Novel opposed-piston free-piston linear generator for hybrids seeks to overcome drawbacks of earlier designs

Design of opposed-cylinder free-piston linear generator from earlier works with single, centered linear generator. Click to enlarge.

A researcher at the Nanjing University of Science & Technology is proposing a novel opposed-piston free-piston linear generator for use in series hybrid electric vehicles. According to his simulations, Ling Huang says, the linear generator could deliver 15 kW average output with a generating efficiency of 42.5%.
A number of research groups have explored the development of a free-piston linear generator, including work at West Virginia University (WVU), Sandia National Laboratory (SNL), Chalmers University of Technology, and Shanghai Jiaotong University (earlier post), among others. Most of this work focused on a similar design: a single linear generator placed between two opposed cylinders, as in the diagram at right from Shanghai Jiaotong.

Although most of the [earlier] work was focused on a similar design [as above], few designs have been successfully developed up to now. This is mainly because the motion of the free-piston in this kind of design is difficult to control under the interaction of the combustion in the two opposed cylinders.

To overcome the drawbacks in the reported opposed-cylinder free-piston liner generator...a novel opposed-piston free-piston linear generator is proposed...The main parts of the new designed system are a central combustion chamber and two linear generators. The central combustion chamber has two opposed pistons.

Firstly, inherent mechanical balance of the system can be realized when the two linear generators are synchronized. Secondly, the central combustion chamber is benefit for highly efficient scavenging via intake and exhaust ports without using valves. Besides, higher compression ratio can be obtained for the same stroke length of linear generators.

—Ling Huang 2012

Sketch of novel opposed-piston free-piston linear generator with two linear generators. Huang 2012. Click to enlarge.

In an SAE paper, Huang describes a modeling and simulation study of the new free-piston linear generator. Each of the two linear generators in the system is equipped with a mechanical spring, which is used for storing mechanical energy in the power stroke and releasing mechanical energy in the compression stroke.
One working cycle of the proposed free-piston linear generator includes two strokes (compression and combustion). In the compression stroke, driven by the fore of the mechanical spring, the piston moves to the middle of the cylinder; combustion then drives the piston backward.
One of the important control variables is ignition timing. As there is no crankshaft or rotational motion to provide a crank angle for timing, Huang proposes a piston position for use for ignition timing. Ignition advance (piston at 4.4 mm) provided a higher peak pressure in the cylinder (67 bar) and better generating efficiency (42.5%).

For further study, an accurate model of the linear generator and combustion will be developed. The control method of linear generator and the movement of free-piston will be investigated, and special attention will be [paid to] the synchronization of th left and the right pistons.

The effects of [HCCI] operation will be investigated by future accurate combustion model. Furthermore, natural gas will be investigated as the fuel of the system.

—Huang et al.