Advanced Low Temperature Combustion Research Articles

Effect of split injection strategy of diesel fuel on multi-stage heat release and performance of a RCCI engine fueled with diesel and natural gas

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.

Advanced hybrid combustion systems as a part of efforts to achieve carbon neutrality of the vehicles

Global goals, including those of the EU, are focussed on transition to the green, circular and low-carbon economy. The intention within the framework of EU is to achieve the zero level of CO2 emissions for the new cars till the year 2035. An important part of this plan is the agreement between Germany and the European Union that the new vehicles equipped with the internal combustion engines can continue to be sold only in such a case if they will use solely the climate-neutral fuel. Therefore, the future will probably belong to application of the advanced low-temperature technologies in combination with the climate-sustainable fuels (e.g. synthetic fuels, hydrogen). The presented scientific-research work introduces two new low-temperature combustion systems, which were developed and patented at the national level. At the same time, these new combustion systems are tested in cooperation with a reputable automotive producer. It is necessary to emphasize such a positive fact that the obtained results prove relevance of the innovative combustion systems as well as their possible future applicability in the real vehicles as a part of the advanced hybrid drive system.Graphical abstractHighlightsNowadays, the significant efforts are focussed on reduction of gaseous emissions generated by the transport, what is resulting in development of electromobility.CO2 neutrality in the transport sector cannot be reached by the electromobility alone. A potential solution is a combination of the sustainable fuels and advanced low-temperature combustion technologies.These two systems, which are presented in the article, were the subject of experimental research and development.DiscussionWithin the EU, there are strong efforts for a total restriction of internal combustion engines. The German Association of Automotive Industry (VDA) declares that “CO2 neutrality in the transport sector cannot be solved by the electromobility alone”. The official statement of VDA continues: “Even if, in ideal case, we had 15 million electric cars on the roads in 2030, most of the vehicles will still be equipped with combustion engines. Nowadays, 280 million cars are using combustion engines in everyday operation in the EU and there are 1.5 billion of them worldwide. Climate-neutral transport is impossible without new structure of global vehicle fleet and its decarbonisation”. According to VDA, the only solution, how to reduce emission footprint, using the current vehicle fleet, is application of the synthetic fuels. Therefore, still more resources should be invested into research and development of new sustainable fuels.

Ultra-low emission power generation utilizing chemically stabilized waste plastics pyrolysis oil in RCCI combustion concept

Emission standards in European Union, designed to reduce the environmental impact of power generation, present a significant challenge for fast-response distributed power generation systems based on internal combustion engines. Regulated emissions, such as NOx and particulate matter present a major concern due to their adverse number of environmental and health effects. Simultaneously, European Union strives towards sustainable management of plastic waste and seeks the ways for its upcycling and production of new fuels and chemicals. As an answer to the presented challenges, the present experimental study addresses the potential for use of chemically stabilized Waste Plastics Oil (WPO), a product of pyrolysis process of waste plastics in a Reactivity Controlled Compression Ignition (RCCI) combustion concept. To establish a reactivity-controlled combustion, the study uses a combination of methane (a model fuel for biomethane) and WPO to a) simultaneously reduce NOx and particulate matter emissions due to low local combustion temperatures and a high degree of charge homogenization and b) address waste and carbon footprint reduction challenges. Through experiments, influence of direct injection timing and energy shares of utilized fuels to in-cylinder thermodynamic parameters and engine emission response were evaluated in engine operating points at constant indicated mean effective pressure. Acquired results were deeply investigated and benchmarked against compression ignition (CI) and RCCI operation with conventional diesel fuel to determine potential for WPO utilization in an advanced low-temperature combustion concept. Results show that chemically stabilized WPO can be efficiently utilized in RCCI combustion concept without adaptation of injection parameters and that with suitable control parameters, ultra-low emissions of NOx and PM can be achieved with utilized fuels. For diesel/methane mix, NOx and PM emissions were reduced compared to conventional CI operation for 82.0% and 93.2%, respectively, whereas for WPO/methane mix, NOx and PM emissions were reduced for 88.7% and 97.6%, respectively, which can be ascribed to favourable chemical characteristics of WPO for the utilized combustion concept. In the least favourable operating point among those studied, indicated mean effective pressure covariance was kept below 2.5%, which is well below 5% being considered the limit for stable engine operation.

Development and evaluation of a skeletal mechanism for EHN additized gasoline mixtures in large Eddy simulations of HCCI combustion

Advanced Low Temperature Combustion modes, such as the Sandia proposed Additive-Mixing Fuel Injection (AMFI), can unlock significant potential to boost fuel conversion efficiency and ultimately improve the energy conversion of internal combustion engines. This is a novel improved combustion process that is enabled by supplying small (<5%) variable amounts of autoignition improver to the fuel to enhance the engine operation and control. Common, diesel-fuel ignition-quality enhancing additive, 2-ethylexyl nitrate (EHN), is doped into gasoline to enable Sandia LTGC + AMFI combustion. This manuscript focuses on the development of a reduced sub-mechanism for EHN chemical kinetics at engine relevant conditions that is implemented into a skeletal mechanism for chemical kinetic studies of gasoline surrogate fuels. The mechanism validation utilized zero-dimensional numerical simulations and comparison to shock tube ignition-delay data of pure and EHN-doped n-heptane. Additional validation is presented with Homogeneous Charge Compression-Ignition (HCCI) engine data of pure and EHN-doped research-grade E10 gasoline. Then, the mechanism was deployed in a 3-D computational fluid dynamics (CFD) using Large Eddy Simulations (LES) to model the HCCI engine experiments of 0.4% vol EHN additized E10 gasoline at several equivalence ratios. Simulations showed a very good performance of the mechanism, and the model accurately reproduced (a) the ignition point, (b) combustion phasing, (c) combustion duration, and (d) the peak of the heat release rates of the engine experiments. The results show that EHN promotes Low-Temperature Heat Release, ultimately driving the gasoline to autoignite at thermodynamic conditions where the fuel would not otherwise ignite. Overall, this work demonstrates a viable reduced chemical-kinetic mechanism for EHN and shows that it can be combined with a skeletal gasoline mechanism for CFD-LES analysis of well-mixed LTGC that matches well with experimental results. The CFD-LES analysis also shows the spatial distribution of EHN-fuel interactions that control the autoignition throughout the combustion chamber.

Experimental and kinetic modeling study of low-temperature oxidation of n-pentane

Attractiveness in advanced low-temperature combustion engines drives a constantly updated understanding of low-temperature oxidation chemistry. In this work, the low-temperature oxidation chemistry of n-pentane in two jet-stirred reactors at atmospheric pressure and in the temperature range of 500–825 K was investigated using combined analysis methods of synchrotron vacuum ultraviolet photoionization mass spectrometry, gas chromatography, and Fourier transform infrared spectroscopy. Furthermore, the gaseous mixture from JSR was collected in acetonitrile for subsequent product characterization using flow injection analysis, high-pressure and ultra-high-pressure liquid chromatography coupled to a Thermo Scientific™ Orbitrap® Q-Exactive high-resolution mass spectrometry. Numerous intermediate species were identified by these analytical methods, which contributed to unraveling the low-temperature oxidation reaction network of n-pentane. A detailed n-pentane model was tentatively developed to reduce deviations between experimental measurements and model predictions by updating the rate constants of C5 keto-hydroperoxide decomposition, C5 hydroperoxy cyclic ether decomposition, and Korcek reactions of C5 keto-hydroperoxide, and by introducing pressure-dependent rate constants for the reaction classes of Q˙OOH + O2, Q˙OOH decompositions, concerted HȮ2-elimination of RȮ2, C5 keto-hydroperoxide decomposition, C5 hydroperoxy cyclic ether decomposition, and Korcek reactions of C5 keto-hydroperoxide, and by adding more detailed sub-mechanisms for C5 cyclic ethers and C5 keto-hydroperoxides. This updated model was validated against a set of available experimental data, including jet-stirred reactor species data and ignition delay times. These exploratory updates of the kinetic model reveal the considerable influence of the rate constants of hydroperoxide decomposition and the pressure-dependent rate constants of key reaction classes on the kinetic model predictions, highlighting the future demands for high-precision quantum chemistry calculations of the pressure-dependent rate constants of the aforementioned reaction classes to reduce mechanism uncertainties and to develop accurate and robust chemical kinetic models.

Parametric optimization with Biodiesel-water emulsion under premixed lean combustion to achieve high efficiency and clean combustion

Advanced low temperature combustion (LTC) aids in simultaneously controlling exhaust emissions of oxides of nitrogen (NOx) and particulate matter (PM). However, LTC strategies are constrained by achievable load range restriction and higher levels of unburned hydrocarbon (HC) and carbon monoxide (CO). The present investigation envisages combining renewable biodiesel-water emulsions and an advanced premixed lean combustion strategy to curtail the higher HC and CO emissions without sacrificing engine performance. Non-edible Karanja oil was the source of biodiesel and the emulsion stabilizer. A production light-duty diesel engine is modified to run under premixed lean combustion conditions based on late injection using a fully-flexible electronic fuel injection system. The engine operating parameters were optimized, including fuel injection timing, EGR flow rate, and emulsion water concentration. Emulsions of neat biodiesel were prepared to contain 6, 12, and 18% water on a mass basis using 1 and 2% surfactant by mass. The fuel injection timing was varied from 2 to 4 degrees after the top dead centre, and up to 15% EGR flow rate was utilized. Design of experiments aided in minimizing the number of engine tests. The performance and emission parameters were the output responses. Taguchi Grey Relational Analysis established the optimal combination of input factors to improve engine performance and reduce exhaust emissions. Engine experiments at optimized operating conditions revealed that compared to conventional diesel combustion, the brake thermal efficiency and brake specific fuel consumption improved by 33 and 16%, respectively, at 5.08 bar BMEP. The NOx, smoke and HC emissions were reduced by 68, 73 and 46%, while CO emissions increased nearly two-fold.

Experimental Evaluation of Pilot and Main Injection Strategies on Gasoline Compression Ignition Engine—Part 2: Performance and Emissions Characteristics

<div>Internal combustion (IC) engines play an important role in the global economy by powering various transport applications. However, it is a leading cause of urban air pollution; therefore, new combustion strategies are being developed to control emissions. One promising advanced low-temperature combustion (LTC) technology is gasoline compression ignition (GCI). This experimental study assesses the performance of a two-cylinder engine, emissions, and exhaust particulate characteristics using G80 (80% v/v gasoline and 20% v/v diesel) blend operating in GCI mode vis-à-vis baseline conventional diesel combustion (CDC) mode using diesel. The effects of double pilot injection, Pilot-1 proportion (10–30%), and main injection timing were investigated on the GCI combustion. Experiments were performed at different engine loads (3, 4, and 5 bar brake mean effective pressure [BMEP]) at a constant engine speed (2000 rpm). GCI combustion showed higher brake thermal efficiency (BTE) than CDC mode at medium loads. Hydrocarbon (HC) and carbon monoxide (CO) emissions increased in GCI mode, but oxides of nitrogen (NOx) were reduced than the baseline CDC mode. High pilot ratio and late main injection timing tests showed higher HC and CO emissions in the GCI mode at low engine loads. The GCI mode engine emitted higher nucleation mode particles and nanoparticles than baseline CDC mode at high engine loads. Using a triple injection strategy, GCI engines simultaneously reduced NOx and particulate matter (PM) emissions, especially at high loads. Controlling these emissions in baseline CDC mode engines is otherwise quite challenging.</div>

RESEARCH ON NUMERICAL SIMULATION OF PCCI ENGINE: A REVIEW

The global environment is being affected by the use of fossil fuels to provide energy for internal combustion engines (ICE) due to a large amount of CO2 emission. Specific toxic substances are NOx, and CO, soot, particulate matter that exists in the environment at high densities while having a global impact. The automotive industry of countries worldwide is constantly changing and evolving to discover new solutions that reduce fuel use while developing cleaner and more advanced combustion methods, which as a low-temperature combustion engine (LTC). This study uses computational fluid dynamics (CFD) to understand partial pre-mixed compression combustion (PPCI), with HCCI and RCCI being advanced low-temperature combustion concepts. Lowering the average combustion temperature is an advantage of PCCI; it optimizes CO and NOx emission rates while maintaining high thermal efficiency. Investigate the intake pattern, including flame length, mixing pulse, and intake air volume, to determine the importance of that factor for oxidation using the LES. Studies have been performed using RANS and LES models to stimulate mixing and combustion.

Study on cool flame radical index and oxygen concentration dependence of oxygenated fuels

The growing utilization of alternative fuels has sparked an interest in understanding the combustion characteristics of oxygenated fuels at low temperature. Cool flame extinction limits provide direct measures of the low-temperature reactivity of fuels and critical information for advanced low-temperature combustion engine design. This paper investigates the effects of fuel and oxygen concentrations on the extinction limits of diffusion cool flames for oxygenated fuels, including dimethyl ether, methyl decanoate, and 1-dodecanol in an atmospheric counterflow burner. The cool flame radical indexes of these oxygenated fuels are developed by isolating the thermal and transport effects from the chemical contribution to diffusion cool flame extinction. The results show that the ranking of low-temperature reactivities of long carbon chain oxygenated fuels, compared with n-alkane, is ether > n-alkane > alcohol > ester for a similar carbon number. Furthermore, due to the critical role of multiple oxygen addition reactions in the low-temperature chemistry, the relationship between the cool flame extinction limit and the oxygen concentration is also explored. The results show that the cool flame extinction limits of the oxygenated fuels are proportional to a nth power of the oxygen concentration, [O2]n, due to the combined effects of multiple oxygen addition reactions and the negative temperature coefficient in low-temperature chemistry. Additionally, the measured n number of 1-dodecanol is found to be larger than 2, which suggests the existence of the third oxygen addition reactions in low-temperature chemistry for large alcohols.

Cool flame wave propagation in high-pressure spray flames

Cool flame has an important impact on the performance and emission of advanced low-temperature combustion (LTC) internal combustion engines (ICEs) in which the liquid fuel is injected earlier at a lower ambient temperature (Tam) than that in the conventional diesel combustion engines. However, the cool flame characteristics of spray combustion under ICEs conditions are not fully understood, e.g., the effect of cool flame on the spray ignition and flame stabilization is not well studied. In this paper, the so-called cool flame wave propagation (CFWP) in the Engine Combustion Networks Spray A flames at three ambient temperatures Tam (800 K, 900 K and 1000 K) is analyzed using the data from large eddy simulation with an improved Eulerian based transported probability density function sub-grid combustion model. A good agreement between the LES results and experimental data is obtained for the spray liquid penetration length, vapor fuel penetration length, mean pressure rise profile, and flame liftoff length. It is shown that CFWP in the spray ignition process promotes the ignition of the fuel-rich and cold reactant mixture, leading to the most reactive mixture shifting toward fuel-richer locations due to the spatial stratification of temperature and concentration, and turbulent mixing. As a result, the high temperature ignition (HTI) can be shortened compared to the ignition in the homogeneous mixture. At Tam = 800 K, the HTI kernels are consistently formed following the CFWP propagating toward the spray head region. However, as Tam increases, the spatial correlation between HTI kernels and CFWP fronts is weakened. On the other hand, the turbulent mixing (quantified using local scalar dissipation rate) contributes more to the formation of HTI kernels at higher Tam. The present results indicate that CFWP is more profound at lower Tam in the spray ignition process. Finally, it is found that cool flame propagates mainly into pre-reacted fuel-rich mixture in an ignition wave propagation mode from the spray upstream region toward downstream region, whereas the ignition assisted flame mode is found in the spray upstream region where the combustion heat release is negligible.

Effects of Carbon Chain Length on N-Alkane Counterflow Cool Flames: A Kinetic Analysis

An in-depth understanding of the low-temperature reactivity of hydrocarbon fuels is of practical relevance to developing advanced low-temperature combustion techniques. The present study aims to study the low-temperature chemistry of several large n-alkanes with different carbon chain lengths in counterflow cool diffusion flames by kinetic analysis. The large n-alkanes that were chosen are n-heptane (NC7H16), n-decane (NC10H22) and n-dodecane (NC12H26), which are important components of practical fuels. Firstly, the thermochemical structure of a typical cool diffusion flame is understood through its comparison with that of a hot diffusion flame. The boundary conditions, including the ozone concentration, fuel concentration and flow velocity—where cool flames can be established—are identified with a detailed chemical mechanism that evaluates the low-temperature reactivity of the investigated n-alkanes. The results show that the n-alkane with a longer carbon chain length is more reactive than the smaller one, thereby indicating the order of NC12H26 > NC10H22 > NC7H16. This trend is qualitatively similar to the findings from non-flame reactors. The reaction pathway and sensitivity analysis are performed to understand the effects of carbon chain length on the low-temperature reactivity. The contribution of an n-alkane with a longer carbon chain to the dehydrogenation reaction, oxidation reaction and isomerization reaction is greater than that of a smaller n-alkane, and abundant O and OH radicals are generated to promote the fuel low-temperature oxidation process, thereby resulting in an enhanced low-temperature reactivity. The effects of ozone addition on the low-temperature reactivity of n-alkanes are also highlighted. It is found that the addition of ozone could provide a large number of active O radicals, which dehydrogenate with the fuels to generate OH radicals and then promote fuel low-temperature oxidation. The present results are expected to enrich the understanding of the low-temperature characteristics of large n-alkanes.

Experimental investigation on the effect of EGR rate variation on emissions in optimized PCCI-DI diesel engine

Combustion in diesel engines is focusing towards unexplored combustion concepts where innovative combustion ideas are being developed following several engine control factors. Advanced low temperature combustion engines for automotive applications are one of the improvements in this regard. The test was conducted on a single cylinder, four stroke and air cooled diesel engine. The scope of this study is to investigate the impact of EGR rate variation (25%, 35% and 45 % EGR rates) on emission reductions in PCCI-DI diesel combustion approach showing a reduction of NOx–soot trade-offs than the conventional diesel combustion approach. The PCCI-DI diesel combustion in this setup is optimized applying the algorithm of grey relational analysis together with Taguchi method. This investigational setup used a methanol port injection and advanced injection timing for PCCI formation. Emission results from different EGR settings in the optimized PCCI-DI combustion scheme were analyzed and compared using surface and contour plots. From the experiment higher EGR rate resulted a decrease in NOx productions but an increase in HC, CO and smoke due to the combustion mode of the experimental setup. The study indicated that excess EGR application should be avoided to have an optimum PCCI-DI combustion having lower emission results.

A systematic study of the influence of 1-pentanol as the renewable fuel blended with diesel on the reactivity controlled compression ignition engine characteristics and Trade-off study with variable fuel injection pressure

Compared to conventional CI engines, the analysis on the advanced low-temperature combustion technologies like RCCI engines produces better thermal efficiency with a simultaneous reduction in smoke and oxides of nitrogen emissions. The purpose of this study is to examine the reactivity-controlled compression ignition (RCCI) engine characteristics using diesel as the high reactivity and 1-pentanol as low reactivity fuel. To analyse the influence of operating parameters (i.e., fuel injection pressure (FIP) (500, 750, and 1000 bar), an experiment was performed on a water-cooled single-cylinder engine (modified) for brake mean effective pressure (BMEP) of 2 and 4 bar at a constant speed of 2000 rpm. During the initial suction stroke, 1-pentanol (10%, 20%, and 30% energy share) is injected into the port. Tests were conducted at varying proportions (energy share) to calculate the optimum proportion of diesel and 1-pentanol. The findings were analyzed and mapped under the same working conditions as diesel fuel, and a 2.5% gain (max) in thermal efficiency is observed while utilizing a 1-pentanol blend in contrast to diesel. Smoke opacity (7.82%) and oxides of nitrogen (NOx) (3.42%) emissions were also lowered while employing the 1-pentanol/diesel blend. The ideal combination for minimizing emissions is the D70A30 blend at BMEP of 4 bar with a FIP of 1000 bar. According to this study, standard fossil fuel can be efficiently substituted with a 1-pentanol/diesel fuel combination in an RCCI engine.

Influence of 1-pentanol as the renewable fuel blended with jatropha oil on the reactivity controlled compression ignition engine characteristics and trade-off study with variable fuel injection pressure

Compared to conventional compression ignition (CI) engines, the analysis on the advanced low-temperature combustion technologies like reactivity-controlled compression ignition (RCCI) engines produces better thermal efficiency with a simultaneous reduction in smoke and oxides of nitrogen (NOx) emissions. Instead of petroleum-based fuels, biofuels obtained from jatropha oil are anticipated to be employed in the current research. The purpose of this study is to examine the RCCI engine characteristics using Jatropha oil as the high reactivity and 1-pentanol as low reactivity fuel. To analyze the influence of operating parameters (i.e., fuel injection pressure (FIP) (400, 500, and 600 bar), an experiment was performed on a water-cooled single-cylinder engine (modified) for medium load and full load operations at a constant speed of 2000 rpm. The biodiesel (B20) obtained from jatropha oil is inducted into the chamber directly which is produced by transesterification. During the initial suction stroke, 1-pentanol (10%, 20%, and 30% energy share) is injected into the port. Tests were conducted at varying proportions to calculate the optimum proportion of biodiesel and 1-pentanol. The findings were analyzed and mapped under the same working conditions as diesel fuel, and a 2.5% gain (maximum) in thermal efficiency is observed while utilizing a 1-pentanol-biodiesel blend in contrast to diesel. Smoke opacity (7.82%) and NOx (3.42%) emissions were also lowered while employing the 1-pentanol-biodiesel blend. The ideal combination for minimizing emissions is the BD70P30 (70% biodiesel + 30% 1-pentanol) blend at maximum load with a FIP of 600 bar. According to this study, standard fossil fuel can be efficiently substituted with a 1-pentanol-biodiesel fuel combination in an RCCI engine.

Leveraging hydrogen as the low reactive component in the optimization of the PPCI-RCCI transition regimes in an existing diesel engine under varying injection phasing and reactivity stratification strategies

This research illustrates the novel interposed zone of partially premixed and reactivity-controlled combustion in conjunction with injection phasing and reactivity phasing strategies to understand the performance emission and stability characteristics of a single-cylinder common rail direct injection diesel engine. However, this study determined the optimum interposed zone of operation using Response Surface Methodology (RSM). The findings indicate that raising the rate of hydrogen induction enhances the stability of interposed zone of combustion. The interposed optimum regime has a 31.58% exergy efficiency and 99.1% desirability. Compared to the similar trial run, the optimization study revealed a 45.09% reduction in Soot, a 14.29% reduction in total unburnt hydrocarbon (TUHC), and a 39.83% reduction in NOx emission levels. Thus, experimental and predicted values have been compared. Hence, this variable injection and reactivity phasing under hydrogen enrichment strategies are feasible enough to achieve the goals of advanced low-temperature combustion (LTC) concepts in an existing diesel engine without making significant design modifications.

Evaluation of reactivity controlled compression ignition mode combustion engine using mineral diesel/gasoline fuel pair

Stricter emission norms, scarce availability of conventional fuel resources, and moderate engine efficiency are primary concerns for both spark ignition (SI) and compression ignition (CI) engines. The use of advanced low-temperature combustion technologies can resolve these issues, amongst which reactivity-controlled compression ignition (RCCI) mode combustion has emerged as an important one. RCCI mode combustion exhibits significantly lower exhaust emissions (oxides of nitrogen [NOx] and particulate matter [PM]) compared to CI mode combustion and delivers higher BTE compared to SI mode combustion. In this study, RCCI mode combustion has been investigated in a single-cylinder research engine using gasoline and mineral diesel as low reactivity fuel (LRF) and high reactivity fuel (HRF), respectively. Experiments have been performed at different premixed ratios of LRF (premixed ratio [rp] = 0, 0.25, 0.50, and 0.75) and four-engine loads (1 bar to 4 bar BMEP) at a constant engine speed of 1500 rpm. The use of gasoline in RCCI mode combustion resulted in relatively smoother RCCI mode combustion compared to baseline CI mode combustion. The presence of low reactivity gasoline led to a slightly retarded start of combustion (SoC) and combustion phasing (CP), which improved the performance of the RCCI mode combustion engine. RCCI mode combustion produced relatively lower NOx emissions compared to baseline CI mode combustion. However, hydrocarbon (HC) and carbon monoxide (CO) emissions from RCCI mode combustion were significantly higher than the CI mode combustion. Lower PM emissions from RCCI mode combustion was an important observation of this investigation. Qualitative correlations between different parameters such as PM number and mass-size distributions, total particulate mass (TPM)-NOx, etc., revealed that RCCI mode combustion was capable of simultaneous reduction of NOx and PM, which is otherwise difficult in conventional CI mode combustion engines

A comprehensive experimental investigation of low-temperature combustion with thick thermal barrier coatings

Thick thermal barrier coatings (TBCs) have a significant potential to increase thermal efficiency by reducing heat transfer losses. However, in conventional combustion modes, the drawbacks associated with charge heating and higher propensity to knock have outweighed the efficiency benefits. Since the advanced low-temperature combustion (LTC) concepts are fundamentally different from the conventional combustion modes, these penalties do not exist in most of LTCs.The current experimental study shows the feasibility and benefits of thick TBCs with advanced LTC enabled by two different fuels: conventional gasoline and wet ethanol 80 (WE80, i.e., 80% ethanol and 20% water by mass). A total of five pistons were tested, including two metal baselines and three TBC-coated pistons with different thicknesses or surface finishes. A load sweep was conducted with each fuel on each piston within the same constraints. The thick TBCs extends the low load limit by about 15% for both gasoline and WE80 cases. A deterioration of the high load limit was not observed, which implies that the charge heating penalty does not occur in LTCs. The combustion efficiency increased for the thicker TBC by up to 2% points, and the fuel conversion efficiency was increased by up to 4.3%. The gasoline cases experience the largest benefits at low load, while the wet ethanol experiences the largest benefits at mid-to-high load. The intake temperature requirement is successfully reduced by 10–15 K. It is also observed that the dense sealing layer results in a significant improvement to UHC emissions. All of the coated pistons survived the 10–20 h of engine operation with no visual failure.

Two-stage ignition behavior and octane sensitivity of toluene reference fuels as gasoline surrogate

Current approaches to improve the efficiency of Spark-Ignition (SI) gasoline engines have been focusing on turbocharging, increasing the compression ratio, and pursuing advanced low-temperature combustion concepts. In order to maximize these strategies, it is important to optimize the knock resistance of the fuel, and therefore knowledge of the sensitivity of the ignition process under a wide range of engine operating conditions is required. Octane sensitivity (OS), which is defined as the difference between Research Octane Number (RON) and Motored Octane Number (MON), has been introduced to represent how fuel's ignition reactivity changes relative to the primary reference fuels (n-heptane/iso-octane) within RON/MON conditions. Previous works have indicated that OS is intimately related to low temperature reactivity of the fuel, which can be revealed as two-stage heat release characteristics during an ignition event. Prompted by these findings, in this paper, we investigate the relationship between two-stage ignition behavior and OS, using chemical kinetic simulations of 24 Toluene Reference Fuels (TRFs)/ethanol blends. TRFs are ternary mixtures of n-heptane/iso-octane/toluene, which is capable of capturing aromatic content and positive values of OS of real gasoline fuels. Simulation results show that fuels with weak or no two-stage ignition behavior tend to have high OS, due to their lack of Negative Temperature Coefficient (NTC) effect and high sensitivity in ignition delay time. Leveraging such observations, we develop a correlation between two-stage behavior and OS as an OS prediction method. Two metrics that represent the strength of the two-stage ignition behavior are proposed and used as OS predictors, which are Low Temperature Heat Release percentage (LTHR%) and Heat Release Rate at the end of first stage (HRRinf) calculated from a simple kinetic simulation. Regression analysis shows a clear trend between decreases in the proposed two-stage behavior metrics and increases in the value of OS of the fuel. We also test the new metric (LTHR%) using simulation results of 0-D reactors with imposed pressure time histories obtained from engine experiments, as well as using different TRF kinetic mechanisms. The results demonstrate the effectiveness of the metric as a representation of the two-stage ignition behavior in practical combustion systems, highlighting the importance of the proposed relationship, and its potential as a simple and effective OS predictor.

Kinetic effects of n-propylbenzene on n-dodecane counterflow nonpremixed cool flames

The development of alternative drop-in fuels and fuel blends with petroleum-derived fuels necessitate an understanding of the kinetic interactions between aromatics and alkanes for the development of advanced low-temperature combustion engines. In the present study, the role of aromatic chemistry on n-alkane low-temperature chemistry is investigated by using nitrogen-diluted nonpremixed cool flames of n-dodecane/n-propylbenzene blends in an atmospheric counterflow burner. Effects of aromatic addition on n-alkane reactivity are examined by fixing the concentration of n-dodecane while varying the ratio of nitrogen/n-propylbenzene. The cool flame structure is probed using formaldehyde laser-induced fluorescence and thermocouple thermometry, and measurements of the cool flame and hot flame extinction limits are obtained. Fundamental differences between the response of cool flame and hot flame heat release and radical production to stretch are also discussed. The results show that, while the specific heat release rate characterizes the burning intensity of a hot flame, the same is not true for a cool flame due to the competition between reactant leakage and negative temperature coefficient reactivity. It is also found that, contrary to hot flames, in cool flames the addition of n-propylbenzene promotes n-dodecane cool flame extinction. Kinetic analysis reveals that aromatics act as a sink of hydroxyl and hydroperoxyl radicals in cool flames by forming stable oxygenated aromatics due to the low flame temperatures. As a result, the low-temperature reactivity of n-dodecane is significantly inhibited by the addition of aromatics, even relative to inert substitution. The present experimental results are valuable for comprehensive chemical kinetic model development and validation.

Establishment of an improved heat transfer model based on an enhanced thermal wall function for internal combustion engines operated under different combustion modes

The heat transfer from the in-cylinder gases to the wall dramatically affects the combustion and emission characteristics of internal combustion engines, particularly for the engines operated with advanced low-temperature combustion modes. In the present work, an improved heat transfer model was established based on an enhanced thermal wall function using the piecewise functions, in which only the laminar Prandtl number is considered in the viscous sublayer, and the competition between the laminar Prandtl number and the turbulent Prandtl number is taken into account in the buffer layer and turbulent core regions. By implementing the improved heat transfer model into a computational fluid dynamics code, the predictions of the combustion processes and the heat transfer behaviors of an engine operated with conventional diesel combustion, homogeneous charge compression ignition, and reactivity controlled compression ignition modes were validated by the experimental data. Moreover, the computational results from two previous heat transfer models, i.e., Han and Reitz model and Rakopoulos et al. model, were presented for comparison. The results indicate that the heat flux characteristics under different combustion modes can be more satisfactorily reproduced using the improved heat transfer model than Han and Reitz model and Rakopoulos et al. model by avoiding the flaws in the thermal wall function. Finally, the influence of the dimensionless distance of the computational grid adjacent to the wall on the predicted heat flux was discussed to study the dependence of the heat transfer model on the size of the near-wall computational grid.