Alcohol combustion chemistry

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<div class="section abstract"><div class="htmlview paragraph">Pre-chamber combustion is well known for the effective way to improve thermal efficiency in internal combustion engine. An active pre-chamber can accomplish super lean burn while a passive pre-chamber can easily improve combustion with low-cost. Therefore, various studies have been carried out. However, since its combustion characteristics are very complicated, the sequence of events for torch ignition and flame propagation in main-chamber from ignition and flame propagation inside pre-chamber have not been well clarified. Especially, investigation on the process from torch ejection to ignition of mixture in main-chamber has been carried out using combustion vessels and rapid compression machines, but this phenomenon has not been clarified.</div><div class="htmlview paragraph">In this study, three types of optically accessible passive pre-chamber with different orifice patterns (normal six orifices, asymmetric five orifices and tangential five orifices) were designed and installed to a single-cylinder gasoline spark ignition engine. Also, an optical-scope was designed uniquely and installed to main-chamber. The flame propagation inside pre- and main-chambers was taken simultaneously by using two high speed cameras. In addition, the temperature distribution around the torch ejection in main-chamber was taken by high speed IR (Infrared) camera. As a result, cyclic variation in flame propagation direction inside pre-chamber and variation in timing of torch ejection in main-chamber were observed. In the cycles when torch ejected to the intake side first, the flame propagation was shifted to the same side inside pre-chamber. This variation of flame propagation direction changed by the orifice pattern of pre-chamber, the flame propagation direction of pre-chamber with tangential orifices were the most stable inside both pre-chamber and main-chamber. Visualizing the temperature distribution during torch ejection showed that the mixture in main-chamber was ignited locally around the torch.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Alcohol fuels, produced from renewable energy sources, are considered a crucial solution for achieving life-cycle carbon neutrality in internal combustion engines. The Boosted Uniflow Scavenged Direct-Injection Combustion Engine (BUSDICE) exhibits significant potential for high thermal efficiency with an aggressive downsizing design. In this study, a computational investigation was carried out to assess the spray mixing and combustion characteristics of BUSDICE fuelled with methanol and ethanol, compared with gasoline, under a high-load condition. The injection duration of methanol and ethanol is significantly longer than that of iso-octane, leading to incomplete evaporation. The mixture exhibits an “outer-rich, central-lean” stratification pattern due to the short mixing time and swirl flow transportation for all three fuels. However, the prolonged injection of methanol induces stronger turbulence, which can enhance the local mixing. The spatial mixture stratification, particularly near the spark-local area, has a strong influence on the initial kernel development and flame propagation. Consequently, methanol exhibits a shorter ignition delay than ethanol under the same spark timing, leading to faster flame propagation attributed to a richer equivalence ratio around the spark plug. Nevertheless, the ignition and combustion performance of ethanol can be improved by advancing the spark timing. The spark timing study reveals that alcohol fuels can operate under high load without knocking, whereas iso-octane requires retarded ignition timing to prevent knocking. As a result, methanol and ethanol provide a better IMEP and ITE than iso-octane under high-load conditions. From an emissions perspective, due to their low carbon-to-hydrogen (C/H) ratio and high oxygen content, unburnt hydrocarbon emissions decrease significantly when using alcohol fuels, especially methanol, for which these emissions are almost zero. However, the soot of ethanol shows a slight increase than iso-octane, due to the highly stratified mixture and incomplete combustion. Additionally, the NOx of ethanol and methanol increases due to the higher combustion temperatures than iso-octane. Overall, the results highlight the strong potential of alcohol-fuelled BUSDICE engines as compact and sustainable solutions for small-displacement powertrains, offering high thermal efficiency and substantially reduced pollutant emissions.</div></div>

Autoignition of cyclohexane at low-to-intermediate temperatures: Rapid compression machine experiments and improved comprehensive chemical kinetic model

The environmental and air pollution brought about by the increasing energy consumption has increased the interest in the use of renewable energy sources in the transportation sector, where internal combustion engines are used, which is responsible for a large part of the exhaust emissions. Alcohol fuels have been evaluated as renewable energy sources for utilising in internal combustion engines and they are reported to have lowering exhaust emissions and costs. This study analyses four monohydric aliphatic alcohol fuels (methanol, ethanol, propanol and butanol) by reviewing the available literature to represent their applicability as an alternative fuel in internal combustion engines. In the study, researches on the directly use of acetone–butanol–ethanol and isopropanol–butanol–ethanol as alcohol fuel were also examined because the production of butanol by acetone–butanol–ethanol and isopropanol–butanol–ethanol distillation is costly, and isopropanol–butanol–ethanol is more preferred due to the corrosive feature of acetone. The higher fuel consumption of alcohol fuels than fossil fuels was the most common result, with reductions in NOx and smoke emissions except for isopropanol–butanol–ethanol, which had higher NOx emissions. It has been reported that less carbon and high oxygen content, low cetane number and high latent heat of alcohol fuels are responsible for the above results. The increase in thermal efficiency with the use of acetone–butanol–ethanol and isopropanol–butanol–ethanol in contrast to other alcohol fuels was a notable result. A comparison among the alcohol fuels showed that methanol was more effective than ethanol in reducing CO, unburned HC and smoke emissions while isopropanol–butanol–ethanol demonstrated high NOx emissions. A simultaneous reduction of NOx and Smoke emissions, which was commonly reported for most of alcohol fuels, makes a significant contribution to the development of internal combustion engines. This study gains importance in terms of comparing the individual effects of the use of alcohol fuels on exhaust emissions and better understanding the current status of these fuels.

Flame speed scaling in autoignition-assisted freely propagating n-heptane/air flames

<div class="section abstract"><div class="htmlview paragraph">Substantial effort has been devoted to utilizing homogeneous charge compression ignition (HCCI) to improve thermal efficiency and reduce emission pollutants in internal combustion engines. However, the uncertainty of ignition timing and limited operational range restrict further adoption for the industry. Using the spark-assisted compression ignition (SACI) technique has the advantage of using a spark event to control the combustion process. This study employs a rapid compression machine to characterize the ignition and combustion process of Dimethyl ether (DME) under engine-like background temperature and pressures and combustion regimes, including HCCI, SACI, and knocking onsite. The spark ignition timing was swept to ignite the mixture under various thermodynamic conditions. This investigation demonstrates the presence of four distinct combustion regimes, including detonation, strong end-gas autoignition, mild end-gas autoignition, and HCCI. The observation indicates that HCCI exhibits a relatively low-pressure rise rate and a prolonged combustion duration.</div><div class="htmlview paragraph">On the other hand, the detonation case can achieve a fast flame propagation velocity of up to 2.4 km/s, generating high-frequency pressure oscillation. Pressure traces were processed using the Fast Fourier Transform (FFT) method to characterize the different end gas autoignition regimes under various spark timing. Moreover, hydrogen fuel blends with DME to reduce the auto-ignition tendency of DME fuel but increase the flame propagation speed. The combustion characteristics of the autoignition-initiated flames are compared with that of using neat DME fuel via pressure measurement and high-speed images. The results demonstrated that deploying hydrogen into the fuel exhibits enhanced knock resistance and reductions in pressure oscillations.</div></div>

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

This interdisciplinary research program at Michigan State University, in collaboration with Ford Motor Company, has explored the application of tailored or designed biofuels for enhanced vehicle performance and reduced emissions. The project has included a broad range of experimental research, from chemical and biological formation of advanced biofuel components to multicylinder engine testing of blended biofuels to determine engine performance parameters. In addition, the project included computation modeling of biofuel physical and combustion properties, and simulation of advanced combustion modes in model engines and in single cylinder engines. Formation of advanced biofuel components included the fermentation of five-carbon and six-carbon sugars to n-butanol and to butyric acid, two four-carbon building blocks. Chemical transformations include the esterification of the butyric acid produced to make butyrate esters, and the esterification of succinic acid with n-butanol to make dibutyl succinate (DBS) as attractive biofuel components. The conversion of standard biodiesel, made from canola or soy oil, from the methyl ester to the butyl ester (which has better fuel properties), and the ozonolysis of biodiesel and the raw oil to produce nonanoate fuel components were also examined in detail. Physical and combustion properties of these advanced biofuel components were determined during the project. Physical properties such as vapor pressure, heat of evaporation, density, and surface tension, and low temperature properties of cloud point and cold filter plugging point were examined for pure components and for blends of components with biodiesel and standard petroleum diesel. Combustion properties, particularly emission delay that is the key parameter in compression ignition engines, was measured in the MSU Rapid Compression Machine (RCM), an apparatus that was designed and constructed during the project simulating the compression stroke of an internal combustion engine under highly instrumented conditions. Simulation of and experimentation on combustion in single and multicylinder engines was carried out in detail throughout the project. The combustion behavior of biofuel blends neat and in petroleum were characterized in the MSU optical engine, in part to validate results obtained in the RCM and to provide data for comparison with simulations. Simulation of in- cylinder, low-temperature combustion included development of an extensive fuel injection model that included fuel spray breakup, evaporation, and ignition, along with prediction of cylinder temperature, pressure, and work produced. Single cylinder and multicylinder engine tests under advanced low-temperature combustion conditions conducted at Ford Motor Company validated experimental and simulation results obtained in the MSU engine and in MSU simulations. Single cylinder engine tests of an advanced biofuel containing biodiesel and dibutyl succinate, carried out under low-temperature combustion conditions, showed similar power generation and gas-phase emissions (CO, HC, NOx), but a reduction in particulates of as much as 60% relative to neat biodiesel and 95% relative to petroleum diesel at the same operating conditions. This remarkable finding suggests that biofuels may be able to play a role in eliminating the need for particulate removal systems in diesel vehicles. The multicylinder engine tests at Ford, carried out using butyl nonanoate as an advanced biofuel, also gave promising results, showing a strong decline in particulate emissions and simultaneously a modest decrease in NOx emissions relative to standard petroleum diesel at the same conditions. In summary, this project has shown that advanced biofuels and their blends are capable of maintaining performance while reducing emissions, particularly particulates (soot), in 3 compression ignition engines. The interdisciplinary nature of biofuel production and testing has identified fuel properties that are capable of producing such performance, thus providing direction for the implementation of renewable fuels for U.S. transportation. The testing and simulation studies have deepened our understanding of combustion 1) by advancing the rigor with which simulations can be carried out and 2) by illustrating that differences in biofuel and petroleum fuel properties can be used to predict differences in combustion behavior in engines. The future viability of biofuels for compression ignition (diesel) engines is now subject to economic (cost) uncertainty more so than to technical barriers, as the advanced biofuel blends developed here can improve cold-weather fuel properties, provide similar engine performance, and reduce emissions.

In recent years, global warming caused by the greenhouse effect has become one of the greatest threats to mankind. This will have a serious impact on the environment and human body, such as land desertification, increase in ocean acidity, sea level rise, and increase in pests and diseases; affect people’s normal work and rest; and make people feel dizzy and nauseated. Excessive emissions of carbon dioxide (CO2), the main component of the greenhouse gas, have contributed to the continued rise in Earth’s temperature. Although the world is vigorously developing clean energy to reduce carbon emissions, it will not replace fossil fuels in the short term. The conversion of biomass into energy is the most important way of energy utilization. Biomass energy refers to the solar energy stored in biomass in the form of chemical energy and is the fourth major energy source after oil, coal, and natural gas. At present, biofuels have gone through three developmental stages, which can be divided into first-generation biofuels, second-generation biofuels, and third-generation biofuels according to the types of raw materials and development history. The first generation of biofuels produced from food crops, such as bioethanol derived from sucrose and starch, has already entered the energy market. However, because the first generation of biofuels uses food crops as raw materials, there is a phenomenon of “competing with people for food,” and it is difficult to achieve large-scale application. To avoid the problem of food shortages, second-generation biofuels produced from nonfood crops, such as wood fiber, have been developed. Microalgae biomass energy is favored by governments and scholars all over the world because of its unique advantages of fast reproduction speed and high oil content. The cultivation of microalgae does not occupy traditional farmland, and the marginal land such as mountains, oceans, and deserts can cultivate microalgae, or develop microalgae cultivation in the air through the innovation of microalgae photosynthetic reactors. When municipal wastewater, food industry wastewater, and aquaculture wastewater are used as the medium for large-scale cultivation of microalgae, and waste gas from biogas power generation, flue gas from coal power plants, and industrial waste gas from fermentation are used as the CO2 gas source for large-scale cultivation of microalgae, it can be further reduced. The comprehensive production cost of microalgae bioenergy plays a significant emission reduction effect. Combining the above advantages, the use of microalgae to produce first- or second-generation bioenergy has become a new research direction. This study focuses on the review of microalgal biomass in fuel, nonfuel, wastewater treatment, and fuel cell.

Proper orthogonal decomposition has been utilized for well over a decade to study turbulence and cyclic variation of flow and combustion properties in internal combustion engines. In addition, proper orthogonal decomposition is useful to quantitatively compare multi-cycle in-cylinder measurements with numerical simulations (large-eddy simulations). However, the application can be daunting, and physical interpretation of proper orthogonal decomposition can be ambiguous. In this paper, the mathematical procedure of proper orthogonal decomposition is described conceptually, and a compact MATLAB® code is provided. However, the major purpose is to empirically illustrate the properties of the proper orthogonal decomposition analysis and to propose practical procedures for application to internal combustion engine flows. Two measured velocity data sets from a motored internal combustion engine are employed, one a highly directed flow (each cycle resembles the ensemble average), and the other an undirected flow (no cycle resembles the average). These data are used to illustrate the degree to which proper orthogonal decomposition can quantitatively distinguish between internal combustion engine flows with these two extreme flow properties. In each flow, proper orthogonal decomposition mode 1 is an excellent estimate of ensemble average, and this study illustrates how it is thus possible to unambiguously quantify the cyclic variability of Reynolds-averaged Navier–Stokes ensemble average and turbulence. In addition, this study demonstrates the benefits of comparing two different samples of cycles using a common proper orthogonal decomposition mode set derived by combining the two samples, the effect of spatial resolution, and a method to evaluate the number of snapshots required to achieve convergence.

A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines

<div class="section abstract"><div class="htmlview paragraph">Advanced combustion engines, as power sources, dominate all aspects of the transportation sector. Stringent emission and fuel efficiency standards have promoted the research interest in advanced combustion strategies and alternative fuels. Owing to the comparable energy density to the existing fossil fuels and renewable production, alcohol and ether fuels may be a suitable replacement, or an additive to the gasoline/diesel fuels to meet the future emission standards with minimal modification to current engine geometry. Furthermore, lean and diluted combustion are well-researched pathways for efficiency improvement and reduction of engine-out emissions of modern engines. However, lean-burn or EGR dilution can introduce combustion inefficiencies in the form of excessive hydrocarbon, carbonyl species and carbon monoxide emissions. In this study, the total energy loss to the exhaust in the form of emission species due to incomplete/inefficient combustion of alcohol (butanol and ethanol) and ether (dimethyl ether) fuels in SI engine has been investigated. The impact of both dilution strategies, fresh air (lean) and EGR dilution on the exhaust gas components for different alternative fuels has been studied. A detailed analysis of the combustion products has been conducted to determine the contribution of different exhaust components to the fuel energy loss to the exhaust species. Additionally, the results have also been compared to gasoline combustion as a baseline. Preliminary test results indicate that as the charge is diluted (either lean or EGR dilution), the contribution of longer chain hydrocarbon species to the exhaust energy increases. DME, primarily following HCCI combustion, exhibits distinct emission speciation.</div></div>

The influences of pressure and temperature on laminar flame propagations of n-butanol, iso-octane and their blends

Evaluation of the combustion process of directly injected methane in a rapid compression machine with a laser-based ignition system and an electrical ignition system