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[Display omitted] • Wall impingement can shorten ignition delay by stagnation when H / D is larger than 7. • Ignition initiates from near wall region or the jet root among different H / D. • Secondary jet induced by swirl-shape wall is favorable for ignition. Pre-chamber turbulent jet ignition is a promising high-efficiency combustion technique to achieve stable lean-burn operation for spark-ignition engines. Wall impingement by a high momentum turbulent jet could exert a critical influence on jet ignition behavior. In this work, the passive jet ignition characteristics of premixed methane/air mixture under wall impinging condition was investigated based on optical experiments in a constant volume chamber and numerical simulations. Two ignition modes were found for flat wall impinging cases across a range of non-dimensional impinging distances (H / D) from 5.7 to 20. In mode 1, ignition initiates from near wall region. Under lean conditions of λ = 1.3, compared with that in the free jet case, the ignition delay time characterized by a 5 % mass fraction burned (MFB-05), decreases by 3.3 ms at H = 30 mm and increases by up to 6.6 ms at H = 20 mm. In mode 2, ignition initiates from the jet root. The ignition delay times are shorter for all impinging distances (H = 20, 30, 40 mm), with MFB-05 decreasing by up to 3.4 ms. Simulations reveal that when the wall is located in the jet developing zone with H / D below 7, wall impinging deteriorates the ignition, which is attributed by higher turbulent kinetic energy (TKE) and lower Damköhler (Da) number near the stagnation zone, resulting in extended ignition delay. Conversely, as H / D increases, ignition is promoted by the stagnation effect with a shorter ignition delay. Additionally, the effect of wall shapes was analyzed. Using sharp-tip V-shaped wall, the stagnation region with high TKE is minimized. A blunt-tip V-shaped wall shows enhanced impinging but no obvious increase of Da number at stagnation point. Using M-shaped wall, the ignition mode with a "lift-off" flame was found. Besides, ignition is more favorable in the secondary jet zone with active radicals and higher Da number. [ABSTRACT FROM AUTHOR]

• Laminar burning speeds of NH 3 /n-butanol blends under wide conditions are measured. • Markstein length and Markstein number are measured and analyzed in detail. • Flame thickness and burning flux are calculated and discussed. Blending n-butanol with ammonia has been shown to effectively improve the combustion performance of ammonia and enable carbon–neutral combustion systems. This study investigated the laminar burning speed and Markstein length of n-butanol/ammonia/air mixtures at various equivalence ratios (0.8–1.4) using the spherical flame method with the nonlinear extrapolation model (NQ) in a cylindrical constant-volume combustion bomb. The initial temperatures varied from 393 K to 453 K, and the initial pressures ranged from 0.1 MPa to 0.6 MPa, with ammonia fractions ranging from 50% to 80%. Results illustrate that the unstretched laminar propagating speed and laminar burning speed have a negative correlation with the ammonia fraction and initial pressure, but are positively correlated with the initial temperature, with peak values occurring at around ϕ = 1.0 or ϕ = 1.1 under all initial conditions. Adiabatic flame temperature is negatively related to the ammonia fraction and positively correlated with the initial temperature, but varies minimally with increasing initial pressure. Hydrodynamic instability is negatively related to the ammonia fraction and initial temperature but positively correlated with the initial pressure. When x N H 3 = 80 % , the Markstein length L b and Markstein number M a firstly decrease and then slightly increase as the equivalence ratios increase. The results of L b and M a indicate that the ammonia addition plays a primary role in the flame instability of ammonia/n-butanol blends at both ϕ ≤ 0.9 and ϕ ≥ 1.3 , while the effect of n-butanol matters more at ϕ = 1.0 - 1.2 under x N H 3 = 80 %. Both R cr and Pe cr increase with increasing ammonia fractions under T in = 423 K, P in = 0.6 MPa and ϕ = 1.4. The findings indicate that the burning flux has a negative correlation with the ammonia fraction, whereas it is positively related to the initial pressure and initial temperature, with peak values occurring at around ϕ = 1.0 under all tested conditions. [ABSTRACT FROM AUTHOR]

• Multi-time ignition mixed-mode combustion strategy is proposed. • The impacts of auto-ignition position were studied by optical method. • Combination of temporal and spatial factors in MIMC control. In this study, multi-time ignition mixed mode combustion (MIMC) strategy is proposed to improve the dual fuel combustion, based on observation of the ignition and flame development in conventional dual fuel combustion. Gasoline and polyoxymethylene dimethyl ethers (PODE) serve as the premixed and pilot fuel through port fuel injection and direct injection (DI). As PODE have good auto-ignition property and extremely low sooting tendency, MIMC strategy can realize multi-time ignition in conditions which should be avoided by conventional dual fuel strategies. Firstly, the combustion of gasoline-PODE in typical single DI mode is investigated under different conditions. It is found that the ignition location in the intermediate region of radius causes the fastest combustion among the test cases. The independent auto-ignition events during the flame development are also discussed. Then, the idea of MIMC mode is proposed, considering both the spatial and temporal aspects of different batches of ignition. Different with the typical reaction-controlled compression ignition (RCCI) strategy, the MIMC strategy can directly controls the ignition time with the in-cylinder DI time, providing more flexibility for the dual fuel combustion. Optical and thermal data indicate that the MIMC strategy can fasten or slowdown the combustion by combining the control of ignition position and ignition time. When the locations of the ignition events are spatially alternated and their heat release are overlapped temporally, the combustion will be the fastest under the same DI mass. [ABSTRACT FROM AUTHOR]

• The laminar burning speeds for PODE 3 under different initial pressures are tested. • The laminar burning speeds for PODE 3 / iso -octane blends are investigated. • The accuracy of mechanism for PODE 3 and PRF/PODE n is validated. • The Markstein length and laminar flame instability are analyzed in detail. Polyoxymethylene dimethyl ether (PODE n) is a promising alternative fuel for diesel due to its high oxygen contents (>40%) and high cetane number, thereby improving the thermal efficiency and reducing the pollutant emissions caused by the internal combustion engine. This study investigated the laminar burning speeds and Markstein length for PODE 3 and PODE 3 / iso -octane blends at wide equivalence ratios (0.7–1.6) and initial temperature of 408 K under elevated and sub-ambient initial pressures (0.5–4 bar) in a cylindrical constant-volume combustion vessel with high-speed Schlieren photography method combined with the nonlinear extrapolation model. The numerical simulation was also carried out to validate the accuracy of the mechanisms of PODE 3 and primary reference fuel (PRF)/PODE n blends and analyze the reaction sensitivities and the mole fraction profiles of key species. In addition, flame instability was discussed experimentally and theoretically. The results show that the laminar burning speed for PODE 3 decreases with the initial pressure, and the laminar burning speed for PODE 3 / iso -octane basically accords with the linear relationship to the PODE 3 volume fractions. The findings indicate that the kinetic effect has a dominant impact on the laminar burning speed for PODE 3 when compared with the thermal effect. The results also reveal that the Markstein length is negative only at an equivalence ratio between 1.5 and 1.6 under P = 2 bar, indicating that PODE 3 has more enhanced flame stability. Furthermore, the flame instability decreases with the increase of PODE 3 volume fractions, increases with the increase of initial pressure, and increases with the increase of the equivalence ratio. The differences between experimental results and theoretical results including the Markstein length, the critical radius and the critical Peclet number might be explained by the stretch rate results. [ABSTRACT FROM AUTHOR]

• A strategy of reaction kinetic model reduction and validation is developed for detonation. • A reduced kinetic model with 55 reactions and 26 species is proposed for ethylene. • The proposed model is applicable to the detonation simulation under wide conditions. • Verified the applicability of three typical reduced models in detonation simulation. The selection of chemical reaction kinetic model is essential in detonation simulation. However, how to obtain an appropriate kinetic mechanism to describe chemical reaction process is still an open question, even for the common ethylene detonation simulation. Since the complete detailed mechanism is extremely time consuming in detonation simulation, single-step or several-step simplified models are widely used, although not capable to realistically reproduce unsteady processes in some detonation studies, especially for detonation initiation and failure. Therefore, a series of studies on the reduction, validation and applicability of chemical reaction kinetic model for ethylene detonation are carried out. First, a reduced kinetic model with 55 reactions and 26 species is proposed for ethylene, based on the detailed kinetic model of USC 2.0. A combination method of path flux analysis and sensitivity analysis is used to initially develop a skeleton model, which is employed to acquire a reduced kinetic model by the sensitivity analysis method. Then, predictive performance of the proposed reduced kinetic model is extensively investigated for one-dimensional steady detonation, one-dimensional and two-dimensional unsteady detonation, as well as zero-dimensional constant volume explosion. The results show that the present reduced model is well suitable for detonation simulations within a broad range of conditions. Compared with three typical reduced models in the literature, the present model performs much better overall, particularly in unsteady ethylene detonation simulation. This work provides a valuable roadmap and solution for the development and selection of chemical reaction kinetic models in detonation simulation. [ABSTRACT FROM AUTHOR]

• Intermolecular interactions in two dominant mixing modes have been investigated. • Transition characteristics of the fuel are decided by intermolecular interactions. • Fuel clusters form in the mixing layer where the transition happens. For a multi-component hydrocarbon mixture under supercritical conditions, especially for fuels injected into compression ignition engines, the mechanism for the transition of the dominant mixing mode from evaporation to diffusion is not well established. In this paper, evaporation processes of a six-component hydrocarbon fuel (13.16 mol% toluene, 13.81 mol% n-decane, 22.30 mol% n-dodecane, 24.60 mol% n-tetradecane, 14.66 mol% n-hexadecane and 11.47 mol% n-octadecane) droplet in nitrogen environments were studied using molecular dynamics (MD) simulations, in comparison with those of three-component and single-component fuel droplets. The ambient pressure ranged from 2 MPa to 16 MPa and the ambient temperature ranged from 750 K to 1350 K. Results indicated that the transition characteristics of the mixed fuel were not the linearly weighted average of the physical properties of individual components in the mixture based on their mole fractions. The reason why there is a limitation on the maximum transition temperature when diffusion dominates the fuel-ambient gas mixing process under high pressures has been discussed. The average resultant force on a fuel atom of an individual component increases with increasing pressure or decreasing temperature at the supercritical temperature, and diffusion will gradually dominate the mixing process of the fuel. The clustering behavior of fuels under supercritical conditions has also been discussed. [ABSTRACT FROM AUTHOR]

• The shock-to-shock interactions were observed in GDI flash-boiling jets. • The shock-to-shock interactions were mainly influenced by R p and ambient pressure. • The shock-to-shock interactions caused different jet-to-jet interaction behaviors. • The detailed process of shock-to-shock interactions was numerically investigated. Through recent numerical studies, the shock-to-shock interactions between flash-boiling jets are believed to be the reason for multi-jet spray collapse in gasoline direct injection (GDI) engines. But this sort of interaction was not well understood through the experimental investigation. In the present study, a two-hole GDI injector was used to examine this process via optical diagnostics and CFD simulations. With the increase in superheat level (R p), the shock waves (primary cells) in individual jets became observable. Further increase in R p could cause shock-to-shock interactions, and a secondary cell could be formulated between two primary cells. The different intensity of shock-to-shock interactions would result in different jet-to-jet interaction behaviors. In the moderate interaction region, the secondary jet appeared inside and downstream of the secondary cell, but the sprays did not collapse. In the highly interaction region, both the secondary jet and spray collapse could be observed, where the secondary jet would laterally develop perpendicular with the plane of the two jets. The detailed numerical results showed that the secondary cell was small in the moderate interaction region, but became much larger in the highly interaction region. The occurrence of secondary cell directed the fuel from primary cells into the secondary cell, contributing to the generation of secondary jet. After flowing out of the secondary cell, the incipient secondary jet could subsequently entrain the nearby low-velocity fuel. Further, the strong shock-to-shock interactions would alter the shape of primary cells, reducing the transverse velocity component of jets and promoting collapse. Based on the experimental and numerical results, the existence of shock-to-shock interactions and its important role in jet-to-jet interactions and spray collapse were fully illustrated, and this understanding would be of help in nozzle optimization. [ABSTRACT FROM AUTHOR]

• Shock waves were observed in n -hexane flashing jets at R p around and above 10. • A linear correlation was found between shock width and Δμ·P amb −0.5. • A quadratic polynomial relationship was found between shock length and η inj. • Mach reflection was not necessary for the formation of shock structure. Flash-boiling sprays issued from multi-hole gasoline direct injection (GDI) injectors could collapse, which is recently proved to be caused by under-expansion, but very few experimental data are available on this sort of under-expansion. In this study, high-speed microscopic Schlieren photography was used to capture the near-field shock structures in flash-boiling n -hexane jets. The tests were carried out in a constant volume vessel with ambient pressures (P amb) from 10 to 101 kPa, fuel temperatures (T fuel) from 30 to 170 ℃, and injection pressures (P inj) from 2 to 20 MPa. The shock structure was carefully analyzed in terms of shock width and length. Generally, both the shock width and length decreased with the increase in P amb. With the increase in T fuel , the shock length was relatively stable and the shock width overall increased, but a decreasing trend in shock width could be observed at 170 ℃, possibly caused by the near-critical behaviors. Further analysis showed that the shock width shows a strong linear correlation with Δμ·P amb −0.5 (Δμ denoting chemical potential of phase change) for a given P inj , and the shock length shows a strong quadratic polynomial relationship with η inj (injection pressure ratio, defined as the ratio of P inj to P amb). Additionally, some new shock structures without Mach disk were observed, indicating that the phase change induced under expansion is different from the typical gaseous under-expansion. [ABSTRACT FROM AUTHOR]

• Particle size distributions of nucleation mode change remarkably along exhaust system. • Particle number concentration reduces more significantly with higher accumulation mode. • The constructed coagulation model can well catch the measured particle variation. • The coagulation history was provided to explain the particle evolution. In this article, Brownian coagulation of particles in the exhaust system of a gasoline engine were investigated by measuring particle size distribution (PSD) at different sampling points and Monte Carlo simulation. The measurement results show that the particle size distributions of nucleation mode (NM) change remarkably after the exhaust gas passes through the exhaust plenum chamber. The constructed coagulation model based on the classic coagulation theory can well catch the measured PSDs, particle number (PN) concentrations, and count median diameters (CMDs) along the exhaust system under both the in-cylinder direct injection and port fuel injection modes, indicating that the particle process in the exhaust system is dominated by coagulation. The NM PN concentration decreases, and the NM CMD increases in the downstream exhaust gas, and the changes are more significant at higher accumulation mode (AM) PN concentration conditions. The coagulation history provided by modelling shows that coagulation mainly occurs between two NM particles (NM-NM) or one NM and one AM particles (NM-AM). At higher AM PN concentration conditions, the NM-AM coagulation dominates and quickly eliminates NM particles, resulting in more significant NM particle reduction. The PN concentration variation as a function of time indicates that PN concentration decreases quickly at first and then the decreasing rate slows down. [ABSTRACT FROM AUTHOR]

• Effects of spark timing, Lambda, injection timing and pressure were examined. • MF exhibits better combustion stability and efficiency at lean burn conditions. • Combustion characteristics of MF have low sensibility to the injection timing. • High injection pressure reduces injection duration differences between MF and gasoline. • Achievable spark timing range for MF is larger than that of gasoline. 2-Methylfuran (MF) is considered as a promising second-generation biofuel for spark ignition engines. MF has better anti-knock ability, and the lower heating value of MF is close to that of conventional fuels. The production methods of MF have been largely improved over the last decade. However, the studies of MF on direct injection spark ignition (DISI) engines are quite limited, and little is known about the effects of engine control parameters on MF combustion at typical DISI engine operating conditions. In the present work, the effects of engine control parameters including spark timing, air–fuel ratio, injection timing, and injection pressure on combustion and engine performances of MF were studied at typical medium load and high load conditions on a single-cylinder spray-guided DISI engine, compared to gasoline and ethanol. The results show that the achievable spark timing range considering combustion phasing and knock limit for MF is larger than that of gasoline especially at high load. MF has more robustness to lean burn than gasoline and ethanol. The combustion characteristics and heat release of MF have low sensibility to the variation of injection timings. To achieve the same engine load MF requires up to 25% longer injection durations than gasoline, however, increased injection pressure can mitigate the injection duration differences between MF and gasoline. [ABSTRACT FROM AUTHOR]