The GM Whirlfire gas turbine engines were developed in the 1950s by the research division of General Motors Corporation and fitted to concept vehicles, including the Firebird concept cars, Turbo-Cruiser buses, and Turbo-Titan trucks through the 1960s. They are free-turbine turboshaft machines with two spools: one compressor/gasifier turboshaft and one power/output turboshaft sharing a common axis without a mechanical coupling between them. Fuel consumption of the first-generation GT-300 was high compared to piston engines, so thermal wheel regenerators were added to the second-generation GT-304, cutting consumption by approximately .
Initially, the engines were built by GM Research, but starting with the third generation GT-305, the Allison Engine division took over responsibility for commercializing gas turbine technology. This division, later merged with Detroit Diesel and renamed Detroit Diesel Allison, would produce approximately one hundred of the final design GT-404 engines, which incorporated ceramic components. Cost, driven by exotic turbine alloys and materials, and fuel consumption proved to be insoluble issues compared to conventional piston engines and further development of gas turbine engines at General Motors was discontinued in the early 1980s.
Design
Because the Whirlfire engines are free-turbine machines, maximum torque is developed when the output shaft is stalled (not turning), and is approximately double the torque developed at full power output. In addition, the lowest fuel consumption is achieved at full power.
Gas path
In the third-generation GT/GMT-305, the air intake is arranged axially with the turboshafts, which share a common horizontal axis. The single-stage rotary compressor draws air at atmospheric conditions through the intake and expels compressed air out radially into the side compartments, where the rotating drum-shaped regenerators preheat the compressed air using heat extracted from the exhaust gases. The compressed air is channeled through the combustors, where it is mixed with fuel and burned, and the resulting combustion gases are expanded through first the gasifier turbine, which is on the same shaft and is used to drive the rotary compressor, then through the power turbine, which is on the output shaft.
At the gasifier turbine inlet, the design temperature is . For the GMT-305, the rotary speed of the gasifier/compressor shaft is 33,000 RPM, while the power shaft turns at 24,000 RPM at full power; the power shaft speed is stepped down to 3,500 RPM through reduction gearing at the output to make it compatible with automotive components. A governor allows the output shaft to turn up to 4,500 RPM. An accessory shaft is driven from the gasifier/compressor shaft for engine ancillaries, including a gear-type lubrication oil pump.
Regeneration
Initially, the first engines developed (GT-300 and 302) did not have a regenerator, but adding regeneration to recapture heat from the exhaust gases was found to reduce fuel consumption by for the second-generation GT-304, so subsequent generations of GM Whirlfire gas turbine engines incorporated a regenerator.
For the GT/GMT-305, two drum regenerators are arranged to either side of the turboshafts in large compartments; the regenerators turn at approximately 30 RPM. Within each side compartment, a vertical bulkhead divides the regenerators into low-pressure exhaust (occupying approximately of the regenerator) and high-pressure inlet (the remaining ) sections. As a regenerator rotates through the exhaust section, it picks up waste heat from the exhaust gases, then as it continues to rotate into the inlet section, the heat is transferred to the compressed air, preheating it before fuel is added in the combustors.
In addition to improving thermodynamic efficiency, the regenerators serve to muffle engine noise and heat, reducing exhaust temperatures. The exhaust section operates at a lower pressure than the inlet section, so regenerator sealing is important to minimize loss of high-pressure compressed air.