Piston Ring Friction Losses in a Free Piston Engine with Variable Frequency

Free piston linear engines (FPLE) directly convert the piston reciprocating motion into electricity using an integrated linear alternator. Unlike conventional crankshaft engines, the FPLE’s motion is variable and is not restricted between the predefined or fixed dead centers. The variable FPLE motion is governed by the system of forces acting on the translator (reciprocating) mechanism. In some cases, energy storage devices like stiff mechanical springs are used in the FPLE system for increasing frequency and power density. Variations in the forces acting on the reciprocating mechanism will significantly influence the dynamics, in-cylinder thermodynamics, and mechanical friction losses of FPLE. While the researchers til today focused on finding the piston ring frictional characteristics for one design and operating point, no investigation was performed to understand how different design and operating variables impact the frictional characteristics of a free piston engine. Furthermore, no investigation was carried out so far for a free piston engine with a dominant energy storage system (i.e., stiff mechanical springs). The novelty of this article lies in analyzing and understanding the effect of the alternator’s moving mass and spring stiffness on FPLE dynamics and piston ring frictional characteristics. Two different cases are considered. The first case deals with the calculation of piston ring frictional losses for different spring stiffness at the same operating frequency. The second case deals with the calculation of piston ring frictional forces for different spring stiffnesses with constant translator moving mass. The piston rings’ power losses on average remained constant for all the spring stiffness values in Case I. This loss value corresponds to 3.01% of the total fuel energy input. In Case II, the frictional power losses increased as the spring stiffness was raised. In this case, the frictional losses increased from 2.12% to 3.37% of the fuel energy with spring stiffness.