Abstract
Friction work in free-piston engines is expected to be lower than in crankshaft engines due to the elimination of the crank mechanism. In this paper, friction mechanisms were reviewed and compared between a free-piston and crankshaft engine of similar size. The main friction mechanisms were identified to be the piston assembly including piston rings and piston skirt, valve train system, the crank and bearing system for the CSE, and the linear electric generator for the FPE. The frictional loss of each friction mechanism was estimated and discussed. A Stribeck diagram was used to simulate the piston ring friction during hydrodynamic lubrication, mixed lubrication, and boundary condition. It is found that the FPE doesn’t show advantage on piston ring friction force over the CSE, and the frictional loss from the piston ring is even higher. While the elimination of the crankshaft system reduces the frictional loss of the FPE, and the total friction loss of the FPE is nearly half of the CSE.
Highlights
While the elimination of the crankshaft system reduces the frictional loss of the free-piston engine (FPE), and the total friction loss of the FPE is nearly half of the crank shaft engines (CSEs)
In order to improve the model accuracy and have some representation of frictional losses, a constant value was used to simulate the friction force during the movement of the piston with the mover of the linear electric generator, and the direction of the friction force was assumed to be opposite with the piston velocity [30,31,32,33]
For the FPE, piston skirt is not necessarily to be used to guide the piston motion as the piston movement in a FPE is linear without any rotation around the piston pin, and even if a piston with a skirt is used, it will not contribute to frictional losses as there is no side forces from the connecting rod acting on the piston in the FPE
Summary
The free-piston engine (FPE) is considered a promising alternative to the conventional crank shaft engines (CSEs) [1,2,3,4,5]. A single piston FPE consists of a combustion chamber, and a load and rebound device. The load and rebound devices could be electric generator, gas spring chamber or hydraulic pump [9,10,11,12]. The opposed piston FPE was used almost exclusively in the early stage of the FPE development (1930–1960). It served successfully as air compressors and later as gas generators in large-scale plants [16,17]. This kind of FPE configuration essentially consists of two opposed pistons with a shared combustion chamber [18]. A number of dual piston designs have been proposed and a few prototypes have emerged, both with hydraulic and electric power output [19,20,21,22,23,24,25,26]
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