Low-temperature flames such as cool flames, warm flames, double flames, and auto-ignition assisted flames play a critical role in the performance of advanced engines and fuel design. In this paper, ...an overview of the recent progresses in understanding low-temperature flames and dynamics as well as their impacts on combustion, advanced engines, and fuel development will be presented. Specifically, at first, a brief review of the history of cool flames is made. Then, the recent experimental studies and computational modeling of the flame structures, dynamics, and burning limits of non-premixed and premixed cool flames, warm flames, and double flames are presented. The flammability limit diagram and the temperature-dependent chain-branching reaction pathways, respectively, for hot, warm, and cool flames at elevated temperature and pressure will be discussed and analyzed. After that, the effect of low temperature auto-ignition of auto-igniting mixtures at high ignition Damköhler numbers at engine conditions on the propagation of cool flames, warm flames, and double flames as well as turbulent flames will be discussed. Finally, a new platform using low temperature flames for the development and validation of chemical kinetic models of alternative fuels will be presented. Discussions of future research of the dynamics and control of low temperature flames under engine conditions will be made.
Unraveling the low-temperature chemistry of ammonia is still an open challenge in combustion kinetics, yet of primary importance because of the novel combustion concepts operating in these ...conditions, as well as of the rising interest on ammonia as an energy carrier. In this work, a fundamental investigation of the H-abstraction reactions from H2NO by O2, NO2, NH2, and HO2 was performed. These reactions, which belong to the radical-radical abstraction class, associate a high sensitivity to the key low temperature ammonia combustion parameters, to a high uncertainty in rate constant values. Theoretically, the investigation of reactions belonging to this class is complicated by their intrinsic multireference nature. To address this issue, a structured theoretical methodology that relies heavily on the use of CASPT2 calculations was devised. The predicted rate constants highlighted significant deviations from the rates commonly adopted in the state-of-the-art mechanisms, most often based on analogies and estimations. In order to understand their impact on ammonia low-temperature kinetics, the obtained rates were integrated into a kinetic model, which was used to investigate ammonia oxidation and ignition at low-temperature and oxygen-rich conditions. It was found that O2 and NH2 play the major role, as abstractors, in regulating ammonia oxidation and ignition. In particular, ignition delay time predictions proved extremely sensitive to the adopted rates: modifying each of them within their theoretical uncertainty caused deviations by even an order of magnitude, and totally changed the predictive features of the mechanism. The kinetic analysis highlighted then the need of a targeted optimization of the critical rates, downstream of the present work and within their uncertainty boundaries, to further refine the mechanism capability over a wide range of operating conditions.
Display omitted
•Need for HCCI engine and classification of homogeneous charge preparation techniques are reviewed.•Engine output characteristics of biofuels powered by HCCI engine are critically ...reviewed.•The impacts of varied operating parameters on HCCI engine characteristics are discussed.•Predicaments and probable solutions in paradigm shift from conventional to HCCI engine are explored.•Future perspectives of biofuels powered HCCI engine are proposed.
Depletion of fossil fuels and its enormous exhaust emission with conventional engines are the major concerns in the current auto industry. Therefore, there is a need to curtail the exhaust emission with lower specific fuel consumption, which gives a great accomplishment in improvisation of IC engines. After-treatment technologies of an IC engine bring in better control over the exhaust emissions but it obtains poor performance by back pressure. Moreover, integration of various after-treatment techniques bring out additional operating condition control and maintenance cost. However, in-cylinder emission control strategies like advanced combustion mode engines owned a greater significance in acquiring exceptional performance and emission characteristics. Among the advanced combustion mode engines, homogeneous charge compression ignition (HCCI) engine has the capability to meet future emission norms with enhanced performance. Moreover, it has great ability to run with distinct physicochemical characteristics biofuel by its homogeneous charge preparation techniques. Therefore, improvisation of biofuel powered HCCI engine secures a great potentiality in current auto industry. Accordingly, efforts are required to enhance the characteristic features by optimizing the engine parameters with respect to various biofuels like biodiesel, alcohol, alkane, and ether. The current review article deliberates the insight view on biofuel powered HCCI engine with its homogeneous charge preparation techniques, which critically reviewed their characteristic features. Moreover, an interpretive investigation on HCCI engine with other advanced combustion mode engines disclosed their greater significance and potential outcomes. Besides, the predicaments in the paradigm shift of conventional to HCCI engine is discussed with probable solutions for future enhancements.
Universal concerns about degradation in ambient environment, stringent emission legislations, depletion of petroleum reserves, security of fuel supply and global warming have motivated research and ...development of engines operating on alternative combustion concepts, which also have capability of using renewable as well as conventional fuels. Low temperature combustion (LTC) is an advanced combustion concept for internal combustion (IC) engines, which has attracted global attention in recent years. LTC concept is different from the conventional spark ignition (SI) combustion as well as compression ignition (CI) diffusion combustion concepts. LTC technology offers prominent benefits in terms of simultaneous reduction of both oxides of nitrogen (NOx) and particulate matter (PM), in addition to reduction in specific fuel consumption (SFC). However, controlling ignition timing and combustion rate are primary challenges to be tackled before LTC technology can be implemented in automotive engines commercially. This review covers fundamental aspects of development of LTC engines and its evolution, historical background and origin of LTC concept, encompassing LTC principle, its advantages, challenges and prospects. Detailed insights into preparation of homogeneous charge by external and internal measures for mineral diesel and gasoline like fuels are covered. Fuel requirements and fuel induction system design aspect for LTC engines are also discussed. Combustion characteristics of LTC engines including combustion chemistry, heat release rate (HRR), combustion duration, knock characteristics, high load limit, fuel conversion efficiencies and combustion instability are summarized. Emission characteristics are reviewed along with insights into PM and NOx emissions from LTC engines. Finally, different strategies for controlling combustion rate and combustion timings for gasoline and mineral diesel like fuels are discussed, showing the way forward for this technology in future towards its commercialization.
Rate coefficients for the reactions of OH with n, s, and iso‐butanol have been measured over the temperature range 298 to ∼650 K. The rate coefficients display significant curvature over this ...temperature range and bridge the gap between previous low‐temperature measurements with a negative temperature dependence and higher temperature shock tube measurements that have a positive temperature dependence. In combination with literature data, the following parameterizations are recommended:
k1,OH + n‐butanol(T) = (3.8 ± 10.4) × 10−19T2.48 ± 0.37exp ((840 ± 161)/T) cm3 molecule−1 s−1
k2,OH + s‐butanol(T) = (3.5 ± 3.0) × 10−20T2.76 ± 0.12exp ((1085 ± 55)/T) cm3 molecule−1 s−1
k3,OH + i‐butanol(T) = (5.1 ± 5.3) × 10−20T2.72 ± 0.14exp ((1059 ± 66)/T) cm3 molecule−1 s−1
k4,OH + t‐butanol(T) = (8.8 ± 10.4) × 10−22T3.24 ± 0.15exp ((711 ± 83)/T) cm3 molecule−1 s−1
Comparison of the current data with the higher shock tube measurements suggests that at temperatures of ∼1000 K, the OH yields, primarily from decomposition of β‐hydroxyperoxy radicals, are ∼0.3 (n‐butanol), ∼0.3 (s‐butanol) and ∼0.2 (iso‐butanol) with β‐hydroxyperoxy decompositions generating OH, and a butene as the main products. The data suggest that decomposition of β‐hydroxyperoxy radicals predominantly occurs via OH elimination.
Rapid compression machines (RCMs) are widely used to acquire experimental insights into fuel autoignition and pollutant formation chemistry, especially at conditions relevant to current and future ...combustion technologies. RCM studies emphasize important experimental regimes, characterized by low- to intermediate-temperatures (600–1200K) and moderate to high pressures (5–80bar). At these conditions, which are directly relevant to modern combustion schemes including low temperature combustion (LTC) for internal combustion engines and dry low emissions (DLE) for gas turbine engines, combustion chemistry exhibits complex and experimentally challenging behaviors such as the chemistry attributed to cool flame behavior and the negative temperature coefficient regime. Challenges for studying this regime include that experimental observations can be more sensitive to coupled physical-chemical processes leading to phenomena such as mixed deflagrative/autoignitive combustion. Experimental strategies which leverage the strengths of RCMs have been developed in recent years to make RCMs particularly well suited for elucidating LTC and DLE chemistry, as well as convolved physical-chemical processes.
Specifically, this work presents a review of experimental and computational efforts applying RCMs to study autoignition phenomena, and the insights gained through these efforts. A brief history of RCM development is presented towards the steady improvement in design, characterization, instrumentation and data analysis. Novel experimental approaches and measurement techniques, coordinated with computational methods are described which have expanded the utility of RCMs beyond empirical studies of explosion limits to increasingly detailed understanding of autoignition chemistry and the role of physical-chemical interactions. Fundamental insight into the autoignition chemistry of specific fuels is described, demonstrating the extent of knowledge of low-temperature chemistry derived from RCM studies, from simple hydrocarbons to multi-component blends and full-boiling range fuels. Emerging needs and further opportunities are suggested, including investigations of under-explored fuels and the implementation of increasingly higher fidelity diagnostics.
•Iso-butanol and n-pentanol were used to enable a partially premixed LTC in a diesel engine.•Combustion-phasing and charge-dilution were controlled by EGR and injection timing.•Test fuels presented ...enhanced premixed combustion with high peak pressures and HRR.•Simultaneous reduction of NOx/PM was realized with test fuels under moderate EGR & late injection.•Iso-butanol blends offered better EGR tolerance than n-pentanol blends.
This study attempts to achieve simultaneous reduction of smoke and NOx emissions using a combination of low EGR, retarded injection timing and diesel fuel reformulation (with low cetane number alcohols) to enable a partially premixed low temperature combustion (LTC) mode in DI diesel engine. Two higher alcohol/diesel blends, B40 (40% iso-butanol–60% diesel) and P40 (40% n-pentanol–60% diesel) blends were prepared and tested under the combination of three EGR rates (10%, 20% and 30%) and two injection timings (23° and 21° CA bTDC) at high loads and constant engine speed. The performance and emission characteristics of the engine under these conditions are investigated. Results indicate that B40 gives a longer ignition delay, higher peak pressure and higher premixed heat release rate than P40. B40 has superior EGR tolerance and better influence on NOx-smoke trade-off when compared to P40. At retarded injection timing (21° CA bTDC) and 30% EGR, B40 presented simultaneous reduction of NOx (↓ 41.7%) and smoke (↓ 90.8%) emissions with diesel-like performance while P40 presented simultaneous reduction of NOx (↓ 39.3%) and smoke (↓ 15%) emissions with a small drop in performance. It was found that B40 presented better smoke suppression characteristics than P40. Smoke emissions of both blends increased drastically beyond 30% EGR. HC emissions increased and CO emissions remained low for both blends at all EGR rates. The combination of low EGR, late injection and higher alcohol/diesel blends can achieve partially premixed LTC and reduce smoke and NOx emissions simultaneously.
•A new method to identify and quantify the multi-stage heat release of RCCI engines was proposed;•Coupling effect of split diesel injection parameters of on combustion and performance was ...examined;•Impact significance of the diesel injection parameters on heat release at each stage was analyzed;•Correlations between heat release at each stage and emissions were revealed;
Reactivity controlled compression ignition (RCCI) combustion is a advanced low temperature combustion concept to reduce CO2 and brake the trade-off between NOx and PM emissions for next generation compression ignition (CI) engines. However, it leads to the more complex multi-stage heat release in diesel/natural gas (NG) dual fuel engines and is criticized for extra-high methane and carbon monoxide emissions. Using the existing methods, it is difficult to effectively quantify the contribution of the accumulated heat release (AHR) at each stage to the whole combustion process and to create their relevance to the performance and emissions in the RCCI engines.
In this paper, experiments were conducted on a ship diesel/NG dual-fuel engine at a load of 25 %. A new identification and quantitative analysis method of multi-stage heat release is proposed, and the coupling effects of the diesel split injection parameters on the multi-stage heat release and performance in the RCCI engine were revealed. The results show that the new method is not only available for analyzing the combustion of traditional CI engines but also is more effective for identifying the multi-stage combustion in RCCI engines. For the increased pre-injection ratio (PR), as the start of first injection (SOI-I) and the start of second injection (SOI-II) advance, the AHR in Stage-I (Q-Stage-I) decreases, and the increased Q-Stage-III lead to the improved indicated thermal efficiency (ITE > 49 %). Meanwhile, methane (CH4), non-methane hydrocarbon (NMHC) and carbon monoxide (CO) emissions can be reduced by more than 50 %, while nitrogen oxides (NOx) emissions increase. The impact significance of SOI-II on Q-Stage-I and Q-Stage-III is higher, their values are + 65.88 % and −51.04 %, respectively, and the Q-Stage-II is mainly controlled by SOI-I. The impact significance of Q-Stage-III on ITE is the highest about + 69.43 %. Q-Stage-I shows a higher positive correlation with CH4, NMHC and CO emissions although the proportion of Q-Stage-I is far smaller than that of Q-Stage-II and Q-Stage-III. Therefore, the simultaneous reduction of incomplete combustion products and NOx emission can be achieved by reasonably adjusting split injection parameters.
•An experimental study of iso-octane/prenol blends is performed inside a RCM.•Validated kinetic models of the oxidation of the neat fuels are merged.•IDTs reflect the strong inhibiting behavior of ...prenol on low-temperature reactivity.•This effect is suggested to originate from combined gas-phase and catalytic effects.
A detailed experimental and kinetic modeling study was dedicated to understand the reported octane hyperboosting effect of prenol, by means of the measurement of the ignition delay times of its blends with iso-octane, and measurement of the mole fraction profiles of the fuels and intermediates inside the ULille rapid compression machine. These results show that prenol addition leads to a reduction of the first-stage ignition phenomena and negative temperature coefficient behavior, which is only qualitatively captured by the model and is consistent with knock resistance improvement. It is suggested that this behavior is caused by two different factors. The first originates from gas-phase reactivity of prenol, and spans from the formation of unreactive unsaturated species through resonance-stabilized radicals, thereby constituting a competitive pathway for the radical pool generated by iso-octane. The second is of catalytic nature and cannot be captured by means of gas-phase kinetic modeling, but could also play an important role in the behavior of prenol in internal combustion engines.
PDF
