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Effects of injection pressure and timing on low load Low Temperature Gasoline Combustion using LES
... Combustion modes are principally categorized into spark ignition and compression ignition technologies. Compression ignition technologies encompass homogeneous charge compression ignition (HCCI) [44,64], partially premixed compression ignition (PPCI) [20,21,52], reactivity-controlled compression ignition (RCCI) [14,26], stratified charge compression ignition (SCCI) [13] and intelligent charge compression ignition (ICCI) [88,89]. Spark ignition technologies are further subdivided into thermal plasma ignition and non-thermal plasma ignition. ...
Review Key Technologies to 50% Brake Thermal Efficiency for Gasoline Engine of Passenger Car Xinke Miao, Bingxin Xu, Jun Deng, and Liguang Li * School of Automotive Studies, Tongji University, Shanghai 201804, China * Correspondence: liguang@tongji.edu.cn Received: 13 August 2024; Revised: 6 December 2024; Accepted: 17 December 2024; Published: 20 January 2025 Abstract: As fuel consumption and emissions regulations become increasingly stringent, various advanced strategies have been proposed to achieve higher efficiency in internal combustion engines. This paper reviews the advancements in thermal efficiency of gasoline engines and analyzes the key technological methods to achieve over 50% brake thermal efficiency (BTE). The technological routes proposed for high-efficiency gasoline engine are primarily focused on high compression ratios and lean combustion combined with novel combustion technologies. Supporting technologies mainly include Atkinson/Miller cycles, intake boosting, exhaust gas re-circulation (EGR), water injection, thermal barrier coatings, friction reduction, structural optimization, and combustion diagnostics and control.
... Modeling ϵ SGS is extremely important for LES, which concludes a misunderstanding of the flow topology with artificial fluctuations by incorrect magnitude of C ϵ [56]. The Dynamic-Structure Large-Eddy Simulation (DS-LES) is used in this work, because this closure of the Leonard stresses has been shown to be highly accurate for mixing related flow physics compared with direct numerical simulation (DNS) data [57,58] and has a broad literature history of success for modeling flows with sharp temperature and density gradient [59,60,61,62,63]. Other LES subgrid closures are not evaluated in this work. ...
Existing literature on ejectors, different researchers utilized different turbulence models by their choice, neither analyzing the physics of turbulence nor providing any sufficient optical diagnostics of the flow field. In this article, the physics captured by the Unsteady Reynolds Averaged Navier–Stokes (URANS) and Large Eddy Simulation (LES) are compared based on well-posed boundary conditions obtained experimentally from a carbon dioxide (CO2) vapor compression cycle with an error
. The experimental results utilized for this study are at Reynold’s number (Re) 1.2e5 at the high-pressure inlet (motive flow) maintaining CO2 in subcritical state. The low-pressure inlet (suction flow) is maintained at Re = 9.8e4 with CO2 in vapor state, providing a pressure lift ratio of 1.2 at the diffuser outlet. Comparison of flow topologies with different URANS models reveals that only standard, Shear Stress Transport (SST) k-omega, and Reynold’s Stress Model-Shear Stress Gradient (RSM-SSG) are the suitable URANS models capturing a linearly increasing anisotropy tensor component with increasing strain in the flow. LES also predicted the similar physics. However, standard k-omega overpredicted the suction pressure beyond the acceptable uncertainty limits, which made it inapplicable. For the first time, a spatio-temporal representation of the flow topology with LES revealed the actual jet morphology of a CO2 ejector. Specifically, finger-like structures are key features to entrain more surrounding warm fluid into the colder dense core fluid at the cost of destabilizing and breaking up the jet by stretching the streamwise vortices and obstructing the increase in pressure-lift ratio.
... Compositional stratification is an integral part of new combustion technologies such as Low-Temperature Combustion (LTC), Homogeneous Charge Compression Ignition (HCCI), Moderate and Intense Low-oxygen Dilution (MILD), etc. The compositional stratification is generally achieved in engines using direct injection inside the combustion chamber (Agarwal et al. 2023;Tan et al. 2016;O'Donnell et al. 2023) or by dilution of the fuel-air mixture (Telli et al. 2022;Fooladgar and Chan 2017;Zhao et al. 2019). With stratification, stoichiometric or slightly rich mixtures can be used near the ignition source, and ultra-lean mixtures can be used near the chamber walls. ...
Gasoline direct injection engines can provide higher thermal efficiency and lower emissions than that for engines using conventional combustion techniques. Compositional stratification inside the combustion chamber opens possibilities for ultra-lean and low-temperature combustion. To explore this further, 2D direct numerical simulation (DNS) has been performed to investigate the propagation of syngas flame in an equivalence ratio (ϕ) stratified medium. Several aspects of flame propagation, such as effect of integral scale of mixing (lϕ) on the non-monotonic behavior of flame propagation, contribution of each chemical reaction to heat release rate (HRR), and the effect of differential diffusion were analyzed using DNS-data. A spherically expanding flame has been initiated with a hotspot at the center of the square domain of size 2.4 × 2.4 cm². The variations in the degree of stratification were simulated varying lϕ and fluctuations ϕ for initial mixture distribution. Further this DNS-data has been used to analyze effects of stratification on flame displacement speed (Sd) and its components, viz. reaction rate (Sr), normal diffusion (Sn), tangential (St), and inhomogeneity (Sz). The results reveal that stratification-induced variations in thermal diffusivity resulted in thermal runaways. These thermal runaways influence the extent of burning for simulated cases. The increase in degree of stratification resulted in flame preferably propagating towards leaner ϕ, causing reduction in components of Sd. The preferential propagation of flame also resulted in shifting of peak reaction rate for fuel species (c*) to a higher reaction progress variable (c). This shifting of c*, lead to a reduction in the HRR contribution of reactions that attain their peak near the production zone of H and OH species. For unity Le simulations, Sn was observed to be reduced drastically compared to cases with differential diffusion, resulting in an overall reduction in Sd.
... Compositional stratification is an integral part of new combustion technologies such as Low-Temperature Combustion (LTC), Homogeneous Charge Compression Ignition (HCCI), Moderate and Intense Low-oxygen Dilution (MILD), etc. The compositional stratification is generally achieved in engines using direct injection inside the combustion chamber [4][5][6] or by dilution of the fuel-air mixture [7][8][9]. With stratification, stoichiometric or slightly rich mixtures can be used near the ignition source, and ultra-lean mixtures can be used near the chamber walls. ...
Gasoline direct injection (GDI) engines can provide higher thermal efficiency and lower emissions compared to conventional combustion techniques. The direct charge injection near the ignition source forms compositional stratification inside the combustion chamber. Compositional stratification inside the combustion chamber opens possibilities for ultra-lean and low-temperature combustion. In this paper, a 2D direct numerical simulation (DNS) has been performed to investigate the propagation of syngas flame in an equivalence ratio stratified medium. A spherically expanding flame has been initiated with a hotspot at the center of the domain. An open-source PENCIL code [Babkovskaia, 2011] is used to analyse the effect of stratification by simulating cases with varying integral scales of mixing ( l ϕ ) and fluctuations of equivalence ratio ( ϕ´ ). Effects of differential diffusion of species on flame propagation have also been examined by comparing results with cases with unity Lewis number ( Le =1). The results show that with an increase in l ϕ , flame propagation shows a non-monotonic behavior. With an increase in l ϕ , the flame speed and extent of burning increase first and then decrease. With an increase in ϕ´, the flame speed and extent of burning decreased consistently. The peak reaction rate of fuel species is also observed to be shifted to a higher reaction progress variable ( c ) with increased stratification. The effect of stratification and differential diffusion has been analysed for four identified components of flame displacement speed ( S d ) viz. reaction ( S r ), normal diffusion ( S n ), tangential ( S t ), and inhomogeneity ( S z ). S r and S n are observed to be major contributors to S d . The magnitude of S r shows reductions with an increase in stratification. In comparison, S n does not show significant change with increased stratification. The variation of the contribution of chemical reactions to heat release rate with stratification is also analysed in this study. The results show that shifting of peak reaction rate of fuel species to higher c values results in variation in heat release rate contribution for chemical reactions.
div class="section abstract"> Low-temperature gasoline combustion (LTGC) engines can provide high efficiencies with very low NOx and soot emissions, but rapid control of the combustion timing remains a challenge. Partial Fuel Stratification (PFS) was demonstrated to be an effective approach to control combustion in LTGC engines. PFS is produced by a double-direct injection (DI) strategy with most of the fuel injected early in the cycle and the remainder of the fuel supplied by a second injection at a variable time during the compression stroke to vary the amount of stratification. Adjusting the stratification changes the combustion phasing, and this can be done on cycle-to-cycle basis by adjusting the injection timing.
In this paper, the ability of PFS to control the combustion during wide engine load sweeps is assessed for regular gasoline and gasoline doped with 2-ethylhexyl nitrate (EHN). For PFS, the load control range is limited by combustion instability and poor combustion efficiency at low loads. However, late single-DI stratification was demonstrated to allow robust control at low loads with good combustion efficiency by concentrating the fuel in the middle of the chamber, avoiding overly lean regions. Stratification is more effective with EHN-doped gasoline than with straight gasoline because EHN enhances the reactivity and φ–sensitivity of the fuel. Thus, lower intake temperatures and less stratification are required when working with EHN-doped gasoline. The combination of PFS at higher loads and late single-DI at lower loads allows load control ranges from 1.0 to 4.8 bar IMEPg using regular gasoline and from idle to 5.6 bar IMEPg using EHN-doped gasoline at naturally aspirated conditions. Combustion control using only stratification is also demonstrated at boosted conditions, allowing the IMEPg to be varied from idle to 7.5 bar by combining two boost levels (1.3 and 1.0 bar intake) and two EHN flow levels.
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div class="section abstract"> Methanol emerges as a compelling renewable fuel for decarbonizing engine applications due to a mature industry with high production capacity, existing distribution infrastructure, low carbon intensity and favorable cost. Methanol’s high flame speed and high autoignition resistance render it particularly well-suited for spark-ignition (SI) engines. Previous research showed a distinct phenomenon, known deflagration-based knock in methanol combustion, whereby knocking combustion was observed albeit without end-gas autoignition.
This work studies the implications of deflagration-based knock on noise emissions by investigating the knock intensity and combustion noise at knock-limited operation of methanol in a single-cylinder direct-injection SI engine operated at both stoichiometric and lean (λ = 2.0) conditions. Results are compared against observations from a premium-grade gasoline. Experiments show that methanol’s end-gas autoignition occurs at lean conditions, leading to the typical autoignition-based knock as that occurring with premium-grade gasoline. However, at stoichiometric conditions, knock-limited operation is achieved with deflagration-based knock. Noise of deflagration-based knock has lower variability than that of autoignition-based knock and it does not seem to be an issue at the engine speed tested experimentally in this paper (1400 rpm). However, computational fluid dynamic large eddy simulations show that deflagration-based knock may lead to high noise levels at 2000 rpm. Deflagration-based knock is insensitive to changing spark timings, so new knock mitigation strategies are required, such as adjusting the spark energy and/or adding dilution. Finally, this study shows that deflagration-based-knock may be directly impacted by the flame speed, occurring more frequently with faster-burning fuels or under conditions that elevate flame speeds, like rich-stoichiometric operation. The finding bears implications on renewable e-fuels, such as ethanol, methanol and hydrogen.
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Flash boiling is the sudden change from liquid to vapour phase and in IC engines it typically occurs during the injection process with a gasoline direct injection (GDI) setup and when high fuel temperatures result in saturation pressures higher than the in-cylinder ones. The flash boiling, due to the complex evolution of a multi-phase flow and a rapid droplet vaporisation, promotes spray atomisation and affects significantly the spray structure and fuel-air mixture formation , with consequences for the engine performance and pollutant emissions. Hence, a deep knowledge of such phenomenon is required to improve the combustion process and reduce the pollutant production. The breakup mechanism of a superheated spray is quite different from a regular one, because it features the nucleation and growth of bubbles within liquid droplets. In this paper, an innovative flash-boiling breakup model was embedded in a Eulerian-Lagrangian two-phase solver using OpenFOAM library. The model assigns to the droplets a radial velocity component due to the bubble explosion, making it capable of reconstructing the spray expansion. The code was tested using experimental data of the ECN Spray G injector covering various thermodynamic conditions. The numerical model achieves a good level of agreement with the experimental data and, in particular, it is able to reconstruct the spray collapse. Besides, it provides useful information regarding parameters that are intrinsically difficult to measure experimentally.
Partial fuel stratification (PFS) is a low temperature combustion strategy that can alleviate high heat release rates of traditional low temperature combustion strategies by introducing compositional stratification in the combustion chamber using a split fuel injection strategy. In this study, a three-dimensional computational fluid dynamics (CFD) model with large eddy simulations and reduced detailed chemistry was used to model partial fuel stratification at three different stratified conditions. The double direct injection strategy injects 80% of the total fuel mass at −300 CAD aTDC and the remaining 20% of the fuel mass is injected at three different timings of −160, −50, −35 CAD to create low, medium, and high levels of compositional stratification, respectively. The PFS simulations were validated using experiments performed at Sandia National Laboratories on a single-cylinder research engine that operates on RD5-87, a research-grade E10 gasoline. The objective of this study is to compare the performance of three different reduced chemical kinetic mechanisms, namely SKM1, SKM2, and SKM3, at the three compositional stratification levels and identify the most suitable mechanism to reproduce the experimental data. Zero-dimensional chemical kinetic simulations were also performed to further understand differences in performance of the three reduced chemical kinetic mechanisms to explain variations in CFD derived heat release profiles. The modeling results indicate that SKM3 is the most suitable mechanism for partial fuel stratification modeling of research-grade gasoline. The results also show that the autoignition event progresses from the richer to the leaner compositional regions in the combustion chamber. Notably, the leaner regions that have less mass per unit volume, can contribute disproportionately more toward heat release as there are more cells at leaner equivalence ratio ranges. Overall, this study illuminates the underlying compositional stratification phenomena that control the heat release process in PFS combustion.
With the objective of enhancing the effectiveness of late fuel injection strategy in extending the low-load limit of homogeneous charge compression ignition engines, a numerical study is conducted to investigate the effects of fuel injection parameters, such as the injection pressure and spray cone angle, on the overall combustion efficiency and CO/NOx emissions. Closed-cycle engine simulations are performed incorporating detailed iso-octane reaction kinetics and combustion submodel based on the spray-interactive flamelet approach. Extensive parametric studies are conducted to provide a detailed map of the combustion efficiency and emission performance. In general, it is found that the in-cylinder charge stratification can be reduced by both an increased injection pressure and a wider spray cone angle, resulting in substantially lower NOx emissions and reasonably high combustion efficiency simultaneously. The present study demonstrates that an optimal adjustment of the two fuel injection parameters can result in significant extension of the low-load limit of homogeneous charge compression ignition through delayed fuel injection strategy.
A comparative cold flow analysis between Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) cycle-averaged velocity and turbulence predictions is carried out for a single cylinder engine with a transparent combustion chamber (TCC) under motored conditions using high-speed particle image velocimetry (PIV) measurements as the reference data. Simulations are done using a commercial computationally fluid dynamics (CFD) code CONVERGE with the implementation of standard k-epsilon and RNG k-epsilon turbulent models for RANS and a one-equation eddy viscosity model for LES. The following aspects are analyzed in this study: The effects of computational domain geometry (with or without intake and exhaust plenums) on mean flow and turbulence predictions for both LES and RANS simulations. And comparison of LES versus RANS simulations in terms of their capability to predict mean flow and turbulence. Both RANS and LES full and partial geometry simulations are able to capture the overall mean flow trends qualitatively; but the intake jet structure, velocity magnitudes, turbulence magnitudes, and its distribution are more accurately predicted by LES full geometry simulations. The guideline therefore for CFD engineers is that RANS partial geometry simulations (computationally least expensive) with a RNG k-epsilon turbulent model and one cycle or more are good enough for capturing overall qualitative flow trends for the engineering applications. However, if one is interested in getting reasonably accurate estimates of velocity magnitudes, flow structures, turbulence magnitudes, and its distribution, they must resort to LES simulations. Furthermore, to get the most accurate turbulence distributions, one must consider running LES full geometry simulations.
As engine technologies become increasingly complex and engines are driven to new
operating points, understanding transient phenomena is important to ensure reliable engine
operation. Unlike Reynolds Averaged Navier-Stokes (RANS) studies that only provide
cycle-averaged information, Large Eddy Simulation (LES) studies are capable of simulating
cycle-to-cycle dynamics. In this work, a finite difference based structured methodology
for LES of IC engines is presented. This structured approach allows for an efficient mesh
generation process and provides potential for higher order numerical accuracy. An
efficient parallel scalable block decomposition is done to overcome the challenges
associated with the low ratio of fluid elements to overall mesh elements. The motion of
the valves and piston is handled using a dynamic cell blanking approach and the Arbitrary
Lagrangian Eulerian (ALE) method, respectively. Modified three-dimensional Navier-Stokes
Characteristic Boundary Conditions (NSCBC) are used in the simulation to prescribe
conditions in the manifolds. The accuracy of the simulation framework is validated using
various canonical configurations. Flow bench simulations of an axisymmetric configuration
and an actual engine geometry are done with the LES methodology. Simulations of the gas
exchange in an engine under motored conditions are also performed. Overall, good agreement
is obtained with experiments for all the cases. Therefore, this framework can be used for
LES of engine simulations. In the future, reactive LES simulations will be performed using
this framework.
Two-dimensional direct numerical simulation (2D-DNS) is used to investigate the effect of turbulence intensity and composition stratification on the ignition of H2/air mixture in a constant volume enclosure relevant to homogeneous charge compression ignition (HCCI) engines. A comprehensive H2 chemical kinetic mechanism is adopted, involving 9 species and 21 elementary reactions. Different turbulence levels and composition fluctuations are simulated to investigate their influence on the combustion behavior. The results show that the ignition delay time tends to be prolonged and the heat release rate increased under higher turbulence intensity. Turbulence can affect the reaction zone, e.g., through wrinkling of reaction front. Higher composition stratification can smoothen the heat release rate. It is found that in HCCI engine flame propagation can co-exist with volumetric auto-ignition.
This article covers key and representative developments in the area of high efficiency and clean internal combustion engines. The main objective is to highlight recent efforts to improve (IC) engine fuel efficiency and combustion. Rising fuel prices and stringent emission mandates have demanded cleaner combustion and increased fuel efficiency from the IC engine. This need for increased efficiency has placed compression ignition (CI) engines in the forefront compared to spark ignition (SI) engines. However, the relatively high emission of oxides of nitrogen (NOx) and particulate matter (PM) emitted by diesel engines increases their cost and raises environmental barriers that have prevented their widespread use in certain markets. The desire to increase IC engine fuel efficiency while simultaneously meeting emissions mandates has thus motivated considerable research. This paper describes recent progress to improve the fuel efficiency of diesel or CI engines through advanced combustion and fuels research. In particular, a dual fuel engine combustion technology called “reactivity controlled compression ignition” (RCCI), which is a variant of Homogeneous Charge Compression Ignition (HCCI), is highlighted, since it provides more efficient control over the combustion process and has the capability to lower fuel use and pollutant emissions. This paper reviews recent RCCI experiments and computational studies performed on light- and heavy-duty engines, and compares results using conventional and alternative fuels (natural gas, ethanol, and biodiesel) with conventional diesel, advanced diesel and HCCI concepts.
The implementation and the combination of advanced boundary conditions and subgrid scale models for Large Eddy Simulation (LES) in the multi-dimensional open-source CFD code OpenFOAM® are presented. The goal is to perform reliable cold flow LES simulations in complex geometries, such as in the cylinders of internal combustion engines. The implementation of a boundary condition for synthetic turbulence generation upstream of the valve port and of the compressible formulation of the Wall-Adapting Local Eddy-viscosity sgs model (WALE) is described. The WALE model is based on the square of the velocity gradient tensor and it accounts for the effects of both the strain and the rotation rate of the smallest resolved turbulent fluctuations and it recovers the proper y3 near-wall scaling for the eddy viscosity without requiring dynamic procedure; hence, it is supposed to be a very reliable model for ICE simulation. Validation of the implemented models has been performed separately on two steady state flow benches, where experimental data were available: a backward facing step geometry and a simple IC engine geometry with one axed central valve. A method to evaluate the turbulence resolution of LES simulation based on a Length Scale Resolution (LSR) parameter has been proposed and a discussion about the completeness of the LES simulation (i.e. LES simulation quality) is presented. Finally, the implementation of a fully parallel algorithm for the management of topologically changing meshes in the OpenFOAM®-2.1.x technology is described. The aim is to implement a reliable framework to perform LES simulation of internal combustion engines by the OpenFOAM® technology.
A modified Redlich-Kwong equation of state is proposed. Vapor pressures of pure compounds can be closely reproduced by assuming the parameter a in the original equation to be temperature-dependent. With the introduction of the acentric factor as a third parameter, a generalized correlation for the modified parameter can be derived. It applies to all nonpolar compounds. With the application of the original generalized mixing rules, the proposed equation can be extended successfully to multicomponent-VLE calculations, for mixtures of nonpolar substances, with the exclusion of carbon dioxide. Less accurate results are obtained for hydrogen-containing mixtures.
A review of using large-eddy simulation (LES) in computational fluid dynamic stud-ies of internal combustion engines is presented. Background material on turbulence model-ling, LES approaches, specifically for engines, and the expectations of LES results are discussed. The major modelling approaches for turbulence, combustion, scalars, and liquid sprays are discussed. In each of these areas, a taxonomy is presented for the various types of models appropriate for engines. Advantages, disadvantages, and examples of use in the litera-ture are described for the various types of models. Several recent examples of engine studies using LES are discussed. Recommendations and future prospects are included.
The development of gasoline compression ignition engines operating in a low temperature combustion mode depends heavily on robust control of the heat release profile. Partial fuel stratification is an effective method for controlling the heat release by creating a stratified mixture prior to autoignition, which can be beneficial for operation across a wide load range. In this study, three-dimensional large eddy simulations were used to model a double direct injection strategy for which 80% of the fuel was injected during the intake stroke, and 20% of the fuel was injected at varying timing during the compression stroke. The simulations replicated a set of experiments performed at Sandia National Laboratories on a 1-L single-cylinder research engine using E10 gasoline (gasoline fuel containing 10% vol. ethanol). The objective of this study is to analyze the effects of the double direct injection strategy on the compositional and thermal stratification of the mixture, and understand the best use of this operating strategy. The modeling results indicated that by retarding the start of the second injection, the mixture stratification increases, which can be used to control the autoignition timing and the combustion phasing. Ignition and CA50 (crank angle of 50% mass fraction burned) are dictated by the mass concentration of the richest zones in the combustion chamber, as well as their location. The richer zones have the lowest temperatures before ignition primarily due to evaporative cooling from direct fuel injection. Overall, this study enhances the understanding of partial fuel stratification that can be used for controlling the heat release in gasoline compression ignition engines.
The utilization of biofuels in Low Temperature Combustion has shown great potential to decrease emissions and improve overall lifecycle energy efficiency. In particular, wet ethanol (a mixture of ethanol and water) as a domestically sourced biofuel has shown such potential. This study aims to determine what blend of wet ethanol would optimize combustion properties under HCCI operating conditions, both naturally aspirated and boosted. Four different blends are tested, and it is determined that WE80 (80% ethanol and 20% water by mass) exhibits optimal combustion characteristics when examining upstream intake temperature, combustion efficiency and thermal efficiency, regardless of intake boost level.
This work then studies how the combustion process of wet ethanol is affected by the addition of the cetane improver di-tertiary-butyl peroxide (DTBP). It was found that DTBP did not elicit any cool flame reactions from the ethanol, which is a single stage ignition fuel, regardless of boost level. DTBP was able to lower the required intake temperature of wet ethanol without significantly affecting the combustion process, i.e. no changes in burn duration or combustion efficiency. It is found that WE80 with 5% DTBP at 1.8 bar intake pressure required an intake temperature of ~320 K. While there is some disagreement about whether the effects of cetane improvers can be attributed to thermal or chemical causes, this study suggests that the effects are more likely attributable to chemical kinetics. DTBP is also shown to apparently increase wet ethanol’s equivalence ratio sensitivity slightly, which could have implications for other advanced combustion modes.
Despite the high technical relevance of early flame kernel development for the reduction of cycle-to-cycle variations in spark ignition engines, there is still a need for a better fundamental understanding of the governing in-cylinder phenomena in order to enable resilient early flame growth. To isolate the effects of small- and large-scale turbulent flow motion on the young flame kernel, a three-dimensional DNS database has been designed to be representative for engine part load conditions. The analysis is focussed on flame displacement speed and flame area in order to investigate effects of flame structure and flame geometry on the global burning rate evolution. It is shown that despite a Karlovitz number of up to 13, which is at the upper range of conventional engine operation, thickening of the averaged flame structure by small-scale turbulent mixing is not observed. After ignition effects have decayed, the flame normal displacement speed recovers the behavior of a laminar unstretched premixed flame under the considered unity-Lewis-number conditions. Run-to-run variations in the global heat release rate are shown to be primarily caused by flame kernel area dynamics. The analysis of the flame area balance equation shows that turbulence causes stochastic flame kernel area growth by affecting the curvature evolution, rather than by inducing variations in total flame area production by strain. Further, it is shown that in local segments of a fully-developed planar flame with similar surface area as the investigated flame kernels, temporal variations in flame area rate-of-change occur. Contrasting to early flame kernels, these effects can be exclusively attributed to curvature variations in negatively curved flame regions.
Thermally stratified compression ignition is a new advanced, low-temperature combustion mode that aims to control the heat release process in a lean, premixed, compression ignition combustion mode by controlling the level of thermal stratification in the cylinder. Specifically, this work uses a mixture of 80% ethanol and 20% water by mass, referred to as ‘‘wet ethanol’’ herein, to increase thermal stratification via evaporative cooling of areas targeted by an injection event during the compression stroke. The experiments conducted aim to both fundamentally understand the effect that a late cycle injection of wet ethanol has on the heat release process, and to use that effect to explore the high-load limit of thermally stratified compression ignition with wet ethanol. At an equivalence ratio of 0.5, injecting just 8% of the fuel during the compression stroke was shown to reduce the peak heat release rate by a factor of 2, subsequently avoiding excessive pressure rise rates. Under pure homogeneous charge compression ignition using wet ethanol as the fuel, the load range was found to be 2.5–3.9 bar gross indicated mean effective pressure. Using a split injection of wet ethanol, the high-load limit was extended to 7.0 bar gross indicated mean effective pressure under naturally aspirated conditions.
Finally, intake boost was used to achieve high-load operation with low NOx (oxides of nitrogen (NO or NO2)) emissions and was shown to further increase the high-load limit to 7.6 bar gross indicated mean effective pressure at an intake pressure of 1.43 bar. These results show the ability of a split injection of wet ethanol to successfully control the heat release process and expand the operable load range in low-temperature combustion.
In the past 20 years, Large Eddy Simulation methods have continuously increased their popularity among the Internal Combustion Engines modeling community, due to their intrinsic potential in the description of the unsteady and randomly generated in-cylinder flow structures. Such capability has gained further relevance in the simulation of modern turbocharged GDI engines, where the high-fidelity resolution of cycle-to-cycle variability phenomena is crucial for the evaluation of the engine performance and emission trends. Nonetheless, even after many years of development the application of standard LES methods to full-scale engine geometries is still not straightforward, due to: the need for specific, turbulence-generating boundary conditions at open ends; more severe grid resolution/quality and time step requirements compared to unsteady RANS; the need for high-order (at least second-order accurate) numerical schemes. Therefore, a viable alternative might be found in hybrid URANS/LES turbulence modeling, which has the potential to achieve adequate scale-resolving levels wherever actually needed, but mitigating at the same time some of the aforementioned concerns. In the present work we discuss the current status and perspectives of URANS/LES hybrids in the ICE field, based on the scientific literature state-of-the art and on a series of previous computational studies made by the authors. Outcomes from this study essentially confirm that this class of methods deserve further attention and will likely support URANS and standard LES in the near future as an effective computational tool for the ICE development and optimization.
Advanced combustion concepts, like homogeneous charge compression ignition, are limited by their narrow operating range, which stems from a lack of control over the heat release process. This study explores a new advanced combustion mode, called thermally stratified compression ignition, which uses a direct water injection event to control the heat release process in low-temperature combustion. A three-dimensional computational fluid dynamics model coupled with detailed chemical kinetics is used to better understand the effects of direct water injection on thermal stratification in the cylinder and the resulting heat release process. Previous results showed that increasing the injection pressure results in a significantly broader temperature distribution due to increased evaporative cooling. In this way, direct water injection can control low-temperature combustion heat release and extend significantly the operable load range. In this study, simulations were performed over a range of start of injection timings in order to determine its effect on thermal stratification and heat release. The results show that for both low and high injection pressures advancing the start of water injection results in increased thermal stratification and reduced peak pressure and heat release rate for injections occurring after −60 °CAD. Before −60 °CAD, advancing the water injection has a varied effect on thermal stratification and heat release depending on the injection pressure and mass of the injected water.
Homogeneous Charge Compression Ignition (HCCI) combustion has the potential for high efficiency with very low levels of NOx and soot emissions. However, HCCI has thus far only been achievable in a laboratory setting due to the following challenges: 1) there is a lack of control over the start and rate of combustion, and 2) there is a very limited and narrow operating range. In the present work, the injection of water directly into the combustion chamber was investigated to solve the aforementioned limitations of HCCI. This new advanced combustion mode is called Thermally Stratified Compression Ignition (TSCI).
A 3-D CFD model was developed using CONVERGE CFD coupled with detailed chemical kinetics to gain a better understanding of the underlying phenomena of the water injection event in a homogeneous, low temperature combustion strategy. The CFD model was first validated against previously collected experimental data. The model was then used to simulate TSCI combustion and the results indicate that injecting water into the combustion chamber decreases the overall unburned gas temperature and increases the level of thermal stratification prior to ignition. The increased thermal stratification results in a decreased rate of combustion, thereby providing control over its rate. The results show that the peak pressure and gross heat release rate decrease by 37.8% and 83.2%, respectively, when 6.7 mg of water were injected per cycle at a pressure of 160 bar. Finally, different spray patterns were simulated to observe their effect on the level of thermal stratification prior to ignition. The results show that symmetric patterns with more nozzle holes were generally more effective at increasing thermal stratification.
RCCI (reactivity controlled compression ignition) engines are found to be capable of achieving higher thermal efficiency and ultra-low NOx and PM emissions. The reactivity controlled combustion is accomplished by creating reactivity stratification in the cylinder with the use of two fuels characterized by distinctly different cetane numbers. The low reactivity (i.e., low cetane number) fuel is firstly premixed with air and then charged into the cylinder through the intake manifold; later, the high reactivity (i.e., high cetane number) fuel is injected into the charged mixture through a direct injector. Subsequently, the reactivity stratification is formed. By strategically adjusting the ratio of two fuels and injection timings, the produced reactivity gradient is able to control the combustion phasing and mitigate the pressure rise rate, as well as the heat release rate. Alternatively, structural factors such as CR (compression ratio) and piston bowl geometries can also affect the combustion characteristics of RCCI. Besides the engine management, the fuels that could be utilized in RCCI engines are also crucial to determine the evaporation, mixing, and combustion processes. To gain a comprehensive knowledge on the state-of-the-art of RCCI combustion, detailed review on the management of RCCI engines has been presented in this paper. This review covers the up-to-date research progress of RCCI including the use of alternative fuels and cetane number improvers, and the effects of fuel ratio, different injection strategies, EGR rate, CR and bowl geometry on engine performance and emissions formation. Moreover, the controllability issues are addressed in this article.
Thermal stratification of the unburned charge prior to ignition plays a significant role in governing the heat release rates in a homogeneous charge compression ignition (HCCI) engine. A deep understanding of the conditions affecting thermal stratification is necessary for actively managing HCCI burn rates and expanding its operating range. To that end, a single-cylinder gasoline-fueled HCCI engine was used to characterize the relationship between key operating conditions, such as intake temperature, residual gas fraction, air-to-fuel ratio, and swirl, and thermal stratification. The recently developed Thermal Stratification Analysis was applied to calculate the unburned temperature distribution prior to ignition from heat release.
This work explores the potential of partial fuel stratification to smooth HCCI heat-release rates at high load. A combination of engine experiments and multi-zone chemical-kinetics modeling was used for this. The term "partial" is introduced to emphasize that care is taken to supply fuel to all parts of the in-cylinder charge, which is essential for reaching high power output. It was found that partial fuel stratification offers good potential to achieve a staged combustion event with reduced pressure-rise rates. Therefore, partial fuel stratification has the potential to increase the high-load limits for HCCI/SCCI operation. However, for the technique to be effective the crank-angle phasing of the "hot" ignition has to be sensitive to the local φ. Sufficient sensitivity was observed only for fuel blends that exhibit low-temperature heat release (like diesel fuel). In contrast, for a single-stage ignition fuel (like typical gasoline) the timing of the hot ignition is relatively insensitive to the local φ, and this renders partial fuel stratification ineffective for creating a staged combustion event. The effects of partial fuel stratification were demonstrated experimentally for PRF80 and PRF83 fuels using a hollow-cone DI injector and two fuel-injection pulses. Suitable fuel stratification was created by injecting the main portion of the fuel during the intake stroke, and the remainder during the compression stroke. This smoothed the heat-release and lowered the pressure-rise rate significantly compared to fully-premixed fueling. For naturally aspirated operation with CA50 at 371°CA, partial fuel stratification allowed IMEP g to be increased from 537 to 597 kPa while maintaining acceptable ringing intensity (PRRmax < 8 bar/°CA) and NOx emissions below US 2010 standards. However, partial fuel stratification must be implemented carefully since too much stratification can quickly lead to unacceptable NOx emissions.
When oxygen reacts with pyro graphite at high flow rates and at pressures of about 0.2 atm, the reaction rate increases steeply with temperature from 1050°C to 1700°C; above this temperature the rate ceases to rise and the reaction curve becomes nearly horizontal. Since much faster rates of reaction are observed with other forms of carbon under otherwise identical reaction conditions, the pyro graphite rates must be genuine chemical rates not substantially influenced by gas phase mass transfer. In the past the reactions of oxygen at very low pressures with carbon filaments have shown that the reaction curve under these conditions reaches a maximum at a temperature of about 1100°C, which is 600°C less than the temperature at which the maximum rate with pyro graphite at a pressure of 0.2 atm was observed. The theory of Blyholder, Binford and Eyring, slightly modified, is shown to account satisfactorily for this difference between the low and high pressure results. The reactions of reactor quality graphite with oxygen have also been studied. The reaction curves reach a maximum at about 1600°C in a manner very similar to those of pyro graphite, and the rate then falls appreciably between 1600°C and 2000°C. The rates in this case are considerably higher than with pyro graphite, and they are certainly partly mass-transfer controlled: but the similarity in shape with the pyro graphite results suggests that, although the absolute values of the rates may be in need of considerable correction for mass transfer, the curves for reactor graphite are in broad outline of a shape determined by chemical factors.
Unlabelled:
The U.S. Environmental Protection Agency (EPA) established strict regulations for highway diesel engine exhaust emissions of particulate matter (PM) and nitrogen oxides (NOx) to aid in meeting the National Ambient Air Quality Standards. The emission standards were phased in with stringent standards for 2007 model year (MY) heavy-duty engines (HDEs), and even more stringent NOX standards for 2010 and later model years. The Health Effects Institute, in cooperation with the Coordinating Research Council, funded by government and the private sector, designed and conducted a research program, the Advanced Collaborative Emission Study (ACES), with multiple objectives, including detailed characterization of the emissions from both 2007- and 2010-compliant engines. The results from emission testing of 2007-compliant engines have already been reported in a previous publication. This paper reports the emissions testing results for three heavy-duty 2010-compliant engines intended for on-highway use. These engines were equipped with an exhaust diesel oxidation catalyst (DOC), high-efficiency catalyzed diesel particle filter (DPF), urea-based selective catalytic reduction catalyst (SCR), and ammonia slip catalyst (AMOX), and were fueled with ultra-low-sulfur diesel fuel (~6.5 ppm sulfur). Average regulated and unregulated emissions of more than 780 chemical species were characterized in engine exhaust under transient engine operation using the Federal Test Procedure cycle and a 16-hr duty cycle representing a wide dynamic range of real-world engine operation. The 2010 engines' regulated emissions of PM, NOX, nonmethane hydrocarbons, and carbon monoxide were all well below the EPA 2010 emission standards. Moreover, the unregulated emissions of polycyclic aromatic hydrocarbons (PAHs), nitroPAHs, hopanes and steranes, alcohols and organic acids, alkanes, carbonyls, dioxins and furans, inorganic ions, metals and elements, elemental carbon, and particle number were substantially (90 to >99%) lower than pre-2007-technology engine emissions, and also substantially (46 to >99%) lower than the 2007-technology engine emissions characterized in the previous study.
Implications:
Heavy-duty on-highway diesel engines equipped with DOC/DPF/SCR/AMOX and fueled with ultra-low-sulfur diesel fuel produced lower emissions than the stringent 2010 emission standards established by the U.S. Environmental Protection Agency. They also resulted in significant reductions in a wide range of unregulated toxic emission compounds relative to older technology engines. The increased use of newer technology (2010+) diesel engines in the on-highway sector and the adaptation of such technology by other sectors such as nonroad, displacing older, higher emissions engines, will have a positive impact on ambient levels of PM, NOx, and volatile organic compounds, in addition to many other toxic compounds.
This study investigates the potential of partial fuel stratification for reducing the knocking propensity of intake-boosted HCCI engines operating on conventional gasoline. Although intake boosting can substantially increase the high-load capability of HCCI, these engines would be more production-viable if the knock/stability load limit could be extended to allow higher loads at a given boost and/or to provide even higher thermal efficiencies. A technique termed partial fuel stratification (PFS) has recently been shown to greatly reduce the combustion-induced pressure-rise rate (PRR), and therefore the knocking propensity of naturally aspirated HCCI, when the engine is fueled with a φ-sensitive, two-stage-ignition fuel. The current work explores the potential of applying PFS to boosted HCCI operation using conventional gasoline, which does not typically show two-stage ignition. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) at 1200 rpm. The engine was equipped with a compression-ratio 14 piston, and combustion phasing was controlled by EGR addition. PFS is produced by premixing the majority of the fuel and then directly injecting the remainder (up to about 20%) in the latter part of the compression stroke. For PFS to be effective, the fuel's autoignition chemistry must vary with the local equivalence ratio (φ) to produce a staged combustion event. Accordingly, tests were conducted to determine the φ-sensitivity of gasoline. They show that at naturally aspirated conditions (P in = 1 bar), gasoline is not φ-sensitive, and PFS is not effective for reducing the PRR. However, with sufficient intake boost (e.g. P in = 2 bar), gasoline is found to become highly φ-sensitive, and PFS very effectively reduces the PRR. Varying the amount of PFS, by adjusting either the timing or amount of DI fuel, allows control of the PRR reduction. Applying PFS to high loads at P in = 2 bar substantially shifts the knock/stability limit and increases the maximum IMEP g from 11.7 (premixed) to 13.0 bar (PFS). Maximum load improvements with PFS are also seen for other intake pressures ranging from 1.6 to 2.4 bar. Finally, because it allows more advanced combustion phasing without knock, PFS is also effective for increasing the thermal efficiency of boosted HCCI over a range of loads for each P in, yielding typical fuel economy improvements of 2 - 2.5%. Overall, PFS has a strong potential for improving gasoline-fueled boosted HCCI operation.
Recent measurements by Siebers et al. have shown that the flame of a high pressure Diesel spray stabilizes under quiescent conditions at a location downstream of the fuel injector. The effects of various ambient and injection parameters on the flame “lift-off” length have been investigated under typical Diesel conditions in a constant-volume combustion vessel. In the present study, the experiments of Siebers et al. have been modeled using a modified version of the KIVA-3V engine simulation code. Fuel injection and spray breakup are modeled using the KH-RT model that accounts for liquid surface instabilities due to the Kelvin-Helmholtz and Rayleigh-Taylor mechanisms. Combustion is simulated using Convergent Thinking's recently developed detailed transient chemistry solver (SAGE) that allows for any number of chemical species and reactions to be modeled. While detailed chemistry is believed to be an accurate methodology for modeling Diesel combustion, in the past the extensive run times rendered it impractical. To expedite the calculations, SAGE has been implemented into KIVA using the Message-Passing Interface (MPI). This implementation allows for the chemical reactions to be simulated in parallel on multiple CPUs. An n-heptane mechanism was used to model Diesel fuel ignition and combustion.
The improved KIVA-3V code was used to simulate the spray-bomb cases over a wide range of injection pressures, nozzle hole sizes, ambient temperatures, and ambient densities. In general, excellent agreement was obtained between the measurements and simulation results for liquid length and flame lift-off length.
To further validate the chemical kinetics model, simulations of a heavy-duty direct injection Diesel engine were conducted. The results indicate that the detailed chemistry model is able to accurately predict ignition delay and cylinder-averaged pressure for a range of start of injection timings.
Engine experiments and multi-dimensional modeling were used to explore Reactivity Controlled Compression Ignition (RCCI) to realize highly-efficient combustion with near zero levels of NOx and PM. In-cylinder fuel blending using port-fuel-injection of a low reactivity fuel and optimized direct-injection of higher reactivity fuels was used to control combustion phasing and duration. In addition to injection and operating parameters, the study explored the effect of fuel properties by considering both gasoline-diesel dual-fuel operation, ethanol (E85)-diesel dual fuel operation, and a single fuel gasoline-gasoline+DTBP (di-tert butyl peroxide cetane improver). Remarkably, high gross indicated thermal efficiencies were achieved, reaching 59%, 56%, and 57% for E85-diesel, gasoline-diesel, and gasoline-gasoline+DTBP respectively.
Using conditions based on CFD simulations, engine experiments were performed using a heavy-duty test engine and the modeling was further used to explain the experimentally observed results. The experiments confirmed that by optimizing the fuel reactivity based on the specific operating conditions, combustion phasing can be optimized in order to minimize fuel consumption. Additionally, it was found that highly efficient operation (greater than 50% indicated thermal efficiency) could be achieved with all three fuel blending strategies over a wide range of loads. This study showed that, compared to gasoline-diesel, significantly higher quantities of diesel fuel were required to maintain optimal combustion phasing with the E85-diesel fuel blends. This result is due to a combination of the lower reactivity and higher enthalpy of vaporization of ethanol (compared to gasoline) and combustion chemistry effects of ethanol diesel blends. Secondly, the single fuel gasoline-gasoline+DTBP yielded near identical emissions and ISFC results to gasoline-diesel operation. Although the emissions and ISFC of all three strategies were similar, the low temperature heat release (LTHR) were different with all three fuels, and the high temperature heat release (HTHR) was different with E85-diesel blends. Fuel chemistry effects for all three fuels were investigated and their effect on the reactivity gradient was found to be responsible for the combustion differences.
Reactivity Controlled Compression Ignition combustion (RCCI) has been demonstrated at mid to high loads [1, 2, 3, 4, 5, 6] as a method to operate an internal combustion engine that produces low NOx and low PM emissions with high thermal efficiency. The current study investigates RCCI engine operation at loads of 2 and 4.5 bar gross IMEP at engine speeds between 800 and 1700 rev/min. This load range was selected to cover the range from the previous work of 6 bar gIMEP down to an off-idle load at 2 bar. The fueling strategy for the low load investigation consisted of in-cylinder fuel blending using port-fuel-injection of gasoline and early cycle, direct-injection of either diesel fuel or gasoline doped with 3.5% by volume 2-EHN (2-ethylhexyl nitrate). At these loads, engine operating conditions such as inlet air temperature, port fuel percentage, and engine speed were varied to investigate their effect on combustion. Results show that at the 4.5 bar gIMEP operating condition it was possible to maintain 54% gross indicated thermal efficiency with NOx and PM emissions below US EPA 2010 limits. The results also show that it is possible to operate at a near idle load of 2 bar gross IMEP load with a gross indicated thermal efficiency of 49% at 1300 rev/min and 44% at 800 rev/min.
A reduced PRF mechanism was proposed for combustion simulations of PRF and diesel/gasoline fuels based on the latest LLNL mechanism. The reduced PRF mechanism consists of 73 species and 296 reactions. The major reaction pathways of the detailed mechanism were mostly retained in the reduced mechanism, which ensures its predictive capability, the ability to be extended to other fuels, and the high computational efficiency of the reduced mechanism. The important reaction pathways and reactions in the reduced mechanism are identified and discussed. Furthermore, the reaction rates of two reactions, HO2 + OH = HO2 + O2 and HO2 + HO2 = H2O2 + O2, in the hydrogen submechanism are discussed and updated. The reduced mechanism was validated with measured ignition delays, laminar flame speeds, premixed flame species concentrations, jet stirred reactor and shock tube species profiles, and PRF fuel HCCI and PPCI combustion and diesel/gasoline direct injection spray combustion data. The reduced mechanism predicts well the ignition timings, flame speeds, and important species concentrations under various validation conditions and shows reliable performance under different engine validation conditions. The overall results suggest that the current mechanism can provide reliable predictions for PRF and diesel/gasoline combustion CFD simulations.
Fuel stratification is a potential strategy for reducing the maximum pressure rise rate in HCCI engines. Simulations of Partial Fuel Stratification (PFS) have been performed using CONVERGE with a 96-species reduced mechanism for a 4-component gasoline surrogate. Comparison is made to experimental data from the Sandia HCCI engine at a compression ratio 14:1 at intake pressures of 1 bar and 2 bar. Analysis of the heat release and temperature in the different equivalence ratio (φ) regions reveals that sequential auto-ignition of the stratified charge occurs in order of increasing φ for 1 bar intake pressure but in order of decreasing φ for 2 bar intake pressure. Increased low- and intermediate-temperature heat release at 2 bar intake pressure compensates for decreased temperatures in higher-φ regions due to evaporative cooling from the liquid fuel spray and decreased compression heating from lower values of the ratio of specific heats. At 1 bar intake pressure, the premixed portion of the charge auto-ignites before the highest-φ regions and the sequential auto-ignition occurs too fast for useful reduction of the maximum pressure rise rate compared to HCCI. Conversely, at 2 bar intake pressure, the premixed portion of the charge auto-ignites last, after the higher-φ regions. More importantly, the sequential auto-ignition occurs over a longer time period than at 1 bar intake pressure such that a sizable reduction in the maximum pressure rise rate compared to HCCI can be achieved.
Recent direct numerical simulations of constant density isobaric turbulent flame-wall interaction have lead to the development of a wall model that can easily be implemented in turbulent combustion models used in conventional CFD codes (as e.g. flamelet models). An essential point of this model is the estimation of the mean heat loss of the turbulent flame brush to the cold combustion chamber walls, emphasizing the need for an accurate description of the boundary conditions on solid walls in terms of wall heat transfer and turbulence. With regard to mesh size and computing time, most industrial CFD codes use high-Reynolds number k - ε turbulence models in conjunction with a law-of-the-wall to describe near wall flow conditions. One important assumption for the validity of the law-of-the-wall is that the near wall flow is isothermal, the fluid properties in this region thus being regarded as constant. This assumption is obviously erronous in flows combining hot gases (generated by combustion and/or compression) with cold walls, as in IC engines. We present a formulation of the law-of-the-wall that is equivalent to the classical one when the wall flow is isothermal and takes into account effects of variable fluid properties for non-isothermal conditions. A modification of the above cited turbulent combustion model is proposed to describe non-isobaric flame-wall interaction in SI engines. Both models are implemented in the KIVA-II code and are first validated on a simple axisymmetric pancake SI engine. The new models are shown to more accurately reproduce experimental local wall heat fluxes and pressure/time histories than the original ones. Finally, computations of intake and combustion in a 4-valve SI engine geometry show the ability of the new models to reproduce global engine characteristics quite satisfactorily over a range of operating parameters.
Chemiluminescence experiments were conducted in a single-cylinder HCCI engine to provide a comprehensive understanding of the effects of temperature and mixture stratification on HCCI combustion. Chemiluminescence spectrum and CFD coupled kinetics models were used to analyze the combustion mechanism under different stratification. Results indicated that the different port injection strategies resulted in different mixture stratifications, thus affected the auto-ignition timing and combustion processes. Under higher intake temperature conditions, injection strategies had less effect on the combustion processes due to improved evaporating and mixing. Some of the chemical kinetic reactions were “frozen” due to the low local temperature in the whole combustion process for lower coolant temperature. Increased temperature stratification was undesirable to low load conditions because more quenching would occur at the cooler regions in the cylinder. It can be concluded that the port injection timing, turbulence generated by higher engine speed, different intake and coolant temperatures can affect combustion processes in HCCI engines. The essence of these factors was their impact on the temperature distribution in the cylinder during combustion. Larger local temperature stratification can reduce the pressure rise rate through smoothing the reaction rates and extend the operating range in HCCI engines.
Homogeneous charge compression ignition (HCCI) combustion with fully premixed charge is severely limited at high-load operation due to the rapid pressure-rise rates (PRR) which can lead to engine knock and potential engine damage. Recent studies have shown that two-stage ignition fuels possess a significant potential to reduce the combustion heat release rate, thus enabling higher load without knock.
In this study, a one-equation LES sub-grid model from Menon, et al. (5) is used in simulating the diesel combustion process. In addition, based on the one- equation methodology of Menon et al., a new one- equation LES scalar transport model is formulated. These models allow for the turbulent transfer coefficients for both momentum and scalar flux to be determined independent of each other. The turbulent viscosity, t µ , is determined as a function of the sub-grid kinetic energy, which is in turn determined from the one- equation model. The formulation for the scalar transfer coefficient, s µ , is similar to that of the turbulent viscosity, yet is made to be consistent with scalar transport. Results for the LES momentum transfer are compared to experimental data of a backward facing step. This model, in conjunction with the LES scalar flux model, is verified by comparing with experimental data for a non- reacting turbulent jet. Finally, these models are used with a Probability Density Function (PDF) combustion model to model the diesel combustion process. The presented results indicate that that LES is a viable option for simulating the turbulent reacting processes that occur within the diesel environment.
This work concerns one of the major issues restricting the application of homogeneous charge compression-ignition (HCCI) engines, overly rapid combustion at high-load conditions, which can lead to engine knock and potential engine damage. To overcome this problem, partial fuel stratification was used, where most fuel was premixed with intake air and the rest of the fuel was directly injected during the compression stroke. To be effective, this technique depends critically on the fuel autoignition chemistry. PRF73, a mixture of 73vol% isooctane and 27vol% n-heptane that exhibits two-stage ignition under these conditions, successfully enabled control of the combustion heat-release rate with properly adjusted injection parameters. More than a 70% reduction in the maximum pressure-rise rate (PRRmax) was achieved, compared to the fully premixed case at the same combustion phasing. Meanwhile, combustion remained stable, efficient, and clean. In particular, NOx emissions were kept well below the US-2010 limits. On the other hand, isooctane, which exhibits single-stage ignition under the present conditions, responded much differently to partial fuel stratification. Instead of reducing PRRmax, isooctane partial stratification increased PRRmax and drastically raised NOx emissions and combustion instability. Such differences between single- and two-stage fuels result from the different response of their autoignition rates to the variations in equivalence ratio produced by partial fuel stratification, termed ϕ-sensitivity.
While engineering applications of the large eddy simulation (LES) technique are becoming a common reality in many branches of engineering and science, its application to engine flows has lagged behind due to the relatively more complex nature of both the flow and the geometry relevant to in-cylinder flows. In this paper a review of the limited number of LES applications to engine flows is given, and most significant results from these studies are presented. Also, the LES formulation appropriate for engine applications is briefly described, along with the main characteristics of in-cylinder flows. As expected, engine applications of LES are not of the so-called ‘high-fidelity’ type, but rather they employ formally second-order accurate numerical schemes in conjunction with finite volume formulation. The subgrid scale (SGS) models used are also kept as simple as possible, mostly using a variant of the Smagorinsky model. Nevertheless, this review reveals that even with relatively coarse grids, LES captures much more interesting features of in-cylinder flows, such as the large coherent vortical flow structures developed during the intake stroke. In the opinion of the present authors, a low-resolution LES provides a better solution than RANS (Reynolds averaged Navier-Stokes) with moderate grid resolution because the important features of flow dynamics cannot be reproduced in RANS due to the high level of non-physical diffusion. Of course, some overheads in computational costs must be paid for this benefit obtained from LES.
In the light of rapidly increasing applications of large-eddy-simulations (LES) it is deemed necessary to impose some quality assessment measures for such studies. The validation of LES is difficult because of the fact that both the sub-grid scale (sgs) model contribution and numerical discretization errors are functions of the grid resolution. In this study various index of quality measures, here and after referred to as LES_IQ, are proposed and applied to some case studies. The recommended LES_IQ is based on the concept of Richardson’s extrapolation. It is postulated that in practical applications of LES numerical dissipation will always be a significant part of the overall dissipation and it must be accounted for in any assessment of the quality of LES results. It is further suggested that an LES_IQ of 75% to 85% can be considered adequate for most engineering applications that typically occur at high Reynolds numbers; LES-IQ greater than 90% can be classified as DNS.
A multi-dimensional model was used to calculate interactions between spray drops and gas motions close to the nozzle in dense high-pressure sprays. The model also accounts for the phenomena of drop breakup, drop collision and coalescence, and the effect of drops on the gas turbulence. The calculations used a new method to describe atomization (a boundary condition in current spray codes). The method assumes that atomization and drop breakup are indistinguishable processes within the dense spray near the nozzle exit. Accordingly, atomization is prescribed by injecting drops ('blobs') that have a size equal to the nozzle exit diameter.