The properties of nanoparticles (including shape, size, material, and volume concentration) may significantly influence the thermal properties of nanofluids. Through molecular dynamics simulations, the aim of this study is to investigate the influence of nanoparticle properties on the thermal conductivity of nanofluids and find an effective criterion for predicting thermal conductivity enhancement. By establishing a series of simulation models, thermal conductivities of nanofluids were calculated on the basis of the Green–Kubo formula. It was found that all the nanoparticle properties that have been considered in this work influence the thermal conductivity of nanofluids, and the influence rules were discussed. Furthermore, there is a positive correlation between the distribution of atomic potential energy and the thermal conductivity of nanofluids. Therefore, the ratio of energetic atoms in nanoparticles is proposed to be the criterion for predicting enhancement of the apparent thermal conductivity of nanofluids.

External exhaust gas recirculation (EGR) stratification in diesel engines contributes to reduction of toxic emissions. Weak EGR stratification lies in that strong turbulence and mixing between EGR and intake air by current introduction strategies of EGR. For understanding of ideal EGR stratification combustion, EGR was assigned radically at −30 °CA after top dead center (ATDC) to organize strong EGR stratification using computational fluid dynamics (CFD). The effects of assigned EGR stratification on diesel performance and emissions are discussed in this paper. Although nitric oxides (NO_x) and soot emissions are both reduced by means of EGR stratification compared to uniform EGR, the trade-off between NO_x and soot still exists under the condition of arranged EGR stratification with different fuel injection strategies. A deterioration of soot emissions was observed when the interval between main and post fuel injection increased, while NO emissions increased first then reduced. The case with a 4 °CA interval between main and post fuel injection is suitable for acceptable NO and soot emissions. Starting the main fuel injection too early and too late is not acceptable, which results in high NO emissions and high soot emissions respectively. The start of the main fuel injection −10 °CA ATDC is suitable.

This study explores key technological challenges and innovative strategies for improving the combustion performance and emission characteristics of low-carbon fuel engines, with a focus on natural gas applications. The core bottlenecks of natural gas combustion, including slow combustion speed and high methane slip under lean burn conditions due to wall quenching, crevice effects, and the long distance of flame propagation from the ignition zone to the whole cylinder, are analyzed. The decarbonization of engines further aggravates these issues. Technological solutions are summarized in four categories, including turbulence enhancement, high-energy ignition, fuel reactivity modification, and fuel synergy with zero-carbon fuels. Geometry modifications of the combustion chamber, dual-fuel operation, pre-chamber ignition, and fuel activation are systematically reviewed and evaluated. A fusion technology integrating diesel pilot ignition with jet flame propagation is analyzed as a new combustion concept, termed induced jet flame combustion. This approach demonstrates significant potential in enhancing both combustion efficiency and stability, especially for lean burn conditions. This work highlights the role of natural gas engines as a transitional technology and a support platform for ultralow-emission and high-efficiency power systems fueled with low/zero-carbon fuels in the context of global decarbonization goals.

In-cylinder exhaust gas recirculation (EGR) stratification, generally achieved by supplying EGR asymmetrically into intake ports on a four-valve diesel engine, is sensitive to trapped exhaust gas in the intake manifold and intake ports that is caused by the continuous supply of EGR during the valve-close periods of the intake valves. The subject of this study is to evaluate the distribution of trapped exhaust gas in the diesel intake system using commercial Star-CD software (version 4.22.018). Numeric simulations of the intake flow of fresh air and recycled exhaust in the diesel intake system were initialized following previous experiments that were conducted on a reformed six-cylinder diesel engine by supplying CO₂ instead of EGR to the tangential intake port alone to establish CO₂ stratification in the first cylinder. The distributions of the intake CO₂ in the intake manifold and intake ports under the conditions of 1330 r/min and 50% load with different mass flow rates of CO₂ are discussed. This indicates that CO₂ supplied to one intake port alone would escape to another intake port, which not only weakens the CO₂ stratification by diminishing the mass fraction disparity of the CO₂ between the two intake ports of cylinder 1, but also influences the total mass of CO₂ in the cylinder. There is 4% CO₂ by mass fraction in the intake port without CO₂ supply under the condition that the CO₂ mass flow rate is 5 kg/h during the intake process, and 10% CO₂ under the condition of 50 kg/h.