Comparisons between thermodynamic and one-dimensional combustion models of spark-ignition engines

A one-dimensional combustion model, employing a constant eddy diffusivity and a one-step chemical reaction, has been developed and applied to study the flame propagation in a spark-ignition engine. Calculations have been made at 1600 and 4200 rev min⁻¹ under fuel rich conditions and compared with available engine pressure data. One- and two-zone thermodynamic models have also been developed and applied to study the combustion process in the engine. The thermodynamic models have been compared with the one-dimensional model results and comparisons include the average mixture temperature, the temperatures of the burned and unburned gases and the flame surface area. These comparisons indicate that the one-dimensional model predictions are very sensitive to the eddy diffusivity and reaction rate data. The two-zone thermodynamic model predicts, first, a monotonically increasing flame surface area with time and, then, a monotonically decreasing surface area, whereas the one-dimensional model always predicts a monotonically increasing flame surface area. The average mixture temperature predicted by the one-zone thermodynamic model is higher than those of the two-zone and one-dimensional models during the compression stroke, while that of the one-dimensional model is higher than the temperatures predicted by the one- and two-zone models during the expansion stroke. The one-dmensional model predicts an accelerating flame even when the front approaches the cold cylinder wall. This yields a faster fuel consumption rate than those predicted by the one- and two-zone thermodynamic models which predict smoother burned fuel mass profiles.     

Mathematical modeling and validation of mass transfer phenomenon in homogeneous charge compression ignition engines based on a thermodynamic multi zone model

The main purpose of the current study is mathematical modelling and validation of mass transfer phenomenon in homogeneous charge compression ignition engines. A validated multi-zone model coupled to a semi-detailed chemical kinetics is used to predict homogeneous charge compression ignition combustion and emissions. Heat and Mass transfer submodels are linked to the multi-zone model. Bulk flow and diffusion mass transfer between zones are considered. The results indicate that the diffusion mass transfer is negligible in homogeneous charge compression ignition engines. Bulk flow mass transfer plays a critical role in homogeneous charge compression ignition simulation and applying it in the multi-zone model leads to accurate prediction of the start of combustion, peak pressure and exhaust emissions. The results show that the maximum error changes from 90% to 5% in carbon monoxide prediction and from 98% to 14% in unburned hydrocarbons prediction, using the mass transfer submodel.     

Turbulent flame propagation with large chemical-heat release

A model is developed to describe the dependence of the turbulent-flame speed on the intensity of an isotropic excitation turbulence prescribed far upstream from the flame for arbitrarily large gas expansion within the flame. For the limit of negligible gas expansion within the flame the new prediction of the present study reduces to an established and recently verified result for isothermal front propagation. It is shown that the turbulent-flame speed varies inversely with the square of the temperature ratio across the flame when the temperature ratio is very large. For typical hydrocarbon flames the results predict generally less substantial rates of decrease of the turbulent-flame speed with increasing heat release. Variations in turbulence kinetic energies and vorticity across the flame and hydrodynamic zones upstream and downstream from the flame are determined as well, accounting for influences of gas expansion and the structure of the excitation turbulence. The results of the present work, which are valid for flame propagation in weakly turbulent flow (where the propagation speed is proportional to the square of the intensity of the excitation turbulence prescribed far upstream from the flame) extend earlier predictions that were limited to relatively small chemical-heat release. The model presented herein does not account for effects of intrinsic flame instability and is appropriate for conditions where influences of buoyancy and flame stretch on flame dynamics are not substantial.     

Effects of engine design and operating parameters on the performance of a spark ignition (SI) engine with steam injection method (SIM)

The effects of engine design and operating parameters such as equivalence ratio (ER), compression ratio (CR), cycle pressure ratio (CPR), cycle temperature ratio (CTR), bore-stroke length ratio (D/L) inlet pressure, inlet temperature, friction coefficient (FC), mean piston speed (MPS) and engine speed on the performance characteristics such as brake thermal efficiency (BTE) and brake power output (BPO) are investigated for a steam injected gasoline engine (SIGE) with a simulation model validated with experiments using a realistic finite-time thermodynamics model (FTTM). Moreover, the energy losses arising from exhaust output (EO), heat transfer (HT), friction (FR) and incomplete combustion (IC), are illustrated by using graphs. The optimum values of engine speed, compression ratio, equivalence ratio, cycle temperature ratio and pressure ratio are presented by grid curves. Also, they are called performance maps. The results showed that the performance characteristics improve with enhancing inlet pressure, cycle pressure ratio and cycle temperature ratio; with diminishing inlet temperature and friction coefficient. The BPO can be increased up to 42%, 55% and 62% by using the optimum values of cycle pressure ratio, cycle temperature ratio and inlet pressure, respectively. Also, the BTE can be increased up to 8%, 12% and 15%, by the same way. On the other hand, the performance characteristics can improve or deteriorate with respect to different conditions of compression ratio, engine speed, equivalence ratio, stroke length and mean piston speed. Therefore, the optimum values should be determined to obtain the maximum performance conditions.     

Effects of mean piston speed,equivalence ratio and cylinder wall temperature on performance of an Atkinson engine

The performance of an air standard Atkinson cycle is analyzed using finite-time thermodynamics. In the model, the linear relation between the specific heat ratio of the working fluid and its temperature, the friction loss computed according to the mean velocity of the piston, the internal irreversibility described by using the compression and expansion efficiencies and the heat transfer loss are considered. The relations between the power output and the compression ratio and between the power output and the thermal efficiency are derived by detailed numerical examples. The results show that if the compression ratio is less than a certain value, the power output decreases with increasing mean piston speed, while if the compression ratio exceeds a certain value, the power output first increases and then starts to decrease with increasing mean piston speed. With further increase in the compression ratio, the increase of mean piston speed results in decreasing the power output. Throughout the compression ratio range, the power output increases with increasing cylinder wall temperature while it first increases and then starts to decrease with the increase of equivalence ratio. The conclusions of this investigation are of importance when considering the designs of actual Atkinson engines.     

A two-dimensional intake manifold flow simulation

A computer flow model of an intake manifold of a four cylinder engine has been developed using a computational fluid dynamic code. This code is base on the Conchas-Spray code developed at the Los Alamos National Laboratory. The flow inside the intake manifold is assumed to be two-dimensional, unsteady, compressible, and turbulent. A simple subgrid scale (SGS) turbulence model and hypothetical boundary conditions are employed in the simulation. Atmospheric pressure is specified at the inlet and the velocities are specified at the outlets of the manifold, together with the law of the wall at all the wall boundaries. Numerical results of the simulation are presented in the form of velocity, pressure, density, and temperature fields. The model is designed in such a way that different manifold geometries may be simulated with ease. A simulation of a concept manifold “the loop-manifold” is also presented.     

Buoyancy-assisted flow of yield stress fluids past a cylinder: Effect of shape and channel confinement

The aiding-buoyancy mixed convection heat transfer in Bingham plastic fluids from an isothermal cylinder of elliptical and circular shape in a vertical adiabatic channel is numerically investigated. For a fixed shape of the elliptical cylinder E = 2 (ratio of major to minor axes), the effect of confinement is studied for three values of blockage ratio, B, defined as the ratio of the channel width to the circumference of the cylinder/π, as 6.5, 2.17 and 1.3. In order to delineate the role of cross-section of the cylinder, results are also presented here for a circular cylinder of the same heat transfer area as the elliptical cylinder. The results presented herein span the range of conditions as: Bingham number, 0 ≤ Bn ≤ 100, Reynolds number, 1 ≤ Re ≤ 40, and Prandtl number, 1 ≤ Pr ≤ 100 over the range of Richardson number Ri = 0 (pure forced convection) to Ri = 10. Extensive results on drag coefficient, local and surface averaged values of the Nusselt number and yield surfaces are presented herein to elucidate the combined effects of buoyancy, blockage ratio and fluid yield stress. The morphology of the yield surfaces shows that the unyielded plug regions formed upstream and downstream of the cylinder grow faster at low Reynolds numbers with the increasing yield stress effects under the weak buoyancy forces, i.e., small values of Grashof or Richardson number. The heat transfer enhancement is observed with the increasing channel-confinement due to the sharpening of the temperature gradients near the surface of the cylinder. The average Nusselt number shows a positive dependence on the Reynolds number, Prandtl number and Richardson number irrespective of the shape of the cylinder or the type of fluid. By employing the modified definitions of the dimensionless parameters (based on the two choices of the overall effective fluid velocity), predictive correlations have been established for estimating the value of the average Nusselt number in a new application.     

Mathematical modeling and comparison of air standard Dual and Dual-Atkinson cycles with friction,heat transfer and variable specific-heats of the working fluid

Based on finite-time thermodynamics, a comparative performance analysis of air standard Dual and Dual-Atkinson cycles with heat-transfer loss, friction like term losses and variable specific-heats of the working fluid have been performed. Also the effects of heat loss, as characterized by a percentage of the fuel’s energy, friction and variable specific-heats of the working fluid, on performance of the mentioned irreversible cycles are analyzed. Moreover, detailed numerical examples show the relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the thermal efficiency of cycles. Results show the importance of consideration of heat loss effects on the both cycles’ performance. Also performance comparison of two cycles show that heat efficiency and power output of a Dual-Atkinson cycle are higher than a Dual cycle’s ones. The results obtained from this paper will provide guidance for the design of Dual-Atkinson engines.     

Premixed Turbulent Combustion Modelling Using Level Set Approach

The paper deals with a level set approach application to SI engine combustion modelling, which is based on solving an additional transport equation to determine the flame front propagation. The presented work is an extension of the paper [6]. The influence of engine speed, air excess, swirl number, engine load as well as application of different turbulence model, in.uence of mesh coarseness and model fine-tuning constants are investigated and the results are presented. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Performance analysis of an irreversible Miller cycle with considerations of relative air–fuel ratio and stroke length

The performance of an air standard Miller cycle is analyzed using finite-time thermodynamics. The results show that if compression ratio exceeds certain value, the power output first increases and then starts to decrease with increasing relative air–fuel ratio, while if compression ratio exceeds certain value, the power output decreases with increasing relative air–fuel ratio. The results also show that if compression ratio is less than certain value, the power output decreases with increasing stroke length, while if compression ratio exceeds certain value, the power output first increases and then starts to decrease with increasing stroke length. With further increase in compression ratio, the increase of stroke length results in decreasing the power output. The results obtained from this work can be helpful in the design and evaluation of practical Miller engines.     

Numerical simulation of two-stage combustion in SI engine with prechamber

Burning lean mixture in spark ignition (SI) engine leads to decrease in temperature of combustion process and is one of the methods of limiting nitric oxide emission and increasing the engine efficiency. The two-stage combustion system of stratified mixture (engine with prechamber) can be an effective method of lean mixture combustion. The paper presents the results of three-dimensional modeling of fuel mixture preparation and combustion in SI engine with sectional combustion chamber powered by liquefied fuel. Three dimensional modeling was performed in KIVA-3V code. The modeling results were compared with results obtained from the analysis of experimental measurements of two-stage combustion test engine operating at the Institute of Internal Combustion Engines and Control Engineering (Czestochowa University of Technology). The performed simulations of the combustion process provided data concerning the spatial and temporal distributions of turbulent kinetic energy, pressure, temperature and nitric oxides concentration in the combustion chambers of the engine. The engine model with two-stage combustion system properly represents the real processes which occur in the combustion chambers of the test engine. Pressure and temperature courses in function of CA obtained from the experiment and modeling were in good qualitative and quantitative consistence. Comparison of modeled and measured nitric oxide emissions revealed relatively significant discrepancies. In case of λ = 1.4, the measured values of NO_x concentration were 1.75 times higher than the modeled values. In case of λ = 2.0, the modeled and measured values were close to each other and were within the range of measurement error.     

Finite time exergoeconomic performance for six endoreversible heat engine cycles: Unified description

The operation of a universal steady flow endoreversible heat engine cycle model consisting of two constant thermal-capacity heating branches, a constant thermal-capacity cooling branch and two adiabatic branches is viewed as a production process with exergy as its output. The finite time exergoeconomic performance optimization of the universal endoreversible heat engine cycle is investigated by taking profit optimization criterion as the objective. The analytical formulae for power, efficiency and profit rate function of the universal endoreversible heat engine cycle with heat resistance loss are derived. The focus of this paper is to search the compromised optimization between economics (profit) and the utilization factor (efficiency) for endoreversible cycles. Moreover, analysis and optimization of the model are carried out in order to investigate the effect of cycle process on the performance of the universal endoreversible heat engine cycle using numerical examples. The results obtained herein include the performance characteristics of six endoreversible heat engines, including Carnot, Diesel, Otto, Atkinson, Brayton and Dual cycles.     

Application of Enthalpy Deficit Flamelet Model in Spray Combustion Simulation北大核心CSCD

An OpenFOAM-based solver for spray combustion simulation with the large eddy simulation (LES) and the flamelet generated manifold (FGM) method, was developed. A simple reduction of the temperature was employed to account for the evaporative heat loss. The solver was firstly validated against the Sydney piloted ethanol spray flame benchmark EtF7. The predicted mean gas temperature and droplet statistics correspond well with the experimental data and have similar accuracy to the spray flamelet model. The turbulence-chemistry interaction modeling may have a larger influence on the simulation accuracy. Then a realistic gas turbine slinger combustor was simulated with 2 sets of operating conditions. The simulation results reveal different flame characteristics of the 2 working conditions. The predicted total pressure losses are close to the measured values. © 2023 Editorial Office of Applied Mathematics and Mechanics. All rights reserved.     

Optimization of the design of recuperative heat exchangers in the exhaust nozzle of an aero engine

Today the needs for safer, cleaner and more affordable civil aero engines are found to be of great importance. Five years ago, the EU initiated an action for the design and the construction of efficient and environmentally friendly aero engines (EEFAE). One of the major European gas turbine industries, MTU, has presented a new technology for an advanced aero engine design, which uses an alternative thermodynamic cycle. The basis of this cycle is the adoption of a recuperation part with the use of a system of heat exchangers, installed in the exhaust nozzle of the aircraft engine. Thermal energy in the turbine exhaust is used in the recuperator to pre-heat the compressor outlet air before combustion. The benefits of this technique are focused on reduced pollutants and decreased fuel consumption. In this work, the procedure of the optimization of this installation, by means of the imposed pressure drop downstream the aircraft engine and the balanced mass inflow to the heat exchangers is presented. The optimization is based on experimental measurements in laboratory conditions and preliminary 2D CFD modeling for the flow inside the exhaust duct and through the heat exchangers. It is shown that with a careful approach, a better arrangement of the heat exchangers can be achieved in order to have a minimum pressure drop in the exhaust nozzle which can positively affect the engine’s performance.     

Simulation of Premixed Turbulent Combustion in a Gas Engine using the Immersed Boundary Method and a Level Set Flame Model

An efficient simulation approach for turbulent flame brush propagation is a level set formulation closed by the turbulent flame speed. A formulation of the level set equation with the corresponding treatment of the turbulent mass burning rate that is compatible with standard Finite Volume discretization schemes available in computational fluid dynamics codes is employed. In order to simplify and to speed up the meshing process in complicated geometries (here in gas engines) the immersed boundary method in a continuous formulation, where the forces replacing the boundaries are introduced in the momentum conservation equations before discretization, is employed. In our contribution, aspects of the numerical implementation of the level set flame model combined with the immersed boundary formulation in OpenFOAM are presented. First representative simulation results of a homogeneous methane/air mixture combustion in a simplified engine geometry are shown. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A study on torsional vibration attenuation in automotive drivetrains using absorbers with smooth and non-smooth nonlinearities

The automotive industry is predominantly driven by legislations on stringent emissions. This has led to the introduction of downsized engines, incorporating turbocharging to maintain output power. As downsized engines have higher combustion pressures, the resulting torsional oscillations (engine order vibrations) are of broadband nature with an increasing severity, which affect noise and vibration response of the drive train system. Palliative devices, such as clutch pre-dampers and dual mass flywheel have been used to mitigate the effect of transmitted engine torsional oscillations. Nevertheless, the effectiveness of these palliative measures is confined to a narrow band of response frequencies. The nonlinear targeted energy transfer is a promising approach to study vibration mitigation within a broader range of frequencies, using nonlinear vibration absorbers (or nonlinear energy sinks – NESs). These devices would either redistribute vibration energy within the modal space of the primary structure, thus dissipating the vibrational energy more efficiently through structural damping, or passively absorb and locally dissipate a part of this energy (in a nearly irreversible manner) from the primary structure. The absence of a linear resonance frequency of an NES, enables its broadband operation (in contrast to the narrowband operation of current linear tuned mass dampers). Parametric studies are reported to determine the effectiveness of various smooth or non-smooth nonlinear stiffness characteristics of such absorbers. A reduced drivetrain model, incorporating single and multiple absorber attachments is used and comparison of the predictions to numerical integrations proves its efficacy.     

Thermal noise engines

In this paper, electrical heat engines driven by the Johnson-Nyquist noise of resistors are introduced. They utilize Coulomb’s law and the fluctuation–dissipation theorem of statistical physics. In these engines, resistors, capacitors and switches are the building elements. For best performance, a large number of parallel engines must be integrated to run in a synchronized fashion, and the size of an elementary engine must be at the 10 nm scale. At room temperature, in the most idealistic case, a two-dimensional ensemble of engines of 25 nm size integrated on a 2.5 × 2.5 cm silicon wafer with 10 °C temperature difference between the warm-source and the cold-sink would produce a power of about 0.5 W. Regular and coherent (correlated-cylinder states) versions of these engines are shown and both of them can operate in either four-stroke or two-stroke modes. In the idealistic case, all these engines have Carnot efficiency, which is the highest efficiency possible in any heat engine without violating the second law of thermodynamics.     

Numerical Investigations of Process Heterogeneities during High Hydrostatic Pressure Treatment with Turbulent Inflow Conditions

During the compression phase of high pressure treatment of aqueous food, the temperature locally increases approx. 2-3 K per 100 MPa in the isentropic case as a result of the external work done by the volume change of the fluid. It could be concluded in the authors' previous contributions that heat transfer induces undesired process non-uniformities, and therefore, significantly influences the product quality. However, the turbulent inflow conditions existing in larger autoclaves are expected to change homogeneity of the thermofluiddynamical fluid. In this contribution, the presence of the turbulence generated during the compression phase of the High -Pressure-treatment and its influence on process non-uniformities throughout the high pressure processing are investigated numerically. For the numerical simulations, a three-dimensional model of a cylindrical autoclave with a net volume of 3.3 Litre is created. To investigate the influence of the turbulence on product quality, a model of the inactivation of the enzyme Bacillus Subtilis α-Amylase is employed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Performance of irreversible heat engines at minimum entropy generation

A thermodynamic analysis is presented by means of mathematical formulation to examine the performance of the most common types of heat engines including Otto, Diesel, and Brayton cycles, at the regime of minimum entropy generation. All engines are subject to internal and external irreversibilities. It is shown that minimum entropy production criterion neither correlates with maximum thermal efficiency design nor with maximum work output criterion. The results demonstrate that the production of entropy is not necessarily equivalent to the energy losses taking place in real devices.