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Lean burning as one of the low-temperature combustion strategies has the potential to reduce pollution emissions and improve fuel efficiency. However, highly diluted mixtures cause internal combust...

In proton exchange membrane fuel cells (PEMFC), the design of the cathode flow field is very important, because an excellent flow channel design can not only accelerate the transmission rate of liquid water, but also affect the distribution of electrode reactants and electrode products which influence the electrochemical performance of the fuel cell. This study presents three new channels (models 1,2 and 3), which were created using two unilateral slopes and a bilateral slope structure with tapered tube lengths of 0.4, 1.2 and 0.8 mm, respectively. The dynamic behavior of liquid water under the three design schemes is numerically studied based on the volume of fluid method. And the influence on the performance of fuel cell was analyzed synthetically. The results indicate that the introduction of a tapered and sloping structure can improve the transmission efficiency of the droplets in the flow channel, and the maximum droplet removal time of the new channel can be reduced by 24.4%compare with standard conventional flow channel. The slope structure guides the flow path of water droplet and reduces the occurrence of droplet spatter. Influenced by the slope and tapered structures, the turbulence of airflow near the bottom surface (gas diffusion layer)of the flow channel is enhanced and Oxygen concentration in the cathode is raised, which improves the mass transfer capacity and average current density of reactive surface. In conclusion, the new type of channel with a tapered and sloping structure has a potential to improve the performance of water management in the cathode channel of PEMFC.

The high-pressure injection strategy for di-n-butyl ether (DBE)-diesel fuel blends reduces particulate matter emissions. Exhaust gas recirculation (EGR) technology was used to decrease NOx ...

This study investigated the effects of cooperative-control of the start of injection (SOI), excess air ratio (λ), internal exhaust gas recirculation (I-EGR), and intake air temperature (IAT) on the combustion and emission characteristics of gasoline compression ignition (GCI) engines, especially regards to the combustion stability and knock characteristics. And optimizing the GCI engine combustion and emissions through the cooperative control of multiple parameters is the innovation of this research. The results showed that advancing the SOI and increasing the I-EGR ratio can significantly expand the low-load limit, but the heating effect of 20% I-EGR only worked when the SOI was earlier. An appropriate increase of λ could increase the maximum brake thermal efficiency (BTE) to 40.06%, but resulted in high-knock probability and high NOx emissions. Rising the IAT was more effective than advancing the SOI in improving combustion fluctuations, but the knock probability and knock intensity were more sensitive to the early SOI. When the SOI varied from 26 °CA BTDC to 30 °CA BTDC, λ was 1–1.5, the I-EGR ratio was 5%–20%, and the IAT was 40–50 °C; the GCI engine can obtain the balance among high thermal efficiency, high combustion stability, low knock probability, and low emissions.

As a highly nonlinear system, the performance of proton exchange membrane fuel cell (PEMFC) is controlled by various parameters. If the effects of all parameters are considered during the performance optimization, low working efficiency and waste of resources will be caused. The development of sensitivity analysis for parameters can not only exclude the parameters which have slight effects on the system, but also provide the reasonable setting ranges of boundary values for simulation of performance optimization. Therefore, sensitivity analysis of parameters is considered as one of the methods to optimize the fuel cell performance. According to the actual operating conditions of PEMFC, the fluctuation ranges of seven sets of parameters affecting the output performance of PEMFC are determined, namely cell operating temperature, anode/cathode temperature, anode/cathode pressure, and anode/cathode mass flow rate. Then, the control variable method is used to qualitatively analyze the sensitivity of main parameters and combines with the Monte Carlo method to obtain the sensitivity indexes of the insensitive parameters under the specified current density. The results indicate that among these parameters, the working temperature of the fuel cell is the most sensitive to the output performance under all working conditions, whereas the inlet temperature is the least sensitive within the range of deviation. Moreover, the cloud maps of water content distribution under the fluctuation of three more sensitive parameters are compared; the results verify the simulated data and further reveal the reasons for performance changes. The workload of PEMFC performance optimization will be reduced based on the obtained results.

The combination dimethyl carbonate (DMC)/diesel-blended fuels and the exhaust gas recirculation (EGR) can decrease nitrogen oxide (NOX) and soot emissions simultaneously emitted from the compressio...

n-Pentanol exhibits some advantageous fuel properties, such as a higher energy density, lubricity, viscosity, and low hygroscopicity, when compared to short-chain alcohols. It is miscible with pure diesel in large proportions and burns directly on diesel engines. Therefore, n-pentanol is considered to be a promising diesel fuel additive. However, the differences between the chemical and physical properties of n-pentanol and commercial diesel affect the spray, combustion, and emissions performance of commercial diesel engines. In this study, visualization and engine experimental test methods were used to study the spray, combustion, and emissions performance of diesel/n-pentanol mixtures. The results showed that the atomization characteristics of diesel/n-pentanol mixtures were better than those of diesel. As the engine load increased, the maximum in-cylinder pressure and heat release rate increased. The brake specific fuel consumption (BSFC), nitrogen oxide (NOX), hydrocarbons (HC), and carbon monoxide (CO) emissions decreased, but soot emissions increased. Compared to diesel fuel, adding n-pentanol to pure diesel resulted in a decrease in soot emissions, while the BSFC, HC, and NOX emissions increased. Upon adding 50% n-pentanol to pure diesel (P50), the soot emissions decreased by up to 77.15% and the brake thermal efficiency (BTE) decreased by 1.86%. In summary, P50 could be burned directly on the diesel engine without any modifications, which can significantly reduce soot emissions; in this case, the BTE experienced only a slight decrease.

Strong knocking combustion has become the greatest challenge for advanced internal combustion engines to pursue thermal efficiency limits at high power density conditions. Arising from enclosed space and extreme combustion situations, the fundamental mechanism for strong knocking combustion has still not been fully understood. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed direct photography, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical rapid compression machine (RCM) with flat piston design. Strong knocking phenomena were reproduced through varying initial thermodynamic conditions, and localized autoignition (AI) initiation and reaction wave evolutions were visualized, companied by synchronous pressure and temperature trajectories. The results show that compared with initial temperature, initial pressure and equivalence ratio exhibit greater influence on the variations of knocking severity. The weighting of different contributors can be further quantified by an effective energy density that shows positive but nonlinear correlations with knocking severity. However, the distinctions between CI and SI knocking characteristics at identical effective energy density also reflect the essential role of the interplay between primary flame propagation and end-gas AI progress. Visualized combustion images show that through improving end-gas thermodynamic state and reactivity sensitivity, the primary flame propagation can enhance localized AI initiation and secondary intensive AI evolutions, facilitating combustion mode transitions into developing detonation. The significant influence of primary flame propagation is diminished until ignition delay time becomes sufficiently short. Finally, with estimated thermal heterogeneities in flat-piston RCM configurations, the ignition modes of strong knocking cycles are quantified by a non-dimensional ignition regime diagram, and favorable scaling agreements with strong and mixed ignition regimes are observed.

Particulate matter (PM) emitted from conventional diesel engines has significant deteriorating effects on natural environment and human health. Among alternative oxygenated fuels, polyoxymethylene dimethyl ether (PODE) is a novel renewable fuel with high cetane number, oxygen content and volatility, and has prominent potential for reducing the PM emissions. In this study, the influences of fuel injection parameters, including injection pressure and injection timing on the reduction of PM emissions were explored for a four-cylinder diesel engine fueled with PODE/diesel blends at various engine loads. The three blends were the pure diesel (denoted as P0), a mixture of 80% diesel and 20% PODE (denoted as PD20), and the mixture of 70% diesel and 30% PODE (denoted as PD30). The results showed that adding PODE to diesel resulted in a reduction in both the total particle number concentration (PNC) and the particle mass concentration (PMC) at all loads. For the PODE/diesel blends, at low load and higher injection pressures, the PMC further decreased, although the value of PNC drastically increased. For the PODE/diesel blends, at medium load and higher injection pressures, both the PNC and PMC values decreased. When the injection pressure was higher than 120 MPa, it could not continuously reduce PNC. Furthermore, for the PODE/diesel blends, at high load and high injection pressures, both the PNC and PMC values greatly decreased. The PODE/diesel blends coupled with the retarded injection timing could reduce the PNC and PMC values, but when the injection timing occurred too close to the top dead center (TDC), the PNC value increased. In addition, as the injection timing was closer to TDC, the effect of the addition of PODE in diesel fuel on reducing PNC and PMC became more significant.

A diesel/natural gas (NG) dual-fuel engine is regarded as an appealing option to reduce emissions while maintaining high thermal efficiency. In this study, a reduced n-heptane–n-butylbenzene–NG–polycyclic aromatic hydrocarbon (PAH) mechanism with 746 reactions and 143 species was developed for predicting the combustion characteristics and emission in dual-fuel engines. A mixture of methane, ethane, and propane was used to model the NG, and a mixture of n-heptane and n-butylbenzene was used to model the diesel. This mechanism was based on a reduced n-heptane–PAH mechanism, and the detailed mechanisms of n-butylbenzene and NG were reduced using the methods of directed relation graph with error propagation (DRGEP), rate of production (ROP), and sensitivity analysis. The key kinetic parameters of the model were optimized and adjusted according to the results of sensitivity analysis. The final optimized dual-fuel mechanism was verified against ignition delay, laminar flame speed, and homogenous charge compression ignition (HCCI) engine combustion, and a good prediction was obtained. Finally, the present mechanism was coupled into the CFD software to simulate the combustion characteristics and emission of a dual-fuel engine under four different NG substitution rates. The simulation results are consistent with the experimental data of emissions and combustion characteristics, indicating that the current mechanism can be applied to simulate practical diesel/NG dual-fuel engines.

Alcohol fuels can effectively reduce particle emissions of diesel engines. However, the effects of lower and higher alcohols on the microscopic physicochemical properties of the particles, such as the graphitization degree, surface aliphatic C–H functional groups, O/C ratio, and the carbon atom hybridization is still unclear. Moreover, the relationship between microscopic physicochemical properties and particle oxidation activity has not been systematically studied. Therefore, the alcohol-diesel mixtures with different carbon chain lengths and the same oxygen content were used as the fuel for commercial diesel engines in this research, such as methanol/diesel mixture (M10), n-butanol/diesel mixture (NB25), and n-octanol/diesel mixture (NO45), and the pure diesel (D100) was used as a reference. The physicochemical properties of different fuel particles under two loads were analyzed using Raman spectroscopy, Fourier Transform Infrared, and X-ray photoelectron spectroscopy. The results showed that as the load increased, the AD1/AG of the particles decreased, which meant that the graphitization degree increased. Moreover, the addition of alcohol fuel will increase the graphitization degree of the particles. In addition, the addition of alcohol fuel will reduce the aliphatic C–H functional groups and the O/C ratio on the surface of the particles. However, with the use of M10 to NO45, the aliphatic C–H functional groups continued to increase, while the O/C ratio first decreased and then increased, and the NB25 particle had the smallest O/C ratio. Through the Partial Least Square analysis, it was found that the fringe length and fringe separation distance had the most significant impact on the activation energy of the particles. Interestingly, among the characteristic parameters of Raman spectroscopy, the D1-full width at half maximum had the largest contribution to the activation energy.

Physicochemical properties of soot, such as graphitization degree and surface functional groups of dimethoxymethane (DMM)–diesel blends, have been conducted with Raman spectrum, FT-IR (Fourier transform infrared spectroscopy), and XPS (X-ray photoelectron spectroscopy), respectively. The experimental samples are collected in a four-cylinder, turbocharged engine at two different speeds (2200 and 1400 rpm), different loads (1.2 and 0.6 MPa), and different fuels (D100, DMM6.4, and DMM13). The results show that with the increase in loads, the difference of graphitization degree is gradually disappeared with different fuels, and a higher graphitization degree can be obtained. Moreover, the use of DMM reduces the proportion of unsaturated fuels, thereby leading to lower amounts of C H groups. In terms of oxygenated surface functional groups’ content of particulate matter (PM), the C O functional group accounts for 4%-12%, the COO functional group accounts for 1%-4%, and the C O functional group accounts for 3%-8%. Sensitivity analysis of the microscopic properties of soot found that the O/C ratio and sp3/sp2 ratio have the lowest effect on the apparent activation energy (Ea) of soot. And AD1/AG, AD2/AG, and AD4/AG significantly affect Ea.

Dimethyl carbonate (DMC) is a promising alternative fuel for diesel engines. Comparing with the traditional diesel fuel, the DMC application can lead to a different combustion process where the soot is synthesized, yielding variation in chemistry of engine-out soot particles. Soot chemistry is highly related to the concentration and accessibility of radical sites for the reactions regarding to soot oxidation on the exhaust systems or during DPF regeneration. Therefore, the study on the chemical feature of soot emissions is of great necessity. Nevertheless, comprehensive determination on the soot chemical property for DMC-contained fuel and its instinctive explanation is not well understood in the previous studies. In this study, to reveal the effect of DMC addition on the chemical feature of soot emissions, the surface functional groups and their bonding states of soot particles from various DMC-diesel blends were characterized using FT-IR and XPS technologies. Results showed that the DMC addition exerts different influences on the surface oxygen content, oxygenated groups and their bonding states with various DMC content. Therein, DMC addition in large amount can significantly increase the C sp3/sp2 ratio and the concentration of C-O epoxies on soot surfaces. At relatively low engine speed and load conditions, the low-content addition of DMC leads to slight increase in the aromatic C–H/aromatic C C ratio in soot bulk structure. However, the large-content DMC addition distinctly reduces the aromatic C–H/aromatic C C and aliphatic C–H/aromatic C C ratios. The results are useful for the prediction of the subsequent soot oxidation behavior during DPF regeneration on diesel engines.

The n-pentanol/diesel dual-fuel coupled with EGR (exhaust gas recirculation) technology could simultaneously reduce soot and nitrogen oxide (NOX) emissions discharged from the compression ignition engine. However, under high EGR rates, the low cetane number of the n-pentanol/diesel dual fuel resulted to combustion deceleration behavior. Because the combustion rate has a significant influence on the thermal efficiency of the engine, it was necessary to add 2-ethylhexyl nitrate (EHN) as a cetane improver to fuel mixtures to ensure that the n-pentanol/diesel fuel has appropriate combustion characteristics. In this study, it was attempted to investigate the performance and emission of a four-cylinder, turbocharged diesel engine with EHN fueled with n-pentanol/diesel blends at varying EGR rates. The five tested fuels included pure diesel (P0), and a mixture of 50% n-pentanol and 50% diesel (P50). Moreover, EHN was added to P50 at ratios of 0.5, 1, and 2%. The results showed that P50 could reduce soot emission, nucleation and accumulation mode particles. However, the burning speed and brake thermal efficiency (BTE) were notably reduced, and emissions of hydrocarbon (HC) and carbon monoxide (CO) significantly increased. With the EGR technology, NOX emission was significantly reduced. When EHN was added to P50, engine ignition delay was shortened, BTE increased, and HC and CO emissions were substantially reduced. The use EGR technology combined with n-pentanol and EHN simultaneously reduced soot and NOX emissions, and only slightly reduced BTE.

In recent years, n-pentanol has attracted much attention because it can reduce the dependence on fossil fuels and the pollution emissions of engines. As important components of diesel, aromatic hydrocarbons play a significant role in soot generation. In this work, a new reduced chemical kinetic mechanism of n-heptane–n-butylbenzene–n-pentanol–polycyclic aromatic hydrocarbon (PAH) was established, containing 178 species and 746 reactions for diesel engine applications. On the basis of the reduced mechanism of n-heptane–PAH, the proposed mechanism was developed by adding the reduced mechanisms of n-butylbenzene and n-pentanol. Direct relation graph with error propagation (DRGEP), sensitivity analysis, and rate of production (ROP) methods were employed to reduce the detailed mechanisms of n-butylbenzene and n-pentanol, respectively; then, the sensitivity analysis method was employed to optimize the simplified model. In order to verify the reliability of the current mechanism, it was used to extensively verif...

Effects of increasing compression ratio (CR) and intake boosting when operating engine with cooled EGR are characterized and compared in a single cylinder, port-fuel injection SI engine. CR and intake pressure is independently increased form 8:1 to 10:1, and 1.0 to 1.4 bar when operating EGR from 0% to 20%. Experiments are performed at constant speed 1200 r/min, full load and stoichiometric air fuel ratio. As increase of EGR, the MBT (minimum ignition advance for best torque) spark timing has to be advanced to compensate retarded combustion phasing caused by the cooling effect of EGR. Increasing intake pressure to 1.4 bar gains more restore in maximum cylinder pressure and peak heat release rate compared with those by increasing CR to 10:1. At full load, increase of EGR produces a significant drop in IMEP. Increasing intake pressure and CR can both effectively restore egnine IMEP while achieving a reduction in ISFC (indicated specific fuel consumption) and COVIMEP (coefficient of cyclic variation of IMEP). Increase of CR and intake pressure also alleviates the deterioration in HC emissions, where the decrease of HC is up to 34% at 20% EGR. However, slightly higher NOX is produced at higher CR and intake pressure.

Huge thermal losses through the exhaust system of the refrigerated truck offer an opportunity for lowering fuel consumption by using waste heat recovery system. To meet the demand of mixed load of refrigerated truck, a basic cycle integrating the supercritical CO2 cycle and dual-temperature refrigeration cycle (BSRC) is proposed and evaluated from the thermodynamic and economic perspective. Besides, a two-phase ejector is introduced into the BSRC to improve cycle performance (defined as SRCE). The proposed cycles can conveniently change operation modes in response to different forms of energy demand, including combined power generation, refrigeration and freezing (mode 1), refrigeration and freezing (mode 2), power generation (mode 3). Detailed parametric analysis and performance comparison between the BSRC and the SRCE are conducted. Thereafter, multi-objective optimization is employed to obtain the optimal operating condition. Comparison results show that the SRCE presents superiority than BSRC owing to pressure lift provided by the ejector. Besides, the advantage of employing ejector is more pronounced under mode 2 than under mode 1. The SRCE can provide an extra power of 28.19 kW under mode 1 when the basic need for freezing and refrigeration (1.635 kW) is satisfied, accounting for 11.23% of the engine power output. And the exergy efficiency and sum unit cost of product of the SRCE are 57.186% and 13.78 $/GJ, respectively. Compared with the BSRC, the exergy efficiency and sum unit cost of product of the SRCE under mode 2 are respectively increased by 15.92% and decreased by 14.85%. While under mode 3, the SRCE can be regarded as a typical regenerative SCO2 cycle, which can provide considerable net power output of 28.362 kW.

Among various supercritical carbon dioxide cycles, the supercritical recompression carbon dioxide cycle can well adapt to the high temperature of the exhaust gas of the solid oxide fuel cell-gas turbine system to augment power generation. Nevertheless, even after the recovery by the supercritical recompression carbon dioxide cycle, the exhaust gas still contains a large amount of unutilized waste energy. Few studies introduce low-temperature cycles to build cascade cycle systems, which are very likely to address this issue effectively. From the perspectives of energy, exergy, environmental and economic indexes, this article analyzes and compares the improvement potential of integrating four common low-temperature cycles, including organic Rankine cycle, transcritical carbon dioxide cycle, Kalina cycle, and organic flash cycle. Different key operating parameters are considered in-depth and optimized by a genetic algorithm. The results illustrate that in terms of efficiency, the introduction of the organic Rankine cycle is the most outstanding since it can reach the highest energy efficiency of 72.74–73.55% (exergy efficiency of 70.22–71.01%) across wide operation conditions. In terms of cost, the coupling of Kalina cycle is suggested due to the lowest capital cost of 19.94 $/h. The environmental penalty of the four systems all accounts for 14.73% of the total cost. As a consequence, the pros and cons of four common low-temperature cycles are fully demonstrated, which can provide references for the power plant planning. The work is supported by Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (No. 2020K009).

The chemistry of soot particles has high affinity with the concentration and accessibility of potential radical sites for the reactions regarding to soot growth and oxidation behaviors during in-cylinder combustion, and thus affects the particle emissions of the engines. The addition of methanol in diesel fuel is widely believed to transform the soot formation and oxidation behaviors. However, the instinctive explanation on these transformation by soot chemistry is not well understood. In this study, the chemical feature of soot emissions from a methanol-diesel blend fueled diesel engine was characterized, using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR), respectively. Results showed that epoxy, ether and C vacancy as well as ester groups were the dominant surface species, while the carboxyl and carbonyl groups exerted less contribution amongst the carbon–oxygen groups on soot surfaces for both of pure diesel fuel and methanol-diesel blends. At low engine load conditions, the increase in methanol blending content led to an increase–decrease-increase behavior of C-O epoxy concentration, which was opposite with the C vacancy contribution. At the higher engine load, increasing methanol content in fuel blends led to higher increment in the hydroxyl species with respect to the epoxy groups on soot surfaces, especially for the 5% and 10% methanol addition. The predominant groups within the bulk of soot particles consisted of aliphatic C–H (methyl and methylene), carbonyl-like C O, aromatic C–H groups and C–O bonds in phenols, anhydrides, esters and ether-like groups for both diesel and methanol blending fuels. Relative to the aromatic C C, the concentration of aliphatic C–H reduced with the increasing methanol content in the fuel blends. The lower-volume-proportion addition of methanol at low engine load led to an increase in the aromatic C–H with respect to aromatic C C. While different, the addition of methanol exerted no significant influence on the aromatic C–H concentration within the soot particles at high engine load condition.

Polyoxymethylene dimethyl ethers (PODEn) are promising alternative biofuels with high cetane number and oxygen content. In this study, effects of PODE3-4 addition to diesel on the macroscopic spray characteristics were studied in a constant volume spray chamber under different injection pressures. The engine performance and emission characteristics of n-butanol/PODE3-4/diesel blends were investigated in a diesel engine under high injection pressures. The results indicate that the spray penetration of BD20 or PD20 was greater than that of D100; the addition of PODE3-4 to BD20 led to a further increase of the spray penetration without significant effects on the spray cone angle. Under an equal fuel injection pressure, the combustion of BD20 showed the greatest Maximum Pressure Rise rate (MPRR), which decreased after the addition of PODE3-4 to the blend. The soot and NOx emissions of BD20 were lower than that of pure diesel. Compared with BD20, the reduction of CO and HC emissions from the combustion of BDP20 were as high as 45.65% and 28.24%, respectively. The soot, NOx, CO and HC emissions of BDP20 were all lower than that of PD20. As the fuel injection pressure increased, a decreasing trend was observed in the number concentration as well as in the mass concentration of total particles. The lowest particle emissions in terms of total particle mass concentration were found for BDP20, followed by PD20, BD20 and D100. The ratio of sub-50 nm particles to total particles was highest for BD20 regardless of the injection pressure, and its value decreased upon adding PODE3-4 to BD20.

End gas auto-ignition and transition of flame front are considered as the main causes of severe pressure oscillation in spark-ignition engines, which is one of the major features of knock and super-knock. The knowledge of characteristics of auto-ignition, flame front development, propagation of pressure wave and relations between them, still needs to be maintained. In this study, flame front transition induced by pressure wave and auto-ignition are investigated using one-dimensional simulation with detailed chemistry in an enclosed space Calculation cases with different initial thermodynamic conditions are investigated. Mass fraction of OH is employed as indicator of auto-ignition progress under variable conditions caused by pressure wave. Different propagation modes of flame front, including subsonic deflagration, detonation and supersonic deflagration, are developed under the effects of both pressure wave and auto-ignition. Results show that mass fraction of OH could successfully reflect auto-ignition progress, thus indicating occurrence and sequence of auto-ignition at different locations. Transitions from deflagration to detonation and detonation to supersonic deflagration are found to be triggered by sequential auto-ignition with different gradient of auto-ignition progress ahead of flame front induced by pressure wave.

Since the technology of n-butanol/diesel blending fuel coupled with post-injection (PI) strategy increases the complexity of engine combustion process, the disputes that the interpretation of the formation mechanism of pollutants are still existed. Moreover, studies on the effects of this coupling-strategy on fine-particles (DP

Dimethoxymethane (DMM) is considered a potential oxygenated fuel due to its high oxygen content (42.1%) and considerable heat value. However, little research emphasizes the impact of oxygenated fuels on small-particles (DP

The flow field structure of a proton exchange membrane fuel cell (PEMFC) is a key factor that affects its performance. In this study, based on the structural characteristics of the superior mesenteric artery and branches of the human body, a new type of bionic flow field was designed for a PEMFC. The effects of the conventional serpentine flow field (CSFF) and new bionic flow field (NBFF) on fuel cell performance were studied via simulation and experiment. Compared with the CSFF, the NBFF significantly improves droplet dynamic transmission characteristics, reduces droplet aggregation at bends, shortens the retention time of droplets in the flow field, and increases the droplet removal rate by 36.3%. The output performance of the NBFF-based fuel cell is the best under an air flow rate of 4 L/min and a hydrogen flow rate of 2.5 L/min. The NBFF exhibits better heat dissipation than the CSFF does. When the current density is high and the operating temperature exceeds 50 °C, the performance of the NBFF fuel cell is improved by 30.03% compared with that of the CSFF-best fuel cell. Moreover, the actual working temperature of PEMFC is 60–80 °C. Therefore, the NBFF can enhance the practicability of fuel cells.

Recently, the way that taking municipal solid waste (MSW) as fuel combustion for power generation is proposed to deal with increasing MSW, based on the method of waste-to-energy (WTE) technology. However, the energy efficiency of WTE plant is only 20% approximately, due to the energy loss of boiler and exhaust gas. A novel waste heat recovery (WHR) system has been developed to improve the thermodynamic and economic performance of WTE plant in this study. Rankine cycle (RC) is utilized to recover the waste heat (WH) of high-temperature boiler slag to generate power. Organic Rankine cycle (ORC) and absorption refrigeration cycle (ARC) are used for cascade recovery of WH from boiler exhaust gas to provide electricity and space cooling, respectively. Aiming to promote the overall performance of the WTE plant, comprehensive thermodynamic, economic, and environmental analysis are performed. Different environmental-friendly organic working fluids of the ORC are studied and compared based on the air pollutant emissions standards. Subsequently, several crucial parameters of proposed system are studied. Eventually, a comparison of the thermodynamic, economic, and environmental performance between original and new WTE plant is carried out. The results indicate that the use of butane can obtain the highest electric energy for ORC, compared with other working fluids. As well, the energy and exergy efficiency of WTE plant increase by 37.66% and 35.65%, respectively, with the choice of the WHR system. Furthermore, the dynamic payback period of new WTE plant is 4.63 year, with a decrease of 4.79 year, and the net present value (NPV) increases from 5.21 M$ to 18.12 M$. From the perspective of environmental analysis, the sustainability of the new WTE plant increases slightly, but the ecological efficiency has an increment of 11.28%.

An engine waste heat driven combined power and refrigeration system, comprised of a regenerative supercritical CO2 Brayton cycle (RSCBC) and an ejector expansion refrigeration cycle (EERC), is proposed. In this system, the RSCBC is adopted as the topping cycle to generate power by recovering the high-temperature waste heat of engine. Meanwhile, the power is utilized by the compressor in the EERC. Such a waste heat recovery system can not only decrease the specific fuel consumption, but also provide refrigeration for refrigerated trucks to realize food preservation. Energy and exergy analysis are conducted on the RSCBC/EERC. The performance of four zeotropic mixtures used in EERC and different mixture compositions are compared. Moreover, the effects of several significant operating parameters are discussed in detail, including turbine inlet pressure and temperature, compressor inlet pressure and temperature, pressure drop in the ejector, evaporating temperature, and condensing temperature. To investigate the influence of the installation of the RSCBC/EERC system, weight estimation analysis is conducted. The results show that the refrigerating capacity and COPcomb of the system with R32/CO2 (0.9/0.1) are up to 225.5 kW and 2.05, respectively. And the equivalent power loss due to the additional weight is estimated to be 5.21 kW. In general, the RSCBC/EERC has proven its application potential in recovering waste heat to provide refrigeration through thermodynamic analysis.

This study presents the physical properties and oxidation reactivity of exhaust soot from a modern direct injection diesel engine fueled with dimethyl carbonate (DMC)-diesel blends and discusses the relationship between physical properties and oxidation reactivity of soot. The soot morphology, size, fractal dimension and nanostructure were analyzed by transmission electron microscopy and Raman scattering spectrometry. The thermogravimetric analysis was carried out to characterize soot oxidation reactivity in the form of apparent activation energy. The results showed that soot aggregates from DMC/diesel blended fuels exhibited fewer and smaller primary particles as well as small-sized clusters than the diesel soot aggregates. Soot primary particles presented shorter fringe length, wider separation distance and greater tortuosity with increasing DMC blending amount, while the degree of graphitization for soot presented an increasing trend. It was also found that soot from DMC/diesel blends, or low loading operation had higher oxidation reactivity than those from pure diesel or heavy loading operation. Among the physical properties, fractal dimension and primary particle size appeared to be insignificant than the nanostructure for governing the soot oxidation reactivity. Further analysis showed the separation distance was more correlated with soot oxidation characteristics than fringe length and tortuosity. These results illustrated that the use of DMC blended diesel fuel affects the physical properties of soot, enhances soot particle oxidation reactivity, which will be significant for improving the regeneration efficiency of after-treatment device.

Waste-to-energy (WTE) technology is regarded as the most promising way to deal with municipal solid waste because it has the advantages of saving land area and reducing the emission of pollutants. However, the electrical efficiency of WTE combined heat and power plant is low. One factor that leads to such result is the large heat loss of the boiler exhaust gas. Besides, the amount of high-temperature steam used for power generation decreases because a part of high-temperature steam is used to provide district heating (DH), resulting in a low electricity output. In this study, a novel combined organic Rankine cycle (ORC) and heat pump cycle (ORC-HP) system is analyzed. The waste heat of the exhaust gas is recovered by the ORC and generates mechanical work to drive the HP system, which absorbs the waste heat of low-temperature and low-pressure vapor for the DH. Considering the environmental compatibility, different organic working fluids are compared to select a suitable working fluid for the combined system. Comprehensive thermodynamic and exergoeconomic analyses are performed to identify the effects of different parameters on combined system. And the economic benefits of the system are considered from the perspective of the investment payback period (PBP). Furthermore, based on sensitivity analysis, multi-objective optimization analysis was applied in the combined system to determine the optimal working conditions. The results indicate that butane and ammonia are the most suitable working fluids. Sensitivity analysis results show that for SIC, the evaporator pressure of ORC and the superheat degree of HP evaporator have a greater impact, and for ORC-HP system efficiency, the evaporator temperature of HP, the superheat of HP evaporator and ambient temperature have a greater impact. Optimization results show that, the optimal PBP and the specific investment cost (SIC) of the new combined system are 0.48 years and 325.94 $/GJ. After optimization, SIC reduces from 333.15 $/GJ to 325.94 $/GJ, with a 2.2% reduction and the product unit cost of DH reduces to 33.97 $/GJ.

In high altitude regions (>4000 m), atmosphere pressure and thus in-cylinder ambient density is decreased significantly, leading to longer spray penetration. Affected by liquid spray impingement on cylinder wall, heavy-duty diesel engines operating at high loads may encounter abnormal combustion like diesel knock. However, with limited research data, the detailed combustion mechanism is not fully understood. To explore the origins of diesel knock and its correlations with spray impingement, optical experiments in a rapid compression machine were employed, and diesel spray impingement and subsequent combustion processes were investigated. High-speed direct photography and simultaneous pressure acquisition were synchronically performed to understand combustion evolutions and pressure characteristics. Meanwhile, the role of injection parameters, ambient conditions, and cylinder sizes were considered. The results show that early injection timing and spray impingement can lead to long ignition delay time. Depending on the premixed mixture formation within longer ignition delay, diesel knock manifesting supersonic detonation-like reaction front propagation can be observed, which results in strong pressure waves with an amplitude approaching hundreds of atmosphere. Further analysis on the critical conditions show that long ignition delay time does not necessarily result in diesel knock. It lies in whether there is sufficient premixed mixture formation before main combustion. The prevalence of diesel knock seems sensitive to large fuel injection pressure and small combustion chamber. Besides, depending on the ambient conditions, there are four kinds of combustion modes under spray impingement conditions.

Supercritical carbon dioxide cascade waste heat recovery has proven to be a promising alternative for energy conversion applications. In this paper, four different CO2 Brayton-based dual-loop cycles are integrated to the dual-fuel engine respectively to recover the engine waste heat, including regenerative supercritical CO2 Brayton cycle/organic Rankine cycle (RSCBC/ORC), regenerative supercritical CO2 Brayton cycle/supercritical CO2 Brayton cycle (RSCBC/SCBC), supercritical CO2 recompression Brayton cycle/organic Rankine cycle (SCRBC/ORC), and supercritical CO2 recompression Brayton cycle/supercritical CO2 Brayton cycle (SCRBC/SCBC). Comprehensive parametric analysis and comparison were carried out. When the engine was operated at 1100 rpm and cycle fuel injection quantity of 14.1 mg, the SCRBC/ORC combined system presents the best performance when cyclohexane is used as the working fluid for the ORC, followed by the RSCBC/ORC, RSCBC/SCBC, and SCRBC/SCBC. The energy efficiency of the whole system is increased by 7.03% with the SCRBC/ORC, compared with the condition that engine without waste heat recovery.

In high altitude regions, affected by the low-pressure and low-temperature atmosphere, diesel knock is likely to be encountered in heavy-duty engines operating at low-speed and high-load conditions. Pressure oscillations during diesel knock are commonly captured by pressure transducers, while there is a lack of direct evidence and visualization images, such that its fundamental formation mechanism is still unclear. In this study, optical experiments on diesel knock with destructive pressure oscillations were investigated in an optical rapid compression machine. High-speed direct photography and simultaneous pressure acquisition were synchronically performed, and different injection pressures and ambient pressures were considered. The results show that for the given ambient temperature and pressure, diesel knock becomes prevalent at higher injection pressures where fuel spray impingement becomes enhanced. Higher ambient pressure can reduce the tendency to diesel knock under critical conditions. For the given injection pressure satisfying knocking combustion, knock intensity is decreased as ambient pressure is increased. Further analysis of visualization images shows diesel knock is closely associated with the prolonged ignition delay time due to diesel spray impingement. High-frequency pressure oscillation is caused by the propagation of supersonic reaction-front originating from the second-stage autoignition of mixture. In addition, the oscillation frequencies are obtained through the fast Fourier transform (FFT) analysis.

Highlights • Morphology, nanostructure, and oxidation reaction of soot particles with different fuels are analyzed. • DMM13 has less structural compactness of aggregates compared with diesel. • Soot of DMM13 has more regular and higher degree of graphitization than that of D100. • The nanostructure influences the oxidation reaction of graphene segments with a line relation., Dimethoxymethane (DMM)–diesel blended fuels can simultaneously reduce exhaust emissions of soot and nitrogen oxide (NOX); several studies have been conducted in this regard. However, the influence of additive DMM on the production of inception and precursors of particulates, especially the relation between oxidation, morphology, and the nanostructure of soot particles has not been extensively investigated. In this study, a transmission electron microscope (TEM) and a thermogravimetric analyzer are introduced to acquire TEM images and conduct temperature-programmed-oxidation experiments. Aiming to study the influence of DMM addition on soot oxidation, morphology, and nanostructure, tests are conducted at different rotational speeds (1400 rpm and 2200 rpm), two engine loads (0.6 MPa and 1.2 MPa), and three fuels (D100, DMM6.4, and DMM13). The results show that the diameter distributions of all samples display a similar distribution, with the range of sample diameters being from 10 to 45 nm, and the addition of DMM reduces the dp (primary particle diameters) and the Df (fractal dimension), indicating a decreased structural compactness of aggregates, compared with diesel. Moreover, with increasing load and speed, La (the length of the fringe) increases and d (the distance between adjacent layer planes) decreases. Furthermore, with the addition of DMM, a more regular and higher degree of graphitization within soot particles can be observed in comparison to D100. The nanostructure influences the oxidation reaction of graphene segments with a line relation, leading to a difference in soot oxidation property.

This paper provides a literature review of the cascade refrigeration system (CRS). It is an important system that can achieve an evaporating temperature as low as −170 °C and broadens the refrigeration temperature range of conventional systems. In this paper, several research options such as various designs of CRS, studies on refrigerants, and optimization works on the systems are discussed. Moreover, the influence of parameters on system performance, the economic analysis, and applications are defined, followed by conclusions and suggestions for future studies.

Biodiesel has a high viscosity, which limits its application to engines. However, n-pentanol has low viscosity and high volatility, which is considered a promising additive. This study examined the influence of n-pentanol/biodiesel/diesel blends on the performance and particulate emissions of a diesel engine at different loads, under the condition of coupling exhaust gas recirculation (EGR) (0–30%). The three test fuels included pure diesel (D100); 80% diesel and 20% biodiesel blended fuel (BD20); 64% diesel, 16% biodiesel, and 20% n-pentanol blended fuel (BDP20). The results indicated that the influence of EGR on the NOx emissions between fuels was greater than that of the physicochemical properties of the fuels. For the same EGR rate, the heat release rate (HRR), peak in-cylinder pressure (IP), and brake thermal efficiency (BTE) increased with the engine load increased. After adding n-pentanol to BD20, the HRR increased. The peak IP and BTE values of BDP20 were similar to those of D100 and BD20. With the addition of n-pentanol, the total particulate number concentration (TPNC), soot, and total particulate mass concentration (TPMC) emissions decreased. Furthermore, the trade-off relationship between TPMC and NOx was improved, and the geometric mean diameter (GMD) of the particles decreased.

Using a transmission electron microscopy (TEM), the morphological characteristic variations of exhaust soot particles have been studied on a modern diesel engine fuelled with a commercial diesel fuel (D100, as a baseline fuel) and three different oxygenated fuel blends with the same oxygen content, including a 11.5% methanol blend (M11.5), a 8.3% dimethyl carbonate blend (DMC8.3), and a 13% dimethyl methane blend (DMM13). The results showed that oxygenated fuel blends emitted smaller soot primary particles with respect to neat diesel, with the most significant decrease in M11.5 followed by DMM13 and DMC8.3. Similar variations with primary particle diameter were found in the decrease of the mean radius of gyration, mean primary particle number and estimated mass for an agglomerate when making comparisons among diesel and the three oxygenated fuel blends. Aggregates shape analysis showed that M11.5 soot had the minimal trend to form a rather circular structure and maximal boundary irregularity among the four fuels, while all soot particles showed larger deviation to the sphere (i.e. higher elongation) regardless of fuel type used. In addition, both mass and perimeter fractal dimension showed consistent results for diesel and the three oxygenated fuel blends, with the particle aggregates being the least compacted morphology for M11.5, followed by DMM13, DMC8.3 and diesel. Therefore, compared to diesel, the exhaust soot particles from the oxygenated fuel blends (especially for M11.5) are more prone to be trapped (lowest fractal dimension) and display a higher reactivity towards oxidation (smallest primary particle size).

This paper compared the soot reactivity and its primarily related factors including soot structure, graphitization degree and surface functional groups, when using mineral diesel fuel (DF) and methanol blended fuel (MBF) at two typical low and high engine loads. Results showed that the soot reactivity increases with increasing methanol concentration in the MBF. Additionally, with the constant methanol blend ratio of methanol, the reactivity of MBF soot degrades as the engine load increases. Correspondingly, the MBF soots had lower aggregate compactness, smaller primary particle size, more disordered structure and more active aliphatic C-H groups in comparison with the DF soot, providing higher concentration and accessibility of active sites for oxidation reactions. The addition of methanol creates variations in combustion kinetics and fuel formulation, which exerts reverse effects on the soot structure and chemistry. Therefore, the factors related to these properties show non-monotonic variations when alter the methanol blend ratio and engine operating mode. Especially, when the methanol blend ratio increases from 10% to 15%, the soot structure and chemistry transform onto the features that are reluctant for soot oxidation.

A gasoline compression ignition (GCI) engine would cause difficulties of ignition stability or even misfired phenomenon at low loads. Internal exhaust gas recirculation (IEGR) technology is used to broaden the operating range and provide a GCI engine with combustion stability. In this paper, the effects of the IEGR rate (10–90%) and excess air ratio (EAR) (1.0–4.0) on the combustion performance and emissions from a GCI were investigated on a fully variable valve engine, with a particular focus on fuel economy and emissions from a GCI within less than or equal to 5% of the coefficient of variation of the indicated mean effective pressure (COVIMEP). And based on the condition of COVIMEP = 5%, the difference values between maximum and minimum cycle pressure and knock are analyzed in detail. The study finds that excessive EGR hinders combustion but improves the ignition stability of a GCI. With the EGR rate from 10% to 60%, and the EAR is 1.5 to 3.0, the combustion efficiency that greater than 90% can be obtained. Moreover, the different values of the peak of maximum and minimum pressure from the 200 continuous cycles decrease as the EGR increases. In addition, the frequency of knock occurred is further greater than that of misfire even though the low loads with the COVIMEP ranges around 5%.

To meet the increasingly stringent emission regulations on CI engines, a new theory that combines post-injection technology and uses dimethyl carbonate (DMC) and a certain amount of EGR to achieve ultra-low emissions of soot and nitrogen oxides (NOX) is proposed. However, DMC causes a delay in combustion phase of the mixed fuel due to its smaller cetane number (CN) value. Because the combustion phase affects thermal efficiency, economic efficiency, and emission performance of compression ignition (CI) engines, 2-ethylhexyl nitrate (EHN) can be used as a CN enhancer, to improve the combustion performance parameters of the mixed fuel. This study verifies this theory by controlling the post-injection rate (0, 5%, 10%, 15%, and 20%) of different experimental fuels for a diesel engine with four-cylinder and turbocharged. The five fuels were pure diesel and a mixture of 80% diesel with 20% DMC (DMC20). As well, EHN was added to DMC20 with a proportion of 0.5%, 1%, and 2%. Through this study, the use of 0.5% EHN allows maximum in-cylinder pressure (MINP) and maximum heat rise rate (MHRR) of the DMC/diesel blend to achieve results similar to those of diesel. And the use of EHN reduced the ignition delay time, but leading to a slightly increase of soot. As the post-injection rate increased, both NOX and carbon monoxide (CO) decreased and soot significantly decreased. The addition of 0.5% EHN with the post-injection rate of >15% has a good combustion and emission effect and extends the trade-off limits of NOX and soot.

Interactions between fuel and engine are basic issues for gasoline compression ignition (GCI) engine, and recent engine experiment results show that the gasoline fuel with low-octane number RON = 60–80 is optimal for GCI engines. However, gasoline fuel with low-octane ratings exhibits significant low-temperature combustion behavior, which will significantly affect the entire combustion process. In this study, different toluene primary reference fuels (TPRF) with identical research octane number (RON) but various octane sensitivities were employed. Optimized start of injection (SOI) was obtained based on indicated mean effective pressure (IMEP), combustion stability limits and engine emissions. Under the optimized SOI, the role of fuel sensitivity in combustion processes was investigated. With addressing low-temperature heat release (LTHR) before main ignition, the effect of intake conditions and fuel sensitivity on combustion phasing was clarified. The results indicate that the LTHR only occurs when the SOI is sufficiently advanced, and an earlier injection timing can achieve stable combustion, higher IMEP and lower NOX emission. More obvious LTHR promotes the advancement of combustion phasing, and the magnitude of LTHR is negatively correlated with intake temperature but positively correlated with intake pressure. The effect of increasing LTHR magnitude on combustion phasing is similar to that of increasing intake temperature. Finally, the effect of octane sensitivity on LTHR was also studied, which shows that the impact of LTHR between different sensitivity fuels is mainly reflected in combustion phasing, and the onset of LTHR is more advanced for low sensitivity fuels.

Natural gas (NG) dual-fuel engines can attain a similar thermal efficiency as that of diesel engines while achieving lower emissions. However, the trade-off relationship between CH4 and NOX emissions limits the development of dual-fuel (DF) engines. In order to resolve this problem, the effects of injection pressure (IP) and exhaust gas recirculation (EGR) ratio on the combustion and emission of diesel/NG dual-fuel engines are investigated in this study. The results show that the diesel/NG dual fuel has a distinct three-stage heat release characteristic in the high-temperature combustion process. As the injection pressure increases, the flame propagation speed of methane and the indicated thermal efficiency (ITE) increase. However, the methane in the crevice region and cylinder wall cannot be ignited because of low temperature; these are the main sources of methane emissions. When the EGR rate increases, the indicated thermal efficiency first increases and there after decreases, and diesel is cleaved through the reaction chain, PC4H9 → C2H4 → CH4, to produce CH4; this becomes one of the reasons for CH4 emissions. When the EGR rate is small ( 30%), CO and CH4 emissions can be significantly reduced by increasing the IP. Accordingly, when the injection pressure is 160 MPa and the EGR rate is 20%, the diesel/NG DF engine can achieve higher ITE and lower emissions.