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Piston 2 forces

Posilion and pressure vectors í L____

Piston 1 forces — Line of position vector

Piston 2 forces

Line of position vector

Line of position vector

(b> Realistic case

Piston 1 forces

Piston 2 forces

Piston 1 forces

(MXJ)j

Fl«. I 1.4. Dynamics of a free-piston engine with two pistons.

absorb work from 2, requiring thai it be driven by work from 1 through, for example, its linear alternator acting as a motor.

The piston-displacer free-piston engine

Fig. 11.5 shows a generalized piston-displacer machine with three moving components—piston, displacer and cylinder. This figure defines the nomenclature to be used in the simplified cases described below.

The simplest special case of the general piston displacer machine of Fig. 11.5 is shown in Fig. 1 1.6, representing the Harwell free-piston electric generator (Cooke-Yarborough 1975). Here the driving energy for I he displacer comes from the spring between the displacer and cylinder (Kj) which, as a result of the phase angle between the displacer and cylinder motion, has a work component adequate to overcome displacer damping. I lie major cycle work flows through the piston damping force (FDl) represented in the case of the Harwell machine by its alternator.

Fig. 11.7 is another special case of Fig. 11.5—a simple, relatively low-speed engine with the displacer sprung to the bounce space. The displacer roil provides a difference in area between hot and cold ends of the displacer which provides the major drive force for the displacer. This is the design of some of the small Sunpower models. In order for this system to operate, it was necessary to have a very light displacer since the acceleration forces available were weak (Agbi 1973).

Displacement and pressure vectors (V

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