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Free Piston Engine Linear Generator (FPEG) with features of thin and compact build, high efficiency and high fuel flexibility is developed. The FPEG consists of a two-stroke combustion chamber, a linear generator and a gas spring chamber. The key technologies to realize stable continuous operation are lubricating, cooling, and control logic. This paper proposes the original structure of the FPEG for enabling stable continuous operation. The main feature is a hollow circular step-shaped piston. The smaller-diameter side of the piston constitutes the combustion chamber, and the larger-diameter side constitutes the gas spring chamber. The larger cross-sectional area of the gas spring chamber leads to lower compression temperature of the gas spring chamber and consequently decreased heat loss. In addition, an oil cooling passage is built in the column stay, which ensures the enough cooling ability of the piston.

Free Piston Engine linear Generator (FPEG) that is thin and compact and has high efficiency and high fuel flexibility has been developed. The developed FPEG consists of a two-stroke combustion chamber, a linear generator, and a gas spring chamber. This paper focuses on the control logic of the linear generator, where the generator can be changed instantly to act as a driving motor, according to demand. Both the position and velocity of the piston are selected as feedback parameters for the control logic. The proposed feedback method realizes stable and robust control behavior with respect to abnormal combustion conditions, such as pre-ignition. In addition, the control logic must satisfy the following requirements. First, in order to achieve stable two-stroke combustion, the position of the piston is precisely controlled, especially near the top dead center (TDC) and the bottom dead center (BDC).

In this paper, a friction estimation method using a tire vibration model is proposed. It is based on the frequency characteristics of the wheel speed vibration which are related to the tire-road friction. The recursive least squares and the instrumental variable method are applied for online estimation. The experimental result shows that the proposed method applied to free rolling tire detects friction change from dry asphalt to iced road at constant speed without braking, accelerating or cornering.