Analysis of the Scavenging Process of a Two-Stroke Free-Piston Engine Based on the Selection of Scavenging Ports or Valves
The free-piston engine generator (FPEG) is a linear energy conversion device with the objective of utilisation within a hybrid-electric automotive vehicle power system. In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventional engine (CE) of the same size, and the difference in the valve timing is compared for both port scavenging type and valve scavenging type, with the exhaust valve closing timing is selected as the parameter. A zero-dimensional
... mulation model is developed in Ricardo WAVE software (2016.1), with the piston dynamics obtained from the simulation model in Matlab/SIMULINK (R2017a). For the CE and FEPG using scavenging ports, in order to improve its power output to the same level as that of a CE, the inlet gas pressure is suggested to be improved to above 1.2 bar, approximately 0.2 bar higher than that used for a CE. If a CE cylinder with exhaust valves is adopted or referred to during the development of an FPEG prototype, the exhaust valve is suggested to be closed earlier to improve its power output, and a higher intake pressure is also suggested if its output power is expected to be the same or higher than that of a CE.        . Most of the reported FPEG prototypes employ a two-stroke thermodynamic process, and the gas exchange process is performed with intake/exhaust ports or valves  . The cylinders adopted in an FPEG prototype at this stage are mainly from commercial products, and the others are designed and manufactured with a commercial engine taken as a reference [21, 23, 24] . For a conventional two-stroke engine, the crank angle is used as feedback to decide the valve timing. On the other hand, for the FEPG, the valve timing cannot be decided according to the crank angle as that of the reciprocating engine due to the elimination of the crankshaft system. However, very little research has been undertaken to examine its scavenging process based on the selection of scavenging ports or valves. Researchers at Beijing Institute of Technology investigated the scavenging process of an FPEG prototype using computational fluid dynamics  . A time-based numerical simulation program was developed in Matlab to describe the movement of the piston, and a parametric analysis was undertaken based on the simulation model. The influence of the engine effective stroke length, valve overlapping timing, system operating frequency, and intake pressure, etc. on the engine scavenging performance was discussed. From the simulation results, it was suggested that in order to obtain high scavenging and trapping efficiencies, a combination of high effective stroke length to bore ratio and long valve overlapping timing with low intake pressure should be employed  . The scavenging process of a two-stroke FPEG prototype was investigated by researchers from Sandia National Laboratories, aiming to improve its thermal efficiency and exhaust emissions . The scavenging system was configured using computational fluid dynamics, 0/1 dimensional modelling, and single step parametric variations. Different design options were analysed and compared, including the application of loop, hybrid-loop, and uniflow scavenging methods, different charge delivery options, etc. Meanwhile, the intake/exhaust port arrangement, valve lift and valve timing setting, and charging pressure selection were adjusted and analysed. The computational results indicated that a stratified scavenging scheme employing a uniflow geometry supplied by a stable, low temperature/pressure charge should best optimise the engine efficiency and emissions characteristics  . In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventional engine of the same size, and the difference in the valve timing is compared for both the port scavenging type and valve scavenging type, with the exhaust valve closing timing being selected as the parameter. A zero-dimensional simulation model is developed in Ricardo WAVE software, with the piston dynamics obtained from the simulation model in Matlab/SIMULINK. Engine performance with different intake pressures and various exhaust valve closing timings are predicted and analysed, aiming to provide guidance for the design and optimisation of the FPEG scavenging process with either scavenging ports or valves.