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Automotive gas turbines are a type of turboshaft engine specifically designed for the purpose of roadgoing automobile or truck propulsion. Although they have generally been considered an evolutionary dead end and there is no major automotive company actively pursuing their development, they are a fascinating area of technology for students of both automobiles and of gas turbines in general.

Generally, automotive gas turbines differ from aircraft or aeroderivative industrial engines in several ways which are common to the needs both of the roadgoing automobile and to the varying market conditions automobiles and turbine aircraft occupy in our world today.

Because aircraft gas turbines are sold in small quantities to price-insensitive markets, they are generally built largely or wholly of light, high energy metals fabricated pieces and often employ intricate complex axial flow or compound compressor sections. Automotive gas turbines almost invariably use single stage centrifugal compressors and are built with structural components of aluminum or iron castings. Therefore, they could be produced at drastically lower costs at a small to moderate increase in weight for a given power. They are also almost all of the free turbine or two-shaft type, where the compressor and accessories are powered by a compressor turbine (or gasifier in GM parlance) whereas the output-transmission, generator, or propeller-is turned by a free spinning power turbine. (This is, for aircraft mechanics, similar to the Pratt & Whitney PT-6 series as oppoosed to the Rolls Royce Dart or Garrett AiResearch TPE-331.)

Straight gas turbine engines-those of simple or open cycle-are generally efficient over a narrow envelope of engine speed, gas flow and power output. This works well in helicopters (and race cars) because they are always at high output, and in fixed wing aircraft which can climb to altitude where the thin air results in lowered gas flow and fuel burn for lower shaft output but higher true airspeed. In a roadgoing auto or truck it would lead to extremely poor part-throttle fuel consumption, as well as high exhaust gas temperatures which would be a serious issue in city traffic.

Automotive gas turbines get around this by using thermal recuperation or regeneration to transfer excess heat from the exhaust immediately aft of the turbine wheels and put it back into the engine's air flow between the diffuser and burner sections. Usually recuperation refers to using a passive heat transfer system, such as an intercooler on a turbocharged piston engine, whereas regeneration uses an active method such as a Cercor porous wheel slowly rotated which is heated by the exhaust and cools off in the diffuser airstream. This makes the engine efficient over a wider power band and reduces total peak gas flow:many engineers compare this to the use of negative feedback in electronic amplifiers. Exhaust gas temperatures are also greatly reduced, as to some extent noise is as well.

Another requirement of automotive engines not found in aircraft service is engine braking. Automotive gas turbines provide engine braking in one of two ways. General Motors designed its engines with a remotely actuable overrunning clutch to link the separate gasifier and power turbine sections of the engine. This allows the compressor to act as a load when the vehicle is at high speed and no power is applied, and also allows the engine to be "locked up" at cruising speed where the engine acts as a single-shaft engine (like a 331 Garrett) for greater cruise efficiency.

Chrysler and most other automotive developers eschewed the clutch system in favor of a variable area turbine nozzle, analogious to the two-pitch stators found in some automotive automatic transmisssion torque converters, or to the petal engine nozzles of afterburning turbojets-or a common garden nozzle. By varying the angle of a number of airfoil-shaped blades around the turbine inlet, as a camera iris diaphragm closes and opens, the pressure and flow to the power turbine could be varied allowing for high torque at low rotor speed or actually a negative torque when the rotor was at high speed. It was considered much cheaper to implement and offered even more torque at high power and low PT (power turbine) speeds, requiring fewer gears in the transmission, although the engine braking was not comparable to the GM technique.

Engine and accessory practice was generally similar to that on small aircraft turboshaft engines, with starter-generators permanently coupled to the power section, and like the earliest aircraft turbines most used standard automotive oil products-ATF or SAE 30 motor oil-for lubrication with wet sump systems. (This in fact led to substantial coking issues in the first two generations of Chrysler engines.) Fuel controllers were standard aircraft pieces (which even then cost more than a new car engine: today, with the pretext of product liability, these same controllers cost more than whole new cars-and, aviation moving at a slothlike pace, are indeed still in production for the most part!) and ignition was usually by AC or Champion igniters fired by automotive or neon sign transformers. (Chrysler designed many of its engines integrally with a modified TorqueFlite transmission,and the engine and power steering gearbox were all lubricated from the transmission gerotor pumps!)

For further information on this obscure but fascinating area of technology, the best reference work is "The Gas Turbine Engine" by Jan Norbye. It was published in 1973-by which time the turbine car engine was almost an extinct notion to American automakers-and is probably the most comprehensive source on the subject. There are also substantial web resources which anyone may locate with any search engine with the obvious criteria.

Is the turbine automotive powerplant concept totally extinct? Commercially, yes. But with the availability of powerful CAD and desktop manufacturing software, the increasing availability of CNC multiaxis machining centers, and a powerful desire on the part of enthusiasts to re-create a unique part of automotive history strongly rooted in a very compelling point in time-the heyday of the turbine car, as an idea, was the era of Kennedy, Monroe, and the Rat Pack with their Dual-Ghias, after all-it's entirely possible someone will deign to turn the pictures and drawings in these old books into sand-casting patterns and centerless-ground shafts and Inconel blades. If a bunch of determined Argintineans can build Grand Prix Bugattis from scratch, it's certainly possible a Chrysler Turbine Car or a BRM-Rover engine might once again breathe air-like the fictional dinosaurs in "Jurassic Park".

Unfortunately, I wouldn't hold my breath, though....

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