Free-Piston Gas Generator

a unit designed to provide a working fluid of the required parameters for a gas turbine; the unit consists of a free-piston internal-combustion engine and a piston compressor. The working fluid is a hot compressed mixture of fuel-combustion products from the engine and scavenging air from the compressor. The engine is a gas generator and drives the compressor. The compressor supplies compressed air to the engine cylinders for scavenging and supercharging. A free-piston gas generator together with a gas turbine forms a power unit.

A unit consisting of a gas generator and an expansion engine was first proposed and constructed by V. I. Grinevetskii. The first model of a free-piston gas generator was developed in 1922 and 1923 by the Soviet engineer E. E. Lontkevich for a gas turbine installed in a vehicle. The first commercial example of a free-piston gas generator, using Pescar’s design, was introduced in France in 1951.

Free-piston gas generators generally use two-stroke uniflow diesel engines with uniflow scavenging and high-pressure supercharging. The combustion products or a mixture of combustion products and excess scavenging air enter the receiver and then the turbine. The gases exiting from a free-piston gas generator have a temperature of 400°-550°C and a pressure of 0.4–0.5 meganewtons/m² (4–5 kgf/cm²). These relatively low parameters for the working fluid make it possible to design inexpensive and economical gas turbines in the range of 10 to 50 megawatts.

Figure 1. Gas turbine power unit with free-piston gas generator: (1) fuel injector, (2) combustion chamber, (3) diesel piston, (4) air delivery valve of the compressor, (5) air intake valve of the compressor, (6) compressor piston, (7) bounce chamber, (8) receiver, (9) gas turbine, (10) exhaust ports, (11) air intake ports

A gas turbine power unit with a free-piston gas generator combines the positive features of the diesel and gas turbine engines, and the efficiency may reach 40 percent. Several free-piston gas generators may be used on one gas turbine. Gas turbine power units with free-piston gas generators are used in various branches of industry, in transportation, and in power engineering.

The disadvantages of free-piston gas generators are the high use of energy at idle and under light loads, the need for bulky gas pipes, and the complexity of the synchronization of the piston operation.