Study on the Control Strategy of Free Piston Expander-Linear Generator Used for Organic Rankine Cycle Waste Heat Recovery

This study presents a free piston expander-linear generator (FPE-LG) for engine waste heat recovery. The control strategies based on displacement (control strategy A) and valve timings (control strategy B) are proposed. The operation and output characteristics with different control strategies are conducted. The results show that control strategy has significant effect on the FPE-LG performance. When FPE-LG operates with control strategy A, the operation frequency is determined by the intake pressure, advanced exhaust duration, fixed position point and external load resistance, whereas it is only related with the intake/exhaust time and advanced exhaust duration (t_a) for control strategy B. The external load resistance has slight influence on the stroke length for these two control strategies. For control strategy A, when advanced exhaust duration is more than 40 ms, the stroke length rises with the intake pressure, whereas it first rises and then reduces when advanced exhaust duration is less than 40 ms. There exist the optimal external load resistance and advanced exhaust duration to achieve the maximum power output with control strategy A, whereas the average power output reduces with the increasing advanced exhaust duration when using control strategy B. Stable operation and high power output can be obtained by using different control strategies and adjusting relevant operation parameters.