Gas Recirculation Ratio Research Articles

Study of injection pressure couple with EGR on combustion performance and emissions of natural gas-diesel dual-fuel engine

Natural gas (NG) dual-fuel engines can attain a similar thermal efficiency as that of diesel engines while achieving lower emissions. However, the trade-off relationship between CH4 and NOX emissions limits the development of dual-fuel (DF) engines.In order to resolve this problem, the effects of injection pressure (IP) and exhaust gas recirculation (EGR) ratio on the combustion and emission of diesel/NG dual-fuel engines are investigated in this study. The results show that the diesel/NG dual fuel has a distinct three-stage heat release characteristic in the high-temperature combustion process. As the injection pressure increases, the flame propagation speed of methane and the indicated thermal efficiency (ITE) increase. However, the methane in the crevice region and cylinder wall cannot be ignited because of low temperature; these are the main sources of methane emissions. When the EGR rate increases, the indicated thermal efficiency first increases and there after decreases, and diesel is cleaved through the reaction chain, PC4H9 → C2H4 → CH4, to produce CH4; this becomes one of the reasons for CH4 emissions. When the EGR rate is small (<10%), CO and CH4 emissions are not sensitive to IP; when the EGR rate is large (>30%), CO and CH4 emissions can be significantly reduced by increasing the IP. Accordingly, when the injection pressure is 160 MPa and the EGR rate is 20%, the diesel/NG DF engine can achieve higher ITE and lower emissions.

The effects of exhaust gas re-circulation and injection timing on combustion performance and emissions of biodiesel and its blends with 2-methylfuran in a diesel engine

In this study, the influences of injection timing and exhaust gas re-circulation on combustion and emissions characteristics of biodiesel/2-methylfuran blends are investigated on a modified water-cooled 4-cylinder four-stroke direct injection compression ignition engine. The experimental conditions are, respectively, to adjust injection timing and exhaust gas re-circulation ratio at 0.38 MPa break mean effective pressure with the engine speed at 1800 rpm constantly. With injection timing in advance, the peak cylinder pressure rose while maximum heat re-lease rate first decreased and next slightly raised. Ignition delay and brake specific fuel consumption reduced first and then raised while combustion duration and break thermal efficiency had the opposite trend. The NOx emissions in-creased, and HC emissions first reduced significantly and then slightly increased, while 1,3-butadiene and acetaldehyde emissions presented a reduction tendency. As exhaust gas re-circulation ratio increased gradually, ignition delay as well as combustion duration was prolonged. brake specific fuel consumption increased and break thermal efficiency declined. HC, CO, 1,3-butadiene, and acetaldehyde emissions raised while NOx emissions reduced significantly. Biodiesel could be-have well in a Diesel engine and thus a feasible alternative fuel for diesel. More-over, methylfuran addition into biodiesel could raise break thermal efficiency and the break thermal efficiency of BM20 is higher than BM10. However, both BM10 and BM20 appeared a combustion deterioration when injection timing at 2.5?CA before top head center.

Optimization of piston bowl and valve system in compression ignition engine fueled with gasoline/diesel/polyoxymethylene dimethyl ethers for high efficiency

Polyoxymethylene dimethyl ethers, with excellent volatility and oxygen content of up to 49%, have great potential to improve engine performance and emission characteristics. In this study, experiments were carried out in a single-cylinder engine fueled with gasoline/diesel/polyoxymethylene dimethyl ethers blend fuel using multiple premixed compression ignition combustion mode along with engine optimization to exploit the high-efficiency potential. The thermal efficiency was increased by 9.4 percentage points after transforming the combustion mode from conventional diesel mode to gasoline/diesel/polyoxymethylene dimethyl ethers multiple premixed compression ignition mode. A fully variable valve system and a redesigned low-heat-transfer piston were used to further improve the thermal efficiency. The low-heat-transfer piston had a 15% lower area–volume ratio compared with the original ω-type piston. By replacing the original ω-type piston with the low-heat-transfer piston, the heat transfer loss was reduced by 2.29 percentage points and thus indicated thermal efficiency could be increased by 2.37 percentage points, which was up to 50.03%. On the basis of the low-heat-transfer piston, indicated thermal efficiency could be further increased to 51.09% with the application of fully variable valve system due to the longer ignition delay and more premixed combustion. At the same time, NOX emissions could be controlled below 0.4 g/kW·h using high exhaust gas recirculation ratio, which equaled the NOX emission limit of Euro VI standard. Although soot emissions could be increased due to weak turbulence and insufficient intake charge using the low-heat-transfer piston and fully variable valve system, it was still lower than those of the original diesel engines.

Effect of addition of hydrogen and TiO2 in gasoline engine in various exhaust gas recirculation ratio

The effects of hydrogen ratios on combustion and emission characteristics of gasoline engine were studied under different exhaust gas recirculation (EGR), ignition timing and ignition pressure. The test performed in a modified gasoline direct ignition engine at different hydrogen ratios of 0%, 5%, 10% and 25%. In addition, the EGR rate set to 0%, 5%, 10% and 20% to study the combustion and emission characteristics. Addition to the different hydrogen fractions, 5% of TiO2 is added to increase the combustion characteristics with reduced emission. Regarding the results of the current study, the engine torque increases by 15% due to the addition of hydrogen in gasoline, while mechanical efficiency is improved by achieving a large throttle opening. At the same time, NOx emission decreased by 62% compared to the unmodified engine due to the influence of EGR, hydrogen ratio and high oxygen concentration TiO2. Moreover, the emission of CO and HC also reduced due to the influence of hydrogen fuel. Additionally, few more tests are taken to monitor the effect of the injection pressure for the hydrogen fuel. Higher injection reports higher effective thermal efficiency at 4 MPa and lower NOx. Reasonable injection pressure results in shorten flame development period.

Assessment of heat-to-power ratio in a bio-oil sorption enhanced steam reforming and solid oxide fuel cell system

A solid oxide fuel cell (SOFC) is a high-efficiency technology for combined heat and power (CHP) generation. Heat and power production can be varied according to changes in heat and power loads, which affect the electrical and thermal efficiencies. This study analyzes changes in heat and power production rates of a bio-oil sorption enhanced steam reforming (SESR) and SOFC system. The variations in the stack-to-burner ratio (bio-oil split fractions), fuel utilization factor (Uf), and recirculation ratio (R) are investigated. The effect of heat-to-power ratio on the system performance and CO2 capture efficiencies is also studied. Moreover, the benefit of the implementation of a bio-oil SESR and SOFC system over a conventional operation to match the energy demand for an industrial case is evaluated. The results reveal that an increase in Uf decreases the operating range of the heat-to-power ratio. On the other hand, the anode gas recirculation ratio affects such a parameter when the system performs at the high stack-to-burner ratio. A higher heat-to-power ratio increases the stack electrical and thermal efficiencies, leading to a higher CHP efficiency. The heat-to-power ratio of the proposed system varies between 0.05 and 7.5. However, to attain a CO2 capture efficiency higher than 50%, the heat-to-power ratio should be lower than 1.5. By comparing the two operational strategies: fixed heat-to-power ratio, and variable heat-to-power ratio, it is found that the heat-to-power ratio is an important factor for energy management. The primary energy saving is enhanced when the system is operated to satisfy the rates of heat and power demand.

The impact of water injection and exhaust gas recirculation on combustion and emissions in a heavy-duty compression ignition engine operated on diesel and gasoline

Steady-state experiments were conducted on a 12.4L, six-cylinder heavy-duty engine to investigate the influence of port-injected water and dilution via exhaust gas recirculation (EGR) on combustion and emissions for diesel and gasoline operation. Adding a diluent to the combustion process reduces peak combustion temperatures and can reduce the reactivity of the charge, thereby increasing the ignition-delay and, allowing for more time to premix air and fuel. Experiments spanned water/fuel mass ratios up to 140mass% and exhaust gas recirculation ratios up to 20vol% for gasoline and diesel operation with different injection strategies. Diluting the combustion process with either water or EGR resulted in a significant reduction in nitrogen oxide emissions along with a reduction in brake thermal efficiency. The sensitivity of brake thermal efficiency to water and EGR varied among the fuels and injection strategies investigated. An efficiency breakdown revealed that water injection considerably reduced the wall heat transfer; however, a substantial increase in exhaust enthalpy offset the reduction in wall heat transfer and led to a reduction in brake thermal efficiency. Regular diesel operation with main and post injection exhibited a brake thermal efficiency of 45.8% and a 0.3% reduction at a water/fuel ratio of 120%. The engine operation with gasoline, early pilot, and main injection strategy showed a brake thermal efficiency of 45.0% at 0% water/fuel ratio, and a 1.2% decrease in brake thermal efficiency for a water/fuel ratio of 140%. Using EGR as a diluent reduced the brake thermal efficiency by 0.3% for diesel operation, comparing ratios of 0% and 20% EGR. However, a higher impact on brake thermal efficiency was seen for gasoline operation with early pilot and main injection strategy, with a reduction of about 0.8% comparing 0% and 20% EGR. Dilution by means of EGR exhibited a reduction in nitrogen oxide emissions up to 15 g/kWh; water injection showed only up to 10 g/kWh reduction for the EGR rates and water/fuel ratio investigated.

Insights into engine autoignition: Combining engine thermodynamic trajectory and fuel ignition delay iso-contour

One of the ultimate goals of chemical kinetic study is to understand and predict autoignition in engines. In this study, utilizing toluene primary reference fuels (TPRF) as a gasoline surrogate and a recently developed multicomponent gasoline kinetic mechanism, we have demonstrated a general approach to analyze autoignition in arbitrary spark-ignition (SI) and advanced compression ignition (ACI) engine conditions by combining thermodynamic pressure-temperature trajectory and the fuel ignition delay iso-contours. This method allows direct evaluation of controlling chemistry, potential involvement of low temperature heat release, and the dependence of autoignition to conventional fuel metrics (research and motor octane rating, i.e., RON and MON, and octane sensitivity OS = RON-MON) and engine operating conditions such as equivalence ratio, exhaust gas recirculation (EGR) ratio and engine intake conditions. Applying the analysis to the pressure-temperature trajectories of the conventional RON and MON tests, as well as those beyond RON and beyond MON, distinct roles of conventional gasoline fuel metrics and engine operating parameters are identified for all representative engine conditions. By comparing the autoignition behavior in ACI and SI engine conditions, the knowledge obtained from SI engine knock cannot be directly transferred to ACI bulk combustion phasing control in general, due to the different mixture equivalence ratios and the associated differences in reactivity and its dependence. This method could be extended to generate an auto-ignition map for arbitrary fuels and arbitrary engine trajectories, and the useful insights and overall evaluations can be used to complement conventional kinetic simulation of engine cycles.

A novel fuzzy logic variable geometry turbocharger and exhaust gas recirculation control scheme for optimizing the performance and emissions of a diesel engine

Variable geometry turbocharger and exhaust gas recirculation valves are widely installed on diesel engines to allow optimized control of intake air mass flow and exhaust gas recirculation ratio. The positions of variable geometry turbocharger vanes and exhaust gas recirculation valve are predominantly regulated by dual-loop proportional–integral–derivative controllers to achieve predefined set-points of intake air pressure and exhaust gas recirculation mass flow. The set-points are determined by extensive mapping of the intake air pressure and exhaust gas recirculation mass flow against various engine speeds and loads concerning engine performance and emissions. However, due to the inherent nonlinearities of diesel engines and the strong interferences between variable geometry turbocharger and exhaust gas recirculation, an extensive map of gains for the P, I, and D terms of the proportional–integral–derivative controllers is required to achieve desired control performance. The present simulation study proposes a novel fuzzy logic control scheme to determine appropriate positions of variable geometry turbocharger vanes and exhaust gas recirculation valve in real-time. Once determined, the actual positions of the vanes and valve are regulated by two local proportional–integral–derivative controllers. The fuzzy logic control rules are derived based on an understanding of the interactions among the variable geometry turbocharger, exhaust gas recirculation, and diesel engine. The results obtained from an experimentally validated one-dimensional transient diesel engine model showed that the proposed fuzzy logic control scheme is capable of efficiently optimizing variable geometry turbocharger and exhaust gas recirculation positions under transient engine operating conditions in real-time. Compared to the baseline proportional–integral–derivative controllers approach, both engine’s efficiency and total turbo efficiency have been improved by the proposed fuzzy logic control scheme while NOx and soot emissions have been significantly reduced by 34% and 82%, respectively.

Investigation of the thermal effects of fuel injection into retained residuals in HCCI engine

Low-temperature combustion in reciprocating engines appears to be a cutting-edge technology which ensures extremely low emissions of nitrogen oxides and particulate matters in parallel with high fuel efficiency. This mode of combustion can be realized in homogeneous charge compression ignition engines. However, this technology poses challenges, and fast response combustion controllability is one of the issues to be solved to make this concept widely applicable. The introduction of direct injection of gasoline into recompressed residuals during negative valve overlap is one of the promising techniques to keep the advantages of having a homogeneous mixture during the main combustion while adding controllability through split ratio and injection timing within the recompression event.The present study aims to thoroughly investigate the thermal effects associated with gasoline injection during the negative valve overlap phase and explain qualitatively and quantitatively their impact on the gas exchange process and, further, on the combustion phasing during the main event. Single cylinder engine experiments are designed to evaluate these effects at different injection timings and mixture compositions. The scope of the research is focused on low-load operation at constant engine speed. The discussion of in-cylinder pressure analysis results and engine operational parameters is complemented by gas exchange simulations to provide more insight into the entire working process.It is confirmed by the results that negative valve overlap gasoline injection can be used as a measure to control combustion phasing in homogeneous charge compression ignition engines. The controllability is assured through balancing between evaporative cooling or heating of the trapped residuals. With this measure effective regulation of internal exhaust gas recirculation ratio, ranging from 41% to 64%, was demonstrated. This further enables fast intake valve closing temperature adjustment in a range of around 50 K, which finally results in superior auto-ignition timing control in a range between 5 °CA and 10 °CA before or after top dead center, depending on the global mixture strength. The work further provides fundamental understanding of the regulation mechanisms and concludes on the possibilities of applying this knowledge in a production engine.

A study on combustion characteristics under enriched-oxygen condition by exergy analysis

The object of this paper is to investigate the effect of operating parameters on Oxygen-enriched combustion (OEC) by exergy balance analysis during methane combustion process. In this work, the specific exergy value of fuel, feed gas and products were calculated on the basis of the second law of thermodynamics in different reaction conditions, so that availability destructions and exergy efficiencies of OEC process can be obtained based on exergy balance, and the influence of initial fuel temperature, oxygen concentration, exhaust gas recirculation ratio and equivalence ratio were addressed. It is observed that during methane OEC process, the exergy efficiency is mainly affected by products temperature, in the case of products temperature lower than 1870K, exergy efficiency of combustion increase with products temperature, and opposite changes was appeared when the products temperature higher than 1870K. Meanwhile, appropriate control of equivalence ratio can also be used to improve exergy efficiency by suppressing the dissociation of product.

Effect of Heavy-Duty Diesel Engine Operating Parameters on Particle Number and Size Distribution at Low Speed Condition

Experiments and simulations were used to investigate the effect of a range of engine operating parameters and fuel characteristics on the particle size and particle number (PN) concentration at low speed and idle speed condition. The occurrence, size, and concentration of particles were tested against a range of parameters including start of injection (SOI), common rail pressure, exhaust gas recirculation (EGR) ratio and load. The results showed that the homogeneity of the mixture had the greatest impact on particle size and number concentration. The performance of particle is different at different levels of load. The particle were of nucleation mode at idle condition, and the cold idle particles had a slightly larger diameter than those produced at hot idle. By using the diesel and under high load, at EGR ratios of less than 20 %, most particles were of nucleation mode. At EGR ratios exceeding 20 %, nucleation-mode particles were gradually replaced by accumulation-mode particles. At EGR ratios above 30 %, most particles were of the accumulation mode. Under the same load, gasoline compression ignition produced particles of smaller size and reduced particulate mass (PM). The use of gasoline extended ignition delay, as the high volatility and octane number of the fuel improved the homogeneity of the mixture. Finally, a linear relationship was found between PM and PN. The relative contribution of the different factors to the formation of nucleationor accumulation-mode particles was investigated.

Ultra-fine particulate matters (PMs) formation during air and oxy-coal combustion: Kinetics study

Although both air (21O2/N2) and oxy-coal (27O2/CO2) combustion are widely adopted in pulverized coal (PC) fired power plants, the formation mechanisms of ultra-fine PMs during PC char combustion under both atmospheres with various flue gas recirculation (FGR) ratios are still unclear. Moreover, conventional experimental measurement devices cannot provide detailed information on the formation and evolution of the size-number of ultra-fine PMs. Therefore, the formation of ultra-fine PMs during PC char combustion under both atmopspheres with and without FGR are studied by a self-developed Char Burning and Particulate Matters Kinetics model (CBPMK). The PC char shows similar burning temperature and thus ash vaporization rate under both atmospheres, whereas the vaporization amount under oxy-coal combustion atmosphere is lower than that under air combustion atmosphere due to the shortened burnout time caused by CO2 gasification reaction under the former. Consequently, during nucleation and condensation stages (before successive coalescence), both the particle size and number under air combustion atmosphere are higher than those under oxy-coal combustion atmosphere. However, after coalescence, the final particle shows fewer but larger size under oxy-coal combustion atmosphere due to the higher cohesion factor between the smaller sized nucleation particles, which improves particle collision and coalescence. Meanwhile, both mean size and number density of the nucleation particles decrease with increased FGR ratio under both air and oxy-coal combustion atmospheres, however, after coalescence the final PMs show increasing number density and decreasing size. As results, oxy-coal combustion advantages PMs removal through an ash collector, but elevated FGR ratio disadvantages PMs removal. Oxy-coal combustion with low FGR ratio should be recommended in PC thermal and power plants.

Self-Humidification of a Polymer Electrolyte Membrane Fuel Cell System With Cathodic Exhaust Gas Recirculation

Water management is critical for the operation of a polymer electrolyte membrane fuel cell (PEMFC). For the purposes of high power and long working-lifetime of PEMFCs, external humidifiers are always utilized as a necessary part of balance of plants to keep the imported air and fuel wet. However, they have several disadvantages, and it is beneficial to remove them so as to reduce system volume and to enhance the cold-starting capability. In this paper, a self-humidified PEMFC of an active area 250 cm2 and cell number 320 is proposed and investigated. The imported dry air on the cathode side is mixed with moisty exhaust gas by using a recirculation valve, and the dry hydrogen on the anode side is humidified by back-diffusion water through the membrane. A nonlinear model is set up based on mass transport and energy conservation equations to capture dynamics of gases in the supply and exhaust manifolds, the gas diffusion layers (GDLs), and the membrane. An analysis is conducted to investigate the influences of parameters on dynamic and stable performances. Simulation results show that system performances can be greatly affected by parameters such as air stoichiometry, current density, exhaust gas recirculation (EGR) ratio, and membrane thickness. By accurately controlling the EGR ratio and carefully selecting design and operation parameters, it is probably for a PEMFC without an external humidifier to have similar system efficiency compared to a traditional system.

Process Simulation of a 620 Mw-Natural Gas Combined Cycle Power Plant with Optimum Flue Gas Recirculation

The main objective of this investigation is to obtain an optimum value for the flue gas recirculation ratio in a 620 MW-Natural Gas Combined Cycle (NGCC) power plant with a 100% excess air in order to have a composition of the exhaust gas suitable for an effective absorption by amine solutions. To reach this goal, the recirculated flue gas is added to the secondary air (dilution air) used for cooling the turbine. The originality of this work is that the optimum value of a Flue Gas Recirculation (FGR) ratio of 0.42 is obtained from the change of the slope related to the effects of flue gas recirculation ratio on the molar percentage of oxygen in the exhaust gas. Compared to the NGCC power plant without flue gas recirculation, the molar percentage of carbon dioxide in the flue gas increases from 5% to 9.2% and the molar percentage of oxygen decreases from 10.9% to 3.5%. Since energy efficiency is the key parameter of energy conversion systems, the impact of the flue gas recirculation on the different energy inputs and outputs and the overall efficiency of the power plant are also investigated. It is found the positive effects of the flue gas recirculation on the electricity produced by the steam turbine generator (STG) are more important than its cooling effects on the power output of the combustion turbine generator (CTG). The flue gas recirculation has no effects on the water pump of the steam cycle and the increase of energy consumed by the compressor of flue gas is compensated by the decrease of energy consumed by the compressor of fresh air. Based on the Low heating value (LHV) of the natural gas, the flue gas recirculation increases the overall efficiency of the power plant by 1.1% from 57.5% from to 58.2%.

Model-based control of diesel engines with multiple fuel injections

We developed a feed-forward controller for a conventional diesel combustion engine with triple fuel injection and experimentally evaluated its performance. A combustion model that discretizes an engine cycle into a number of representative points to achieve a light calculation load is embedded into the controller; this model predicts the in-cylinder gas-pressure-peak timing with information about the operating condition obtained from the engine control unit. The controller calculates the optimal main-fuel-injection timing to control the in-cylinder gas-pressure peak using the prediction result as a controller with a single input and output. The controller’s performance was evaluated by experiments using a four-cylinder diesel engine under changing the target value of the in-cylinder gas-pressure-peak timing during a target-following test and the performance was also evaluated under changing the exhaust gas recirculation ratio at the constant target value of the in-cylinder gas-pressure-peak timing for the disturbance-response test. It was found that the controller could calculate the optimal main-injection timing over a cycle and maintain the targeted in-cylinder gas-pressure-peak timing even when the target value or exhaust gas recirculation changed. The combustion model was also shown to be fast enough at predicting diesel combustion for onboard control.

Emission Characteristics for a Homogeneous Charged Compression Ignition Diesel Engine with Exhaust Gas Recirculation Using Split Injection Methodology

Due to the serious issues caused by air pollution and global warming, emission regulations are becoming stricter. New technologies that reduce NOx and PM emissions are needed. To cope with these social exhaust gas regulation demands, many advanced countries are striving to develop eco-friendly vehicles in order to respond to stricter emissions regulations. The homogeneous charged compression ignition engine (HCCI) incorporates a multi-stage combustion engine with multiple combustion modes, catalyst, direct fuel injection and partial mixing combustion. In this study, the HCCI combustion was applied to analyze and review the results of engines applying HCCI combustion without altering the conventional engine specifications. The optimization of exhaust gas recirculation (EGR) and compression ratio changes provides an optimal fuel economy. In this study, potential for optimum economy within the range of IMEP 0.8 MPa has been evaluated.

NOX reduction in a 40 t/h biomass fired grate boiler using internal flue gas recirculation technology

A decoupled numerical modelling method is developed in this study to simulate the whole combustion process of biomass in a grate firing boiler, which includes the thermochemical conversion of biomass in the fuel-bed and gaseous combustion in the freeboard. With the aid of this modelling method, the objective of this study is to explore the NOX reduction mechanism as well as to investigate the potential of internal flue gas recirculation technology (IFGRT) on the combustion process and emissions formation in a 40 t/h biomass-fired grate boiler. Computational fluid dynamics (CFD) modelling results show that IFGRT can be realized in the grate boiler by establishing intense flue gas recirculation within the boiler, which allows for lower peak combustion temperatures and smaller flame kernel sizes, while improving the overall average gas temperature. Consequently, NOX emission can be reduced mainly via the thermal formation route in comparison with the conventional combustion case. More specifically, the parallel over-fired air (OFA) burner configuration is suggested for implementation to produce even lower NOX emission compared to the staggered OFA burner configuration. To further understand the reasons behind the NOX reduction, NOX formation and destruction mechanisms are also examined through reduced order modelling (ROM) with the help of detailed reaction chemistry. It is revealed that flue gas recirculation inhibits NOX formation from thermal, NNH and N2O routes. Although NOX destruction rate through reburning is suppressed, the net NOX production rate is found to be decreased under the condition of IFGRT. Moreover, as the flue gas recirculation ratio increases, final NOX emission shows a decreasing trend.

Fluid dynamics parameters of a high temperature ejector for SOFC anode gas recirculation

The energy sector embraces solid oxide fuel cells (SOFC) since they are highly efficient and ecologically sound power generators. Yet, maintaining fuel cells sustainability in power units takes a lot of resources. Hence, we need to find the appropriate ways to increase the efficiency of the whole SOFC power system inclusive of its balance-of-plant equipment. This can facilitate our fuel and energy complex green transition through the SOFC technology introduction. As a matter of fact, anode gas recirculation is instrumental for boosting the SOFC power units efficiency. One of the challenges in developing the SOFC power unit with anode gas recirculation is to return the amount of anode gas appropriate for carbon deposition free reforming. The paper contains the anode gas recirculation ratio in the SOFC power units calculation method for various reforming temperatures. The safest operation modes of the SOFC power units with anode gas recirculation were determined. The 3 kW SOFC power unit with high temperature ejector and nickel-based catalyst for methane conversion was designed. For the given flow channel geometry, the high temperature ejector off-design performance was plotted. In addition, fields of temperature, pressure and velocities were plotted for the ejector design point using Ansys computational fluid dynamics software.

Development and validation of a new reduced diesel-n-butanol blends mechanism for engine applications

A reduced n-heptane-toluene-n-butanol-polycyclic aromatic hydrocarbon (PAH) combustion mechanism was developed for a blended fuel of n-butanol and diesel, including 101 species and 531 reactions. A sensitivity analysis was conducted for the optimization of the mechanism. The optimized mechanism was widely validated with ignition delays, laminar flame speeds, species concentrations in premixed flame and counterflow flame, and homogeneous charge compression ignition (HCCI) engine combustion. According to the validation results, the optimized mechanism shows favorable predictions, especially on the important species related to the PAH formation. Moreover, the reduced mechanism was coupled with computational fluid dynamic (CFD) to conduct direct injection compression ignition (DICI) engine tests within a wide range of exhaust gas recirculation (EGR) ratio and also shows favorable prediction results. Therefore, the proposed mechanism can be used to simulate the combustion of diesel or n-butanol-diesel blends in multi-dimensional CFD modeling. Besides, the main formation pathways of benzene (A1), a precursor of soot, were analyzed in a single-zone HCCI engine model. Results show that, in addition to the traditional pathways of ‘C4+C2’ and ‘C3+C3’, toluene was quite crucial to produce A1. The counterflow flame validation tests also demonstrated the importance of the addition of toluene to the PAH and soot formation. In the early combustion period, C6H5OH significantly contributed to the formation of A1, while toluene directly produced a large amount of C6H5CH2 and thus promoted the formation of A1. In the middle combustion period, OC6H4CH3 played an important role in the formation of A1, while toluene made less contribution.

A phenomenological model of knock intensity in spark-ignition engines

In experimental calibration of mass production spark-ignition (SI) engines, knocking cycles are usually determined by a specified threshold of knock intensity (KI). In engine cycle simulation, however, modeling of knock is usually limited to prediction of knock onset (i.e. auto-ignition of end gas), or merely simple parameters such as unburned mass fraction (UMF) for KI are taken into account, causing considerable deviation from actuality. The objective of this study is to develop a phenomenological KI model to improve predictability of engine cycle simulation. The experiments with a turbocharged 1.5L gasoline engine operated by spark sweeps around knock limits over a wide speed range are conducted to generate the data base for the model formulation and evaluation. With assumption of lognormal distribution of KI in consecutive cycles, a knock factor (KF) based on the likelihood ratio is proposed as a criterion for definition of knocking cycles. The model developed in the previous study, which includes the cylinder pressure, end gas temperature, exhaust gas recirculation (EGR) ratio and excess air ratio as variables, is used to predict the knock onset. With in-depth analysis of the physics influencing knock intensity, the energy density and heat release rate (HRR) in hot spots are identified to play critical roles in determination of knock intensity. The heat release rate is related to the end gas temperature, pressure, EGR ratio and excess air ratio, while the energy density is determined by the amount of fresh charge in the cylinder and the charge volume at knock onset. Thus, the correlation of the two factors, including the HRR factor α and the energy density factor β, is formulated to develop the predictive model of knock intensity YKI. Finally, the newly developed KI model is evaluated through comparison with other KI models, and better performance in terms of prediction of knock intensity is obtained.