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.