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.
Design considerations of a linear generator for a range extender application