High Thermal Efficiency Research Articles

Integrated operation and efficiency analysis of CaCO3/CaO in a fixed-bed reactor for thermochemical energy storage

Calcium-based thermochemical energy storage (TCES) has attracted much attention in solar energy utilization and storage. However, the investigations of the CaCO3/CaO system are incomplete and poorly integrated at the reactor scale. In this work, a fixed-bed reactor for calcium looping (CaL) is used to conduct the integrated operation of energy storage and release. The decomposition conversion of CaCO3 in N2 at 850 °C for 8 h is 63.8% and the carbonation conversion of the corresponding decomposition product is 67.2% in CO2 at 750 °C for 4 h. The lower reactor filling increases overall thermal energy storage efficiency but decreases released energy. Furthermore, a simulation model is built to study the key operation parameters that greatly affect reactor performances. According to the orthonormal design, the high calcination temperature and porosity of 0.6–0.7 are key factors to improve both high thermal energy storage efficiency and released energy. The carbonation temperature and thermal conductivity are less important factors than decomposition temperature and porosity, which can be adjusted flexibly to meet the needs of heat utilization and cost reduction. This work provides valuable guidance for optimizing reactor operation and modifying materials to achieve high overall efficiency and released energy in fixed-bed reactors.

Data-driven optimal placement of minichannel-based solar water heater using satellite-derived and ground-telemetry weather information

The aluminum minichannel solar collector is a novel technology for solar water heating. Minichannel-based solar collectors have higher thermal efficiency than conventional flat plate collectors and do not suffer from potential loss of vacuum as evacuated-tube collectors. This technology can play a significant role in reducing natural gas consumption that translates into lower greenhouse gas emissions to the atmosphere. However, the performance of solar collectors depends on the geographical location of the installation due to solar resource availability and weather pattern. The potential reduction in natural gas consumption using aluminum minichannel solar collectors is assessed using solar irradiance, ambient temperature, and wind data obtained from ground weather station and satellite-derived data. A data-driven numerical analysis is performed using a validated solar water heater (SWH) model, population, and natural gas consumption data for the entire state of California to assess the best locations to install these systems. The SWH model is validated based on data collected from an actual SWH system installed at a single-family house in Northridge, California. A K-means clustering method is then applied to select the best regions for installation of this technology. Based on performance, population density, and natural gas consumption, the regions of Southern California and the Central Valley are chosen as having the highest potential for reduction of natural gas consumption. The analysis was performed from weather data obtained based on two full years (2020 and 2022), where the effect of COVID-19 (year 2020) is observed as having higher water tank temperatures and higher solar fractions, which could be associated with lower hot water consumption.

Level of Awareness on the Usage of Thermal Insulation among Building Professionals: Case of a Tropical City in Nigeria

Adoption of buildings’ thermal insulation in Nigeria is still at the developmental stage; evidence of its usage for high thermal performance buildings and energy efficiency connotes a very low rate as there is very little evidence-based research in this regard. This paper seeks to evaluate the level of awareness on the usage of building thermal insulation among professionals in the building construction sector within Akure, a tropical city in Nigeria, to identify factors that deter its adoption and raise the level of commitment of the professionals within the building construction sector to sustainable practices of reducing buildings’ energy consumption. This is to create a resilient built environment against the challenges of climate change. Well-structured questionnaires were administered to gather data for the investigation. Jupyter Notebook version 6.4.12 was employed for the analysis. Results indicated a fairly good level of awareness among the building professionals while the barriers that affect the level of usage of thermal insulation within the study area are the high cost of procuring thermal insulation materials, non-compliance to building regulations, and governmental policy on construction. The research concluded that a good number of professionals have a high level of awareness of the usage of building thermal insulation, but the cost of the materials and government policy deterred its level of usage in the study area. Consequently, propagation of the usage of thermal insulation should be done through education and training of the stakeholders. Furthermore, the government needs to strengthen policies that support the usage and availability of thermal insulation within the study area

CFD simulation of a forced draft biomass cookstove for different airflow conditions

Households in developing countries rely on traditional cooking methods despite low-efficiency and high risks of air pollution and health. Various improved biomass cookstove technologies have been developed to address this issue, but there is still room for improvement in biomass cooking technology. This research aims to enhance the biomass cookstove by investigating its performance under different air inlet conditions using computational fluid dynamics analysis. The impact of air flow rate and the position of the secondary air inlet on the thermal efficiency of the biomass cookstove is examined. The eddy dissipation combustion model and k-epsilon turbulence model are utilized in ANSYS Fluent to simulate the combustion process. The cookstove's performance is assessed at three different air flow rates (0.01, 0.02, and 0.03 kg/s) and at two different secondary air inlet positions. The highest thermal efficiency is observed at secondary air inlet located 50 mm below the outlet at a flow rate of 0.03 kg/s. The computational findings from this investigation show a significant connection between the air inlet conditions and the thermal efficiency of the biomass cookstove. Repositioning the secondary air inlet to a location with more available space for complete combustion improves thermal efficiency compared to traditional designs.

An effective material for solar steam generation applications: Gradient graphene sponge

The efficient use of solar energy for clean water is a renewable and environmentally-friendly route to solve global water scarcity. For this, we report the preparation of light, mechanically durable, and effective photothermal material. A simple acid impregnation method was applied to graphene sponge material (GSM) possessing hydrophobic character to gain partial hydrophilicity to obtain a gradient structure. The morphological and structural properties of the gradient graphene sponge material (GGSM) were carried out using scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray powder diffraction patterns, Fourier-transformed infrared spectroscopy, and water drop contact angle measurements. GGSM showed fast and effective water vapor conversion performance due to the gradient structure, as well as the high rate of sunlight absorption and photothermal conversion thanks to graphene. With the advantageous feature of gradient structure, GGSM achieved a high water vapor conversion capacity (1.79 kg/m2h) and solar thermal efficiency of 57% compared to GSM (1.58 kg/m2h; 45%). This new material provides a novel approach to solar energy applications.

DFT based study of copper calcium halide perovskite nanomaterials for optoelectronic and energy applications

The mechanical, electronic, optical, and thermoelectric analyses of CuCaX3 (X = Cl, Br, I) have been done using DFT based TB-mBJ approach. The stability of CuCaX3 (X = Cl, Br, I) has been confirmed from the enthalpy of formation; values for CuCaCl3, CuCaBr3, and CuCaI3 are −1.07 eV, −1.55 eV, and −2.15 eV, respectively. The phonon spectra plot is also discussed, and the phonon dispersion analysis confirms that the CuCaX3 systems are dynamically stable since no imaginary modes are observed. The modified Becke-Johnson exchange potential is used to evaluate the opto-electronic and thermo-electric properties. By replacing anions (Cl to I), the band gap of the studied material in the visible region has been tailored, from 2.9 eV to 2.65 eV, for renewable energy devices. In addition, various optical properties such as dielectric constant, refraction, absorption, optical conductivity, and optical loss factor have been analyzed for optoelectronic applications. With the replacement of anions (Cl to I), the peaks of maximum intensities shifted to lower energy to values of 7.2 eV, 6.2 eV and 5 eV for CuCaCl3, CuCaBr3 and CuCaI3 respectively. Furthermore, the BoltzTrap code has been used to discuss thermal conductivity, power factor, Hall coefficient, specific capacitance and electron densities in detail for thermoelectric applications. The CuCaX3 family of perovskites has been found to have bandgaps falling in the visible region for light-emitting diodes (LEDs). These also exhibit high thermal efficiency, making them potential multifunctional candidates for optoelectronic and energy harvesting applications.

Experimental Investigation on Knock Characteristics from Pre-Chamber Gas Engine Fueled by Hydrogen

Hydrogen-fueled engines require large values of the excess air ratio in order to achieve high thermal efficiency. A low value of this coefficient promotes knocking combustion. This paper analyzes the conditions for the occurrence of knocking combustion in an engine with a turbulent jet ignition (TJI) system with a passive pre-chamber. A single-cylinder engine equipped with a TJI system was running with an air-to-fuel equivalence ratio λ in the range of 1.25–2.00, and the center of combustion (CoC) was regulated in the range of 2–14 deg aTDC (top dead center). Such process conditions made it possible to fully analyze the ascension of knock combustion until its disappearance with the increase in lambda and CoC. Measures of knock in the form of maximum amplitude pressure oscillation (MAPO) and integral modulus of pressure oscillation (IMPO) were used. The absolute values of these indices were pointed out, which can provide the basis for the definition of knock combustion. Based on our own work, the MAPO index > 1 bar was defined, determining the occurrence of knocking (without indicating its quality). In addition, taking into account MAPO, it was concluded that IMPO > 0.13 bar·deg is the quantity responsible for knocking combustion.

Analysis and optimisation of emissions and fuel economy of biodiesel fuelled CI engine using Taguchi technique

The objectives of the present work are to examine the combined effect of engine operating and injection parameters and to optimise these parameters for better fuel economy, performance, and reduced emissions. The fuels used were diesel and biodiesel blends of Jatropha and Karanja biodiesel. The results indicate that diesel gives higher thermal efficiency at low loads, however, the 20% jatropha blend was slightly more efficient, at high loads. It was observed that the BSFC increases when the EGR is varied from 0% to 20%. Even though the smoke opacity increases with increased EGR, advanced injection at 28o BTDC decreases smoke. Increased EGR also reduced NOx emissions by up to 11%. Blending also has a significant impact on the output parameters. Taguchi analysis was applied to optimise the performance and emission characteristics of the diesel engine and set of input values were obtained for the optimum performance under different conditions.

Investigation of injection parameters coupled with fuel reactivity on combustion and emissions in dual direct-injection GCI engine

Fuel injection parameters and fuel reactivity significantly affect engine performance, combustion, and emissions. The dual direct-injection strategy could compensate for the insufficient heat release rate to improve the gasoline compression ignition (GCI) mode with medium injection pressure for high loads. This research aims to couple injection parameters and fuel reactivity to optimize the combustion, emissions, and engine performance of the dual direct-injection GCI mode at high loads. The effect of research octane number (RON) of gasoline, injection timing, and injection pressure of dual injection system on combustion and emissions was investigated. Results show that the optimized dual direct-injection GCI mode can simultaneously achieve high thermal efficiency, low emissions, and low maximum pressure rise rate (MPRR) at high loads with the maximum indicated mean effective pressure (IMEP) of about 16 bar with indicated thermal efficiency (ITE) of 48.5%. Delaying the injection timing of injector-1 reduced the peak pressure (Ppeak), MPRR, and NOx emissions. Ppeak, CO, NOx, and indicated specific fuel consumption (ISFC) decreased compared to the dual-injector simultaneous injection case when the injection timing of injector-2 was retarded to between 5 and 10°CA ATDC. Increasing the injection pressure of injector-2 reduced CO, THC, ISFC, and particulate emissions. Lower RON gasoline obtains higher ITE while higher RON gasoline obtains lower particulate emissions.

Design of a 130 MW Axial Turbine Operating with a Supercritical Carbon Dioxide Mixture for the SCARABEUS Project

Supercritical carbon dioxide (sCO2) can be mixed with dopants such as titanium tetrachloride (TiCl4), hexafluoro-benzene (C6F6), and sulphur dioxide (SO2) to raise the critical temperature of the working fluid, allowing it to condense at ambient temperatures in dry solar field locations. The resulting transcritical power cycles have lower compression work and higher thermal efficiency. This paper presents the aerodynamic flow path design of a utility-scale axial turbine operating with an 80–20% molar mix of CO2 and SO2. The preliminary design is obtained using a mean line turbine design method based on the Aungier loss model, which considers both mechanical and rotor dynamic criteria. Furthermore, steady-state 3D computational fluid dynamic (CFD) simulations are set up using the k-ω SST turbulence model, and blade shape optimisation is carried out to improve the preliminary design while maintaining acceptable stress levels. It was found that increasing the number of stages from 4 to 14 increased the total-to-total efficiency by 6.3% due to the higher blade aspect ratio, which reduced the influence of secondary flow losses, as well as the smaller tip diameter, which minimised the tip clearance losses. The final turbine design had a total-to-total efficiency of 92.9%, as predicted by the CFD results, with a maximum stress of less than 260 MPa and a mass flow rate within 1% of the intended cycle’s mass flow rate. Optimum aerodynamic performance was achieved with a 14-stage design where the hub radius and the flow path length are 310 mm and 1800 mm, respectively. Off-design analysis showed that the turbine could operate down to 88% of the design reduced mass flow rate with a total-to-total efficiency of 80%.

The experiment of rotating detonating engine using an asymmetric vortex shape on an annulus chamber

Rotating detonating engine is new propulsion technology with promising advantages. Higher thermal efficiency and simple design with higher thrust attracted intention worldwide to make it realistic. The normal design uses a circular shape of an annulus chamber. The circular shape of the annulus chamber was well-known and tested with various conditions. The new interest area of changing a circular shape into an asymmetric shape is still unknown. Exploring the new shape of the annulus chamber was done by applying the new shape of the annulus chamber on RDE. The experiment was successfully done using methane at different conditions. Three modes can be observed from pressure product and thrust result: a mode of detonation, unstable detonation, and failure on transition. The highest thrust happens at an equivalence ratio of 1.2 with 18N. The new shape of the annulus offers the versatility of RDE.

Innovative cinque rib-roughened stimulators on performance improvement in triangular channel solar air heater

Abstract Present study investigates application of cinque-faced rib roughening to improve the efficiency of solar air heaters (SAHs). Research focuses on a novel design, the cinque-faced rib-roughened triangular channel SAH (CFRTCSAH), which incorporates five-faced wire ribs on the absorber plate’s underside. By breaking down the laminar sublayer and enhancing flow separation and reattachment, - ribs significantly enhance heat transfer. It establishes a comprehensive theoretical model to analyze the thermal and hydraulic performance of the CFRTCSAH. The study considers the influence of parameters such as relative roughness pitch (P/e) and relative roughness height (e/Dh) on thermal efficiency and heat removal factor. The results demonstrate that the CFRTCSAH achieves remarkable thermal efficiency, reaching up to 80.7% under optimal roughness parameters (P/e = 12, e/Dh = 0.05). It is found that an increase in mass flow rates positively impacts thermal efficiency but should not exceed 0.09 kg/s for maximum effective efficiency. Additionally, it has observed that the CFRTCSAH performs most effectively when the relative roughness pitch ratio (P/e) is set to 12. A higher relative roughness height (e/Dh) promotes convective heat transfer, enhancing thermal performance, especially at lower mass flow rates. Conclusion presents a novel approach to improving the thermal performance of SAHs by introducing cinque-faced rib roughening. The CFRTCSAH design exhibits significant potential for achieving high thermal efficiency and can contribute to more efficient solar energy utilization in various applications. These findings offer valuable insights for the development of SAHs, promoting sustainable and energy-efficient technologies.

The 4E emergy-based analysis of a novel multi-generation geothermal cycle using LNG cold energy recovery

Over the last decade, the increasing demand for sustainable energy sources has led to the development of innovative approaches in geothermal power plant cycles. In this study, a novel combined geothermal power plant cycle (GTPPC) that integrates the Kalina cycle, transcritical Carbon Dioxide (T-CO2) cycle, with the injections of the Liquid Natural Gas (LNG) stream and geothermal water (GTW) stream is proposed in order to provide cold and hot sources of the system. The performance of the system is evaluated using emergy analysis, an evaluation factor based on thermodynamic principle and solar energy with the aid of emergoeconomic and emergoenvironmental factors. The injection of LNG stream leads to significant improvements: enhancing the net output power and overall emergoeconomic and emergoenvironmental efficiencies of the cycle. By conducting improvement priority and parametric studies, crucial stream points and the system's performance's optimization is identified. Our results indicate that the Kalina sub-cycle has a greater impact on performance, and the exergy destruction in the LNG stream will be minimized. At the optimal point, the GTPPC achieves high thermal efficiency and emergy-based metrics, indicating its sustainability and viability as a geothermal power generation solution. By the injection of LNG stream, the net output power, and overall emergoeconomic and emergoenvironmental efficiencies of the cycle, improved by 1.79 MW, 14 %, and 16 %, respectively. According to the obtained results, the performance of the Kalina sub-cycle has more effect than the Carbon Dioxide sub-cycle, and destructing exergy in the LNG stream will have irreparable effects on the operation of the GTPPC. At the optimum point of the system performance, the GTPPC will operate with the thermal efficiency, ψ, and φ of 15.4 %, 94.3 %, and 98.5 %, respectively.

Thermal performance evaluation of an indigenously designed small-scale solar parabolic trough collector for manufacturing Indian milk products

ABSTRACT A small-scale solar parabolic trough collector system coupled with a jacketed vessel was designed and evaluated for its performance and quality of the products manufactured in it. The prototype having a 7.026 m2 aperture and 88.38° rim angle was installed in Mehsana, Gujarat, India. The study was conducted in May–June 2022 at different flow rates of sigma therm-k. The useful heat rate ranged from 567.82 W to 861.25 W. The daily average thermal efficiency was obtained in the range of 9.81–14.61% and maximum thermal efficiency achieved during the milk trials was 31.51%. Similarly, maximum absorber temperature achieved during stagnation was 193.4°C and during absorber fluid flow condition was 146.5°C. The maximum convective heat transfer coefficient of 49.5 W/m2K was calculated at Reynold number 1822 and Nusselt number 24.12. It was found that the quality parameters of manufactured milk products were at par with domestic food safety standards. Direct normal irradiance was found to be the most influencing factor followed by the initial milk load. The present system is a potential substitute for various small-scale Indian traditional heat-desiccated milk products, where higher thermal efficiency is not the primary requirement and emphasis is more on the importance of ease of operation.

Ammonia combustion using hydrogen jet ignition (AHJI) in internal combustion engines

The application of ammonia and hydrogen in internal combustion engines (ICE) is a promising zero-carbon technology. This paper investigates ammonia combustion in ICE using hydrogen multiple-injection jet-ignition (MIJI). The experiment was conducted on a single-cylinder heavy-duty engine with a compression ratio of 17 and a cylinder bore of 123 mm. In the passive jet ignition mode, the minimum hydrogen energy ratio for stable operation of the ammonia-hydrogen engine was 15 %. Using H2 active jet ignition could improve the combustion stability significantly. It was found that engine performance was gradually improved by H2 injection strategy in order of single-injection, double-injection to triple-injection. The triple-injection strategy could achieve a more ideal distribution of ammonia-hydrogen mixture: the first injection forms a homogeneous mixture to increase fuel reactivity; the second injection creates a stratified hydrogen mixture, facilitating rapid flame propagation; the third injection forms a relatively rich mixture in the jet chamber conducive to spark ignition. Through the optimization of the hydrogen injection strategy and spark timing, stable combustion (COV = 1 %) and high thermal efficiency (ITE = 42.5 %) in ammonia-hydrogen engines were achieved, with a minimum hydrogen energy ratio less than 3 %.

Comparing the partial cooling and recompression cycles for a 50 MWe sCO2 CSP plant using detailed recuperator models

Supercritical carbon dioxide (sCO2) power cycles are attractive for renewable energy technologies as they exhibit high thermal efficiencies, compact components, and reduced cycle complexity and levelized cost of energy (LCOE) relative to traditional steam power cycles. Consequently, numerous studies have investigated different layouts and operating parameters in search of an optimal cycle configuration. Two cycle layouts provide promising results, namely the partial cooling with reheating (PCRH) cycle, and the recompression with intercooling and reheating (RCICRH) cycle. Both cycles utilise recuperators with high heat transfer rates and literature suggests that the cost for the recuperators may account for a significant portion of the total plant capital expenditure. When modelling the recuperator, most cycle comparative studies employ simplified recuperator models where heat exchanger effectiveness or conductance is utilised to approximate heat transfer. These models do not consider the effect of recuperator size and geometry on heat transfer and pressure drop in detail. In this work, for a concentrated solar power (CSP) application, the performance and component size requirements of the cycles are evaluated parametrically for different cycle mass flow split ratios, pressure ratios, and ratio of pressure ratios across the compressors using detailed discretised one-dimensional (1D) recuperator models. Furthermore, the geometry of these models are optimised (by volume) for straight and zigzag channel printed circuit heat exchangers (PCHEs), widely accepted as the most suitable for this application. Finally, a high-level cost comparison is conducted. The results show that if appropriate pressure drop assumptions are made, a simplified model approach yields valid cycle level results when compared with the results obtained using detailed models which consider practical recuperator geometry. However, the pressures and temperatures within the recuperator may not be predicted to a sufficient level of accuracy. In addition to providing clarity regarding the interplay between key cycle process parameters and cycle performance, straight channel PCHEs provide better thermofluid performance than zigzag channel PCHEs. Furthermore, the RCICRH cycle requires larger turbomachinery and a marginally higher capital outlay for the power cycle, but offers superior thermal efficiencies and requires smaller heat exchangers. For sCO2-CSP applications employing dry cooling, this suggests that the RCICRH cycle requires a smaller solar field and cooling system, and may therefore offer increased revenue in adverse weather conditions where direct normal irradiance (DNI) is low and/or ambient temperatures are high. However, the PCRH cycle requires a smaller solar receiver system, a smaller thermal energy storage (TES) system, and smaller turbomachinery, the cost savings of which may outweigh the benefits of the RCICRH cycle.

Dynamic performance of supercritical CO2 Brayton cycle and its relationship to the correction of turbomachinery performance maps: A comparative analysis

Due to its numerous advantages, including the capability to achieve high thermal efficiencies and compactness, the supercritical carbon dioxide Brayton cycle (SCO2BC) has recently been considered as an attractive next-generation heat-to-power conversion cycle. Dynamic modelling of the SCO2BC is essential to simply grasp and analyze its performance in either design or off-design conditions. Turbomachines are key components in the SCO2BC, which are typically modelled by adopting their design-point performance map. However, during off-design operations, the performance maps become inaccurate, and correction relations must be integrated into these models to enhance their validity. Despite the development of multiple performance map correction, reduction, and normalization methods, there is a scarce literature that implemented highly accurate correction relations in SCO2BC transient models, which impacts the validity of the obtained results. Therefore, there is a need to assess the accuracy of these results compared to the results of a SCO2BC model adopting an accurate correction method. However, some information needs to be provided first, which include how turbomachinery correction impacts the dynamic behavior of SCO2BC compared to using uncorrected turbomachinery models, and if the correction of one turbomachine (i.e., the compressor) is more significant than the other. This work aims to provide this data and more by developing dynamic SCO2BC models (CPP models), comprehensively evaluating their transient performance, and comparing them to SCO2BCs with uncorrected turbomachinery models (FPP models). This work possesses novelties of utilizing the most accurate turbomachinery correction model found for SCO2BC and building the model in Simcenter Amesim. It is found that FPP models may be viable when the dynamic responses of the components near the fluctuation source are desired. Otherwise, at least a compressor model correction is required to obtain reliable results. Moreover, all turbomachinery models should be corrected for accurate mass flow rate and power responses.

The CO2 Capture System with a Swing Temperature Moving Bed

Abstract The reduction in CO2 emissions is now a very popular topic. According to the International Energy Agency, CO2 emitted in 2021 was 6% more than that emitted in 2020. Carbon capture and storage (CCS) is gaining popularity as a possible solution to climate change. Experts estimate that industry and power plants will be responsible for 19% of total CO2 emissions by 2050. This paper presents the design of a semi-industrial-scale system for CO2 capture based on the moving bed temperature swing adsorption technology. According to the results of laboratory tests conducted by the SINTEF industry, this technology demonstrates high capture efficiency (>85%). The CO2 capture medium involved in adsorption is activated carbon passing through individual sections (cooling, heating, adsorption), where CO2 is bonded and then released. The heat and mass transfer processes are realised on the developed stand. The heat exchangers use steam and water as the heating/cooling medium. The paper reviews the existing solutions and describes the developed in-house design of heat exchangers that will ensure heat transfer conditions being a trade-off between economic and efficiency-related issues of the CO2 capture process. The designed test stand will be installed in a Polish power plant and is expected to meet the method energy intensity target, set at ≤ 2.7 MJ/kg CO2, with a capture efficiency exceeding 85%. The aim of the work was to develop and solve technical problems that would lead to the construction of a CO2 capture station with parameters mentioned above. This stand uses an innovative method where CO2 is captured by contacting the fluid (gases) with solid particles. The heat exchange associated with the heating and cooling of the adsorbent had to be solved. For this purpose, heat exchangers were designed with high thermal efficiency and to prevent the formation of mounds.

Machine learning approaches for identification of heat release shapes in a low temperature combustion engine for control applications

This paper presents the application of machine learning classification algorithms to identify and classify different heat release rate (HRR) shapes to control the combustion for an optimal multi-mode low-temperature combustion (LTC) engine operation. Low-temperature combustion engine produces low nitrogen oxides (NOx) and soot emissions and offers high thermal efficiency. But high in-cylinder pressure rise rates limit the operating range of the LTC engine. Therefore, it is imperative to control combustion in the LTC engine for safe operation. To this end, the HRR traces for over six hundred engine operating conditions are classified using supervised (i.e., Decision Tree, K-Nearest Neighbors (KNN), and Support Vector Machines (SVM)) and unsupervised (i.e., Kmeans clustering) machine learning approaches to segregate different combustion regimes based on HRR shape. Kmeans clustering was not successful in classifying the HRR shapes. Among different supervised machine learning techniques, SVM has proved to be the best method, having an overall classifier prediction accuracy of 92.4% for identifying the distinct shapes using normalized HRR data. In addition, three classifiers have been trained based on the combustion parameters and control inputs. These classifiers are then used as scheduling variables to develop predictive models. A model predictive control (MPC) framework is developed to control multi-mode LTC engine on cycle-to-cycle basis. The MPC framework achieved the simultaneous reference tracking of combustion phasing (CA50) and indicated mean effective pressure (IMEP) while constraining maximum pressure rise rate (MPRR) below 8 bar/CAD.

Experimental and numerical investigation of methane combustion in a HCCI engine under enriched oxygen conditions

The challenge in the energy transition lies in the intermittent nature of renewable energy, requiring the development of effective storage methods. E-fuels are considered a promising solution, with potential applications to Homogeneous Charge Compression Ignition (HCCI) engines known for their high thermal efficiency and low nitrogen oxide emissions. However, HCCI engines face limitations such as low power density and a narrow operating range. To overcome these issues, oxygen-enriched combustion is proposed, aiming at enhancing the mixture reactivity and expanding the operating range by decreasing the intake temperature. As the potential of the expansion of the operating range has never been investigated, we performed an experimental campaign using a methane-fuelled HCCI engine at variable oxygen fractions in the oxidizer. To further elucidate the effects of oxy-fuel combustion, we also performed a kinetic analysis of the reactions driving the combustion behaviour. It has been experimentally found that increasing the oxygen content in the mixture, up to 90%, has a significant potential to decrease the needed intake temperature (up to 70 °C) of the charge to keep the Maximum Pressure Rise Rate (MPRR) below the safety threshold of 8 bar/CAD. Despite the notable IMEP increase achieved, operating the engine at high oxygen content presents significant technical challenges. Therefore, to make HCCI engines competitive with other combustion modes, it is essential to combine oxygen enrichment with additional power density enhancement techniques. The kinetic analysis has highlighted that the mixture is less sensitive to intake temperature changes at higher oxygen percentages (above 50%) and that the rates of the main reactions involved in methane and OH consumption are highly enhanced by oxygen addition.