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Textron Lycoming AGT 1500 Turboshaft
The Textron Lycoming AGT 1500 turboshaft is somewhat of a unique engine here because it was designed and developed solely to power a land vehicle; namely the M1A1 Abrams Main Battle Tank, which is the United States' front line battle tank, weighing in at nearly 70 tons. A land turbine, especially one to power a 70 ton tank, has many different design requirements than a helicopter engine. Weight is not a tremendous concern, because the weight of the engine is a mere fraction of the overall vehicle weight, and obviously high altitude operation is not considered. Instead, part power fuel consumption, engine responsiveness, and resistance to FOD are some of the most critical design factors. Nevertheless, a properly designed turboshaft is well suited to a heavy land vehicle application such as this due to the many positive characteristics of gas turbine engines. High power to weight ratio, high torque, multi-fuel capability, ease of maintenance, compact packaging, vibration free operation, ease of starting, and smokeless exhaust all make the AGT 1500 a very effective powerplant for the Abrams Tank. The AGT 1500 was developed specifically for the Abrams, with an output in the 1,500 horsepower class as an alternative to the Diesel engine. Textron Lycoming used the experience gained in developing the T53 turboshaft engine from the Huey Helicopter, in the development of the AGT 1500, even though they are two very different engines.
One of the major concerns in the design of the engine was its operation in very sandy and dusty enviroments, such as those experienced during Operation Desert Storm and the more recent conflict in Iraq. The airflow first passes through dual external cyclonic air cleaners, then is drawn through an engine mounted intake screen, and finally into the engine intake bellmouth. Beyond the bellmouth is an axial flow engine inlet with variable geometry inlet guide vanes. The engine features a twin spool, mixed flow compressor, with a total of 10 stages of compression to assure a high pressure ratio. The engine features a 5 stage axial low pressure compressor, which raises the intake air pressure before feeding it to the counter-rotating high pressure compressor, which consists of 4 axial stages and a single centrifugal stage. An interstage bleed valve just before the centrifugal compressor, as well as variable stator vanes on the low pressure compressor control excess airflow.
From the compressor, the airflow is fed through an annular duct which travels around the outside of the combustor, where air is fed to the large stainless steel recuperator. The recuperator consists of a stacked plate core arranged in a cylindrical shape with a hollow center. The compressor discharge air passes through small holes in the core, makes its way toward the back of the recuperator, and then turns back around and comes back towards the engine. The exhaust flow travels perpendicular to the airflow, exiting the turbine in an axial direction and turning 90 degrees upwards to exit into the exhaust duct through one large square shaped outlet. The recuperator is one of the elements that sets this engine apart from a typical aircraft turbine. It is massive, actually as large again as the rest of the engine itself, which allows for a large surface area and maximum heat transfer, but also adds a considerable amount of weight. The purpose of the recuperator, of course, is to transfer some of the waste heat of the exhaust gas to the compressor discharge air, so that less fuel can be burned to acheive a desired turbine inlet temperature.
The heated compressor discharge air makes its way back towards the engine where it enters the large annular diffuser. The compressed air surrounds the toroidal (scroll) combustor, and it enters the combustor through holes in the liner. Fuel is administered through nozzles in the top of the combustor, where the fuel air mixtures burns at very high temperatures. The hot combustion gas makes a 90 degree turn through the burner nozzle and impinges upon a cooled single stage axial high pressure turbine, which drives the high pressure compressor and the accessory gearbox through a bevel gear and jack shaft in front of the high pressure spool. Because the low pressure turbine is counter-rotating, there is no need for a nozzle between the high pressure and low pressure turbine. The low pressure turbine is a cooled single stage axial turbine, and drives the low pressure compressor. The hot gas then passes through a variable geometry nozzle before it drives the two stage, uncooled, shrouded, axial free power turbine. Variable free turbine nozzles are essential on a land vehicle, because they improve engine response dramatically and also provide engine braking. The AGT 1500 features something called tactical idle, which idles the engine at a higher speed than the normal idle; the turbine nozzles are moved to a neutral position by the engine control unit to reduce the amount of residual torque. When the driver applies the power, the vanes immediately snap into position providing immediate response while the engine accelerates. The twin spool compressor allows for quick engine response as well. The power turbine drives a shaft that runs rearward through the center of the regenerator core and connects to an output reduction gearbox with an approximate 10:1 reduction to drive the output shaft at 3,000 rpm.
The accessory geartrain operates in a perpendicular plane to the engine axis, eliminating the need for a second bevel gear set in the accessory gearbox. Accessories include a starter, a generator, a fuel pump and fuel control, oil pumps, and a hydraulic pump.
The Allison transmission, though technically not part of the engine, is noteworthy as well. The engine couples to the transmission through a lock up torque converter, which provides a fluid coupling at low speeds, but once the tank is on the move locks up with a rigid mechanical coupling to reduce losses. The transmission itself is a transverse cross-drive planetary gearbox with 4 automatically shifted forward speeds and two reverse speeds. The transmission splits the drive up to the left and right, and drives a hydrostatic steering system consisting of a combination variable displacement hydraulic pump hydraulically connected to a fixed displacement hydraulic motor which is designed to accelerates either track individually to allow for steering; This allows for more positive steering than the traditional way to steer a mechanically driven track vehicle, which is to brake the inside track to steer. The system uses the hydrostatic pump and motor to apply additional torque to the outside track to effect a change in direction, which allows for much more positive steering control and less likelyhood of bogging. An integrated hydraulic braking system in the transmission helps to bring the tank to a rest. The final drive units are integrated planetary drives on each side which in turn drives the road wheels. The cogged road wheels lock into the rubber padded steel treads on the Abrams, providing the necessary forward thrust to accelerate the 70 ton tank from 0-20 mph in 6 seconds, to an unofficial, ungoverned top speed of some 70 miles per hour. Honeywell is currently developing the successor to the AGT 1500, called LV100, which features a simplified design based on the General Electric T700 helicopter turboshaft engine. The engine will be more powerful, lighter, more fuel efficient, and more responsive.
Textron Lycoming AGT 1500 Turboshaft
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