Research Presentation Extended Abstract Submission Project 2 B . 3 Free Piston Engine Hydraulic Pump

The goal of this project is to provide a compact and efficient fluid power source for mobile applications (10 kW-500 kW), including on-road vehicles and off-road heavy machineries. This is achieved through the development of a hydraulic free-piston engine (HFPE). Previously, a virtual crankshaft mechanism is proposed by the authors which regulates the pistons to follow any reference trajectories precisely. As a result, continuous combustion performance was achieved in the HFPE. In this presentation, the author will show that the continuous combustion performance of the HFPE is even improved by integrating a supercharge system and adding a transient controller to the existing virtual crankshaft mechanism. Additionally, a novel combustion control method enabled by the HFPE with virtual crankshaft mechanism, namely the trajectory-based combustion control, is also proposed. The simulation results from a dynamic model with high fidelity not only demonstrate the effectiveness of this control method but also show its benefits on the engine efficiency and emissions performance. Introduction For mobile applications including both on-highway vehicles and off-road heavy equipment, fluid power is currently generated onboard using a crankshaft-based internal combustion engine (ICE) with a rotational hydraulic pump. The main drawbacks of this configuration are the relatively low efficiency and the complex design of both the ICE and the hydraulic pumping system. An alternative approach is to supply fluid power using a free piston engine (FPE) with a linear hydraulic pump. This configuration has the potential to significantly improve the ICE and pump efficiency by simplifying system structure and increasing system flexibility and modularity. However, previous work on free-piston engine has shown limited success mainly due to the complex dynamic interactions between the combustion and the load in real-time and the lack of systematic active control that can precisely regulate the piston motion. Thus, we propose to design an active controller, acting as a virtual crankshaft by balancing the in-cylinder gas pressure and the hydraulic loading pressure in real-time, which forces the piston to follow the desired trajectory [1]. Additionally, this controllable piston trajectory can further act as an extra control means to improve the combustion performance in the FPE which realizes the piston trajectory-based combustion control [2]. Such a novel control method enables us to tailor the combustion processes in real-time and achieve higher thermal efficiency as well as lower emissions simultaneously. Theory The virtual crankshaft mechanism works as a crankshaft that guides the piston to follow a predefined trajectory, but instead of mechanical linkage, it controls the piston motion via hydraulic servo valve and high pressure fluid in the hydraulic accumulator. The advantage of the virtual crankshaft over mechanical crankshaft lies in its ultimate flexibility of compression ratio control. Specifically, the control algorithm and the reference trajectory of the virtual crankshaft can be altered digitally to achieve a wide range of piston motion profile within a short time period. Given the periodic nature of the piston motion, the developed controller is a robust repetitive controller, which is capable of rapidly tracking of periodic references with known period. Additionally, a feedforward controller [3] and a transient controller [4] are also integrated with the existing virtual crankshaft mechanism to further improve the tracking performance in combustion tests of the HFPE. The feedforward controller is developed accounting to the flatness approach, which involves the inversion of a nonlinear model of the system. The transient controller is achieved by shifting the reference and control signals intellectually after detecting the combustion occurrence. The modified reference and control signals are sent to the virtual crankshaft afterwards and reduce the transient period caused by the combustion force. 2015 Fluid Power Innovation & Research Conference (FPIRC 15) Research Presentation Extended Abstract Submission The virtual crankshaft mechanism not only offers robust and precise piston motion control for FPE, but also provides a new control means to maximize the engine efficiency and minimize emissions by forcing the combustion occurring along the optimal piston trajectory, which forms the basic idea of the trajectorybased combustion control. Generally, combustion is largely driven by the interaction between the fuel chemical kinetics and the gas dynamics inside the combustion chamber through a feedback manner. The chemical kinetics is represented by various element reactions whose reaction rates are directly influenced by the temperature, pressure and species concentration in the combustion chamber. On the other hand, the reaction products and the released thermal energy also affect the gas dynamics inside the chamber. Consequently, the combustion chamber volume is capable of affecting the complex interactions directly and therefore, it is possible to have piston trajectories that generate the optimal combustion chamber volume profile and optimize the combustion process in terms of precise ignition timing control, maximal energy extraction and minimal emissions. Methods In our research, detailed dynamic models are necessary to the design of the virtual crankshaft mechanism [3, 4] as well as the investigation of the piston trajectory-based combustion control [5, 6]. Such dynamic models, involving the gas dynamics in the combustion chamber, heat loss process during the combustion, chemical mechanism of the utilized fuel and hydraulic actuation dynamics, is realized in the Matlab Simulink. Specifically, the gas dynamics in the combustion chamber is presented by the first law of the thermodynamics; the heat loss process is simulated using the modified Woschini equation. Moreover, Cantera is also utilized to implement the detailed chemical reaction mechanism into the physics-based model in order to represent the combustion process with high fidelity. Besides the software part, a free piston engine driven hydraulic pump, donated by Ford motor company, has also been set up in the University of Minnesota (Fig.1). Figure.1 Picture of the HFPE Necessary subsystems which are needed to conduct the experiments in this facility have also been designed and integrated, including a high pressure (1500 psi) fuel injection system and a supercharge intake system. Meanwhile, a rapid prototyping control system (dSPACE® real-time control system) is used to convert the designed Simulink program into the executable code and realize the implementation of the virtual crankshaft mechanism. As a result, the experimental results from the HFPE can be used to demonstrate the effectiveness of the virtual crankshaft mechanism and validate the piston trajectory-based combustion control. Results and Discussion 2015 Fluid Power Innovation & Research Conference (FPIRC 15) Research Presentation Extended Abstract Submission Figure 2. Tracking performance from motoring experiment Figure 2 above and Figure 3 below demonstrate the effectiveness of the virtual crankshaft mechanism in both motoring test and combustion tests. Even the combustion tests exists large cycle-to-cycle variation, the virtual crankshaft mechanism can still handle it and recover the piston trajectory following the predefined reference. Attributed to the improvement on control and the hardware enhancement, we have achieved continuous combustion in the HFPE last year. Figure 3 shows the corresponding experimental result, which offers valuable information for future HFPE research, as no previous experimental results have been published in the literature on FPE operation with opposed-piston-opposed-cylinder (OPOC) architecture. Figure 3. Continuous HFPE combustion operation 2015 Fluid Power Innovation & Research Conference (FPIRC 15) Research Presentation Extended Abstract Submission Figure 4 represents the benefits of the piston trajectory-based combustion control. By changing the piston trajectory, we indeed adjust the volume of the combustion chamber, affect the pressure, the temperature and the species concentrations of the in-cylinger gases and therefore tailor the combustion process for better engine performance. The corresponding simulation proves this idea and shows that higher indicated thermal efficiency and lower NOx emission can be achieved in the HFPE compared to conventional ICEs.