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• IDTs of H 2 /NC 7 H 16 mixture were measured in RCM. • A skeletal H 2 /NC 7 H 16 oxidation mechanism was developed. • Hydrogen diffusion velocity is determined by injection direction. • Hydrogen diffusion velocity influences pre-mixture homogeneity and slipping. • Hydrogen injection timing affects pre-mixture homogeneity and slipping. Achieving efficient lean combustion with homogeneous pre-mixtures is essential in marine low-speed two-stroke hydrogen/diesel engines. However, measures enhancing pre-mixture homogeneity may lead to hydrogen slipping. To address this, the numerical simulation research on injection process was carried out. A skeletal hydrogen/n-heptane oxidation mechanism with 49 species and 174 reactions was developed and validated against experimental ignition delay times in rapid compression machine with temperature range from 600 K to 800 K and pressure of 8 bar and 24 bar. Based on the mechanism, the influence of hydrogen injection parameters on pre-mixture homogeneity and hydrogen slip was investigated by three-dimensional computational fluid dynamic simulation. The angular, radial, and longitudinal velocity of hydrogen diffusion, determined by side and downward angle of hydrogen injectors directly impact the circumferential, radial, and longitudinal homogeneity of pre-mixture. By affecting swirl intensity and friction between fluid and cylinder wall, side angle indirectly influences longitudinal velocity, which dominates hydrogen slipping. Moderate side (10° − 20°) and downward (0° − 20°) angles of hydrogen injectors ensure both pre-mixture homogeneity and prevent hydrogen slipping. Finally, the influences of injection timing on pre-mixture homogeneity and hydrogen slipping were studied. An advanced injection timing (earlier than 226°CA) enhances mixing time but increases hydrogen slip risk. Conversely, delaying injection timing (later than 238°CA) prevents slipping but reduces pre-mixture homogeneity due to shorter mixing time. [ABSTRACT FROM AUTHOR]

In order to avoid the abnormal combustion in high-power hydrogen engine, a 3D CFD numerical model of a direct-injection spark-ignition hydrogen engine was built up based on a large-bore medium-speed four-stroke marine diesel engine using CONVERGE software. To obtain the influence of injection parameters on mixture homogeneity, a dimension reduction optimization method was proposed. The results revealed that the turbulence intensity and the penetration distance varied with the injection parameters, determined the level of mixture homogeneity. The performance comparison between the hydrogen engine and prototype diesel engine showed a great potential of hydrogen in internal combustion (IC) engines. • Process of improving homogeneity can be simplified by optimizing turbulence intensity and injection penetration distance. • Turbulence intensity and injection penetration distance varied with injection system parameters. • The higher the turbulence intensity, the faster the mixing rate. • Neither too long nor too short injection penetration distance was conducive to mixing. [ABSTRACT FROM AUTHOR]

• Auto-ignition properties of ammonia with low n-heptane content were studied using RCM. • The IDTs were measured in the temperature range 620–970 K and pressure range 15–25 bar. • An optimization was conducted based on the existing detailed reaction mechanism. • A small content of n-heptane has a great effect on the IDT and HRR of the mixture. • N-heptane is consumed mainly by the reaction of extracting H atoms by NH2. The low reactivity of ammonia leads to a low combustion efficiency of premixed combustion mode ammonia-diesel engines. In order to overcome this shortcoming, a small amount of diesel is pre-injected into the ammonia pre-mixture to increase the reactivity. Therefore, this study investigated the auto-ignition characteristics of lean-equivalent-rich ammonia/n-heptane mixtures at low n-heptane blending ratios (2 %&5 %&10 %) and wide thermodynamic conditions (Temperature range of 620–970 K, Pressure range of 15–25 bar) via rapid compression machine. The results showed that a small content of n-heptane had a strong ignition-promoting effect: significantly reducing the Ignition Delay Times (IDTs) and increasing the heat release rate. Besides, the IDTs of the mixture decreased with the increase in equivalence ratio, while the stoichiometric mixture showed the highest heat release rate. Moreover, an improved dual-fuel mechanism, which dramatically enhanced the predictive performance of IDTs for ammonia/n-heptane and pure ammonia mixtures, was proposed to illuminate the reaction kinetic properties. N-heptane is consumed prior to ammonia and at a higher rate. The reaction of NH 2 extracting H atoms from n-heptane was the main consumption reaction of n-heptane at low n-heptane content conditions. And the reaction of ammonia consuming OH radicals was the most ignition-inhibiting reaction. Besides, the reaction NH 3 + O 2 = NH 2 + HO 2 plays an important role in the extent of the negative temperature coefficient effect for the mixture under different blending ratios and equivalence ratios. [ABSTRACT FROM AUTHOR]

• Influence of exhaust gases on the ignition of CH 4 /n-C 7 H 16 mixture were revealed. • Thermal, chemical and third-body collision effects are qualitatively compared. • CO 2 suppressed ignition due to its thermal and chemical effect. • H 2 O accelerated ignition due to its third-body and chemical effect. • The addition of CO 2 /H 2 O mixture slightly fastened auto-ignition of CH 4 /n-C 7 H 16. To further optimize the combustion performance of natural-gas/diesel engine under exhaust gas recirculation (EGR) condition, influence of physicochemical impacts of H 2 O and CO 2 on the ignition characteristics of n-heptane/methane mixture (Φ = 0.5/1.0) under various thermodynamic conditions (p = 2 bar/1199 K < T < 1568 K, p = 60 bar/700 K < T < 1200 K) were investigated by shock tube and NUI mechanism. Experiments indicated at p = 2 bar/1199 K < T < 1568 K, CO 2 and H 2 O additions increased and shortened ignition delay times (IDT) respectively. Mixture of CO 2 and H 2 O slightly accelerated ignition. Original and minor modified NUI mechanism well captured the inhibition and acceleration effect of CO 2 and H 2 O on ignition respectively. Both thermal and chemical effects of CO 2 (R36:CO + OH = CO2 + H) were responsible for its ignition inhibition impacts at higher temperatures. Whereas thermal effect of CO 2 became the dominant factor at lower temperatures. Chemical effect of H 2 O (H 2 + OH<=>H + H 2 O&O + H 2 O<=>2OH) promoted OH formation and enhanced whole system reactivity, which suppressed its thermal effect and accelerated ignition process. At p = 60 bar/700 K < T < 1200 K, CO 2 addition significantly retarded ignition due to its thermal effect. At 900 K < T < 1200 K, the higher third-body efficiency of H 2 O promoted ignition (R21:H 2 O 2 (+M) = 2OH(+M)&R34:H + O 2 (+M) = HO 2 (+M)). At 700 K < T < 900 K, thermal effect of H 2 O, which suppressed its third-body effect, inhibited ignition progress. These observations implied that the impact of exhaust gases on the ignition of n-heptane/methane depended on the coupled influence mechanisms of physicochemical effects, the composition of exhaust gases and located thermodynamic conditions. At intermediate-temperature high-pressure conditions (typical thermodynamic conditions of dual-fuel engine), raising the concentration of H 2 O or CO 2 in exhaust gases accelerated or delayed the combustion progress of natural-gas/diesel engine at EGR condition by enhanced third-body effect or thermal effect respectively. [ABSTRACT FROM AUTHOR]