE)—this eliminates the crankshaft to enable unconstrained piston motion. With a FPE, linear hydraulic pumps can be aligned with the pistons of a FPE so that the combustion process directly pumps fluid to the application. This results in a more compact design with significantly reduced frictional loss.
Because there is no mechanical output, an FPE can be designed in a modular fashion. Specifically, we can combine several FPE modules that do not need to operate at the same time or locate at the same space. They can be turned on and off according to the power demand, so that the overall cycle efficiency can be significantly improved.
Another important advantage of an FPE is that the unconstrained piston motion allows us to alter the compression ratio, as well as the piston trajectory shape in real time. This feature makes the engine much more tolerant to the variability of fuel properties, and it also makes the FPE an ideal candidate for advanced combustions such as homogeneous charge compression ignition (HCCI). Therefore, an FPE can be a compact power source that is clean, efficient and capable of operating on a variety of renewable or fossil fuels.
The major technical barrier for the wide spread of FPE technology is the large cycle-to-cycle variation, especially during transient operations. This is due to the fact that piston motion is dependent on the complex dynamic coupling between the combustion and the hydraulic load. To tackle the problem, we have designed a robust and precise piston motion control system. The controller acts as a virtual crankshaft that guides the piston to follow a reference trajectory by seamlessly coordinating the hydraulic load with the combustion force in real-time. The advantage of the active motion controller lies in its ability to precisely track and shape the piston trajectory. Specifically, the reference trajectory of the virtual crankshaft can be altered digitally, in real time, to achieve a wide range of piston motions, and thus obtain maximum engine efficiency and low emissions with respect to various operating points.
The virtual crankshaft has been developed and implemented on a hydraulic free piston engine donated by Ford Motor Co. (see photo and schematic). The engine has an opposed-cylinder and opposed-piston configuration, which has the potential to significantly increase the ICE and pump efficiency while increasing system modularity.
While combustion occurs alternately between the left and the right combustion chamber, the inner piston pair and the outer piston pair are reciprocating in the opposed direction. The linear motion of the piston pairs compresses the oil in the hydraulic pumps to produce fluid power. The effectiveness of the virtual crankshaft has been demonstrated by the engine motoring and engine firing tests. We are currently working on the continuous engine combustion testing, so that the engine performance can be characterized and utilized for future research.
For more information, please visit the University of Minnesota website or the Center for Compact and Efficient Fluid Power (CCEFP) website.
How does the efficiency compare with a rotary hydraulic pump driven by a conventional IC SI engine? If the servo valve must throttle the oil flow into the HP side to keep the piston elements on the prescribed trajectory, friction losses will reduce the system efficiency. The on-off solenoid valves apparently are used only for engine starting, but the operation was not explained. Efficiency can be improved only if the pumping resistance is switched between high and an additional intermediate but useful pressure to keep the piston elements to the prescribed trajectory.
Hi, we are still conducting system testing to determine the efficiency, but please refer to the Innas website for a comparison of the efficiencies between a hydraulic free piston and a rotary engine driven hydraulic pump, http://www.innas.com/Chiron_efficiency.html
The on-off valves are only used for synchronization, the servo valve is used for engine starting and trajectory control. When the engine is running, the piston trajectory is mainly determined by combustion force, and the control is only to assist the piston motion to match with the desired trajectory. And when we design the prescribed trajectory, this factor are considered, it just does not make sense to track a trajectory if you need to throttle the flow all the time.
Hello Ke Li, Have you considered other engine cycles for your projects? I have a design for a free piston brayton cycle engine. I believe the constant pressure piston Brayton engine could be better for the application. I have built a working prototype.
I have an application where the Brayton cycle would fit very well. Contact me at diratiinc@gmail.com
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I was working on a similar concept using a piston Brayton cycle engine about 12 years ago. At the time I didn’t really know enough about the piston Brayton engine to make it work. I’ve recently made some major breakthroughs with the combustion on the piston Brayton engine and now I think I could make a successful free piston version. Some advantages might be that the Brayton is more of a constant pressure engine instead of an explosion engine and because it stores pressurized air it could also be self starting. If you would like to see my engines search youtube for Brayton engine or Braytom Ericsson engine.