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In natural gas/diesel dual-fuel engines, natural gas (NG) still produces some emissions during combustion, especially at high NG substitution rates. In order to reduce the emissions and to clarify the emissions formation mechanism. In this work, the performance and emission characteristics of the dual-fuel engine under different diesel injection strategies, as well as the formation mechanism of emissions (NOx, CO, THC and soot) were analyzed by combining bench test and chemical mechanism. The results showed that compared with diesel single injection, the diesel pilot injection strategy (PIS) could effectively improve the engine BTE (22.6%), CO and THC emissions were reduced by 87.8% and 76.1%, respectively. By advancing the diesel pilot injection timing (PIT), the combustion efficiency and BTE increased by 5.1% and 10.9%, respectively; the soot, THC, and CO emissions reduced by 63.9%, 69.4% and 76.9%, respectively. Increasing the diesel pilot injection ratio (PIR) could improve the combustion efficiency and reduce CO and THC emissions. According to the path analysis, the CO consumption was mainly the reaction R120: HO2+CO OH + CO2, the generation of THC were mainly affected by path 1: CH3→CH3O→CH2O→HCO→CO and path 2: CH3→C2H6→C2H5→C2H4, and the soot was mainly related to C2H2, but it was mainly affected by the synergistic effect of multi-step reaction. Therefore, through the analysis of different diesel injection strategies, advanced the MIT and PIT of diesel and the medium PIR of diesel, the pressure and temperature of the engine were within the appropriate range, and the reactivity was kept in a good state, which could achieve low NOx, soot and THC emissions, high BTE and low BSEC from the engine. • Experimental study on a heavy-duty six-cylinder natural gas/diesel dual-fuel engine. • The diesel PIS can effectively improve BTE and reduce CO and THC emissions. • The BTE increased and the emissions decreased with the diesel PIT advanced. • Reducing the PIR and advancing the MIT can increase NOx emissions with a PIT in the range of −25∼-10 °CA ATDC. • Advancing the PIT can further increase the BTE and further reduce emissions with a PIT in the range of −50∼-25 °CA ATDC. [ABSTRACT FROM AUTHOR]

• A reduced 179-species and 769-reactions diesel/PODE n mechanism was developed. • A mixture of n -butylbenzene and n -heptane was modeled as the diesel. • The CFD prediction capture the diesel engine emission and combustion characteristics. • Soot emission decreased as the substitution rate of PODE n increased. Polyoxymethylene Dimethyl Ethers (PODE n) is a promising diesel alternative fuel, with a high cetane number and oxygen content, which can reduce the emissions of soot. In this study, a new simplified n -heptane– n -butylbenzene –PODE n –polycyclic aromatic hydrocarbon (PAH) mechanism for diesel engine application was developed. In this model, a mixture of n -heptane and n -butylbenzene is regarded as representative substance of diesel. First, the detailed mechanism of PODE n is simplified by using the methods of rate of production (ROP), direct relation graph with error propagation (DRGEP), and sensitivity analysis. The simplified PODE n sub-mechanism was coupled to the simplified multi-component diesel mechanism of n -heptane– n -butylbenzene–PAH to construct a simplified model of n -heptane– n -butylbenzene–PODE n –PAH containing 179 species and 769 reactions. Then the key dynamic parameters of the PODE n sub-model were optimized using the sensitivity analysis method. To confirm the rationality and reliability of the optimized diesel/PODE n reduced mechanism, the experimental values such as laminar flame velocity, component concentration and ignition delay were verified. The verification results show that there was little difference between the simulated values and the test values. Finally, the proposed mechanism was coupled to the computational fluid dynamic (CFD) software CONVERGE to predict the emission characteristics and combustion of an actual diesel engine under different mixing ratios of PODE n. The results for in-cylinder pressure and heat release rate (HRR), carbon monoxide (CO), nitrogen oxide (NO X) and soot emissions modeled by proposed mechanism are well in line with the test data, indicating that the newly developed diesel/PODE n reduced mechanism can simulate the combustion and emission of diesel engines. [ABSTRACT FROM AUTHOR]

Highlights • Experiment and simulation were used to study the combustion of natural gas–diesel engine. • Indicated thermal efficiency increased first and then reduced with advancing injection timing. • Carbon monoxide and methane reduced first and then increased with advancing injection timing. • Methane was mainly distributed at the cylinder wall and slit area at exhaust valve opening. Abstract Natural gas–diesel dual-fuel engines can significantly reduce soot emissions and mitigate the oil crisis. However, some drawbacks exist including low thermal efficiency, as well as high methane and carbon monoxide emissions. Multiple injection plays an important role in improving the performance and reducing emissions in dual-fuel engines. This paper presents an experimental and numerical study of multiple injection effects on the combustion and emission characteristics of a natural gas–diesel dual-fuel engine at low load condition. The results showed that, when the first diesel injection timing is advanced, the indicated thermal efficiency and nitrogen oxide emissions increased first and then decreased, while the maximum pressure rise rate, carbon monoxide and methane emissions reduced initially and then increased. When the first diesel injection ratio increased, combustion rate of natural gas–air mixture increased, methane and carbon monoxide emission decreased, and nitrogen oxide emission increased. When the first diesel injection timing was relatively rearward, the combustion process showed a distinct four-stage heat release characteristic. The proportion of premixed combustion of the first injected diesel and the duration of flame propagation of natural gas had considerable influence on methane emission. When the first diesel injection timing was relatively forward, the first injected diesel played a premixed role, and the start of combustion was controlled by the second injected diesel. A relatively high indicated thermal efficiency can be obtained under this injection strategy. The methane was mainly distributed in the cylinder wall and crevice area at exhaust valve opening due to the limitation of flame propagation under the multiple injection strategy. [ABSTRACT FROM AUTHOR]

• A reduced HXN-HMN-PAH mechanism for diesel from indirect coal liquefaction was proposed. • The reduce mechanism entails 178 components and 650 reactions. • The reduced mechanism is capable of reasonably predicting various experimental conditions. • The current mechanism can excellently predict the combustion and emission performances in practical engines. Diesel from indirect coal liquefaction (DICL) is a kind of extremely promising alternative fuel to alleviate the oil security problem caused by excessive dependence on imported oil and reduce the pollutant emission. However, the mechanism of diesel from indirect coal liquefaction is very few, and the numerical simulation of combustion and emission characteristics of diesel from indirect coal liquefaction is even less. Therefore, a reduced mechanism of DICL, entailing 178 components and 650 reactions, was put forward to research the combustion and emission characteristics of engines fueled with DICL under different loads in this work. n-Hexadecane (HXN) mechanism and 2,2,4,4,6,8,8-heptamethylnonane mechanism (HMN) were respectively considered as representative species of straight-chain paraffin and branched alkane in surrogate model. Firstly, direct relation graph and direct relation graph with error propagation were used in turn. Then, sensitivity analysis coupled with rate of production analysis has been used to further reduce the detail 2,2,4,4,6,8,8-heptamethylnonane mechanism. After that, the skeleton mechanism of n-hexadecane and the reduced polycyclic aromatic hydrocarbon (PAH) mechanism were combined with the simplified HMN mechanism to develop a novel multi-component mechanism of HXN-HMN-PAH. Brute-force sensitivity analyses were respectively conducted at different temperatures to find out these key reactions that have great effects on the ignition delay times (IDTs). Then, the optimizations of the reduced mechanism were made based on the ignition delay times of the experimental datum and the detail mechanism. A proportion of 71.5% HXN and 28.5% HMN by mole fraction was determined according to the properties of practical DICL. Furthermore, the optimized mechanism was employed to verify experimental values including the ignition delay times, the species concentrations on jet-stirred reactors (JSR) and the laminar flame speeds. Eventually, the mechanism was coupled into computational fluid dynamic (CFD) to perform multi-dimensional numerical simulation validation in a directed injection compression ignition (DICI) engine under different loads. [ABSTRACT FROM AUTHOR]

• A detailed urea decomposition mechanism was used in simulation. • Studied the relationship between temperature and substances in the liquid film. • Different mixers lead to different mass and location of deposit. • The injection parameters of a low turbulent system affected the deposits location. In the Selective Catalytic Reduction (SCR) system of a diesel engine, the decomposition of urea will produce the reducing agent NH 3 , and by-products which will form deposits. To reduce deposits, this work studied the relationship between liquid film temperature and reactants. Further, the mixers with different structures and the injectors with different injection parameters were designed and compared with different designs about the location and mass of deposits. The evaporation and pyrolysis process of urea aqueous solution in an SCR system were simulated by using computational fluid dynamics (CFD) coupled with a detailed urea decomposition mechanism. The results indicated that the by-products (biuret, CYA, and ammelide) would be produced at 300, 380, and 437 K, respectively. With increasing liquid film temperature, the mass fraction of NCO− and by-products also increased. The turbulence intensity varied when they were generated by the different mixer structures. In lower turbulence intensity of mixer1, with the decrease of injection angle and pressure, the mass and distribution regions of liquid film and deposits were obviously reduced. In higher turbulence intensity of mixer2, the urea droplets were entrained by strong turbulence. The decrease of the injection angle and pressure did not significantly reduce the liquid film and deposits. [ABSTRACT FROM AUTHOR]

• A reduced n -heptane– n -butylbenzene–methylcyclohexane–PAH mechanism was developed. • The reduced mechanism well predicted various experimental conditions. • The CFD simulations capture the engine combustion process and the soot formation. • The mechanism contains aromatics and cycloalkane components, enhancing soot predictions. Aromatics and cycloalkanes which play important roles in soot formation, are two important components in diesel. This work constructed a reduced multi-component mechanism of n -heptane– n -butylbenzene (BBZ)– methylcyclohexane (MCH)–polycyclic aromatic hydrocarbon (PAH) with 183 species and 777 reactions for diesel engine emissions and combustion prediction. Based on the reduced mechanism of n -heptane–BBZ–PAH, this multi-component diesel mechanism was constructed by merging the reduced mechanism of MCH. First, the detailed mechanism of MCH was reduced based on the methods of direct relation graph with error propagation (DRGEP), sensitivity analysis, and rate of production (ROP) analysis. Next, some most important parameters of kinetics in the mechanism were optimized by sensitivity analysis method. Further, the simplified multi-component diesel mechanism was extensively validated using the experimental values of ignition delays, species concentrations, and laminar flame speeds. The developed mechanism provides favorable prediction results, indicating that it can be used for simulating the combustion of multiple components in diesel. Finally, the developed multi-component diesel mechanism was coupled with three-dimensional computational fluid dynamic (3D-CFD), and multidimensional numerical simulations were performed in a direct-injection compression ignition (DICI) engine at EGR = 0%, 13%, 27%, 37%. The prediction results well coincided with the experimental values of combustion characteristics and emissions of soot and NOx, indicating that this multi-component diesel mechanism could be applied for predicting practical engine simulations. [ABSTRACT FROM AUTHOR]

Natural gas (NG) is a clean and low-carbon fuel and the NG engine is one of the main measures used by the public transportation industry to achieve carbon peak targets. However, NG engines have problems, such as ignition difficulty and low thermal efficiency. The use of diesel to ignite NG is an effective solution to these problems; however, this solution will increase CO 2 emissions compared with NG engines. In this study, the potential of reducing CO 2 emissions from a diesel-pilot-ignited (DPI) NG engine by optimizing the injection parameters (injection pressure and injection timing) under different loads is studied through experiments. Furthermore, the formation mechanism of CO 2 in combination with chemical kinetics is analyzed. The results show that combustion in the DPI mode presents an obvious two-stage heat release and its CO 2 emission is 17.15% lower than that of pure diesel combustion (PDC). Under high-load conditions, as the diesel injection pressure increase, the THC and NO X emissions emissions decrease by 67.53% and 84.32%. As the diesel injection timing (DIT) advances, the NO X emissions increase from 1.84 to 22.96 g/kW·h respectively. According to the analysis of the chemical kinetic mechanism, the formation of CO2 in DPI mode is primarily through the reaction of CO + O 2 = CO 2 + O, whereas in conventional diesel combustion (CDC) mode, CO 2 is formed through the reaction of CO + OH = CO 2 + H. Within the range of −18 to −5°CA ATDC DIT, increasing the diesel injection pressure (DIP) or advancing the DIT can improve the thermal efficiency of DPI NG engines and reduce CO 2 emissions. [Display omitted] • Advancing injection timing, the BTE increases and the CO 2 decreases in DPI mode. • The CO 2 emission in DPI mode is much lower than that in pure diesel mode. • With optimized injection parameters, the performance in DPI mode have been improved. • The reaction path of CO.→CO 2 in DPI mode is different from that in the diesel mode. [ABSTRACT FROM AUTHOR]

• A detailed mechanism of urea decomposition was used to analyze the formation process of urea deposition. • The structure of the mixer was optimized, and the pressure drop of mixer was reduced. • The uniformity indices of NH 3 concentration and velocity were improved. • Optimization scheme I produces urea solid, biuret, CYA, and ammelide that have the lowest qualities. A three-dimensional computational model of a diesel engine exhaust post-treatment system is built in the computational fluid dynamics software to simulate the deposit formation process in the mixer area of the diesel engine Urea-SCR (selective catalytic reduction) system. The SCR inlet uniformity, pressure drop in the mixer, urea solid, and three main urea by-products (biuret (biurea), CYA (cyanuric acid), and ammelide (cyanuric acid monoamide)) are investigated in the exhaust post-treatment system of the diesel engine. The results show that in the whole exhaust post-treatment system, the liquid film only appears in the mixer and nozzle areas. It is found that the production of biuret, CYA, and ammelide, which are by-products of urea decomposition, is mainly affected by the NCO− (isocyanate-group) concentration, and liquid film temperature and height. The thicker the liquid film and the smaller the rate of evaporation, the smaller the mass fraction of urea solid and the larger the mass fraction of urea decomposition by-products. According to the analysis results, the optimization scheme of the mixer is proposed, and the simulation analysis is performed. The analysis results show that the NH 3 concentration uniformity at the SCR inlet increases, and the pressure drop in the mixer decreases. The deposits, such as urea solids, biuret, CYA, and ammelide decrease. Consequently, the performance of the exhaust post-treatment system is improved. [ABSTRACT FROM AUTHOR]