Armor-plated auxiliary power

A new design for an auxiliary power unit that dispenses with a gearbox and oil-lubrication equipment has been field-tested in an M1A1 main battle tank. By Robert Nims

By Robert Nims
The life-cycle cost of auxiliary power units (APUs) has not dropped significantly over the past 20 years, and only minor reductions have been achieved through enhanced aerodynamic performance. Taking a different approach, AlliedSignal Aerospace in Torrance, Calif., has applied air-bearing air-cycle machine technology from its aircraft environmental control system (ECS) and the technology from its high-performance, high-speed permanent-magnet (PM) generator to develop a small gas-turbine engine that operates on air bearings without oil and without a gearbox.

AlliedSignal's design concept significantly reduces APU life-cycle cost by creating a very simple turbomachine and by using a bearing system with nearly unlimited life. These factors plus the elimination of the gearbox and ancillary oil-lubrication equipment result in a significant improvement to APU reliability, maintainability, per-unit initial cost, and total life-cycle cost.

The trade-off is that this simple turbomachine requires electronics and electronic power conditioning that are more sophisticated. Technology in this area is improving very rapidly, however, with the continuous development of higher-power, increased-capability devices that are a fraction of their predecessors' size, weight, and cost.

The development of the oilless, gearless, and bleedable APU was initiated in March 1991 under a contract administered by the U.S. Army Armored Systems Modernization Program Office in Warren, Mich. The underarmor auxiliary power unit (UAAPU) was developed as a major subassembly for an advanced military armored tracked vehicle. This vehicle was originally named the common-chassis advanced-technology transition demonstrator (CCATTD), and its prime contractor was Teledyne Vehicle Systems in Muskegon, Mich.

The UAAPU, a small gas-turbine engine with a combined bleed, has an electrical-power output capability of 32 kilowatts (43 shaft horsepower). The intent was to develop this advanced-technology APU to meet the CCATTD's pneumatic- and electrical-power requirements while the vehicle was stationary. The Army terminated the CCATTD program before the vehicle was completely developed, however, and the UAAPU was never fielded under this program.

As the CCATTD program came to a close, the U.S. Army Armor Center at Fort Knox, Ky., identified the oilless, gearless, and bleedable underarmor APU as a viable technology for field testing. Under the U.S. Army Battle Lab Initiative, the Mounted Warfare Battlespace Laboratory at Fort Knox began a program to demonstrate the UAAPU on an M1A1 main battle tank. This program gave AlliedSignal the opportunity to test this unconventional design in the field.

Technical Features

The oilless, gearless, and bleedable air-bearing gas-turbine APU consists of a single-rotor, simple-cycle gas turbine driving a shaft-speed PM electric generator. (The APU system in its nonrecuperated version is shown on the next page.) With this system, the compressor pulls outside air through a small heat exchanger before compressing it to approximately 3 atmospheres (45 pounds per square inch absolute). The air then enters the combustor, where it is mixed with fuel; this mixture is ignited to produce temperatures of approximately 1,650°F at the combustor exit. This high-temperature, high-pressure air is expanded through the turbine wheel, which extracts the energy in the air and converts it to shaft power.

The shaft power directly drives the generator and compressor, which are mounted on the same shaft as the turbine wheel. The generator absorbs approximately 30 percent of the turbine power, while the compressor absorbs the rest. The electric power developed in the PM generator is three-phase ac at the same frequency as the rotating shaft.

During turbogenerator start-up, the power-electronics unit (PEU) incorporates an inverter that uses dc power to drive the PM machine (rotor and stator) as a motor, thereby eliminating the need for a separate starter. Once the turbogenerator is operating at its design speed (66,000 rpm), the same PM machine acts as a generator supplying power to the vehicle on demand.

The high-frequency ac output (at approximately 1,100 hertz) from the fixed-field PM generator is converted to dc by using the PEU start inverter as an up-chopper to regulate the dc output voltage to match the vehicle bus voltage.

In the United States, all armored military vehicles are now required to incorporate a nuclear, biological, and chemical (NBC) filtration system. This type of system is usually combined with an environmental control system (ECS) to filter and condition the air (for temperature, pressure, and flow) that is sent to the vehicle's crew. An advanced NBC/ECS requires high-pressure air to operate. Gas-turbine APUs are ideal for performing this task because high-pressure "bleed" air is available for takeoff at the compressor discharge without the addition of significant auxiliary equipment. The oilless and gearless APU is designed to deliver 15 pounds per minute of bleed air, the current standard NBC/ECS requirement for the Army's armored-vehicle fleet. The APU can supply a full complement of bleed air and simultaneously deliver 10 kilowatts of electrical power.

Two cooling circuits are incorporated in the APU design. To dissipate the heat from power losses by the turbogenerator's air bearing and generator, a small amount of compressor-discharge air is filtered by a reverse-facing probe at its takeoff point. The air is cooled in a small air-to-air heat exchanger located upstream from the compressor. The cooled air then enters the bearing and stator sections. The heat exchanger's heat-rejection source is the compressor inlet flow.

The semiconductor devices in the PEU also require cooling. These devices are mounted onto a cold plate, and fuel from the vehicle fuel tanks is circulated through the cold plate in a closed loop. This cooling circuit removes the heat generated by semiconductor power losses.

This program developed a new approach to supplying and metering fuel into the APU. Because the gearbox was eliminated, the standard gearbox-driven fuel pump could not be used. An improved fuel-metering system combines the delivery and metering functions into a single component. A conventional gas-turbine APU uses a fuel pump that has a constant speed and output pressure plus a fuel-metering valve to control the input pressure to the combustor fuel nozzle. In the oilless and gearless APU design, an electrically driven variable-speed fuel pump pulls fuel out of the fuel tank and is commanded to vary its speed by the APU digital electronic fuel control.

By varying the speed of the pump, output pressure is varied as well, and the combustor fuel nozzle's input pressure is therefore directly adjusted. Life- cycle cost of the APU is lowered by eliminating the fuel-metering valve, a sophisticated electromechanical component that has been somewhat unreliable in conventional gas-turbine APUs.

Oilless, gearless, and bleedable APUs with and without recuperators have been developed. The primary reason for adding the recuperator was to improve the thermal-cycle efficiency, thereby reducing the APU's specific fuel consumption. The compromise is that the recuperator adds weight and volume to the APU. The recuperated system requires a slight modification to the system, as shown in the above figure, where the compressor discharge flow is ducted directly into the combustor section. In the recuperated system, the air goes first into a high-temperature heat exchanger where it is heated before entering the combustor. The source of heat on the recuperator low-pressure side is the high-temperature turbine-exhaust air. By preheating the combustor inlet air, less fuel is required to produce the same turbine-inlet temperature.

Design Flexibility

The oilless, gearless, and bleedable APU has inherent advantages in three critical areas. First, it has considerable flexibility regarding the type of electrical power it takes and delivers to the vehicle. At voltage levels between 24 and 500 volts, dc power can be accommodated with relatively minor changes to the APU. On the two power units that have been developed to date, only a stator change in the turbogenerator and a semiconductor-device change in the PEU have been required to develop a 28-volt dc APU for the M1A1 tank and a 270-volt dc APU for the CCATTD.

Early design work has been performed on a dual-voltage APU (both 28- and 270-volt dc) using the same simple, single-rotor turbogenerator. Dual voltage is achieved either by putting two sets of stationary stator windings in the same generator housing or by having two choppers in the PEU.

The second advantage is that the oilless, gearless, and bleedable APU is more flexible in the design of its physical packaging arrangement than a standard APU. This flexibility is a result of eliminating the conventional APU gearbox. The new APU has five major components (turbogenerator, PEU, turbine-exhaust muffler, fuel pump, and air-inlet assembly), which can be stacked, laid lengthwise, or installed vertically with very few installation constraints. The longest single component, the turbogenerator, is 24 inches long and 11 inches in diameter, and is the one fixed-geometry component. There are no direct drives between the five major components—only air ducts, fuel lines, and electrical cabling.

The third advantage is that the oilless, gearless, and bleedable APU incorporates a conventional gas-turbine combustor, which operates with most types of liquid fuel. The UAAPU was optimized to operate at high combustion efficiencies on both jet fuel (JP-4 and JP-8) and diesel fuel (DF-1 and DF-2) without hardware modifications.

Fuel consumption is a major concern for military vehicles. In both battlefield and peacetime training environments, providing fuel to the vehicles is difficult. By design, the UAAPU was developed to reduce overall APU fuel use compared with conventional gas-turbine APUs. Conventional APUs typically operate at constant speed and are not designed to operate efficiently off their maximum power design point. Thus their specific fuel consumption is high whenever the vehicle's silent-watch electrical and pneumatic loads move away from peak power demand.

High-part-load thermal-cycle efficiency was recognized as an extremely desirable feature for an APU on a large, armored, tracked vehicle. During silent watch, the vehicle may need pneumatic power (for the NBC/ECS) and electrical power simultaneously. However, the soldiers typically will require only electrical power. This requirement varies widely depending on the battlefield or training situation.

To keep the thermal-cycle efficiency high throughout the APU power-delivery range, the UAAPU is operated at varying speeds. This keeps the turbine-inlet temperature above 1,300°F in nearly all conditions. Operating at reduced speeds is not a problem with the oilless and gearless APU. There are no auxiliaries to be driven through a gearbox and no critical speed concerns in the rotor-bearing system. The power electronics can adapt to reduced or high generator output voltages. Therefore, the APU speed can be varied without impact.

However, because the oilless and gearless APU delivers both pneumatic and electrical power, it requires separate control strategies for the occasions when it operates as an electrical generator only and those when it works as both a generator and an NBC/ECS air source. In the dual-power-delivery mode, the NBC/ECS requires a minimum input pressure to meet vehicle overpressurization as well as heating or cooling requirements. Because the bleed air takeoff comes from the APU compressor discharge, a reduction of the APU speed results in a reduced NBC/ECS input pressure. Therefore, whenever the NBC/ECS is required, the APU is commanded into a high-speed region in which the APU speed is controlled solely to meet the minimum NBC/ECS input-pressure requirement.

In the electrical-power-only mode, the APU speed is greatly reduced, to approximately two-thirds of the APU's maximum design speed. The APU speed is further tuned to meet a specific control schedule for electrical power versus APU speed. This schedule keeps the turbine-inlet temperature high and the specific fuel consumption low. The PEU is designed to provide a regulated dc voltage in both operating modes. The ability to operate the APU over a wide speed range results in excellent overall performance.

APU Components

The mechanical design of the turbogenerator has three basic sections: turbine, compressor, and bearing/PM machine. The turbine section (or power section) incorporates sheet-metal Hastelloy X housings, a can combustor, a Mar-M-247 vaned nozzle, and a Mar-M-247 radial-inflow turbine wheel. This set of parts takes the elevated-pressure (45-pounds-per-square-inch-absolute) compressor-discharge flow, heats it up to an average of 1,650°F, and expands it back to near-ambient pressure so that the necessary power can be generated to drive the electrical generator and compressor impeller. The turbine performance goal was modest under this program, with a peak efficiency of 86.5 percent and a 3.2 pressure ratio.

The compressor section incorporates a cast CRES 347 vaned diffuser welded onto an Inconel 625 formed sheet-metal scroll and a titanium centrifugal compressor impeller. Some compressor leakage into the backside of the impeller must be accommodated to allow adequate thrust bearing cooling flow. In the current design, a large-diameter (5-inch) labyrinth seal is used that offers adequate performance. In the future, a brush seal will be used in this region with expected leakage reductions of up to 50 percent. The compressor performance goal was also modest, with a peak efficiency of 79 percent. The maximum compressor pressure ratio for a standard day condition is 3.5.

The air bearings—more aptly named foil bearings—used in the UAAPU are self-acting hydrodynamic-fluid-film bearings. A leaf-type journal bearing has overlapping foils that wrap around the shaft. The foils contact the shaft while it is stationary. Liftoff occurs when a fluid film develops between the shaft and foil at a minimum speed. No wear occurs above the liftoff speed. A protective coating is used on the foil to minimize wear during start/stop transients. Laboratory tests have shown a coating capability in excess of 100,000 start/stop transients.

Foil bearings are compliant and tolerate misalignment, thermal distortion, and mechanical distortions better than conventional types of bearings. High damping in the foil-bearing system permits stable rotor dynamic operation throughout the UAAPU operating range. The high damping of the bearing system also allows the UAAPU to perform well under severe shock loads and to tolerate rotor-shaft imbalances.

An advanced, shaft-speed, high-performance PM electrical machine is the other key AlliedSignal technology that enabled the oilless, gearless, and bleedable underarmor APU to be produced. The PM machine, now known as the ring-wound, two-pole toothless PM motor/generator, has undergone extensive development since 1984. For reasons of speed and power density, the PM is located in the center of the turbogenerator, between the two impellers and the two sets of journal bearings. The rotor incorporates a two-pole samarium-cobalt magnet with a substantial Inconel 718 hoop to keep the magnet in compression, even at high APU rotational speeds.

A ring-winding-type stator is required to minimize end-turn extension dimensions and facilitate machine integration. The stator assembly requires the use of a laminated flux return stack, resulting in iron losses whenever the machine is operating. The ring-winding stator is simpler, less expensive, and more reliable than the flat, conventional-type stator winding. The specific benefits of a ring-winding stator include simple construction and low parts count, dedicated half-phase sectors, full phase-to-phase isolation and insulation, full turn-to-turn insulation (single-layer winding), full ground isolation and insulation, shorter length for the same tip speed with a higher critical speed margin, and improved stator cooling.

The two-pole toothless electrical machine design has other benefits. It operates with a large magnetic air gap, resulting in a very low magnetic spring rate, minimum rotor heating, and no rotor cog torque. The unvarnished windings use Litz wire (fine-stranded, fully transposed conductors), which controls the eddy-current losses, eliminates circulating current losses in the copper strands, and provides a large effective surface area for cooling.

The PEU, which resides in the UAAPU between the vehicle bus and the turbogenerator, is required to drive the PM machine as a motor during start-up and to condition the electrical power out of the PM machine during its generating mode.

The power-electronic functions for both motoring and generating modes are accomplished by a three-phase full wave bridge of solid-state switches. (The power-circuit schematic is shown on the next page.) In the motoring mode, the bridge is operated as a voltage-source, current-regulated inverter. The current regulation is accomplished by means of pulse-width modulation of the bridge devices. By using rotor-position sensors to derive power-device switching information, the electronic frequency is effectively locked to the speed of the PM machine.

Control of the generating mode is accomplished by designing the PM machine to produce an output voltage that is lower than necessary. This requires the electronics to operate continually in a voltage-step-up mode. The step-up mode is accomplished using the same inverter devices to, in effect, short-circuit the machine windings momentarily to raise the phase current levels, then remove the short circuit to allow the energy stored in the machine reactances to discharge into the bus load and capacitors. The alternate shorting and discharging cycle is controlled by pulse-width modulation similar to the inverter-current-regulation mode.

For the generation mode, however, the internal current regulator is driven by a dynamically compensated voltage error signal. The modulation scheme, therefore, is used to regulate the UAAPU output voltage to the vehicle electrical bus voltage. A useful feature of this scheme is that failure modes of the bridge devices do not result in application of excessive voltage to the vehicle bus. This control scheme also demonstrates excellent control and power-conversion characteristics, and can meet MIL-STD-704E power-quality levels.

This power-conditioning approach was taken to minimize the total number of semiconductor devices and reactive elements required to accomplish the power-conversion tasks. The additional power-circuit elements provide an EMI filter function and a series switch to limit bus-fault currents.

To produce a thermally acceptable design in the smallest possible package, trade-off studies were performed on both air- and fuel-cooled designs. The fuel-cooled cold-plate design was selected because its volume was 30 percent less than the air-cooled version and was less susceptible to contamination.

The APU digital controller performs all the control and equipment safety monitoring functions. It has diagnostics and fault-memory storage capability for APU troubleshooting, and incorporates all the drivers to open and close the various valves in the system, operate the combustor igniter, and read in and monitor all APU sensor data.

The controller is also the interface to the vehicle APU control panel. Inputs to the APU controller from the control panel are the APU on-off and NBC/ECS on-off commands. Feedback from the APU controller to the operator is provided by lamps indicating "APU On," "APU Faulted," and "NBC/ECS On." The digital controller performs the fuel-control task, which varies the fuel pump speed to adjust flow into the combustor so the specific APU control requirement can be met at any time.

The combustion system consists of a very small tangential can combustor in a folded-can, stacked-ring arrangement. The fuel is atomized in a piloted air-blast fuel atomizer. In both recuperated and nonrecuperated APU configurations, compressor discharge air (and not recuperator outlet air) was used as the air-blast source to avoid coking problems.

The APU's key technical advantages include its low part count, simple single rotating component, minimal maintenance, and high part-load performance, as well as its ability to adapt easily to a different installation and electrical system. Turbogenerator robustness, including margins in the rotor critical speed and margins in the many thermal aspects of the design, have been demonstrated through extensive development testing. Field testing has provided an additional guarantee that the UAAPU can withstand the extremely dirty, high-shock, and high-vibration environments that an armored vehicle imposes on an APU.

Other expected advantages of the oilless and gearless approach have yet to be proven. Demonstration of long-term reliability and lower life-cycle cost requires testing over an extended period of time. The design has proven to be extremely robust, however, and reliability goals should be met.

Further performance goals include recovering the lost turbine performance as well as optimizing the compressor seal leakage and bearing cooling flows. The most significant need is for a smaller optimized electronic package (in both weight and volume) that incorporates the power electronics and overall APU control functions.

This article was adapted from a paper presented at the 1995 ASME International Gas Turbine and Aeroengine Congress and Exhibition in Houston.

Robert Nims is a turbomachinery-systems engineer with AlliedSignal Aerospace Equipment Systems in Torrance, Calif.

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