The internal combustion (IC) engine, which is the power system of an automobile, consumes a tremendous amount of energy. As energy shortages are continuously worsening, the energy-efficient utilization of IC engines has attracted increasing attention from researchers for a long time now. Only one-third of the energy from fuel combustion is utilized effectively in IC engines, whereas the rest is wasted in various forms of cooling, exhaust, and mechanical losses; exhaust energy accounts for the largest proportion of wasted energy in IC engines [1
]. Therefore, recovering exhaust energy is an effective means to save vehicle energy. In the case of waste heat recovery (WHR), the Organic Rankine Cycle (ORC) system is an effective technical solution and a promising means for industrialization of energy savings [5
]. To date, research on ORC applications has mostly focused on large-scale heat sources, such as solar energy, geothermal energy, and industrial waste heat, etc.
]. The lack of a suitable expander has caused difficulties in building corresponding ORC systems for recovering small-scale waste heat, such as vehicle exhaust energy, etc.
At present, developing an ideal small-scale expander or power output system that can convert waste heat into mechanical energy or electric energy to recover exhaust energy from vehicle IC engines is a crucial objective of ORC system research [13
An expander is a key component in an ORC system. Compared with a centrifugal expander, a positive displacement expander is characterized by low flow rate, large expansion ratio, and low rotational speed [15
]. A positive displacement expander should therefore be more suitable for a small-scale ORC system [18
]. However, low cost and low rotational speed scroll expanders are frequently used in 0.1–2 kW experimental systems, whereas their performance in situations involving higher power has not been verified yet [20
]. A piston expander has higher thermal efficiency for small steam flow rate conditions and a good power output/size ratio. This type of expander is generally built or renovated based on existing commercial IC engines. If the pressure ratio on the expander is high, then piston expanders are suitable. Moreover, these expanders are more robust than scroll expanders. Gao et al.
] constructed a mathematical model to evaluate the performance of a Rankine cycle system with a reciprocating piston expander and conducted a preliminary experiment. The results showed that the introducing a heat recovery system could increase engine power output by 12% when a diesel engine was operated at 80 kW/2590 rpm. Glavatskaya et al.
] investigated the performance of a reciprocating expander for an automotive WHR application. The results showed that a maximum power output of 7 kW could be obtained at a high operating point, and the isentropic efficiency of the reciprocating expander varied from 55% to 70%. Chiong et al.
] presented a concept for a new piston expander utilizing a nozzle as part of a secondary steam cycle to recover exhaust energy. Through simulation, the nozzle piston expander was found to increase the output power from a minimum of 0.73 kW to a maximum of 4.75 kW. The aforementioned survey of Rankine cycle systems applied to passenger vehicles indicates that the piston expander is a promising technology for WHR applications compared with other types of expander. This technology is innovative in the automotive industry [23
]. However, a comparatively large weight and a complicated structure restrict the extensive application of piston expanders in small-scale vehicle ORC systems.
To overcome the aforementioned shortcomings of piston expanders, the concept of a free piston expander (FPE), which is characterized by cancelling the crank-link mechanism, has been presented [27
]. Zhang et al.
] developed a FPE that could be used in the transcritical CO2
refrigeration cycle to replace the throttling valve. An expansion machine was integrated into a compressor in this device, and thus, the crankshaft became unnecessary. Moreover, a slide valve was designed to replace the traditional inlet/outlet valves to control the charge/discharge process. The design efficiencies of the expander and the compressor were assumed to be 60% and 70%, respectively; the experimental studies using p
diagrams showed that device isentropic efficiency could reach 62%. Han et al.
] constructed a simulation model and a test bench for a similar Rankine cycle WHR system using a FPE. Subsequently, a preliminary experiment was conducted. The power output of the free piston mechanism was eventually consumed through hydraulic buffers and reached 2.08 kJ, whereas piston displacement was 52 mm when the evaporator outlet pressure of the working fluid was 0.7 MPa. Weiss et al.
] designed a FPE to produce power using a low-temperature energy source, and the FPE model was studied under various conditions. The results showed that decreasing piston mass reduced piston stroke length, but increased operation frequency and power output. Power output and energy recovery efficiency reached 25.6 mW and 80%–90%, respectively. Champagne et al.
] conducted a preliminary experimental analysis of a small-scale FPE. The results showed that lubricants significantly affected the seal and leakage of a FPE. A thick lubricant seal was better in static configurations than in dynamic testing. Nickl et al.
] developed a free piston compressor–expander unit with three expansion stages, and the isentropic efficiency of this device could reach 70% during the tests.
In the present research, a novel free piston expander-linear generator (FPE-LG) is proposed based on previous design experiences and the considerable literature on FPE prototypes mentioned earlier [27
]. Compared with the ORC power generation system that uses a conventional expander, an ORC power generation system based on FPE-LG exhibits certain advantages such as compact structure, operation flexibility, and effective power output under the action of a two-phase flow in a partial load condition [30
]. In addition, FPE-LG is more suitable for ethanol and the water Rankine cycle because of its large expansion ratio. A small-scale ORC system with FPE-LG can transform waste heat from IC engine exhaust into electricity and subsequently supply power to the powertrain or auxiliary load of the vehicle.
In this research, a novel FPE-LG was developed, and several preliminary experiments were performed using an air test rig. For the intake and exhaust processes of the FPE, the dynamic characteristics of the in-cylinder flow field were analyzed using a computational fluid dynamics (CFD) method and Fluent software.
Based on this initial work, the FPE-LG shows promising potential for continued development as a simple and efficient thermo-electric conversion unit. This research focuses on the development of the FPE-LG, as well as on the conceptual design and 3D numerical simulation of the FPE. In addition, several preliminary experiments based on an air test rig have been conducted. The main research findings can be summarized as follows:
The working principle of the FPE-LG is proven feasible through the air test rig. However, further test of the FPE-LG in a small-scale ORC system should be performed in the future;
The energy-conversion efficiency of the expander is obviously affected by the intake parameters. The indicated efficiency of the FPE can reach 66.2% and the maximal electric power output of the FPE-LG can reach 22.7 W when the working frequency is 3 Hz and the intake pressure is 0.2 MPa;
Two large-scale vortices are formed during the intake process. To improve the energy conversion efficiency of the FPE, several practical approaches should be adopted to adjust the intake flow and to reduce the energy losses caused by large-scale vortex flow.
Several conclusions may be drawn from this design study. First, a linear generator with a larger thrust force are desirable to increase output power of FPE-LG and avoid collisions between the piston and the cylinder head at low working frequencies. Second, the mechanical strength of related parts in the valve train needs to be improved to further increase the working frequency. Meanwhile, the cam plate intake duration needs to be decreased, whereas the duration of the expansion process needs to be increased. Third, in its final form, installation space is a premium consideration for the FPE-LG. This unit will be designed to be modular, and thus, the number of these units can be flexibly adjusted according to the output power requirement for a wide variety of small-scale vehicle ORC systems.
These results are greatly beneficial to next generation FPE-LG development. More experimental testing of the optimized FPE-LG performance will be conducted using the guidelines established in this work as a foundation. Future CFD simulation with organic working fluid will be conducted in an attempt to further reveal the in-cylinder flow characteristics.