GM Whirlfire engine
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 half.
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, 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 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 and high-pressure inlet 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.
Engine braking
In a conventional piston engine, engine braking can be used to slow a vehicle without use of the friction brakes; because the power turbine is not mechanically connected to the compressor in a free-turbine turboshaft engine, a similar effect cannot be accomplished. During the development of the Whirlfire engines, GM found the gasifier turbine could generate more power than was required to operate the compressor, so for the fifth-generation GT-309, GM and Allison coupled the gasifier and power turboshafts using a clutch to extract some of that surplus power. The resulting system, which Allison branded Power Transfer, gave the GT-309 an engine braking effect and improved fuel economy at partial load.Fuel
As external combustion engines, the GM Whirlfire gas turbines were capable of burning a wide variety of fuels; for example turbine engines burning powdered coal were fitted to a Cadillac Eldorado and Oldsmobile Delta 88 in the early 1980s as a response to the 1979 oil crisis. Other potential sources of fuel included methanol, ethanol, liquefied coal, and shale oil.Models
GT-300/302
The first engine, carrying an internal designation of GT-300, did not have a regenerator. The GT-300 had an output of when the gasifier turbine was spinning at 26,000 RPM and the free turbine was spinning at 13,000 RPM. The weight of the entire engine unit was. The GT-300 was fitted to an "Old Look" transit bus, which was branded "Turbo-Cruiser". To reduce overall size, the single large burner was replaced by two smaller burners and the engine was re-designated GT-302, which was fitted to Firebird I.The GT-300 was designed with a 3.5:1 compression ratio and nominal design turboshaft speeds of 24,000 RPM and 12,000 RPM. Engine accessories are driven by the gasifier turboshaft through a perpendicular bevel gear arrangement; a conventional automotive starting motor is used to crank the accessory drive shaft. A new nickel-base alloy, designated GMR-235, was developed and patented for the turbine blades in the Whirlfire engine.
Externally, the Turbo-Cruiser was distinguished from piston-powered buses by "turbocruiser" script lettering on the sides, blanked-out rear windows, and a large central exhaust stack at the roof. The rearmost seats were replaced by "a complete mobile laboratory with a large instrumentation panel" for two engineers. Operating experience with the Turbo-Cruiser showed the engine's mechanical durability; according to W.A. Turunen, "on several occasions, pieces of instrumentation have passed through the machine. The turbine buckets were bent, but in no instance did they fail even after subsequent running of damaged parts." The bus accumulated in testing.
Brake-specific fuel consumption was a notable issue, which at 1.63 lb/hp·h was significantly greater than that of a comparable Detroit Diesel 8V71 diesel engine, even though the turbine was lighter. Other planned improvements would target throttle lag, which was caused by accelerating the gasifier turbine to peak speed, and lack of engine braking.
GT-304
GT-304 was the first GM gas turbine to include a regenerator, which used exhaust heat to warm intake air, improving fuel consumption to 0.77 lb/hp·h. As fitted to Firebird II, GT-304 output was at a gasifier turbine speed of 35,000 RPM. The gasifier turbine idled at 15,000 RPM and the power turbine operated at up to 28,000 RPM. Overall compression ratio in the gasifier stage was 3.5:1. Turbine inlet temperature was increased to from ; after GM Research re-rated the temperature resistance of the GMR-235 superalloy. With the regenerators, the engine weight increased to ; each regenerator was. 7.27:1 reduction gearing made the output shaft speed compatible with conventional automobile accessories. A fluid input coupling was used between the engine and transmission; in addition, larger accessories were powered from the transmission, not the gasifier turboshaft, as it had been discovered that at idle, accessory power draw could exceed available surplus power.The GT-304 also was fitted to the first Turbo-Titan, a heavy-duty Chevrolet Model 10413 truck-tractor with tandem rear axles; Turbo-Titan was tested with various loads, demonstrating superior acceleration and gradeability compared to the Loadmaster V-8 engine that was removed, a overhead valve V-8 with output.
GT-305
The GT-305 fitted to Firebird III had an output of and weight of. With a regenerator and additional component refinements, GT-305 achieved a brake-specific fuel consumption of 0.55 lb/hp·h, an improvement of 25% compared to the earlier GT-304; similarly, the engine weight of the GT-305 was reduced by 25% compared to the 304. External dimensions were long, high, and wide. Exhaust temperature had been reduced considerably; the GT-305 exhaust was at full power, decreasing to at idle.Firebird III had a two-cylinder auxiliary power unit for accessories and a special grade retarder to simulate engine braking, which Jan Norbye criticized as resulting from "the refusal of the turbine experts to tackle the problems at the base... these two systems seem of dubious value except in an application where cost is no object".
The engine was redesignated GMT-305 in 1959 and further development for regular production was handed off from GM Research to Allison Transmission. As the GMT-305, it incorporated approximately of nickel in alloys, including the turbine blades, turbine wheels, turboshafts, turbine bolts, turbine and bulkhead casings, and combustion chambers. The first GMT-305 prototypes began shipping in November 1959 for fitment to U.S. military vehicles, including the M56 Scorpion and a 28-foot personnel boat. The Whirlfire-powered M56 underwent winter conditions testing and accumulated of service with little trouble. In addition, the GMT-305 was fitted to an ore-hauling truck at an open-pit nickel mine in Sudbury, Ontario.