•A new spray/wall impingement model is purposed based on droplet impact results.•Morphology dynamics of prompt splash and corona splash are analyzed.•Good performance is obtained in estimation of spray impingement.A new spray/wall impingement model is purposed based on droplet impact experiments and droplet morphology dynamics. Various splashing modes are summarized and analyzed based upon earlier research on droplet impact and conservation laws. Corona splash and prompt splash are distinguished in the new spray/wall impingement model taking the incident and wall conditions into consideration. Morphology dynamics of droplet impingement is analyzed and coupled into the conservation equations, which helps to obtain more accurate dynamic results of splashing droplets. The new wall impact model is validated in a constant volume combustion system under different conditions. Two different fuels, including n-tridecane and diesel, are adopted for spray impingement validations. It is found that this new model shows good performance in predicting impinging spray properties, including radius penetration and splashing height, in wide jet conditions.

•Fuel droplet impingement has been studied using Smoothed Particle Hydrodynamics method.•A new splashing sub-model is proposed to predict splashing mass and secondary droplets.•Temporal evolution of the droplet shape after impingement is shown.•The influence of incident energy and wall film on the splashing mass is studied in detail.Fuel droplet impingement on different wall conditions have been studied using Smoothed Particle Hydrodynamics (SPH) method, and a new splashing sub-model is proposed based on numerical results to calculate the splashing mass fraction of the incident droplet and compared with experimental results. Temporal evolution of the droplet shape after impingement is studied with various initial and boundary conditions. It is found that few splashes take place when the drop hits a relatively smoothed dry surface. Wall film plays a great role on droplet splashing. On one hand, the film slows down the droplet spreading process and transmits the energy to the crown part, making it easier to splash; on the other hand, the incident energy is dissipated when the drop moves through the film. The result shows that when non-dimensional wall film (hnd) is less than 0.5, the splashing mass fraction is relatively high and secondary droplets are easier to form. Further increase hnd, the fraction goes down and more film part splashes. The splashing sub-model is fitted and modified using numerical data and experimental data in a wet wall case. The result shows that the splashing mass and secondary droplets can be predicted as a function of We using the proposed sub-model.

A skeletal combustion mechanism with 146 species and 652 reactions of methyl decanoate (MD) as a surrogate for biodiesel fuels was developed for compression ignition engine simulations. The skeletal mechanism of MD was derived by reducing the detailed mechanism based on an integrated reduction method that contains directed relation graph method, sensitivity analysis, and reaction path analysis. A reduced polycyclic aromatic hydrocarbon mechanism was merged into the skeletal combustion mechanism of MD to predict the soot emission. The skeletal mechanism was validated against the experimental data of ignition delays in a shock tube, as well as the mole fractions of the reactants and the intermediate species in a jet-stirred reactor. The skeletal mechanism maintains accuracy with its dramatically reduced size, compared with the detailed mechanism that consists of 2878 species and 8555 reactions. The skeletal mechanism was coupled with the KIVA code for 3-D biodiesel combustion simulation. Compared with the soot measurements in an optical constant volume combustion chamber, the simulation results showed similar soot location and occurrence during the combustion. Engine simulations were conducted with the EGR rate ranging from 0 to 65% at intake temperatures of 25 and 50 °C. The predictions profiles of the pressure and the heat release rate for various conditions agreed well with the experimental data. The skeletal mechanism predicted the emissions, including CO, HC, NOx, and soot accurately.

•A reduced combustion mechanism of n-butanol/biodiesel was developed.•The reduced mechanism contains 170 species and 769 reactions.•This mechanism can be used for n-butanol/biodiesel dual fuel engines simulation.•The interaction of n-butanol/biodiesel during the ignition process was analyzed.•The evolution of oxygen contained in n-butanol and biodiesel molecules was discussed.A reduced chemical kinetic mechanism of the n-butanol/biodiesel blend was developed for dual fuel engine simulations. The reaction flow analysis reduction method was adopted to lump and remove the unimportant species and the related reactions. The reduced mechanism of n-butanol contains 71 species and 349 reactions. The reduced mechanism of n-butanol was merged into a reduced mechanism of biodiesel to construct a combined mechanism of n-butanol/biodiesel with 171 species and 765 reactions. The combined mechanism was validated against the n-butanol experimental data including ignition delays in shock tubes and the mole fractions of species in a jet-stirred reactor.The n-butanol/biodiesel mechanism was further validated against the engine experiments fuelled with the n-butanol/biodiesel dual fuel under multiple operating conditions. The predicted pressure and the heat release rate profiles, as well as CO, HC, NOx, and soot emissions under various conditions agreed well with the experimental data. Analysis of the interaction of n-butanol/biodiesel during the ignition process shows that the biodiesel produces OH radicals at the low in-cylinder temperature, and the produced OH radicals are consumed by n-butanol to produce HO2 radicals after a consecutive reaction. HO2 radicals are then transformed to H2O2, which dissociate to OH radicals at higher in-cylinder temperature. OH radicals in turn facilitate further consumption of n-butanol/biodiesel dual fuel. Analysis of the evolution of oxygen contained in the fuel molecules indicates that the O atom in the n-butanol molecule ends up in CH2O, which is turned into CO and the O atoms in the biodiesel may also give rise to CO2 directly.

•Liquid penetration length of n-butanol is less affected by the downstream flame.•Flame propagation and distribution of n-butanol is less sensitive to temperature.•Biodiesel soot concentrates on the reacting spray jet and near wall region.•The soot of n-butanol is very low and concentrates on reacting spray at 900 K and 1000 K.•B50D50 soot is lower 20–30% than biodiesel at the same fuel oxygen content.The spray, flame natural luminosity and soot quantitative measurement by fueling n-butanol and soybean biodiesel were investigated on a constant volume chamber using multi laser diagnostics with high temporal resolution of 15,037 frames per second. The ambient temperature ranged from 700 K to 1000 K was applied to mimic the diesel in-cylinder environment covering both conventional and low temperature combustion conditions. Results demonstrate that the transient liquid penetration lengths gradually decrease with the development of injection in a burning spray. The transient liquid penetration length of n-butanol is less affected by the downstream flame and is shorter than that of biodiesel under similar conditions. The two fuels also present drastically different flame dynamics and emission characteristics during the combustion process. n-Butanol displays higher normalized combustion pressure indicating higher potential thermal efficiency. The flame can be observed at reacting spray jet and near wall region for biodiesel, while the main n-butanol flame is concentrated on reacting spray jet. Compared to biodiesel, the flame luminosity of n-butanol is lower and its propagation and distribution is less sensitive to ambient temperatures. Ambient temperature is confirmed as the dominant impact on the soot emissions as the net soot increases for both tested fuels with elevated ambient temperature. No soot emission is detected for either n-butanol or biodiesel at low ambient temperature of 700 K. The soot starts to appear at both downstream of the jet and near wall regions from 800 K for biodiesel whereas the soot do not emerge until 900 K for n-butanol. The soot concentration of n-butanol is also much lower and restricted within the downstream of the spray jet even at high ambient temperatures. A particular interesting observation is that the normalized time integrated soot mass for B50D50 (50% n-butanol and 50% low-sulfur diesel fuel in volume) is 20–30% lower than that of neat biodiesel fuel at different ambient temperatures even though the oxygen content in both fuels is nearly the same. The higher soot formation for biodiesel is explained by the fact that fuel-bound oxygen is less effective in reducing soot production and that higher viscosity and boiling point results in more local high equivalence-ratio region in mixing process. In this regard, n-butanol is more effective in suppressing the soot formation and shows stronger soot reduction capability than that of biodiesel.

Biomass is the largest and most important renewable energy option at present. Because of similar physicochemical properties to gasoline and improved production methods, 2,5-dimethylfuran (DMF) has drawn extensive attention of global researchers. But currently, little investigations have been carried out on DMF as the engine fuel, especially on CI engines. In this paper, effects of DMF addition on combustion and emissions were investigated on a modified single cylinder heavy-duty diesel engine with low temperature combustion, and the characteristics of DMF and gasoline were compared specially. The results show that when DMF fraction is up to 40%, the trade-off relationship between NOx and soot disappears and soot emissions are close to zero. DMF addition has little effects on CO and THC emissions. Although the physicochemical properties of DMF and gasoline are similar, due to the difference of ignition delay and DMF is oxygenated, it makes a great difference in combustion and emission characteristics.Highlights• Differences of combustion and emissions for D40 and G40 are great for CI engine. • When DMF fraction is up to 40%, the trade-off between NOx and soot is solved. • DMF or gasoline addition has little effects on CO and THC emissions. • NOx emissions increase for DMF-diesel blend. • D40 may be a better fuel to meet future emissions regulations with medium EGR.

Motivated by the lack of information regarding an appropriate diesel fuel surrogate under low temperature combustion conditions, an experimental work had been carried out in a single-cylinder common-rail diesel engine running with diesel fuel and several diesel fuel surrogates containing the blend of n-heptane/toluene and n-heptane/toluene/1-hexene. The combustion and emission characteristics of the diesel fuel and its surrogates had been investigated in a wide range of intake oxygen concentration ranging from 21% to approximately 10%, covering both conventional diesel combustion and low temperature combustion conditions. The chemical kinetics mechanism had been used to analyze the experimental phenomenon. Results demonstrated that the ignition delay was longer with the increase of toluene in the pure n-heptane. The toluene had larger effects on ignition delay at lower intake oxygen concentrations because less oxidation process resulted in a lower HO2 emission at lower oxygen concentrations and more termination reactions occurred with more benzyl radical. The ignition delay and smoke emission of TRF20 (80% n-heptane/20% toluene in volume) were closer to that of diesel fuel as compared with the pure n-heptane and TRF30 (70% n-heptane/30% toluene in volume) fuel. However, there were still some disparities between TRF20 and diesel fuel in smoke emission at lower intake oxygen concentrations ([O2]in < 16%). Therefore, the further study of fueling TRF20/1-hexene mixture (95/5vv) was conducted. Results showed that the addition of 1-hexene retarded the ignition delay slightly than that of TRF20 and the smoke emission of TRF20/1-hexene mixture was closer to that of diesel fuel. Both experiments and chemical kinetics analysis showed that the acetylene as the soot precursor had a higher emission at lower intake oxygen concentrations for TRF20/1-hexene mixture and thus led to a higher smoke emission compared to TRF20. In addition, other emissions studies demonstrated that NOx emissions were almost identical for diesel and its surrogates, while HC and CO emissions had some disparities at very low intake oxygen concentrations. Finally, it can be concluded that the TRF20/1-hexene mixture had a better match in auto-ignition delay and emissions at both conventional combustion and low temperature combustion processes in the current study.Highlights► Toluene has larger effects on ignition delay at lower intake oxygen concentrations. ► NOx, HC and CO emissions were identical for tested fuels except the low intake oxygen concentration case for HC and CO. ► The choice of surrogates has large effects on smoke emission between 16% and 11% intake oxygen concentrations. ► Addition of 1-hexene retarded ignition and increased the C2H2 and soot at lower intake oxygen concentrations. ► TRF20/1-hexene mixture (95/5vv) matches well to diesel at both conventional and LTC conditions.

This paper experimentally and numerically investigates the effects of injection timing on combustion of a methanol/dimethyl ether (DME) dual-fuel compound at various methanol concentrations. In this dual-fuel compound combustion approach, methanol is directly injected into the cylinder, and DME is injected at the intake port. The experimental results indicate that methanol injection timing has an obvious effect on the heat release process of the dual-fuel compound combustion, and the effect of methanol concentration (i.e., the engine load) on injection timing is also evident. Late injection should be adopted to achieve smooth combustion and low NOx emissions at high methanol concentrations. Moderate injection timing should be adopted to achieve higher indicated thermal efficiency (ITE) at low methanol concentrations. The simulation results, using computational fluid dynamics (CFD) combined with reduced chemical kinetics, show that methanol injected at −26 °CA after top dead center (ATDC) has a remarkable effect on the high-temperature reaction of DME. In the case of injection timing at −26 °CA ATDC, the high-temperature combustion region is concentrated within the combustion chamber, which results in a higher NO concentration. With injection timing at −6 °CA ATDC, by contrast, the high-temperature combustion region is dispersed in the compression clearance and near the chamber wall, which leads to a relatively low NO concentration. Increasing injection pressure is an effective way to shorten the duration of the methanol/DME dual-fuel compound combustion achieved by later injection.

The effect of fuel and operating conditions on the combustion process, load range, and exhaust emissions of a homogeneous charge compression ignition (HCCI) engine was investigated in a modified four-cylinder direct-injection diesel engine through an experimental study. Six fuels were used during the experiments: two primary reference fuels (PRF), two mixtures of PRF and ethanol, and two commercial unleaded gasoline fuels. All research octane numbers (RON) of these fuels are over 90. Six operating conditions were considered, including different intake temperatures (Tin), intake pressures (pin), and engine speeds (n). Experimental results indicate that autoignition of gasoline is earliest under low pin but the PRF is earliest under high pin. That is, the effect of fuel properties on the HCCI combustion process depends upon the operating conditions. It is beneficial to extend the load range with the sensitive fuels and suitable control strategies. For a sensitive fuel, a higher Tin is needed to extend the load range toward light load, while a lower Tin and higher pin is needed to extend the load range toward high load. In addition, the octane index (OI) does not show a correlation with autoignition, HC and CO emissions, and load range when the mixtures of PRF and ethanol are used in some operating conditions.

Effects of oxygen concentration on combustion and emissions of diesel engine are investigated by experiment. The intake oxygen concentration is controlled by adjusting CO2. The results show that very low levels of both soot and NOx emissions can be achieved by modulating the injection pressure, timing, and boost pressure at the low levels of oxygen concentration. However, both CO and HC emissions and fuel consumption distinctly increase at the low levels of oxygen concentration. The results also indicate that NOx emissions strongly depend on oxygen concentration, while soot emissions strongly depend on injection pressure. Decreasing oxygen concentration is the most effective method to control NOx emissions. High injection pressure is necessary to reduce smoke emissions. High injection pressure can also decrease the CO and HC emissions and improve engine efficiency. With the increase of intake pressure, both NOx and smoke emissions decrease. However, it is necessary to use the appropriate intake pressure in order to get the low HC and CO emissions with high efficiency.

The influence of inlet pressure ( P in) and octane numbers on combustion and emissions of a homogeneous charge compression ignition (HCCI) engine was experimentally investigated. The tests were carried out in a modified four-cylinder direct injection diesel engine. Four fuels with different research octane number (RON) were used during the experiments: 90-RON, 93-RON, and 97-RON primary reference fuel (PRF) blend and a commercial gasoline, 94.1-RON(G). The inlet pressure conditions were set to give 0.1, 0.15, and 0.2 MPa of absolute pressure. The results indicate that, with the increase of inlet pressure, the start of combustion (SOC) advances and the cylinder pressure increases. The effects of the PRF octane number on SOC are weakened as the inlet pressure increased. However, the difference of SOC between gasoline and PRF is enlarged with the increase of the inlet pressure. The successful HCCI operating range is extended to the upper and lower load as the inlet pressure increased. The maximum achievable load of gasoline is higher than that of PRF with the cases of supercharging. The HC and NO x emissions of the HCCI engine decrease when supercharging is employed, while CO emissions increase remarkably. The PRF octane number has little effect on HC, CO, and NO x emissions when supercharging is employed. Nevertheless, the HC and CO emissions of gasoline are higher than those of PRF with supercharging.

HCCI combustion has been drawing the considerable attention due to high efficiency and lower nitrogen oxide (NOx) and particulate matter (PM) emissions. However, there are still tough challenges in the successful operation of HCCI engines, such as controlling the combustion phasing, extending the operating range, and high unburned hydrocarbon and CO emissions. Massive research throughout the world has led to great progress in the control of HCCI combustion. The first thing paid attention to is that a great deal of fundamental theoretical research has been carried out. First, numerical simulation has become a good observation and a powerful tool to investigate HCCI and to develop control strategies for HCCI because of its greater flexibility and lower cost compared with engine experiments. Five types of models applied to HCCI engine modelling are discussed in the present paper. Second, HCCI can be applied to a variety of fuel types. Combustion phasing and operation range can be controlled by the modification of fuel characteristics. Third, it has been realized that advanced control strategies of fuel/air mixture are more important than simple homogeneous charge in the process of the controlling of HCCI combustion processes. The stratification strategy has the potential to extend the HCCI operation range to higher loads, and low temperature combustion (LTC) diluted by exhaust gas recirculation (EGR) has the potential to extend the operation range to high loads; even to full loads, for diesel engines. Fourth, optical diagnostics has been applied widely to reveal in-cylinder combustion processes. In addition, the key to diesel-fuelled HCCI combustion control is mixture preparation, while EGR is the main path to achieve gasoline-fuelled HCCI combustion. Specific strategies for diesel-fuelled, gasoline-fuelled and other alternative fuelled HCCI combustion are also discussed in the present paper.

•Different gasoline/diesel dual-fuel combustion modes were compared experimentally.•The mixing status, fuel reaction and emission formation were examined numerically.•The mixture in E-HPCC is basically uniform in both concentration and reactivity.•The combustion efficiency of HPCC can be improved by higher gasoline ratio.In this study, numerical simulation and experiments have been carried out to explore the differences in combustion and emissions characteristics between dual-fuel Highly Premixed Charge Combustion (HPCC, including E-HPCC and L-HPCC) and blended-fuel Low Temperature Combustion (LTC) modes with gasoline and diesel. The results illustrate that, most of the mixture in E-HPCC is uniform in both concentration and reactivity, and there are various degrees of mixture stratification in L-HPCC and LTC. Based on the in-cylinder charge distributions, the combustion occurs in the very center area of combustion chamber and the area closer to the piston bowl wall in the two HPCCs and LTC respectively, and then flame spread to peripheral regions. In the two HPCCs, the substantial heat release is determined by the oxidation of OH radical that derived from the low temperature reaction of diesel fuel, and the staged reaction of diesel and gasoline leads to reasonable MPRR values; the fuel stratification in LTC mode results in a rapid heat release rate and high MPRR because of the coupling combustion reaction of gasoline and diesel taking place in the regions with higher fuel concentration. The observed NOX and soot reductions of E-HPCC are due to the avoidance of high equivalence ratio and high temperature region in the combustion chamber. Compared to LTC, the two HPCCs produce more incomplete combustion products and consequent lower combustion efficiencies, which can be improved by increasing gasoline ratio.