Development of free piston engine linear generator system and a resonant pendulum type control method

Abstract

A free piston engine linear generator, which has the potential of the compact physique, high brake efficiency and high flexibility for fuel, has been developed. The developed free piston engine linear generator consists of a two-stroke combustion unit, an air-bounce chamber and a linear generator. The key technologies to realize the continuous operation are the control method and lubricating and cooling strategies. The proposed structure is featured as a piston shape with two different diameters coaxially, called a “W-shape” piston, which has an empty space inside for the oil cooling path. The performance of the structure is evaluated by a one-dimensional cycle simulation. The result indicates the possible output power of 10 kW and thermal efficiency of 42% using premixed charge compression ignition combustion strategy. The control method is another challenge of the free piston engine linear generator. This work proposes two strategies of the position feedback control method and the resonant pendulum type control method. The first method has the function of the feedback loops for the piston position and velocity so that the piston motion follows the reference profile calculated in advance. The experimental results show the limited range of operation because the fixed profile does not absorb the deviation of the piston motion due to the combustion deviation. The second method is based on the speed control without a fixed reference profile of the piston motion. The experimental result shows the robustness to the change in operating parameters such as ignition position, amount of fuel and desired power output.

Recent progress on performance and control of linear engine generator

“…80 However, the fluctuations in pre-set position cases increase for severe retarded ignition positions or fuel injections, resulting in unstable combustion. 126 The same trend is observed with other operating parameters such as frequency and desired power. This indicates that one pre-set position profile will favour stable operation within limited operating conditions.…”

“…Therefore, the resonant pendulum-type control method is proposed, and its block diagram is shown in Figure 28. 13,126 The resonant pendulum-type control strategy was utilised to mitigate linear generators’ low efficiency in low-speed regions. As indicated in Figure 29, the LG functioned only around the mid-stroke, where the piston speed was highest, allowing the piston to move freely during the remainder of the stroke.…”

“…The test result demonstrated that the resonant pendulum-type control method is robust to changes in operating parameters compared with the targeted position feedback control method. 126…”

Parametric Simulations on Leakage and Performance of a Miniature Free-Piston Generator (MFPG)

“…The modeling of the MFPG system is a multi-physics problem since it couples a linear motor with an internal combustion engine [54]. Therefore, a dynamic model of the MFPG prototype is proposed here, consisting of a heat release model, gas exchange model and electromagnetic force model, etc.…”

Operation of a turbine-compound free-piston linear alternator

“…8,9 Chalmers University of Technology studied a 2-stroke FPE with combustion chambers at both ends utilizing HCCI combustion. 10 New Castle University and Beijing Institute of Technology and others have worked together for investigations on various details of FPE operation [11][12][13][14][15][16][17][18][19] Other groups have contributed with independent research in emissions and control 20,21 and recently a more comprehensive review was done by Zhou et al 22…”