Ground Combat Vehicle Hybrid Electric Systems
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To meet the endurance and logistics requirements of the Future Combat System ("FCS"), Ground Combat Vehicles ("GCV") will require propulsion systems with higher fuel efficiency than current internal combustion engines with mechanical transmissions can provide. One such system is based on an almost-forgotten, yet well-proven thermodynamic cycle which, with modern materials, aerodynamics, combustion and heat transfer design, has the potential of silently generating electric power to propel combat vehicles at significantly higher thermal efficiencies with substantially higher reliability and lower maintenance than other systems being projected. It consists of an externally-fired closed Brayton cycle turbo-generator with a basic power core that is adjustable to serve applications with maximum power requirements ranging from 30 to 130 kW. A second power core is designed to serve the power range from 150 to 300 kW. Each power core can be trimmed with inserts to meet specific applications and is then matched with heat transfer elements that are modularly sized in proportion to the maximum output required. This approach drastically reduces the logistic burdens of a fleet of ground combat vehicles equipped with such power plants.
For hybrid electric propulsion of a ground combat vehicle, the power unit is sized below maximum propulsion needs with the difference in power made up with electricity drawn for a storage system. For the Diesel generator, shown in blue, the engine is turned off at lower loads where it is inefficient and the substitute power drawn from storage is later recharged at a power level where the Diesel is more efficient. The opposite is true for the fuel cell, shown in green, which wants to be on at low loads and wants assistance from storage at higher levels of demand to remain at high efficiency as far as possible. The flat efficiency curve of the closed Brayton cycle makes it much less storage-dependent, except at very low or negative (dynamic braking) loads. Based on a range of analytical test drives with different ground combat vehicles, ranging in weight from 1,500 to 30,000 lb and with various drag and rolling resistance coefficients, the closed Brayton cycle improves upon the fuel economies of the Diesel and of the fuel cell by 15 to 60 percentage points. However, a fuel cell, while suitable for electric passenger cars, is not a suitable match for combat vehicle conditions. Finally, since the closed Brayton cycle creates its own operating atmosphere, it is flat rated to 10,000 ft altitude, whereas the turbocharged Diesel and pressurized fuel cell each suffer power losses of about 1kW for each 1,000ft rise in elevation. System Weight:For a system designed for a maximum output of 80kW, the closed Brayton cycle weight amounts to about 220lb, i.e. 2.75lb/kW (1.25kg/kW). From a vehicle design point of view, the weight of the power plant should also include the weight of storage system. Since the closed Brayton cycle is fundamentally less dependent on storage, it requires less battery weight to support it than its Diesel equivalent for the same set of operating parameters The payoff: The system presented is a highly efficient, stealth power system for second-generation ground combat vehicles associated with the Future Combat System and significantly improves the logistic support requirements for typical missions. This is accomplished through substantial fuel savings, sharp reductions in the need for separate field generators and auxiliary power units, and commensurate reductions in supporting spares and service requirements. |
System Concept:
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Thermodynamic Cycle: The proposed advanced technology demonstration (ATD) closed-loop, power generation system utilizes two state-of-the-art, high-efficiency, turbo compressors operating in an externally heated closed loop Brayton cycle as depicted in the schematic diagram. 1Air at near ambient temperature enters the low-pressure compressor and is compressed to approximately two and a half times the inlet pressure2. The heat of compression is removed by an air-cooled intercooler3 before entering the high-pressure compressor where it is compressed to about three and a half times the low-pressure compressor outlet pressure4. The high-pressure air is passed through the recuperator where it is heated by the exhaust air from the low-pressure turbine5. The partially heated compressed air is heated to 1,800ºF in the first pass through the combustion heat exchanger6 before entering the high-pressure turbine. The turbine drives the compressor, in the course of which the air is expanded and its temperature reduced7 before entering the second pass through the combustion heat exchanger where it is reheated to 1,800ºF8. |
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The air next enters the low pressure turbine which drives the low pressure compressor and direct shaft-mounted, high speed, wild-frequency alternator, in the course of which its pressure is further reduced and temperature decreased9 before passing on to the recuperator. In the recuperator much of the heat in the air is transferred to the cooler air leaving the high pressure compressor10 and the remaining heat is then removed in the heat sink heat exchanger to bring the temperature down to near ambient before repeating the cycle1. The numbers in red correspond with those in the temperature / entropy diagram. A blower drives ambient cooling air through the intercooler, heat sink heat exchanger, and alternator stator. In most FCS ground combat vehicle applications, some of the heat in the loop should be used to drive an absorption chiller to provide cooling for onboard electronics and personnel, if present. The external combustor is charged with a highly diluted mixture of fuel and air that has been partially heated by a combustion air recuperator before entering the combustor. The exhaust gas from the combustor is cooled by the combustion air recuperator before being discharged overboard. The control system consists of a fill and drain unit that either adds small amounts of air under pressure into the low-pressure compressor inlet to increase the pressure in the loop and therefore, the air density, or to discharge air from the high-pressure compressor outlet to ambient to reduce the density of the air in the loop. As the density increases, the power output from the generator increases and as the density is decreased the output declines. A secondary thermostatic control regulates the fuel flow to maintain the turbine inlet temperatures constant at 1,800ºF. Once the density drops to its lowest point, which corresponds to about an 8% full load condition, further reduction in power output requires the fuel flow to be diminished to reduce the turbine inlet temperatures below 1,800ºF. Once the turbine inlet temperatures reach 700°F, there is no electrical output. |
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Physical Design Aspects:
The conical housing upstream of the combustor contains the combustion air recuperator and fuel / air mixer leading to the combustor. The combustion exhaust out of the annulus is used to heat the incoming combustion air in the recuperator.
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The system is designed around a power core consisting of the combustor, combustion heat exchanger and the two stages of turbo compressor. The combustion system is actually contained within the cylindrically shaped annular combustion heat exchanger as depicted in the drawing on the right.
As shown in the cross sectional diagram, hot gas from the internal combustor is directed through a plenum at the end of the cylinder into two cylindrical annular heat exchangers in series which heat the air in the closed loop before entering the low pressure and high pressure turbo compressor turbines which are mounted on plenum chambers built into the core assembly. The combustor and combustor heat exchanger is fabricated from Silicone Carbide. In production, the rotors of the turbochargers as well as turbine inlet nozzles and compressor diffusers will be made of Silicone Nitride. The specific weight of ceramics is about the same as aluminum, which besides reducing the weight of the system has the advantage of reducing the inertia of the rotors.
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The intention is to use the same power core to meet the power requirements of a wide range of applications. In order to fit all FCS GCVs it is anticipated that two power cores will be required. The smaller of the two is illustrated in this description. The proposed design for 80 kW can be easily adapted to a large range of applications with maximum power demands from 30 kW and 130 kW. This is accomplished by modifying the length of the magnet shaft of the low pressure spool and the alternator stator to match the max power required without changing the aerodynamics or housings of the turbo compressor. The high pressure turbo compressor turbine nozzle area needs to be adjusted to match the desired pressure ratio. |
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The power core to cover the output range up to 300 kW will employ a turbo compressor that was developed as the low pressure stage of a turbo-compound diesel research application for TACOM. The high pressure stage will be the low pressure stage of the smaller power core with the alternator removed. The compressor in the TACOM project had the distinction of exceeding 80% efficiency at a pressure ration of 5 to1.
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System Heat Transfer Components:Unlike the power core, the recuperator, intercooler and heat sink heat exchanger all have to be sized for specific applications. To do so as painlessly as possible, the heat transfer elements are designed to be modularly adjustable. The heat exchanger core stack heights are directly proportional the system airflow. In this manner it is easy match heat exchanger sizes with the system flow. Combustion Heat ExchangerThe combustor design is sized to accommodate the maximum power of 130 kW. The combustion heat exchangers are wrapped around the combustion chamber to place the high pressure elements in compression. The circumference of the heat exchanger is the maximum stack height of the heat exchanger. If a smaller stack height is required an empty shell is placed where the stack has been reduced to maintain structural integrity. RecuperatorThe high temperature steel recuperator is of high-density, plate-fin, counter-flow design. It is rectangular in shape and for an 80 kW system has dimensions of 10” flow length, 20”stack height and 6”width. |
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The aluminum intercooler is also of plate-fin design, but uses a cross-flow configuration. For an 80 kW application its dimensions are 20” hot flow length, 4” cold flow length and 15” stack height. A typical example of such an intercooler previously used by the company in a high performance vehicle engine application is shown on the left. Heat Sink Heat ExchangerThe heat sink heat exchanger is also made of aluminum with the same heat transfer configuration as the intercooler. For 80 kW its dimensions are 15” hot flow length, 4” cold flow length and 20” stack height. The proposed ATD program contains an element to evaluate the possibility of building the “generator” part of an absorption chiller into this sink heat exchanger. This would involve incorporating into its heat transfer core a liquid section to boil off the water in the mixture of Lithium Bromide and water arriving from the chiller evaporator. The Lithium Bromide is returned to the evaporator and the water vapor is ducted to the chiller’s condenser. The purpose of the chilled water from the evaporator would be to: • cool the payload and personnel in the combat vehicle; • reduce the infrared signature of any heat leaving the vehicle; and • decrease engine power loss in extreme heat by cooling the inlet air to the closed loop. By using heat that would otherwise be rejected to power the cooling system instead of using some of the power being generated, the overall energy efficiency of the vehicle is improved. |
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Alternator: The permanent magnet alternator in the production version will likely be integral with the low pressure turbo compressor. Its magnetic media is housed within the rotor as illustrated in the earlier rotor drawing. The stator with the windings is attached to the compressor housing and is cooled by air from cooling air blower. In the Phase 1 prototype tests the alternator will be driven by a separate turbine. Conditioning of the high, wild frequency power output from the alternator is done separately by solid state rectifiers and inverters.
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Comparison With Other On-going Research Alternatives.The closed Brayton cycle competes principally with the latest designs of direct injected, turbocharged Diesel engines and PEM automotive fuel cells. Both alternatives have been subject to intensive research efforts by the automotive industry in partnership with the Department of Energy. Other alternatives, including spark ignition internal combustion engines and simple cycle recuperated gas turbines fundamentally operate at efficiency levels too low to be competitive in combat vehicle applications, although they have been used in experimental combat vehicles including the Spinner and the 15 ton platforms designed by United Defense.
Storage System Objectives:While great improvements have been made in electrical storage system technology, they are still heavy in comparison with the specific energy of JP-8. Hence, the objective clearly has to be to manage the state of charge (SOC) to minimize the size of storage system. One of the challenges is that no SOC management system can anticipate on-coming drive conditions, which denies it the luxury of postponing dissipation or replenishment opportunities. Basically, the SOC has to be maintained as close to its mid point as possible.
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Efficiency ComparisonDiesel engines in hybrid electric power systems take advantage of the electrical storage system to satisfy the lower power demands in order for the Diesel to operate mostly at high output power levels where it is more efficient. Closed Brayton cycle generators and fuel reformer equipped PEM fuel cells share the advantage of reaching peak efficiencies at much lower part loads and accordingly employ different control strategies. Whereas the Diesel is turned off at low load levels, the closed Brayton and fuel cell alternatives remain at “hot idle” until significant dynamic break recovery occurs when they too can be shut down. But, whereas the closed Brayton cycle has a flat efficiency curve, the fuel cell characteristics militates in favor of using fuel cells at power levels below 50% of full capacity, as much as possible. Simulation of Possible Drive Conditions:To test the extent of this challenge, operational simulations were used:
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 Surface conditions are shown on the topography graph as a multiplier to adjust the rolling resistance coefficients of the vehicles. Paved roads are shown in black, dirt roads in dark brown, sandy areas in light ocher, and cross country terrain in orange. A number of different CGVs were test driven mathematically around the Pendleton circuit to assess their performance in electrical terms. The analytical models are based on the equations of motion, adjusted for ambient, surface, and altitude conditions and are available on request.
The assumed characteristics of the CGVs were as listed below: |
Camp Pendleton Simulation:The course follows an existing road from the Marine Corps air strip, proceeds across country over a ridge line to rejoin the existing road near Las Flores canyon, proceeding north alongside Interstate 5 and the Union Pacific tracks to Homo Canyon where it leaves the road and proceeds across country over an 1,100 foot pass, through a valley and up to Hommo Summit. The return trip crosses several dry creeks and ridges, following roads where possible, to wind up following the road along the southern perimeter of the base to the air strip.
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Assumed Parameter
Dimension
MULE
Spinner
RST-V
MGCV-W
MGCV-T
CD = Drag Coefficient
Coeff.
0.300
0.350
0.40
0.45
0.45
μ = Rolling Resistance Coefficient
Coeff.
0.0190
0.0210
0.015
0.025
0.035
A = Frontal Area
Sq ft
20.00
38.25
24
54
45
W = Vehicle Gross Weight
lb
1,500
15,000
8,000
28,700
30,000
M = Mass = W/g
46.62
466.21
248.65
892.02
932.43
Power Generator Capacity
kW
10
100*
90
300
300
Percent recovery - deceleration energy
%
70%
70%
70%
70%
70%
Turnaround storage efficiency
%
92%
92%
92%
92%
92%
Vehicle accessory electric load
kW
0.80
8.00
7.20
24.00
24.00
Traction Motor Efficiency
%
94%
94%
94%
94%
94%
Power Conditioning Efficiency
%
96%
96%
96%
96%
96%
Overall fuel economy results were as shown in the table below.
Closed Brayton Cycle
MPG
60.21
6.57
13.87
2.80
2.08
Direct Injection Turbo Diesel
MPG
52.14
5.76
12.16
2.47
1.81
PEM Fuel cell with JP-8 reformer
MPG
36.13
4.05
10.74
1.93
1.28
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For the Spinner GCV, SOC management was structured to maintain levels near constant at 9000 kW-sec (2,500 Wh). The graph demonstrates that this could reasonably consistently be attained for all three power plants. The span of variation ranged from almost zero to 15,000 kW-sec, which indicates the need for a storage capacity of roughly 4,200 Wh. If one were to assume a specific energy of 70 Wh/kg for a NiMH battery pack, this would represent a storage system weight of about 105 lb.
However, as shown next, the rates of charge / discharge to achieve the SOC result reached 100 / 140 kW. This level of discharge would require a NiMH battery pack weighing about 340 lb.
For the Spinner test drive, the manipulation of the closed Brayton cycle plant would have been as reflected in the diagram on the left. Essentially, the engine follows the load up to 100 kW after which power from storage needs to be added. When the vehicle power demand turns negative, the power plant is turned off. A method has been devised to retain most of the thermal mass in the power loop.
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Similar results were obtained from test driving the Spinner UGCV around a high altitude course at Fort Carson on a 95oF day. In this instance the Diesel and fuel cell suffered power loss as result of the lower density air while the closed Brayton, which creates its own loop air density, remained at full output. The proposed closed Brayton power system is flat-rated to 10,000 ft and 120oF. |
Consideration
Closed Brayton
TC DI Diesel
PEM Fuel cell
State-of-the-Art
Unproved for HEV use
Well established
Large development risk
Thermal efficiency
Superior at part-load
Good at peak power
Good at low power
Mechanical complexity
Simple, reliable, lasting
Complex, limited life
Complex accessories
System integration
Best overall potential
Good proven outcomes
Mediocre with reformer
Response – agility
Fast, low inertia
Adequate
Questionable
Emissions
Ultra low
Requires attenuation
Depends on reformer
Maintainability
Excellent, no servicing
Good, much servicing
Promising, unknown
Reliability
Extremely reliable
Good with great care
Promising, unknown
Silent watch
Inherently favorable
Inherently unfavorable
Promising, unknown
Weight
Very competitive
Competitive
Questionable
Volume
Competitive, adaptable
Competitive, rigid
Unknown
Alternative fuel use
Highly adaptable
Modifications required
Inflexible
Alternative uses
Doubles as APU
Somewhat inflexible
Promising, unknown
Logistic burden
Extremely low
Moderate, fixed
Unknown
Important Closed Brayton Cycle AdvantagesJust as tubochargers generally outlast their host engines by multiple overhaul cycles, so too closed Brayton cycle generators should outlast their Diesel engine competitors. Through the use of air bearings and self cleaning loop air filters, closed Brayton cycle engines are liberated from periodic maintenance needs. Since they have no reciprocating stresses and absorb no combustion products, they are vibration free and highly wear resistant. Because they are inherently silent they cause minimal fatigue of human operators. They are very easy to start, and come up to full power rapidly. The high rotational speed of the turbo compressor driving the alternator reduces the alternator to minimal proportions, inertia and cost. The control system is autonomous, self powered, without wearing components.
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Required Power Range: The power requirements for unmanned vehicles ranges from approximately 30 kW to 130 kW and for manned vehicles it ranges probably from as little as 100 kW to 300 kW with a concentration in the area of 200 to 300 kW. It is believed possible to cover the entire range with only three identical frame sizes of turbo compressor which would yield important logistics and technical support benefits.
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