Over the last century and a half engineers and designers have created a wide array of different combustion engine designs. In spite of certain inherent advantages of some of these designs, the traditional four stroke reciprocating engine has continued to dominate the transportation space. Many of these alternatives have worked in principle but have never been adopted for several reasons. The most important reason is typically the problem of converting the chemical energy of combustion into rotating drive torque to turn the wheels. This has been the case with so-called free piston engines. In its most basic form, the piston in a free piston engine simply oscillates back and forth driven by the alternating combustion energy at either end of a cylinder. Such a system has very low friction compared to a traditional engine.

Conventional engines used today are typically about 25 percent efficient. That is, one quarter of the energy in the fuel they consume is available to drive the vehicle. The actual combustion process is actually extremely efficient converting almost all of the available chemical energy in the fuel into heat energy. The problem is that about one third of that energy goes out the exhaust pipe in hot exhaust gases while another third goes into the engine coolant and oil too keep the temperature of the internal components of the engine below their melting point. Read on after the jump to learn how free piston engines can overcome some of this.

[Source: TreeHugger]

A significant chunk of energy is lost to friction in all the moving and rotating parts such as the camshafts and crankshaft. The absence of these parts and lateral forces inherent in a conventional engine means dramatically less friction as well lower weight. The problem of converting the oscillating mechanical force of a free piston engine remains with a conventional car. However the power of a free piston engine could be harnessed as part of an hybrid electric drive system.

Magnets can be mounted on the piston rod inside of the cylinder with electric coils around the outside of the piston. As the pistons and rod move back and forth the magnets induce a current in the coils. This current can then be used to charge a battery in a series hybrid or ER-EV drive system. Such a system could be up to fifty percent efficient in transforming the energy in the fuel into electrical energy. A free piston engine like this could even be used with an HCCI combustion process that could improve the overall efficiency. This compact design could potentially be the best solution to a problem of a range extender for electric vehicles.

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