Powering the U.S. Army of the Future (2021) / Chapter Skim
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8 Fuel Conversion Efficiency and Other Material Driven Opportunities
8 Fuel Conversion Efficiency and Other Material Driven Opportunities
Pages 105-123
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From page 105... ...
Some of these opportunities are described below. PRESENT ARMY POWER PACK FUEL EFFICIENCY AND PERFORMANCE UPGRADES The Army already has a number of active power pack initiatives in this area, which are then balanced against other key performance objectives such as power density and heat rejection.
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From page 106... ...
access and higher vehicle speed power packs using military on-the-shelf (MOTS) components (see Figure 8.1)
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From page 107... ...
, part of the APD, is a 746-kW four-cylinder, two-stroke compression ignition engine with horizontally opposed pistons (see Figure 8.3)
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From page 108... ...
temperatures of 105°C.3 Aggressive targets for these APD powertrain technologies in 2035 and 2050 already have been established by the Army Ground Vehicle Systems Center. Another advanced propulsion system presently being defined by the Army Ground Vehicle Systems Center is simply entitled the "Projected Propulsion System." This hybrid power pack includes the following: (1)
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From page 109... ...
FURTHER EFFICIENCY IMPROVEMENTS IN COMPRESSION IGNITION ENGINES Within the last decade, there have been some very impressive improvements in the efficiency and power density of compression ignition engines, in large part driven by the SuperTruck projects undertaken by Cummins, Navistar, Daimler, Volvo, and PACCAR. Base engine thermal efficiencies exceeding 50 percent at their best speed/load operating point have been demonstrated.7 The brake thermal efficiency (BTE)
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From page 110... ...
SuperTruck advances, including waste-heat recovery concepts, could be leveraged for military applications and provide the potential to significantly improve vehicle range and reduce the JP8 supply line. Also included in Appendix J is a list of the possible design/development actions that might be considered on future horizontally opposed two-stroke compression ignition engine designs to enable some of the aggressive targets in these areas that the Army is setting for 2035 and beyond while maintaining low heat rejection.
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From page 111... ...
Some initial experiments using an aerogel as a thermal barrier coating were unsuccessful due to adhesion problems, which could be solved with further materials development and surface engineering. Ceramic thermal barrier coatings are already commonly used on production aviation turbo-shaft engines where extremely high temperatures are encountered on both moving and stationary components.
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From page 112... ...
promise to improve the thermal performance of future power electronic converters. Because thermal management plays such a critical role in all ground combat vehicles, technical electrification challenges in power density and temperature threshold have been identified by the Army as part of its hybrid studies.
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From page 113... ...
Silicon carbide has made possible semiconductor devices with maximum junction temperatures exceeding 200°C, while Si transistors are generally limited to a junction temperature of 175°C. The thermal limits of current packaging technology prevent fully exploiting the higher thermal ratings of SiC.
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From page 114... ...
Additional background material on the power electronics challenge and how they can be addressed is contained in Appendix L Finding: Although SiC semiconductor devices can operate at higher tem peratures than conventional Si devices, the operating temperature limits of passive components such as capacitors and inductors still establish the upper temperature limit of power electronic systems.
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From page 115... ...
. UNIQUE METAL MATRIX COMPOSITE MATERIALS FOR PISTONS Most modern military diesel engines use steel pistons based on their ability to tolerate higher temperatures and higher peak cylinder pressures.
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From page 116... ...
with piston skirt and rings; • Possible unique skirt materials or coatings -- possibly diamond like coating or higher temperature–capable polymer base coating; • Thermal barrier coatings -- matched to adhere to the MMC crown material and minimize heat transfer needed to undercrown -- possibly used selectively on the outside of a liner to allow more uniform temperatures within the bore; and • AI/ML algorithms to enable further exploration of the materials design space without relying exclusively on testing. Such new piston materials and architecture may provide lower reciprocating mass, enabling higher speeds and increased power at equal peak cylinder pressures.
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From page 117... ...
Another approach used by Honda in their GC-family engines, called monobloc construction, is the integration of the cylinder head and block to eliminate the need for head gaskets, a high warranty item.19 Costs associated with 3D printing versus other manufacturing methods have precluded its widespread adoption in the past. It is often used for low-volume prototype parts that are needed quickly or have significant tooling costs with traditional manufacturing methods, such as casting and machining.
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From page 118... ...
21 S.C. Singhal, 2001, "Zirconia Electrolyte-based Solid Oxide Fuel Cells," pp.
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From page 119... ...
Other possible cathode materials include perovskitestructured oxides such as lanthanum cobaltite and lanthanum nickelates, suitably doped with alkali and alkaline earth ions to tailor the conductivity and thermal expansion coefficient. Selection and development of a suitable cathode material capable of providing high cell performance and performance stability with time is important in developing high power density and lower-cost SOFCs.
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From page 120... ...
. For low-temperature PEM fuel cells for operation from about 60°C to 90°C, the electrolyte membrane is generally a fully fluorinated polymer (such as Nafion manufactured by DuPont)
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From page 121... ...
To improve PEM fuel cell durability, research and development should focus on understanding the fuel cell degradation mechanisms and developing materials and strategies to overcome them. In addition, the practicality of on-board reformation of hydrocarbon fuels to produce CO- and S-free hydrogen for PEM fuel cells for mobile ground and air vehicles should be investigated.
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From page 122... ...
Also, the metal in current reactors degrades as a result of neutron damage, which limits its lifetime. In addition, as mentioned earlier, zircoloy cladding has the deleterious thermal runaway reactions with steam that produce hydrogen gas and reactor core meltdowns.
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From page 123... ...
However, using SiC-SiC fuel-rod elements can reduce the precious volume in the reactor core that is lost by using TRISO fuel. This replacement can enable higher power densities and increased electric power generation without a weight and volume penalty while maintaining safety.
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