Effect of pressure wave disturbance on auto-ignition mode transition and knocking intensity under enclosed conditions

Pressure wave propagation behavior is an essential feature for the combustion under enclosed conditions, e.g. internal combustion engines. Previous work by Pan et al. (2016) and Yu et al. (2015) showed that pressure wave disturbance not only affects hot-spot formation and knocking origin, but also induces detonation wave through a coupling mechanism between pressure wave and flame front. On this basis, this study further investigates the role of pressure wave disturbance in auto-ignition mode and knocking intensity by means of detailed numerical simulations with stoichiometric H2/air mixture. Firstly, the pressure waves with different levels in strength have been obtained by adjusting ignition temperature of hot ignition kernel. It shows that as ignition temperature is raised at each initial temperature, pressure wave strength is decreased monotonously, with declining compression ratio and temperature rise caused by pressure wave disturbance. Secondly, three auto-ignition modes have been observed with the variations of pressure wave strength, i.e. detonation, mixed mode and supersonic deflagration. As the weakness of pressure wave strength, there is an auto-ignition mode transition from detonation to supersonic deflagration, accompanied by rapid decreases in pressure peak, obvious pre-flame partial reaction and significant increases in auto-ignition reaction front speed. These observations are still maintained at elevated initial pressure conditions. Finally, such auto-ignition modes and knocking intensity for the detailed computations are summarized in the non-dimensional Bradley's diagram. The results show that both auto-ignition mode and initial thermodynamic state can affect knocking intensity, and the modifications in knocking intensity by pressure wave disturbance are mainly through auto-ignition mode transition. This is qualitatively consistent with the distribution of combustion regimes in Bradley's diagram, even though some deviations do exist because the diagram is constructed on basis of initially non-reactive flows

Resonant actuation based on dynamic characteristics of bistable laminates

Bistable or multi-stable structures have found broad applications in the fields of adaptive structures, flow control, and energy harvesting devices due to their unique nonlinear characteristics and strong local stability behavior. In this paper, a theoretical model based on the principle of minimum potential energy and the Rayleigh–Ritz method is established to study the dynamic characteristics of a bistable unsymmetric laminate with a fixed center. Numerical results of this theoretical model were obtained and verified by an FEA model using ABAQUS. The nonlinear dynamic characteristics and the structural response under different levels of external excitation were investigated and verified by experiments. The realization conditions of single-well vibration and cross-well vibration of bistable laminates were determined, with which the actuation strategies can be optimized for targeting modal frequencies of bistable laminates

Experimental study on laminar flame characteristics of methane-PRF95 dual fuel under lean burn conditions

The effects of methane addition to PRF95 (primary reference fuel with 95% volume of iso-octane and 5% volume of n-heptane) on the fundamental combustion parameters are experimentally investigated in a cylindrical combustion vessel using classical schlieren technique. In this study, methane is added with three energy fractions of 25%, 50% and 75% to PRF95. The laminar flame propagation, Markstein length and flame instability of dual fuels under different initial pressures and with different equivalence ratios, especially under lean burn condition, are well studied. Spherical flames are experimentally investigated at the initial temperature of 373 K and under the pressures of 2.5 bar, 5 bar and 10 bar. The equivalence ratios vary with 0.8, 1.0 and 1.2. The stretched flame speeds are determined by outwardly spherical flame method. The results show that at low initial pressures, the addition of methane to PRF95 increases the stretched flame speeds with lean equivalence ratios while decreases it in rich region. Laminar flame of methane-PRF95 mixtures burn faster than those of pure methane and PRF95 with equivalence ratio of 0.8 over the whole range of the initial pressures investigated, and this trend is more obvious at low pressure. Comparing the data of 25% methane dual fuel (DF25) with that of base fuels with the equivalence ratio of 0.8 and under the initial pressure of 2.5 bar, it can be seen that the flame speed of DF25 is 57% faster than that of methane and 22% faster than that of PRF95. These results provide important theoretical references to lean burn SI engine with methane-gasoline dual fuels under lean burn conditions

Experimental analysis of super-knock occurrence based on a spark ignition engine with high compression ratio

The super-knock phenomenon is a major obstacle for further improving the power density in SI engines. The objective of this paper is to experimentally investigating the mechanism involved in the occurrence of super-knock. In this work, a high compression ratio (CR = 13) coupled with advanced spark timings were employed to achieving intense or critical thermal-dynamic conditions to easily inducing the super-knock. The results show that super-knock can originate from spark ignition, which is different from previous results regarding pre-ignition. Changing the spark timing super-knock can be induced with very high pressure oscillation at the present high compression ratio. The high compression ratio could generate sufficiently high thermal-dynamic conditions to inducing the abnormal combustion. In this research, four combustion phenomena were observed. The present work indicates that there is a nonlinear relationship between knock intensity and knocking onset in terms of pressure profiles at different cycles. The super-knock or knock phenomena were dominantly induced by spark ignition, which were controlled by the pre-ignition after several cycles. Finally, the analysis of the mechanism of super-knock with severe pressure oscillation was employed based on the thermal explosion theory and cavity resonances. There are two possible auto-ignition combustion modes that can induce the intense pressure oscillation

Experimental study on laminar flame characteristics of methane-PRF95 dual fuel under lean burn conditions

This article was published in the journal Fuel [© Elsevier Ltd.] and the definitive version is available at: http://dx.doi.org/10.1016/j.fuel.2016.07.065The effects of methane addition to PRF95 (primary reference fuel with 95% volume of iso-octane and 5% volume of n-heptane) on the fundamental combustion parameters are experimentally investigated in a cylindrical combustion vessel using classical schlieren technique. In this study, methane is added with three energy fractions of 25%, 50% and 75% to PRF95. The laminar flame propagation, Markstein length and flame instability of dual fuels under different initial pressures and with different equivalence ratios, especially under lean burn condition, are well studied. Spherical flames are experimentally investigated at the initial temperature of 373 K and under the pressures of 2.5 bar, 5 bar and 10 bar. The equivalence ratios vary with 0.8, 1.0 and 1.2. The stretched flame speeds are determined by outwardly spherical flame method. The results show that at low initial pressures, the addition of methane to PRF95 increases the stretched flame speeds with lean equivalence ratios while decreases it in rich region. Laminar flame of methane-PRF95 mixtures burn faster than those of pure methane and PRF95 with equivalence ratio of 0.8 over the whole range of the initial pressures investigated, and this trend is more obvious at low pressure. Comparing the data of 25% methane dual fuel (DF25) with that of base fuels with the equivalence ratio of 0.8 and under the initial pressure of 2.5 bar, it can be seen that the flame speed of DF25 is 57% faster than that of methane and 22% faster than that of PRF95. These results provide important theoretical references to lean burn SI engine with methane-gasoline dual fuels under lean burn conditions

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Improving Thermal Efficiency of Internal Combustion Engines: Recent Progress and Remaining Challenges

Improving thermal efficiency and reducing carbon emissions are the permanent themes for internal combustion (IC) engines. In the past decades, various advanced strategies have been proposed to achieve higher efficiency and cleaner combustion with the increasingly stringent fuel economy and emission regulations. This article reviews the recent progress in the improvement of thermal efficiency of IC engines and provides a comprehensive summary of the latest research on thermal efficiency from aspects of thermodynamic cycles, gas exchange systems, advanced combustion strategies, and thermal and energy management. Meanwhile, the remaining challenges in different modules are also discussed. It shows that with the development of advanced technologies, it is highly positive to achieve 55% and even over 60% in effective thermal efficiency for IC engines. However, different technologies such as hybrid thermal cycles, variable intake systems, extreme condition combustion (manifesting low temperature, high pressure, and lean burning), and effective thermal and energy management are suggested to be closely integrated into the whole powertrains with highly developed electrification and intelligence

Improving Thermal Efficiency of Internal Combustion Engines: Recent Progress and Remaining Challenges

Improving thermal efficiency and reducing carbon emissions are the permanent themes for internal combustion (IC) engines. In the past decades, various advanced strategies have been proposed to achieve higher efficiency and cleaner combustion with the increasingly stringent fuel economy and emission regulations. This article reviews the recent progress in the improvement of thermal efficiency of IC engines and provides a comprehensive summary of the latest research on thermal efficiency from aspects of thermodynamic cycles, gas exchange systems, advanced combustion strategies, and thermal and energy management. Meanwhile, the remaining challenges in different modules are also discussed. It shows that with the development of advanced technologies, it is highly positive to achieve 55% and even over 60% in effective thermal efficiency for IC engines. However, different technologies such as hybrid thermal cycles, variable intake systems, extreme condition combustion (manifesting low temperature, high pressure, and lean burning), and effective thermal and energy management are suggested to be closely integrated into the whole powertrains with highly developed electrification and intelligence

New Insights into Abnormal Combustion Phenomena Induced by Diesel Spray-Wall Impingement under Engine-Relevant Conditions

High altitude and low temperature is the common extreme environment for internal combustion engines. Under such operating conditions, heavy-duty diesel engines often suffer from serious abnormal combustion, such as knocking combustion, which results in piston crown breakdown and cylinder head erosion. Spray-wall impingement and pool fires are considered potential causes; however, the detailed mechanism remains poorly understood owing to the lack of research data. In this study, for the first time, the destructive abnormal combustion induced by diesel spray-wall impingement was identified using an optical rapid compression machine under engine-relevant conditions at high altitudes. Combining instantaneous pressure and temperature measurements with simultaneously recorded high-speed photography gives useful insights into understanding the detailed combustion processes. The experimental results show that depending on the extent of diesel spray-wall impingement, supersonic detonation-like reaction fronts featuring bright luminosity can be observed. The propagation of these reaction fronts in-cylinder results in severe pressure oscillations with an amplitude approaching hundreds of atmospheres, which is like the super-knock events in boosted direct-injection spark-ignition engines. Further parametric analysis indicates that the interplay between the diffusion combustion controlled by diesel spray and the premixed combustion dominated by attached film evaporation results in the formation of abnormal combustion. Destructive reaction fronts tend to occur at a prolonged ignition delay time, which facilitates the mixing between diesel evaporation and hot air