Search Results (192)

The evaluation of heat transfer in a back-pass channel for an industrial circulating fluidized bed (CFB) reactor was studied. The authors proposed a heat transfer model taking into account (i) convective heat transfer on the tube side, (ii) convective heat transfer on the shell side, and (iii) radiative heat transfer on the shell side. The analysis of heat transfer is based on measured data from tubular heat exchangers (superheater SH Ia, reheaters RH Ia and RH Ib, and economizer ECO). Performance tests were conducted over a wide range of CFB reactor loads (from 40% MCR to 100% MCR) and also at a secondary air-to-primary air ratio of 0.11. The experimental data as a function of flue gas temperature, maximum flue gas velocity, CO₂ partial pressure, fly ash concentration, and particle size are discussed. During performance tests, the temperature and velocity of flue gas are no more than 1206 K and 16.3 m/s, respectively. As the CFB reactor load decreases from 100% MCR to 40% MCR, the overall heat transfer coefficient of tubular heat exchangers decreases from 60.61 W/(m² × K) to 30.1 W/(m² × K). The overall heat transfer coefficient was higher when the fly ash concentration was higher (from 0.0412 kg/m³ to 0.0612 kg/m³) in the back-pass channel of the CFB reactor. Mean particle size of fly ash corresponds to the maximum overall heat transfer coefficient at bigger (d₅₀ = 0.038 mm) and smaller (d₅₀ = 0.015 mm) particle diameters. Achieved heat transfer findings were calculated at carbon dioxide partial pressure varied between 8.48 kPa and 11.53 kPa. Research studies conducted on an industrial CFB reactor showed that the thermodynamic parameters of steam also influenced heat transfer. The geometry of the heat exchange surfaces affected heat transfer augmentation in the back-pass channel of the CFB reactor. For practicality, the relationships between the heat transfer data and other operational parameters are proposed using regression analysis. Comparing the operational data and the model results, the average absolute error is 19.23%. The heat transfer findings may be used in data-driven design, scale-up, commissioning, and operation of commercial CFB reactors. Full article

Methane cracking has emerged as a promising route for sustainable hydrogen production because it avoids direct carbon dioxide emissions while simultaneously enabling carbon sequestration in the form of solid carbon. In this work, a comprehensive and systematic experimental investigation of catalytic methane cracking was performed by directly comparing a commercial nickel catalyst (KATALCO™ 25-4MQ), an in-house catalyst prepared by wet impregnation using Fe(NO₃)₃·9H₂O as the iron precursor and Puralox SCFa-160 Ce20 as the support, and non-catalytic thermal conditions under identical operating parameters. Experiments were carried out in a laboratory-scale fixed-bed reactor at atmospheric pressure by varying methane partial pressure (0.1–0.2 atm) and operating temperature (600–800 °C), while maintaining a constant methane-specific WHSV of 0.3 h⁻¹. Continuous online gas analysis was employed to monitor reactor performance, and a dedicated post-processing methodology, including nitrogen-tracer-based carbon balance calculations, was developed to validate the experimental results. The results demonstrated the strong beneficial effects of both temperature and catalytic materials on methane decomposition. The Fe-based catalysts exhibited the highest performance, achieving average methane conversions and hydrogen yields approaching 50%, with peak values exceeding 90% under the most favorable conditions. Commercial Ni catalysts also showed promising activity, although a more pronounced deactivation tendency was observed during prolonged operation. Conversely, non-catalytic tests resulted in substantially lower performance. Overall, this work provides an experimentally assessed and integrated methodology together with benchmark performance indicators that may serve as useful guidance for researchers, process designers, and practitioners involved in the development, optimization, and future scale-up of methane cracking technologies for turquoise hydrogen production. Full article

Mild hyperbaric oxygen (HBO) exposure might influence ventilatory and gas-exchange responses. We investigated the effects of mild HBO exposure at 1.3 atmospheres absolute (ATA) on VE/VCO₂ and related variables at rest in healthy young men. Using a single-group repeated-measures design, pre, steady (20–50 min at 1.3 ATA), and post-phases were compared in 16 participants. The primary outcome was VE/VCO₂, calculated as minute ventilation (VE) divided by carbon dioxide output (VCO₂); VE, VCO₂, and partial pressure of end-tidal CO₂ (PETCO₂) were secondary outcomes. Linear mixed models and Bonferroni-adjusted pairwise comparisons were used. Using the primary bin-wise calculation, VE/VCO₂ increased significantly from 39.9 ± 1.0 in the pre-phase to 43.8 ± 1.0 in the steady phase and decreased to 38.8 ± 1.0 in the post-phase. VE and PETCO₂ showed no phase effects; VCO₂ decreased significantly during the post-phase. However, an alternative calculation based on phase-averaged VE divided by phase-averaged VCO₂ produced the opposite phase-dependent pattern. Among eight participants completing both HBO and normobaric normoxic conditions, the exploratory Condition × Phase interaction was significant for both calculations, although phase patterns differed between methods. Thus, the direction of the VE/VCO₂ change depended on the calculation method, and its physiological significance remains uncertain. Full article

Associated gas in the Permian Basin is a methane-rich but spatially fragmented and intermittently available feedstock. Methane pyrolysis can convert hydrocarbons to hydrogen and solid carbon without forming process CO₂ in the reactor, but its practical value depends on the captured-gas capacity factor, feed composition, high-temperature heat supply, product purification, continuous carbon withdrawal, carbon offtake, and transparent greenhouse-gas accounting. This study presents an implemented screening model for a modular 1 million standard cubic feet per day (MMSCFD) Permian associated-gas unit. A representative Permian composition is evaluated with hydrocarbon cracking stoichiometry, catalytic and thermal conversion envelopes, a net hydrogen recovery assumption, an energy-duty allocation, a levelized-cost model, and a well-to-gate carbon-intensity model. The catalytic base case produces 3.78 t/d of saleable H₂ after 90% pressure-swing adsorption (PSA) recovery and 14.27 t/d of solid carbon; the thermal near-complete conversion bound produces 4.31 t/d of saleable H₂ and 16.15 t/d of solid carbon. At a 0.85 capacity factor, $10 million installed capital expenditure (CAPEX), 8% real discount rate, 20-year life, 10 kWh per kg H₂ energy intensity, and $0.06 per kWh electricity, the deterministic plant-gate levelized cost of hydrogen (LCOH) is $1.81 per kg H₂ at zero carbon value and $1.05 per kg H₂ at a net realized carbon value of $0.20 per kg C. Monte Carlo analysis over capacity factor, CAPEX, energy intensity, electricity price, carbon value, feed/capture cost, and yield uncertainty gives levelized cost of hydrogen values at the 10th, 50th, and 90th percentiles (P10/P50/P90) of $1.32/$1.91/$2.57 per kg H₂. The corresponding screening carbon-intensity distribution is 2.34/4.11/5.89 kg carbon dioxide equivalent (CO₂e) per kg H₂, dominated by electricity carbon intensity and upstream methane loss. Geothermal or waste-heat preheat is treated quantitatively as a partial offset to low- and mid-temperature duties, not as a replacement for high-grade 900–1200 °C trim heat. The pathway is benchmarked against steam methane reforming, autothermal reforming with carbon capture and storage, electrolysis, small-scale liquefied natural gas, and gas-to-liquids conversion. Reported LCOH values are plant-gate production costs; separate hydrogen-logistics and negative-carbon-value stress tests identify conditions under which remote delivery or carbon disposal can erode the apparent economic advantage. Full article

This paper presents a theoretical engineering feasibility analysis of the RK-X photocatalytic process for In Situ Resource Utilisation (ISRU) on Mars. Experimental validation under simulated Martian conditions is the essential next step before any mission deployment claim can be made. The RK-X process converts the two most abundant Martian resources, atmospheric carbon dioxide (CO₂) and subsurface water ice (H₂O), into formic acid (HCOOH) and oxygen (O₂) through a fulvic acid-based photocatalytic cycle validated at the industrial scale in Hungary. A reference module processing 10 tonnes of CO₂ per Earth year yields 10.459 tonnes of formic acid and 3.636 tonnes of oxygen, sufficient to sustain a six-person crew for approximately two Earth years with a 198% safety margin over nominal respiratory demand. The economic analysis indicates that importing equivalent oxygen from Earth costs $1.82–$3.64 million per year; equivalent energy storage (Li-ion) costs $30.5–$61 million for one-time use. Formic acid stores 15.25 MWh of energy in ambient-stable liquid form at a round-trip efficiency of 68.64% without cryogenic infrastructure. A photovoltaic array of 55.37 m² provides the primary energy source; a kilowatt-class nuclear fission reactor constitutes the strategic opportunity for continuous, dust-storm-immune operation with free thermal co-generation. Three critical research gaps have been identified requiring laboratory validation before Mars deployment: (i) catalyst performance at the Martian CO₂ partial pressure (p(CO₂) < 10 mbar, T = 15 °C); (ii) water ice and dry ice extraction at an operational scale; and (iii) integrated closed-loop system demonstration. Built on Earth-proven chemistry with identified, addressable development pathways, the RK-X process theoretically resolves the problems of oxygen supply, seasonal energy storage, water management, and cryogenic infrastructure within a single closed-loop chemical cycle. Full article

Rivers emit substantial amounts of carbon dioxide (CO₂) to the atmosphere, yet its response to heavy rainfall remains unclear with intensive anthropogenic disturbances. To fill the knowledge gap, this study investigated the dynamic variability of CO₂ partial pressure (pCO₂) and CO₂ emissions flux at the Chaohu Lake Basin, a watershed under intensive anthropogenic perturbations, based on field campaigns across diverse river systems during dry season, normal season, and post-rainfall periods. Results demonstrated marked differences in aquatic pCO₂ across river types, with urban rivers (3949 µatm) exhibiting significantly higher levels than non-urban counterparts (1423 µatm). Rainfall events elevated riverine pCO₂, but the effect size varied between river types (urban river versus non-urban river). In non-urban rivers, pCO₂ following heavy rainfall (2461 μatm) was significantly higher (p < 0.05) than those observed during both dry season (1096 μatm) and normal season (712 μatm). In contrast, urban rivers demonstrated only marginal pCO₂ elevation after rainfall (20–30%). Statistical analysis revealed that discharge, total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH₄⁺-N) showed significantly positive correlations with pCO₂, while dissolved oxygen (DO) and pH exhibited significantly negative correlations with pCO₂. Overall, rivers in the Chaohu Lake Basin act as significant sources of atmospheric CO₂, with an annual mean CO₂ emission flux of 297.84 mmol·m⁻²·d⁻¹, and the heavy rainfall events amplify riverine CO₂ emissions (629.91 mmol·m⁻²·d⁻¹), with observed enhancement effects exceeding 300% compared to baseline conditions. To accurately estimate the CO₂ emissions from human-dominated rivers, future research should emphasize the impacts of extreme or heavy rainfall events. Full article

Background: The ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO₂/FiO₂) is central to the classification of acute respiratory distress syndrome (ARDS). However, its assessment requires arterial blood gas analysis, which may be limited by availability, cost, and invasiveness. Consequently, the ratio of peripheral oxygen saturation to fraction of inspired oxygen (SpO₂/FiO₂) has been proposed as a non-invasive surrogate for estimating the degree of oxygenation impairment. Methods: A retrospective cross-sectional study was conducted in adult patients with COVID-19 admitted to the intensive care unit at an altitude of 2600 m above sea level (m.a.s.l.). Spearman correlation coefficients were calculated to assess the association between the SpO₂/FiO₂ and PaO₂/FiO₂ ratios and their corresponding imputation models. A generalized linear model was applied, and the diagnostic performance of the SpO₂/FiO₂ ratio and the imputation models for detecting severe and non-severe hypoxemia (PaO₂/FiO₂ cutoff value of 150) was evaluated using the area under the receiver operating characteristic curve (AUC). Results: A total of 473 patients receiving invasive mechanical ventilation were included, with a mean age of 62.4 years (SD 14.1), and a predominance of males (67.2%). An SpO₂/FiO₂ ratio cutoff value of ≥206 demonstrated excellent diagnostic performance, with an AUC of 0.983 (95% CI 0.97–0.99), high sensitivity (90.6%), high specificity (96.7%), and an overall correct classification rate of 93.9%. This performance remained consistent across multiple clinical scenarios. In patients with positive end-expiratory pressure > 10 cmH₂O, the AUC was 0.982, with a specificity of 97.7%. In the presence of hyperbilirubinemia (total bilirubin ≥ 3 mg/dL), the AUC was 0.951. Among patients with hemoglobin levels < 10 g/dL, sensitivity reached 100%, although specificity was reduced. In the subgroup with arterial partial pressure of carbon dioxide > 35 mmHg, an SpO₂/FiO₂ ratio ≥ 206 showed near-perfect specificity (99.4%) and a positive likelihood ratio of 120.9. Conclusions: The SpO₂/FiO₂ ratio is a reliable and non-invasive surrogate of the PaO₂/FiO₂ ratio in mechanically ventilated patients with COVID-19 living at high altitude, particularly for the identification of non-severe hypoxemia. Full article

Background/Objectives: Abnormalities in the partial pressure of carbon dioxide (PCO₂) can occur during respiratory support and may contribute to adverse neonatal outcomes. This study aimed to assess the incidence of early hypocapnia and hypercapnia in mechanically ventilated preterm infants and their major associated outcomes. Methods: A single-center retrospective cohort study (2017–2024) was conducted in preterm infants < 32 weeks’ gestation who required > 24 h of invasive ventilation within the first 3 days of life. Perinatal–neonatal data were retrieved from the medical database. Admission blood gas values (arterial and capillary–venous) and the maximum and minimum PCO₂ in the first 72 h were evaluated. Normocapnia was defined as PCO₂ 35–45 mmHg, hypocapnia as < 35 mmHg, and hypercapnia as > 45 mmHg. Primary outcomes were the incidence of PCO₂ abnormalities; secondary outcomes included death or severe brain injury (SBI), SBI alone, and bronchopulmonary dysplasia (BPD) among survivors. Logistic regression identified independent predictors of the secondary outcomes. Results: Among the 134 infants evaluated, most experienced both hypercapnia and hypocapnia. Hypercapnia occurred in 81.3% of infants, and hypocapnia in 93.2%. Death or SBI was observed in 51.5%, and SBI alone in 42.5%. Gestational age < 28 weeks, air-leak syndromes, and pulmonary hemorrhage were independent predictors of death or SBI. Among survivors, hypercapnia and gestational age < 28 weeks independently predicted BPD. Infants with adverse outcomes had higher maximum PCO₂ values and greater PCO₂ variability, although these were not independent predictors of SBI or death. Conclusions: PCO₂ instability is highly prevalent in ventilated preterm infants, underscoring the need for individualized ventilation strategies. Extreme prematurity emerged as the primary risk factor for adverse outcomes, while hypercapnia was independently associated with BPD. Full article

Background: Effective management of key physiological parameters, such as blood pressure, temperature, blood glucose, and gas exchange, is central to neurocritical care. However, the clinical impact of variability within guideline target ranges after an acute ischemic stroke, intracerebral hemorrhage, or subarachnoid hemorrhage remains unclear. Methods: In this multicenter observational study of nine German neurocritical care units, we analyzed in-range measurements over 96 h. Of 524 screened patients, 281 met the predefined criteria for sufficient in-range data. Variability in systolic blood pressure, mean arterial pressure, body temperature, blood glucose, partial arterial pressure of oxygen and carbon dioxide was quantified using the coefficient of variation. Associations between in-range variability of each physiological parameter and clinical outcomes including duration of mechanical ventilation, NIHSS score at discharge, and in-hospital mortality were evaluated using multivariable regression models. Results: Variability for all parameters peaked in the first 24 h and then remained largely stable; blood glucose showed a secondary rise after ~60 h. Greater in-range blood glucose variability was associated with in-hospital mortality in hemorrhagic stroke (adjusted OR 1.08; 95% CI 1.00–1.17; p = 0.04), while no other parameter’s variability was associated with the evaluated outcomes. Conclusions: Overall, in-range variability had limited short-term prognostic value, supporting current guideline-based management. Full article

Background/Objectives: Positive end-expiratory pressure (PEEP) is a standardized component of the invasive mechanical ventilation (IMV) settings to improve oxygenation; however, its physiological effects in patients with no documented prior lung disease remain poorly defined. This study evaluated the impact of moderate PEEP variations on macrohemodynamic parameters, gas exchange, and driving pressure (ΔP). Methods: This single-arm, non-randomized, crossover study included adult intensive care unit (ICU) patients with no documented prior lung disease during the early phase of IMV. Sequential PEEP levels of 6, 8, and 10 cmH₂O were applied for 30 min each within the first 24 h of ICU admission, while all other ventilatory parameters were kept constant. Arterial blood gases [partial pressure of oxygen (PaO₂), partial pressure of carbon dioxide (PaCO₂), and arterial oxygen saturation (SaO₂)], oxygenation index [PaO₂/fraction of inspired oxygen (FiO₂)], systolic, diastolic, and mean arterial pressures, ΔP, and static compliance (Cstat) were measured. Friedman and Mann–Whitney U tests were used, with adjustment for multiple comparisons. Results: A total of 150 patients were enrolled (64.7% male). The observed mortality rate was 53.3%; however, mortality was not defined as a primary or secondary outcome, and was used only as a grouping variable for comparative analyses. Intraindividual comparison across PEEP levels of 6, 8, and 10 cmH₂O showed small but significant reductions in systolic and mean arterial pressure at higher PEEP (p-value < 0.05), with Bonferroni-adjusted significance for PEEP 6 vs. 10. No significant differences were observed in oxygenation (SaO₂, PaO₂, and PaO₂/FiO₂), PaCO₂, ΔP, or Cstat. These results suggest that moderate PEEP changes produced limited macrohemodynamic effects without relevant impact on gas exchange or respiratory mechanics. Overall, no clinically relevant or statistically significant differences were observed in gas exchange, macrohemodynamic parameters, ΔP, or Cstat across PEEP levels when mortality was used as the grouping variable. Among survivors, higher PEEP was associated with modest reductions in systolic and mean arterial pressures and higher PaCO₂ values; however, these findings did not translate into consistent physiological benefits. Conclusions: In mechanically ventilated patients with no documented prior lung disease, PEEP may exert divergent effects on macrohemodynamics, gas exchange, and ΔP, supporting a cautious and individualized approach to PEEP selection in this population. Full article

Background: The use of HFNC (High Flow Nasal Cannula) in the management of acute respiratory failure has been fully established in clinical practice. Conversely, less data is available supporting its use in chronic hypoxemic–hypercapnic respiratory failure. The aim of the present study is to evaluate the efficacy of HFNC in chronic hypercapnic respiratory failure associated with stable COPD. Methods: In this retrospective single-center longitudinal observational study, 40 patients treated with HFNC at home followed at the COPD Clinic of Respiratory Diseases (University of Campania L. Vanvitelli Monaldi Hospital, Naples) were included. All patients are re-assessed at our clinic at T0, T3, T6 and T12 months through functional respiratory tests and blood gas analysis. Results: After 12 months, significant reductions in pCO₂ (arterial partial pressure of carbon dioxide) (from 58.5 to 48.0 mmHg) and lactates (from 1.60 to 0.90 mmol/L) were observed, and MIP and MEP improved significantly. Patients receiving HFNC flows ≥50 L/min experienced greater reductions in pCO₂ and fewer exacerbations. Multivariate analysis identified HFNC flow rate (p = 0.0046), hours of use/day (p = 0.0157), lactate levels (p = 0.0301), and FEV₁ (forced expiratory volume in 1 s) (p = 0.0491) as independent predictors of reduction in PaCO₂. Higher BMI and greater airway obstruction were associated with a reduced response. Conclusions: Treatment with HFNC represents a reasonable therapeutic choice to reduce AEs-COPD and reduce PaCO₂ and lactates in stable COPD patients. Full article

Satellite remote sensing has revolutionized the monitoring of atmospheric carbon dioxide (CO₂) concentrations, yet its integration into studies of air–sea CO₂ flux dynamics remains limited. Leveraging high-resolution observations from the Orbiting Carbon Observatory 2 (OCO-2) and Copernicus Marine Environment Monitoring Service (CMEMS), this study investigated the spatiotemporal heterogeneity of atmospheric column-averaged CO₂ (XCO₂) and sea surface partial pressure of CO₂ (pCO₂) between 2015 and 2020. Our analysis reveals pronounced latitudinal gradients, with the Northern Hemisphere exhibiting stronger seasonal XCO₂ variability (5.67 ± 0.42 ppm annual amplitude) compared to the Southern Hemisphere (1.2 ± 0.18 ppm). Notably, the XCO₂ growth rate was marginally higher in the Southern Hemisphere (2.48 ppm yr⁻¹) than the Northern Hemisphere (2.39 ppm yr⁻¹), while coastal regions showed elevated atmospheric CO₂ concentrations, but slower pCO₂ increases relative to the open ocean, suggesting a buffering capacity of marginal seas. Furthermore, we identified distinct seasonal phasing between land and ocean XCO₂, with oceanic signals lagging terrestrial ones by approximately one month. These findings highlight the utility of satellite data in resolving fine-scale air–sea carbon flux dynamics and provide critical insights into how heterogeneous atmospheric CO₂ changes propagate across marine systems. Full article

Background/Objectives. Changes in CO₂-derived variables during a fluid challenge have been proposed as markers of fluid responsiveness. We investigated whether, instead of fluid administration, passive leg raising (PLR)-induced changes in the CO₂-derived variables, namely central venous-arterial carbon dioxide partial pressure (P(cv-a)CO₂) and the ratio between P(cv-a)CO₂ and the arterial-central venous oxygen content (P(cv-a)CO₂/C(a-cv)O₂), could detect preload responsiveness in critically ill patients. Methods. We studied 30 mechanically ventilated patients in whom a PLR test was performed due to acute circulatory failure. Routine hemodynamic variables, velocity-time integral (VTI), in the left ventricular outflow tract, and CO₂-derived variables, were measured before, during, and after a PLR test. A PLR-induced increase in VTI of ≥10% defined preload responsiveness. The differences (Δ) of P(cv-a)CO₂ and P(cv-a)CO₂/C(a-cv)O₂ between PLR and pre-PLR were calculated. The predictive values of PLR-induced changes in the CO₂-derived variables was determined by receiver operating characteristic area under curves (ROC-AUCs). Results. Fifteen patients (50%) were classified as preload responsive. ΔP(cv-a)CO₂ and ΔP(cv-a)CO₂/C(a-cv)O₂ were correlated with VTI changes and differed significantly between responders and non-responders −1.3 (−2–−0.6) vs. 0.6 (−0.1–1.1) mmHg, p < 0.001, and −0.38 (−0.97–−0.34) vs. 0.1 (−0.15–0.57) mmHg/mL O₂, p < 0.001, respectively. The PLR-induced decrease in P(cv-a)CO₂ was significantly associated with preload responsiveness (OR 0.48, CI 0.20–0.89, p = 0.016, bootstrap CI 0–0.85). The AUC curves for both ΔP(cv-a)CO₂ and ΔP(cv-a)CO₂/C(a-cv)O₂ ratio to predict preload responsiveness were 0.89 (CI 0.74–1), p < 0.001, and 0.85 (CI 0.70–1), p < 0.001, respectively. Conclusions. In mechanically ventilated ICU patients with circulatory shock, PLR-induced changes in P(cv-a)CO₂ and P(cv-a)CO₂/C(a-cv)O₂ ratio were correlated with VTI changes. The change in P(cv-a)CO₂ was the only variable detecting preload responsiveness assessed by PLR; therefore, it could serve as an indirect marker, useful to guide fluid resuscitation when cardiac output measurement is not feasible. Full article

The use of sustainable carbon-free energy sources is becoming a priority in the field of transport so that it becomes sustainable. Sustainable transport can also be achieved with vehicles equipped with diesel engines fuelled by alternative fuels that do not contain carbon, like hydrogen. The paper presents an analysis of the experimental results obtained at the fuelling with diesel fuel and hydrogen of a modern diesel engine, operating at 50% partial load and 2500 rev/min speed. For H₂ energy substitution degrees of up to 43%, the combustion process is improved: the specific energy consumption is reduced, the combustion duration is reduced, the heat release rate is increased, the maximum pressure is increased, the carbon-based pollutant emissions are decreased and the cyclic dispersion is reduced. For 33% H₂ energy substitution degree, the maximum pressure increases by 16.4%, the indicated mean effective pressure increases by 7.5%, the specific energy consumption is reduced by 5.36% and the level of greenhouse gases emission is reduced by 34.5% for carbon dioxide. In case of pollutant emissions, the smoke level is reduced by 58.6% and the unburned hydrocarbons level is reduced with 18%. For higher percentages of H₂, emissions reductions can be accentuated. At H₂ use, the combustion cyclic variability is reduced, the values of the COV variability coefficients determined for the parameters of interest and the combustion duration being reduced. As a novelty aspect, the optimal adjustment between engine load-speed-diesel fuel flow-hydrogen flow-maximum combustion pressure-smoke emission level-exhaust temperature level is presented. The use of hydrogen at the diesel engines can provide the beginning of sustainable transportation solutions in the future. Full article

Background/Objectives: Acute respiratory failure (ARF) is associated with a high mortality. This study aimed to explore the association of arterial partial pressure of carbon dioxide (PaCO₂) in relation to survival outcomes in mechanically ventilated patients with ARF. Methods: This multicenter retrospective cohort study integrated the data from the eICU Collaborative Research Database (eICU-CRD; n = 10,946), the Medical Information Mart for Intensive Care IV (MIMIC-IV; n = 6683), and clinical records from two university-affiliated intensive care units in China (n = 410). The patients were categorized into low, normal, and high PaCO₂ groups using a restricted cubic spline model to explore the relationship between PaCO₂ and mortality. The 28-day survival distributions among the three groups were compared using Kaplan–Meier curves, with statistical significance assessed via the log-rank test. A multivariable Cox proportional hazards model was constructed to evaluate the independent prognostic value of PaCO₂ for multiple complications. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated for the low and high PaCO₂ groups relative to the normal PaCO₂ group. Results: A U-shaped relationship was observed between PaCO₂ and mortality, with both low PaCO₂ (<36.4 mmHg) and high PaCO₂ (>57.9 mmHg) associated with an increased mortality risk. Kaplan–Meier survival analysis demonstrated that patients in the intermediate PaCO₂ range (36.4–57.9 mmHg) exhibited the highest survival rate (65.2%), whereas those in the low and high PaCO₂ groups had significantly lower survival rates (60.0% and 63.2%) (log-rank test, p < 0.001). Adjusted survival analyses further revealed that complications such as sepsis and chronic kidney disease significantly influenced the mortality across PaCO₂ strata. Compared with the intermediate PaCO₂ group, the hazard of death increased by 25.5% in the low PaCO₂ group and by 18.9% in the high PaCO₂ group. Conclusions: This retrospective analysis indicates that arterial PaCO₂ levels within the optimal range are associated with improved survival in patients with acute respiratory failure (ARF) on mechanical ventilation, but prospective studies are needed to establish causality and consider potential confounding factors. Full article