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a What is a free-piston engine and is an engine of this type employed as a road-vehicle unit?
Page 55
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AA free-piston engine is more correctly a free-piston gasifier that is used in conjunction with a turbine. The free-piston section takes the place of the burner, or burners, of a conventional gas turbine.
Because all the heat produced by a gasifier is converted into hot gas instead of mechanical energy, it does not require a crankshaft. A Ford unit was described in an SAE Paper published in 1957 that was developed for application to a farm tractor. So far as is known, there have been no examples of applications to road vehicles and no commercial applications are currently extant.
The Ford unit has two pistons, each of which has a small-bore inner section and a largebore outer section, the inner section being the power piston and the outer section the compression piston. The two power pistons operate in a common cylinder having inlet ports at one end and exhaust ports at the other, the two-stroke cycle being either diesel or spark-ignition. A pair of synchronizing racks ensure that the pistons operate in unison.
The compressor pistons close discharge ports at about mid-stroke and further movement compresses the remaining air in the ends of the cylinders. The compressed air acts as a spring and causes the pistons to bounce back to their inner positions. Compressed air from an external source is used for starting.
Like the two-shaft gas turbine, the gasifier can run up to any speed irrespective of turbine speed and a good low-speed torque can therefore be produced. The unit will run on practically any type of hydrocarbon fuel.