Determination of TDC in a hydraulic free-piston engine by a novel approach

•A new approach to determine TDC for indicator diagram of HFPE was investigated.•Test measurement method and error cause of HFPE test system was researched.•The piston dynamic equations were given during the HFPE operation.•The efficiency from the thermal energy to the hydraulic energy is 77% was obtained.

In indicator diagram analysis, precise determination of the top dead center (TDC) position is vital. However, in a hydraulic free-piston engine (HFPE), the piston is not restricted by any mechanism, and it is in the “free” state. This means that the methods used to determine the TDC position in a conventional engine are not applicable to an HFPE because its different structures. Thus, an HFPE indicator diagram has been developed on the basis of the piston displacement and in-cylinder gas pressure measured with respect to time. However, the vibration of engine block during the HFPE operation will lead to the errors between the real minimal volume of cylinder and the measurement TDC position by measuring the piston displacement. Therefore, it is necessary to find a new approach for determining the TDC position for indicator diagram analysis of an HFPE. In this paper, we discuss a novel solution to the problem for determining the TDC position using a one-to-one correspondence between the geometrical characteristics of the pressure diagram and the maximum pressure point position relative to the TDC position. The results of the experiments we conducted show that the proposed method can determine the TDC position.