Air-fuel Ratio Research Articles

Experimental study of CO2 and pollutant emission at various altitudes: Inconsistent results and reason analysis

How altitudes affect vehicle CO2/pollutant emission has long been studied, while different, sometimes conflicting conclusions are reported. To evaluate how altitudes impact vehicle CO2/pollutant emission, 21 vehicles were tested in six cities with altitudes ranging from 26.7 m above sea level (masl) to 2264.7 masl. Three of the vehicles were tested in the altitude chamber at five different heights (from 0 to 2400 masl) following the Worldwide Harmonized Light Vehicles Test Procedure (WLTP). CO2/pollutant emissions, conformity factor, and extended factor were evaluated based on the results obtained. No uniform patterns were found between altitudes and CO2/pollutant emission in both laboratory and real driving tests. For laboratory tests, the different CO2 emission results could be explained by air-fuel ratio variations during the transient cycles and engine operation variations between tests. More variables were introduced to the real driving tests, which brings more uncertainty to the CO2 emission. The average CO2 emission of the turbocharged and naturally aspirated vehicles at various cities share similar patterns, indicating other parameters (e.g., driving patterns, road conditions, ambient conditions, et al.) might have a larger impact on the CO2 emission than the altitude growth. These reasons could account for the irregular pollutant results. Besides, the cylinder-to-cylinder variations in equivalence ratio about the mean value have more enormous impacts on the emission results. Discrete results mean that single engine/vehicle tests couldn’t reflect the real situations at various heights. The current conformity factors and the extended factors might be too loose.

A Numerical Analysis of the Effects of Equivalence Ratio Measurement Accuracy on the Engine Efficiency and Emissions at Varied Compression Ratios

Increasingly stringent regulations to reduce vehicle emissions have made it important to study emission mitigation strategies. Highly accurate control of the air-fuel ratio is an effective way to reduce emissions. However, a less accurate sensor can lead to reduced engine stability and greater variability in engine efficiency and emissions. Additionally, internal combustion engines (ICE) are moving toward higher compression ratios to achieve higher thermal efficiency and alleviate the energy crisis. The objective of this investigation was to analyze the significance of the accuracy of air-fuel ratio measurements at different compression ratios. In this study, a calibrated 1D CFD model was used to analyze the performance and emissions at different compression ratios. The results showed that carbon monoxide (CO) and nitrogen oxides (NOx) were sensitive to the equivalence ratio regardless of the compression ratio. With a slight change in the equivalence ratio, a high compression ratio had little effect on the change in engine performance and emissions. Moreover, with the same air-fuel ratio, an excessively high compression ratio (CR = 12) might result in knocking phenomenon, which increases the fluctuation of the engine output parameters and reduces engine stability. Overall, for precise control of combustion and thermal efficiency improvement, it is recommended that the measurement accuracy of the equivalence ratio is higher than 1% and the recommended value of the compression ratio are roughly 11.

An explicit robust stability condition for uncertain time-varying first-order plus dead-time systems

First-order plus dead-time (FOPDT) models are broadly used in process control to represent damped dynamic processes with time delays. An explicit condition for parameter- and delay-dependent robust stability of FOPDT systems with varying uncertain parameters and delay is derived in this paper. An internal model control (IMC) approach is proposed to parameterize stabilizing controllers that satisfy the output tracking objective in time-varying FOPDT systems represented by an uncertain first-order dynamic model with a time-varying delay in the control input. The small-gain theorem is used to derive an explicit necessary and sufficient parameter-dependent robust stability condition as a function of the nominal system gain, nominal varying delay, nominal time constant, and the bounds of the parameter uncertainties. An equivalent proportional-integral-derivative (PID) controller is then extracted to facilitate the implementation of the proposed IMC-based robust control. The application of the proposed explicit robust stability condition is studied in the context of air-fuel ratio (AFR) control in lean-burn spark ignition (SI) engines with a large time-varying transport delay in the control loop due to the placement of the universal exhaust gas oxygen (UEGO) sensor downstream the catalytic converter.

Entropy generation analysis of premixed hydrogen–air combustion in a micro combustor with porous medium

Porous medium with a high heat capacity, conductivity and emissivity effectively enhances the heat transfer in the combustor chamber. In order to improve the thermal performance and efficiency of the micro-TPV (thermophotovoltaic) system, premixed hydrogen-air combustion in the combustor inserted with porous medium is numerically simulated and analyzed. Results indicate that the combustor with partial porous medium filled and a step length 4 mm obtains a better thermal performance. It is also found that SiC is a better material as the porous medium and Al2O3 is more suitable for the combustor wall to achieve a higher wall temperature and low entropy generation, thereby presenting a better thermal performance. The combustor, which is made by Al2O3 and inserted with porous medium SiC, is able to achieve an efficiency of 2.22% and a maximum power of 3.10W from a micro-TPV system with InGaAsSb PV cells at fuel-air ratio Φ=0.9 and mH2=1.13 × 10−6kg/s.

EXPERIMENTAL ANALYSIS OF THE COMBUSTION CHAMBER COATED WITH Mo AND Al2O3-TiO2 IN A DIESEL ENGINE

The effect of thermal-barrier coatings (TBCs) reduces fuel consumption, effectively improving the engine efficiency. This research focused on a TBC with a thickness of 300 µm insulating the combustion chamber of a direct ignition (DI) engine. The piston crown, inlet and exhaust-valve head were coated using air-plasma-spray coating. Ceramic powder materials such as molybdenum (Mo) and aluminum oxide titanium dioxide (Al2O3-TiO2) were used. A performance test of the engine with the coated combustion chamber was carried out to investigate the brake power, brake thermal efficiency, volumetric efficiency, brake specific fuel consumption and air-fuel ratio. Also, an emission-characteristic test was carried out to investigate the emissions of unburned hydrocarbon (HC), carbon monoxide (CO), nitrogen oxides (NO, NO2, NO3) and smoke opacity (SO). The results reveal that the brake thermal efficiency and brake specific fuel consumption show significant increases because of these coating materials. The effect of the Al2O3-TiO2 coating significantly reduces the HC and CO engine emissions.

Anode-Oxidizing Critical Operating Conditions of Large Planar Solid Oxide Fuel Cell

In a solid oxide fuel cell power system, large planar cells are common central parts for electric generation. In practice, the operating conditions of a planar cell may have significant impact on the electrical efficiency and the lifespan of the cell, thus the operating and maintenance cost of the power system. Given varying electric load, the power system would dynamically adjust the operating conditions of each cell or cell module, so that the energy and heat balance in the system is maintained, the fuel consumption is minimized and the cells are protected from reactant starvation, local overheating etc. To realize this functionality, we seek a fundamental understanding of the quantitative connections among fuel cell properties, operating conditions and optimal electrical efficiency.We established a quasi-2D model for quick evaluation of fuel and oxidant concentration and current density along the flow direction. For fuel cells with very low charge transfer impedance i.e. 0.02 Ω*cm^2 operating with low fuel flow rates, the fuel concentration at the exit may reach an equilibrium where the local Nernstian potential across the cell equals the cell voltage. In such cases, the fuel utilization remains almost constant regardless of varying fuel flow rate, given a certain cell voltage. For fuel cells with local charge transfer impedance of 0.5 Ω*cm^2 operating with high fuel flow rates, such phenomenon was not observed because the fuel concentration at the exit depends strongly on the fuel flow rate. Given the fuel molar fraction at the fuel inlet, the electrical efficiency of a fuel cell has its maximum at certain cell voltage, which depends strongly on the charge transfer impedance of the fuel cell. At constant fuel flow rates, the optimal cell voltage increases with lowering charge transfer impedance. The optimal electrical efficiency appears lower with higher inlet fuel molar fraction. Given fixed and uniform fuel cell temperature, the air flow rate has little impact on the electrical efficiency for air-fuel stoichiometric ratios above 1.3.Besides the above findings, further studies are necessary on the effects of temperature distribution as well as porous electrode diffusion resistance. Figure 1

Study on the feasibility of the micromix combustion principle in low NOx H2 burners for domestic and industrial boilers: A numerical approach

The purpose of the present investigation is to explore the feasibility of applying the micromix combustion principle (MCP) to design low NOx burners using 100% H2 as fuel for domestic and industrial boilers. Previous investigations studying the MCP on stationary and aero gas turbine applications, showed low NOx emissions without flashback risk, which represent the two main issues when burning 100% H2. Since boiler burner operating conditions differ from gas turbine combustors, the present paper studies the MCP through CFD calculations under re-defined conditions for domestic and industrial burners, adapting the energy densities, air-fuel equivalence ratios and pre-heated air temperatures. A reference geometry was built to validate the selected numerical models with experimental results from literature. Afterwards, burners were re-dimensioned following an existing geometry-scaling methodology. The obtained results evidenced that MCP characteristics were still maintained for the defined cases, keeping low NOx values between 4 and 14 ppm. The numerical results were also validated through experimental NOx measurements in a laboratory scale micromix burner prototype. In order to assess the benefits of using hydrogen micromix burners in domestic and industrial boilers, the present work includes a final discussion with practical design considerations. The present study lays the groundwork for complementary experimental research work, which is being carried out in laboratory-scale prototypes.

Research on ammonia emissions from three-way catalytic converters based on small sample test and vehicle test

In this paper, a combination of catalyst sample evaluation and vehicle test is used to deeply study the formation mechanism of ammonia in the process of three-way catalytic reaction, and further explore the influence of catalyst formulation and aging on ammonia emissions. The catalytic sample test shows that CO reacts with terminal hydroxyl and bridging hydroxyl on the surface of the catalyst to generate H2 at low temperature, which then reduces NO to generate NH3. At high temperatures, CO reacts with water to generate H2, or hydrocarbon compounds in exhaust react with steam to generate hydrogen, and then H2 reacts with nitrogen oxides to generate NH3. On the one hand, the presence of water vapor can be prompted catalytic hydroxylation of materials and promote the reaction of the hydroxyl and bridging hydroxyl to improve the selectivity of NH3, on the other hand, as the competitive adsorption of H2O molecules and NO on the catalyst surface inhibits the reduction reaction between NH3 and NO, the consumption of NH3 molecules is reduced, and more NH3 vaporizes from the catalyst surface to the gas phase. The combination of Pd/Rh can effectively reduce the NH3 generation compared with the single Pd formulation. Ammonia emission can be effectively reduced by precisely controlling the air-fuel ratio of the engine and combining it with the catalytic converter which optimizes the ratio of precious metals.

Energy Consumption Calculation and Analysis of Gas Suspension Calciner

Gas suspension calciner is a kind of ideal fluidization process equipment for energy economy. In order to improve the effective of gas suspension calciner, a series of effecting factors such as producer gas component, air-fuel ratio, products moisture and heat dissipation on calciner energy consumption were discussed in this paper, which is based on the research of thermal balance calculation of gas suspension calciner. The study of thermal balance calculation has significant meaning on the energy-saving and the whole refinery energy consumption.

Thermodynamic analysis of a combined gas-steam cycle without and with afterburner

Currently in power plants what is sought are higher thermal efficiencies, which is why combined cycle plants have been chosen, since they make better use of fuel, producing greater net power, all this It has led to innovative modifications to combined cycle power plants, improving their performance. In this article the thermodynamic analysis of a combined cycle (Gas-Steam) without and with Afterburner is carried out, in said analysis adequate thermodynamic indices have been used and the calculations have been carried out taking into account the state of the art for gas turbines and the typical values for steam cycle quantities. The purpose of this study is to analyze the combined plants where the exhaust gas at the outlet of the gas turbine is used in a waste heat recovery boiler to produce steam that is expanded in a turbine and discharged in a condenser. Since the air-fuel ratio in the combustion chamber of a gas turbine is higher than the stoichiometric, fuel can be added in the boiler (Afterburner) to increase steam production or to improve the quality of the steam produced.

HARDWARE AND SOFTWARE OF AUTOMATIC CONTROL SYSTEM OF FUEL COMBUSTION PROCESS IN LOW AND MEDIUM POWER BOILERS. PART 1. METHOD AND HARDWARE

In Ukraine today there are more than 6000 boiler plants with a heating capacity of up to 1 Gcal/h with an efficiency of about 70 %, requiring replacement or modernization, 40 % of boilers are operated with an efficiency of less than 82 %, about 11000 boilers with a capacity of 100 kW to 1 MW have been in operation for over 20 years. Although the part of these boilers in the municipal heat power system of Ukraine does not exceed 14 %, the projected savings in natural fuel in these boilers is more than 130 million cubic meters per year. Thus, increasing the efficiency of the fuel combustion process in small and medium power boilers is an urgent task at the present time.
 The article presents the results of creating a method and hardware that implements it, to increase the speed and reliability of monitoring the process of fuel combustion in boiler units based on measuring the concentration of residual oxygen in exhaust gases. The developed method is implemented by stepwise correction of the ratio of the air-fuel mixture entering to the furnace of the boiler for combustion, according to feedback signals from a broadband oxygen sensor manufactured by Bosch, located in the outgoing channel.
 The air-fuel ratio control with automatic adjustment of the blower fan speed depending on the amount of fossil fuel supplied for combustion ensures low-toxic combustion of fuel with low emissions of nitrogen oxides and carbon monoxide, and high efficiency. Additional use of a variable frequency drive in the combustion control system allows to reduce energy consumption by 30-40 %, eliminate starting currents and motor overloads, reduce mechanical wear of equipment, increase the service life of contact switching equipment. In general, the developed fuel combustion control system allows to optimize the fuel combustion mode, taking into account the actual conditions, operating modes of the boiler unit and fuel characteristics; to reduce specific fuel consumption by at least 10%; to reduce the level of emissions of nitrogen oxides up to 40 % and carbon monoxide up to 50 %; to increase the efficiency by at least 5 %; qualitatively to simplify the work of the boiler maintenance personnel.

The effect of premixed stratification on the wave dynamics of a rotating detonation combustor

Typical injection schemes of rotating detonation combustors inject fuel locally into the combustion channel, creating stratified fuel-rich and fuel-lean mixing regions. In this study, premixed hydrogen and air rotating detonations are explored in a rotating detonation combustor through premixing part of the fuel into the oxidizer flow. The objective is to investigate the effect of premixing on the operation of the combustor. Three premixing schemes are examined where the detonation wave speeds are analyzed. The results show that in premixing, the fuel-lean regions became more favorable for continuous detonation propagation when premixed with the bypass fuel, resulting in higher detonation wave speeds. This phenomenon is shown to be independent of the global fuel-air equivalence ratio and the amount of fuel premixed into the oxidizer. As such, combustor performance and the operational regime could be improved with lean hydrogen premixing amounts in the main flow oxidizer.

Local entropy generation and entropy fluxes of a transient flame during head-on quenching towards solid and hydrogen-permeable porous walls

Premixed H2-air flames are studied in a one-dimensional wall-bounded configuration. The laminar flame propagates towards and quenches at a wall that is either solid or permeable. Entropy generation by each of 19 elementary reactions is evaluated. Their total contribution remains the most important up to the quenching instance. Close to quenching, the conduction entropy generation grows considerable. Mass diffusion has a modest contribution, which decreases towards quenching. Viscous forces are negligible as a source of entropy. Effects of unburnt-mixture temperature and fuel-air ratio are investigated, and also dilution with nitrogen (inert) and water vapour. The diffusive entropy flux changed direction away from the permeating wall compared that of the solid wall. A major finding is that fuel permeation through the wall tends to decrease the entropy generation per unit of converted fuel, in particular for initially lean mixtures.

Kaji Eksperimen Terhadap Penggunaan Bahan Bakar Emulsi Pada Burner

The purpose of this study was to observe the combustion of diesel fuel combined with the percentage volume of water and emulsifier, namely alkyl benzene sulfonic acid (ABS). The emulsion fuel composition used in this experimental study is a mixture of diesel fuel, emulsion and water. The composition of the emulsion fuel is designed according to the percentage of water volume and the emulsifier in diesel fuel with a variation of the percentage of 10%, 20%, and 30%. The parameters observed were combustion flame temperature, combustor annulus wall temperature, hot gas temperature, air fuel ratio (AFR), heat loss, and flame shape from differences in the composition of the percentage volume of emulsion fuel. In this test using Combustion Laboratory Unit C 491. The results showed that the heat absorbed due to cooling (Qa) in the AFR stoichiometry of diesel fuel (diesel) 26.334 kW was smaller than emulsion fuel (30.096 kW), so the use of emulsion fuel on the Burner Combustion Laboratory Unit C 49 is very suitable. The shape of the flame produced by the combustion of emulsion fuel is short, turbulent, and covered in water vapor.

Developing a model for prediction of the combustion performance and emissions of a turboprop engine using the long short-term memory method

In this study, the exhaust emissions index and combustion efficiency of the single shaft T56-A-15 engine are modeled using the Long-Short Term Memory (LSTM) method, one of the recent artificial intelligence algorithms. For this purpose, emissions data based on air–fuel ratio (AFR), engine speed (RPM) and different fuel flow rate parameters are used experimentally under different loads. In the designed LSTM models, fuel flow, engine speed and AFR are used as input parameters for the prediction of exhaust emission indices, engine speed and AFR data is used as an input parameter for the prediction of combustion efficiency. In the designed system, the experimental data is divided into two, 80% training and 20% test, by crossing according to the k-fold 5 value. Mean Absolute Error (MAE), Mean Square Error (MSE), Root Mean Square Error (RMSE), Mean Absolute Percentage Error (MAPE) error functions and the R squared (R2) function are used in the evaluation of the designed LSTM models. The originality of this study is the prediction of the exhaust emissions index and combustion efficiency values for the T56-A-15 turboprop engine using the LSTM method, which is an artificial intelligence method. This study attempts to address the literature gap in the calculation of the CO, CO2, UHC, NO2 exhaust emissions index and combustion efficiency values with an accuracy of over 95%, without the need for hundreds of experimental studies required for intermediate values.

Combustion analysis of a stratified pre-chamber ignition system by means of a zero-dimensional turbulence and flame speed model

In the current work, the combustion process inside a stratified pre-chamber ignition system is analyzed. The pre-chamber is installed on a Rapid Compression-Expansion Machine (RCEM) and runs at different combinations of fuel-air equivalence ratio in the main chamber as well as in the pre-chamber, which is controlled by a fuel injection during the compression stroke. In order to analyze the results in detail, a zero-dimensional model for the pre-chamber is proposed. First, a K-k-ϵ model is developed and calibrated according to three-dimensional simulations in motoring conditions performed in a Computational Fluid Dynamic solver (Converge). Then, a thermodynamic model including the mass exchange between both chambers as wells as the heat transfer losses to the walls is used to compute the instantaneous heat release rate in the pre-chamber. Both pieces of information are combined to evaluate the effective flame propagation speed, and also to decouple the effects in terms of laminar flame speed, turbulence-flame interaction and gas velocity due to expansion effects in the burned products. The flame speed values obtained are consistent when the equivalence ratio in the pre-chamber is maintained, regardless the conditions in the main chamber, while a significant deterioration is seen once lean operation appears in the pre-chamber. Finally, the flame speed is compared to an average propagation speed estimated from broadband chemiluminescence visualization tests, showing good consistency with the model predictions.

Application of machine learning tools for constrained multi-objective optimization of an HCCI engine

Because of strict emission regulations, significant efforts are made by researchers to reduce the pollutants from internal combustion engines. Homogeneous mixture of fuel and air in Homogeneous Charge Compression Ignition (HCCI) engine results in lower amounts of thermal NOx while CO and HC emissions would be higher in this process. Besides, performance instability due to knock and misfire is another problem that HCCI engines are facing. In this paper, machine learning tools have been implemented to learn the behavior of a single-cylinder engine using three different fuels with the HCCI strategy. Changes in six objective functions with engine Inlet Valve Closing (IVC) temperature and air-fuel equivalence ratio are studied with the help of an Artificial Neural Network (ANN). Those objective functions are thermal efficiency, combustion efficiency, IMEP, together with NOx, CO, and HC emissions. A multi-objective optimization algorithm based on a decomposition method and genetic algorithm is used to identify the best operating condition of studied fuels. Results show that HCCI engine fueled with Ethanol and Methanol could produce significantly lower HC and CO emissions at their optimum point. However, the produced NOx would be higher in these cases comparing to the engine fueled with CNG. The genetic algorithm indicates that the optimum point of operation for CNG-fueled engine is TIVC ≈ 445 K while λ ≈ 2.25. In contrast, for the Ethanol-fueled case, this optimum point is located in the IVC temperature of ≈450 K and λ ≈ 2.75. The TOPSIS point is located in minimum air-fuel equivalence ratio and lower IVC temperatures (TIVC ≈ 420 K) when the engine is being operated with Methanol as fuel.

Effect of compression ratio on engine knock, performance, combustion and emission characteristics of a bi-fuel CNG engine

Compressed natural gas (CNG) is considered one of the most promising alternative fuel used in the transport sector. CNG has high octane number, wide flammability limit, and high H/C ratio, brings in important aspects regarding its feasibility as a fuel for internal combustion engines. It is necessary to have a dedicated engine rather than retrofitted, bi-fuels or dual-fuel ones to exploit the full potential of CNG properties. Most vehicles use the CNG in bi-fuel mode, where the fuel system is modified to operate either on gasoline or CNG. Therefore, the engine's compression ratio (CR) needs to be determined according to gasoline fuel requirement. However, CNG can be used at a higher CR, and the performance of the engine can be improved. The aim of this study is to investigate the effect of CR on knock intensity, performance, combustion, and emissions of a spark ignition (SI) engine fuelled with gasoline and CNG. Gasoline and CNG engine's performance were compared at different CRs and stoichiometric air-fuel ratio. The results show that the knock intensity was high at CR 12 and 7 bar indicated mean effective pressure (IMEP) for the gasoline-fueled engine. The maximum CR was limited to 12 for the gasoline engine. However, Knock was not observed even at CR 16 for the CNG engine. The engine was successfully operated at CR 16 with CNG. The performance and emission of the engine were also compared and presented. Fuel consumption and thermal efficiency were improved for CNG at higher CR compared to the gasoline engine.

Physics-based linear model predictive control strategy for three-way catalyst air/fuel ratio control

The Current practice of air-fuel ratio control relies on empirical models and traditional PID controllers, which require extensive calibration to maintain the post-catalyst air-fuel ratio close to stoichiometry. In contrast, this work utilizes a physics-based Three-Way Catalyst (TWC) model to develop a model predictive control (MPC) strategy for air-fuel ratio control based on internal TWC oxygen storage dynamics. In this paper, parameters of the physics-based temperature and oxygen storage models of the TWC are identified using vehicle test data for a catalyst aged to 150,000 miles. A linearized oxygen storage model is then developed from the identified nonlinear model, which is shown via simulation to follow the nonlinear model with minimal error during nominal operation. This motivates the development of a Linear MPC (LMPC) framework using the linearized TWC oxygen storage model, reducing the requisite computational effort relative to a nonlinear MPC strategy. In this work, the LMPC utilizing a linearized physics-based TWC model is proven suitable for tracking a desired oxygen storage level by controlling the commanded engine air-fuel ratio, which is also a novel contribution. The offline simulation results show successful tracking performance of the developed LMPC framework.

Evaluation of the Effects of Diesel Fuel Injection Timing in a High-Speed RCCI Engine at Low to High Load Conditions

This numerical research mainly aimed to investigate the impacts of diesel fuel direct injection timing on the combustion characteristics, emission formation, and performance in a high-speed diesel-gasoline Reactivity Controlled Compression Ignition (RCCI) engine under low, medium, and high load operating conditions. The numerical achievements indicated that by late diesel injection timing (32 Crank Angle (CA) Before Top Dead Center (BTDC)), regions with a higher temperature and equivalence ratio were formed in the combustion chamber and caused a simultaneous increase in both Nitrogen Oxides (NOx) and soot emissions. On the contrary, early Diesel Injection Timing (DIT) under low load conditions concurrently reduced NOx and soot due to appropriate air-fuel ratio, more homogeneous air-fuel mixture formation as a result of longer Ignition Delay (ID) period, and the absence of high-temperature regions inside the combustion chamber. Also, when DIT was postponed under high load conditions, the combustion process became unstable and noise emission, as well as detonation tendency (sudden auto-ignition), were dramatically increased due to a considerable increase in in-cylinder maximum temperature and pressure rise rate. Furthermore, under low load conditions, because of low flame temperature and its incompetent propagation in the engine cylinder, a substantial amount of unburnt mixture (gasoline) was formed and accumulated at the center of the combustion chamber.