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Thermal insulation coating is considered a low heat rejection technology for improving the thermal efficiency of engines in the future. A thermal-insulation-coated piston was used to reduce an engine's heat loss; however, the piston temperature was increased notably for high-power engines. This increased wall temperature causes burning of the diesel spray. In order to analyze and improve the combustion quality of a high-power engine compounded with a thermal-insulation-coated piston, experiments and simulation methods were used in this research. The ultrahigh-pressure fuel injection system (up to 250 MPa) was applied to study the effect on combustion under high wall temperature and high-power conditions. A high wall temperature environment combustion image was achieved by visualization experiment in the constant volume combustion bomb. The thermal-insulation-coating and high-power engine condition can lead to ultrahigh wall temperatures. To study this kind of high wall temperature environment, the constant volume combustion bomb model was founded and calibrated. The effects of ultrahigh-pressure fuel injection and spray impingement distance on the combustion of high wall temperature were studied. The simulation results showed that the ignition delay is shortened, leading to more soot and longer combustion duration, which caused the combustion deterioration. With an increased fuel injection pressure and longer impinging distance, the combustion was improved. Moreover, the effect of increasing injection pressure on combustion is more obvious. [ABSTRACT FROM AUTHOR]

Low-temperature polymer electrolyte fuel cell systems (FCSs) need to reject large amounts of low temperature heat. Often a mobile FCS's cooling capacity limits the FCS power output. Cryogenic hydrogen is typically utilized as a direct heat sink using heat exchangers (HXs), even though HXs destroy most hydrogen exergy. This paper investigates synergies between FCS thermal management and cryogenic hydrogen exergy utilization in terms of their benchmark performance: the FCS coolant circuit supplies heat at coolant temperature level to a so named reversible cryogenic exergy utilization system (rCEUS) comprised of thermodynamically ideal heat engine processes. The rCEUS converts this heat partly to electrical energy (the value of which equals the hydrogen exergy) and rejects remaining heat to hydrogen to heat it to coolant temperature. The rCEUS output power is used to support the FCS, so the FCS rejects less heat and a significant fraction of this heat is utilized by the rCEUS. As a result, significantly less heat has to be transferred to ambient and the fuel demand decreases. In this paper, three hydrogen storage options are compared: liquid hydrogen, subcooled liquid hydrogen and cryo-compressed hydrogen. Different para- and orthohydrogen compositions are evaluated. For typical FCS operating points, rejected FCS heat to ambient is reducible by 40–67%. FCS power demand is reducible by 14–31%. FCS rejected heat to ambient reduction is 4.5–8 times larger than that of conventional HXs. Calculations are based on hydrogen's lower heating value. [ABSTRACT FROM AUTHOR]

[Display omitted] • 7-Milligram miniature engine using shape-memory alloy wires coated with platinum. • Engine operates at 6 Hz, lifts 650 times its weight, generates 39.5 μ W average power, • Chemical fuels and catalytic combustion for energy dense microrobotic actuation. • Thermal and dynamic modeling predict engine frequency and actuation output. • Fast custom-built fuel delivery system demonstrates engine's applicability. Microrobots at the subcentimeter scale have the potential to perform useful complex tasks if they were to become energy independent and could operate autonomously. The vast majority of current microrobotic systems lack the ability to carry sufficient onboard power to operate and, therefore, remain tethered to stationary sources of energy in laboratory environments. Recent published work demonstrated that chemical fuels can react under feedback control on the surfaces of tensioned shape-memory alloy (SMA) nickel-titanium (NiTi) wires coated with platinum (Pt) catalyst. Combining catalytic combustion of fuels with high energy densities with the high work densities of SMA wires is a promising approach to provide onboard power to microrobots. In this article, we present a novel 7-mg SMA-based miniature catalytic-combustion engine for millimeter-scale robotic actuation that is composed of a looped NiTi-Pt composite wire with a core diameter of 38 μ m and a flat carbon-fiber beam with a length of 13 mm. This beam acts as a leaf spring during operation. The proposed design of the engine has a flat and narrow geometry, functions according to a periodic-unimorph actuation mode, and can operate at frequencies as high as 6 Hz and lift 650 times its own weight while functioning at 1 Hz, thus producing 39.5 μ W of average power in the process. For the purposes of design and analysis, we derived a model of the heat transfer processes involved during actuation, which combined with a Preisach-model-based description of the SMA wire dynamics, enabled us to numerically simulate the response of the miniature system, and thus predict its performance in terms of frequency and actuation output. The suitability for microrobotics and functionality of the proposed approach is demonstrated through experimental results using a custom-built fast-response high-precision system of fuel delivery. [ABSTRACT FROM AUTHOR]