In-cylinder Flow Field Research Articles

Spray formation and spray-wall-flow interaction in a gasoline direct-injection (GDI) engine under early-injection conditions: A flow bench study

The work presented in this study aims to understand the spray-wall-flow interaction within a gasoline direct-injection (GDI) engine flow bench under simulated early-injection conditions. The Engine Combustion Network (ECN) Spray G injector is installed in the Darmstadt optically accessible engine flow bench. Under simulated early-injection conditions, the formation of a multi-hole spray and the interaction with characteristic intake flows, such as the intake jet and central tumble flow, are extensively discussed. By reducing the complexity in the number of variables inherent in engine flow and whole-engine simulation, an engine flow bench operating under various mass flow rates is applied in this study. The numerical simulation is carried out using Large Eddy Simulation (LES) under the Eulerian-Lagrangian framework for spray simulation. Experimental data, acquired through particle image velocimetry (PIV) measurements, provides 2-D flow fields on both the central tumble and valve planes, facilitating the validation of in-cylinder flow fields. Furthermore, experimental data obtained through Mie scattering is utilized to investigate spray formation and evolution within the GDI engine, providing the liquid penetration length and liquid spray angle. Comparison between the numerical and experimental data demonstrates several agreements. Moreover, the variation of different spray plumes under different mass flow rates is observed in the case of both experimental and numerical data. Increasing the mass flow rate distorts the overall plume shape and shifts it away from the intake port. This phenomenon is examined by extracting the liquid volume fraction and vapor fields of each plume. Spray plumes encounter different convective disturbances and evaporation due to their local characteristic in-cylinder flow. Furthermore, spray-wall-flow interaction and wall film deposits are observed during the injection. Lastly, the influence of the spray-induced turbulence is analyzed under different mass flow rates.

Swirl-induced motion prediction with physics-guided machine learning utilizing spatiotemporal flow field structure

PurposeHigher energy conversion efficiency of internal combustion engine can be achieved with optimal control of unsteady in-cylinder flow fields inside a direct-injection (DI) engine. However, it remains a daunting task to predict the nonlinear and transient in-cylinder flow motion because they are highly complex which change both in space and time. Recently, machine learning methods have demonstrated great promises to infer relatively simple temporal flow field development. This paper aims to feature a physics-guided machine learning approach to realize high accuracy and generalization prediction for complex swirl-induced flow field motions.Design/methodology/approachTo achieve high-fidelity time-series prediction of unsteady engine flow fields, this work features an automated machine learning framework with the following objectives: (1) The spatiotemporal physical constraint of the flow field structure is transferred to machine learning structure. (2) The ML inputs and targets are efficiently designed that ensure high model convergence with limited sets of experiments. (3) The prediction results are optimized by ensemble learning mechanism within the automated machine learning framework.FindingsThe proposed data-driven framework is proven effective in different time periods and different extent of unsteadiness of the flow dynamics, and the predicted flow fields are highly similar to the target field under various complex flow patterns. Among the described framework designs, the utilization of spatial flow field structure is the featured improvement to the time-series flow field prediction process.Originality/valueThe proposed flow field prediction framework could be generalized to different crank angle periods, cycles and swirl ratio conditions, which could greatly promote real-time flow control and reduce experiments on in-cylinder flow field measurement and diagnostics.

Effects of injection timing on mixture formation, combustion, and emission characteristics in a n-butanol direct injection spark ignition engine

The impact of injection timings on the in-cylinder flow field intensity, mixture formation process, combustion, and emission characteristics are analyzed. The range of injection timing is set between −320°CA and −260°CA. The research results indicate: Delaying the injection timing causes the spray plumes increasingly align with the tumble direction, consequently enhancing the in-cylinder tumble intensity and the turbulence near the spark timing. The mixture homogeneity is influenced by both the formation time and the in-cylinder flow field intensity. Moreover, combustion performance is linked to the homogeneity of the mixture and turbulence intensity near the spark timing. Despite the fact that postponed injection may augment both turbulence intensity and the equivalent ratio around the spark plug at the moment of spark, achieving higher explosion pressure, but the higher viscosity of n-butanol impedes droplet break-up, and its low volatility decelerates the evaporation rate, leading to poorer mixture homogeneity, reduced BMEP, and increased CO and soot emissions. However, at −300°CA start of injection, the extended mixing time optimizes the fuel-air mixture homogeneity, thus improving combustion quality and reducing CO, soot, and HC emissions, albeit leading to increased NOx emissions.

LES of pilot n-heptane ignition and its interaction with the lean premixed methane–air mixture in a dual-fuel combustion engine

Large eddy simulations of pilot fuel ignited, lean premixed, natural gas engines are performed to study the pilot-ignition process and its subsequent interaction with the premixed charge. The injection pressure (Pinj) and injection duration (Δinj) are varied (i.e. 800 bar/760μs, 800 bar/500μs, and 400bar/760μs) to study the impact of the injection process on the subsequent combustion evolution. Open-cycle simulations considering the full engine geometry are used to predict the in-cylinder flows, while combustion is modeled using a finite-rate chemistry model. In-cylinder methane (CH4) is shown to delay the low-temperature ignition of the pilot fuel, regardless of the pilot injection setting, which subsequently prolongs the overall pilot fuel ignition delay. Moreover, all simulated cases show the occurrence of back-supported combustion (BSC), where the burning of CH4-air mixture is “back-supported” by pilot fuel radicals. Despite both the 800 bar/500μs and 400 bar/760μs cases having the same injected pilot fuel mass, the peak in-cylinder pressure and burning rate of the premixed CH4-air mixture in the former case are higher. Higher Pinj and shorter Δinj lead to better mixing between the pilot fuel and the premixed CH4-air charge. Subsequently, this forms a larger volume of regions with elevated equivalence ratio due to the presence of pilot fuel (ϕ>ϕCH4) which, consequently promotes the formation of BSC. The impact of in-cylinder flow fields on the dual-fuel combustion process is investigated by performing two closed-cycle 800 bar/500μs cases with one assuming solid-body rotation and another without solid-body rotation (i.e. zero velocity field). In-cylinder flow field is shown to have a visible impact on the transition stage between the pilot ignition stage and the premixed flame propagation stage, but have an insignificant effect on the pilot fuel ignition process. In the transition stage, slower flame propagation is observed in the zero-velocity case. The results show that this is not only due to the turbulence effect on premixed flame but also due to differences in the volume and distribution of pilot fuels that impacts BSC.

Optimization of direct injection mixture formation for dual-fuel low speed marine engine using novel compound electric gas injection devices

For two-stroke low speed marine engines, using a natural gas-diesel dual-fuel mode can effectively reduce emissions such as NOX, SOX, and CO2, making it an important approach to promote the low-carbon and environmentally friendly development of the shipping industry. To address the issues of abnormal combustion and gas leakage faced by low-pressure gas direct injection, a novel compound electric gas injection device is proposed, which is designed with a coaxial arrangement of a moving coil actuator and a moving iron actuator. Based on this new device, a three-dimensional computational fluid dynamics (CFD) model of the low speed marine engine operating process is established to simulate the distribution characteristics of the in-cylinder flow field under 16 different scenarios, including different injection side-slip angles and lower deflection angles, and to clarify the impact of these factors on the formation, diffusion, and evolution of the mixture. Based on this, an evaluation system was established from two dimensions: mixture uniformity and gas leakage rate. An improved multi-attribute decision-making TOPSIS algorithm based on the entropy weight method was used to define the attribute importance using information entropy, and to construct an optimization evaluation model for the spatial arrangement of the injection device. The optimal arrangement angle was calculated, in which a side-slip angle was 20° and a lower deflection angle was 10°. This method avoids the problem of unreasonable decisions caused by subjective factors such as designer preferences and experience, effectively improving the in-cylinder mixture uniformity and reducing gas leakage rate.

The interaction of charge motion and EGR on anti-knock performance and efficiency of a medium-duty gasoline engine: An experimental study

Due to the common operating characteristics of medium duty (MD) gasoline engines, achieving higher load at the expense of fuel efficiency is not an acceptable tradeoff. EGR dilution-stoichiometric combustion can suppress the knocking and improve the thermal efficiency, but has problems such as slow combustion rate and poor combustion stability, which have limitations in optimizing combustion for MD gasoline engines with strong swirl. The experimental investigations in this study focus on the interaction of charge motion and EGR on anti-knock performance and thermal efficiency under different load conditions, and verifies the potential to improve the thermal efficiency of inclined swirl combustion system. The results show that at low-medium load, the tumble-dominated inclined swirl scheme can enhance the effectiveness of EGR in knock suppression, thus reducing the introduction of EGR to avoid its adverse effects on the combustion duration. At medium-high load, although the above benefit wears off, it can enhance the anti-knock performance and thus advance the spark timing by increasing the EGR tolerance. The EGR ratio corresponding to the highest thermal efficiency increases with higher load. The difference in the in-cylinder flow field will affect the combustion rate in a high EGR dilution and high load condition. The scheme adopting the compound intake ports and inverse cuneiform-shaped combustion chamber can improve both EGR tolerance and initial combustion rate under high EGR dilution, with the most significant improvement in thermal efficiency, and the improvement range will increase along with the increase of load.

Numerical Simulation of Turbulent Structures Inside Internal Combustion Engines Using Large Eddy Simulation Method

<div>Using two subgrid-scale models of Smagorinsky and its dynamic version, large eddy simulation (LES) approach is applied to develop a 3D computer code simulating the in-cylinder flow during intake and compression strokes in an engine geometry consisting of a pancake-shaped piston with a fixed valve. The results are compared with corresponding experimental data and a standard K-Ɛ turbulence model. LES results generally show better agreement with available experimental data suggesting that LES with dynamic subgrid-scale model is more effective method for accurately predicting the in-cylinder flow field. Representative Fiat engine equipped with moving valve and piston bowl is analyzed as the second case to assess the capability of the method to handle complex geometries and impacts of geometrical parameters such as shape and position of piston bowl together with swirling intake flow pattern on both turbulent structure of in-cylinder flow and engine performance using dynamic version of LES approach over a curvilinear computational meshed geometry. Results indicate that presence of piston bowl leads to eye-catching increment in both turbulent kinematic energy and tumble ratio amounts at the end of compression stroke by around 29% and 33%, respectively. The optimum swirl ratio found to be 4, leading to 67.9% increment in pre-injection turbulent kinetic energy in comparison with non-swirl pattern, whereas 20% eccentricity of cylinder bowl just led to 2% improvement in the pre-injection turbulent kinetic energy, which is not recommended due to small impact compared to noticeable manufacturing expenditures.</div>

Study of engine knocking combustion using simultaneous high-speed shadowgraph and natural flame luminosity imaging

Engine knocking is a classical but significantly impactful combustion phenomenon that arouses unfailing research interests. An in-depth understanding of the knocking mechanism holds the key to the next-generation high-efficiency spark ignition engines adopting a high compression ratio. In this study, a novel 72 kHz imaging system of simultaneous shadowgraph and natural flame luminosity (NFL) was implemented on an optical engine. The key combustion parameters that affect the engine knock intensity were investigated based on the heat release analysis. The shadowgraph image velocimetry (SIV) was utilized to measure the in-cylinder flow field. The end gas autoignition process in the engine knocking combustion was studied under knock intensity as high as 18.1 bar. The results indicate that there is an “optimum” combustion phasing that produces the highest knock intensity. The cases with more end gas mass at knock onset, though having lower mean temperatures, generally produce higher knock intensity. The knock intensity can be significantly different under the same end gas mass and mean end gas temperature, possibly due to the different temperature gradients in the end gas. The simultaneous shadowgraph and NFL imaging succussed in differentiating the density variation zone in the end gas caused by the low-temperature heat release from the hot flame zone before auto-ignition. An intense density variation zone is witnessed around the center of the cylinder under higher engine knock intensity by the shadowgraph images, showing a dark region that oscillates together with the local pressure oscillations. The coupling among the local density variation, pressure oscillation, and NFL intensity oscillation is observed during the engine knocking combustion. The velocity distribution of the flow field indicates that the engine knocking combustion enhances the in-cylinder flow, which may cause more convective heat transfer loss and flow loss.

The effect of intake port shape on in-cylinder flow field and turbulence distribution in a high-tumble production engine with endoscope accesses

Three cylinder heads with varied intake port shapes are experimentally investigated to evaluate intake flow structures and their influence on near top dead centre (TDC) flow fields and turbulence distributions in a motored high-tumble engine. The endoscopic high-speed particle image velocimetry (eHS-PIV) is implemented in multi-cylinder engines with two laser endoscopes and a camera endoscope installed in the cylinder head. For a range of engine load and speed conditions, particle seeded and laser illuminated high-speed movies are recorded for 100 cycles with which ensemble-averaged flow fields and spatial-filtered high-frequency flow magnitude distributions are analysed. The results show that a straighter intake port producing a more lateral flow direction results in a larger swinging arc, which is related to enhanced flow later in the compression stroke. The tumble vortex shows an asymmetric structure; the flow field during the compression stroke exhibits higher magnitude vectors at the leading head. This surging flow head becomes stronger with a straighter intake port, which leads to enhanced tumble vortex formation near TDC. As it becomes very strong, the flow vectors originally directed towards the exhaust valves bounce back towards the intake valves, causing a very complex flow structure involving multiple flow components. The result is enhanced turbulence throughout the late compression stroke including the spark timing. However, the impact of the straighter intake port shape on TDC turbulence becomes less significant at lower engine load and speed conditions as the lower intake air momentum limits the enhancement.

Dynamic Mode Decomposition for the Comparison of Engine In-Cylinder Flow Fields from Particle Image Velocimetry (PIV) and Reynolds-Averaged Navier–Stokes (RANS) Simulations

Validation of Reynolds-averaged Navier–Stokes (RANS) simulation results against experimental data such as flow measurements from particle image velocimetry (PIV) remains a challenge for the development of thermal propulsion systems. This is partly due to cycle-to-cycle variations (CCVs) in the air motion and partly due to uncertainties in the PIV measurement technique, complicating the question of what constitutes a fair validation target for the RANS model. Indeed, an inappropriate validation target can misguide subsequent adjustments of a RANS model. In this work, the ensemble-averaged PIV field is first investigated for its suitability as a validation target for RANS simulations. The relevance index and the velocity histogram distance are used as quantitative metrics to assess the similarity of the ensemble-averaged field to the full dataset of individual PIV cycles. While a high similarity is seen between the average PIV flow field and the individual cycles on the tumble plane, the similarity is lower and more variable on the cross-tumble plane, where there are significant CCVs. Standard (space-only, phase-dependent) proper orthogonal decomposition (POD) is employed as an alternative method of data processing with the aim of providing a fairer comparison to RANS simulations. The cycle-dependence of the standard POD modes is shown to be an aspect that results in many validation targets and an excessively broad validation range, limiting its utility in this context. Dynamic mode decomposition (DMD) and sparsity-promoting dynamic mode decomposition (SPDMD) are then proposed as alternative solutions, capable of extracting flow structures at specific frequencies. The background 0 Hz SPDMD modes exhibit an ability to produce more realistic flow fields with velocity magnitudes that are significantly closer to the individual cycles.

ANALYSIS OF IN-CYLINDER FLOW FIELDS USING PROPER ORTHOGONAL DECOMPOSITION-BASED QUADRUPLE DECOMPOSITION

In order to meet increasingly stringent emission norms coupled with a heightened requirement of performance, there has been an unabated effort toward improvement in the combustion process of modern internal combustion engines. One of the major impediments of enhanced combustion in spark-ignited port fueled engines are combustion variations. These variations are especially dominant at low-load, low-speed operations. Cycle-to-cycle variation (CCV) in in-cylinder flow fields is one of the major contributors of such combustion variations. Therefore, in this work, CCV of in-cylinder flow fields of an optical port fuel injection engine was analyzed at part load (50% throttle opening) and low speed (1200 rpm) with the help of proper orthogonal decomposition. Flow fields were subsequently decomposed into four components, namely, mean, coherent, transition, and turbulent parts. CCV of flow fields was studied using several metrics based on kinetic energy and the relevance index. It was found that the share of mean energy is a better metric for CCV quantification based on kinetic energy. Interestingly, it was observed that the mean part, though consistent in its flow structure for various cycles, has a lot of variation in kinetic energy at early compression stroke. Also, a weak mean flow coupled with a strong coherent flow structure opposing the mean flow produces the largest deviation in a flow field from its corresponding ensemble-averaged field. Furthermore, even though the coherent and transition parts are comprised of comparable energy, it was the coherent part that showed large variations in kinetic energy. Hence, the mean and coherent parts are mainly responsible for CCV in flow fields.

Investigating the Origins of Cyclic Variability in Internal Combustion Engines Using Wall-Resolved Large Eddy Simulations

Abstract Modern internal combustion engines (ICE) operate at the ragged edge of stable operation characterized by high cycle-to-cycle variations (CCV). A key scientific challenge for ICE is the understanding, modeling, and control of CCV in engine performance, which can contribute to partial burns, misfire, and knock. The objective of this study is to use high-fidelity numerical simulations to improve the understanding of the causes of CCV. Nek5000, a leading high-order spectral element, open source code, is used to simulate the turbulent flow in the engine combustion chamber. Multicycle, wall-resolved large-eddy simulations (LESs) are performed for the General Motors (GM), Transparent Combustion Chamber (TCC-III) optical engine under motored operating conditions. The mean and root-mean-square (rms) of the in-cylinder flow fields at various piston positions are validated using particle image velocimetry (PIV) measurements during the intake and compression strokes. The large-scale flow structures, including the swirl and tumble flow patterns, are analyzed in detail and the causes for cyclic variabilities in these flow features are explained. The energy distribution across the different scales of the flow are quantified using one-dimensional (1D) energy spectra, and the effect of the tumble breakdown process on the energy distribution is examined. The insights from this study can help us develop improved engine designs with reduced cyclic variabilities in the in-cylinder flow leading to enhanced engine performance.

Effect of a Taper Intake Port on the Combustion Characteristics of a Small-Scale Rotary Engine

Taper intake ports are effective in improving the charging efficiency of small-scale rotary engines (REs), but it is unclear how their structural parameters affect the in-cylinder flow field and combustion characteristics. For this reason, the effects of the diameter-length ratio (D/L) of an intake port on the in-cylinder flow field and combustion characteristics of a small-scale RE were numerically investigated by utilizing a three-dimensional computational fluid dynamics (CFD) model. The results showed that the in-cylinder pressure of the RE did not follow a simple single-directional trend with the D/L of the intake port, but it was divided into three levels, where the peak in-cylinder pressure was at its maximum at the D/L of 0.6 and at its minimum at the D/L of 0.8. The gas flows in the intake port with different values of the D/L were all unidirectional, and they made a difference in the vortexes formed on the leading side of the combustion chamber of the RE, which was the main factor affecting the in-cylinder combustion performance. The vortexes formed on the leading side of the combustion chamber with D/L = 0.6 were maintained for a long period of time, thus promoting the propagation of flame and advancing the center of gravity of combustion. So, the heat release rate and combustion efficiency in the cylinder were increased at the price of a larger increment in nitrogen oxide formation.

Effect of the Hydrogen Injection Position on the Combustion Process of a Direct Injection X-Type Rotary Engine with a Hydrogen Blend

As a new type of power device, the X-type rotary engine (XRE) is regarded as a major revolution of the internal combustion engine with its special structure and high-efficiency hybrid cycle (HEHC). A 3D CFD model of an XRE with hydrogen–gasoline fuel is firstly built in this paper. The gasoline is premixed with air in the intake of the XRE. The hydrogen is directly injected (DI) into the cylinder with four different injection positions. The effects of the hydrogen injection position on the combustion process, engine thermodynamic performance, and unburned carbon emissions and NOx emissions are investigated. The results show that, due to the interaction between the in-cylinder main flow field and the injected hydrogen gas flow, different hydrogen concentration zones are formed at different injection positions. Furthermore, a larger hydrogen distribution area and being closer to the ignition position led to a faster in-cylinder combustion rate and a higher in-cylinder temperature and pressure. When the injection position is from the front to the back of the combustion chamber such as in position 2, the hydrogen has the widest distribution area and is closest to the ignition position, resulting in its fastest combustion speed. Meanwhile, the peak in-cylinder pressure is 3.73 MPa and the peak temperature is a maximum of 1835.16 K. Especially, the highest indicated thermal efficiency of 26.56% is found in position 2, which is 10.08% higher than that of position 4 (from right to left of the combustion chamber), which was 24.13%. At the same time, due to the best overall combustion effect, position 2 presents the lowest final unburned carbon emission of 0.36 mg, while it produces the highest NOx emission of 9.15 μg. Thus, this study provides important theoretical guidelines for the hydrogen injection strategy of the XRE using hydrogen–gasoline fuel.

Influence of tumble and TKE on combustion, fuel economy, and emissions of a single cylinder motorcycle engine

The impact of gasoline engines on improving fuel consumption and reducing emissions has received increasing attention around the world, and emissions regulations have become stricter. However, complex techniques in engine improvement increase costs. The purpose of this study is to develop or improve the design of the intake geometry without additional cost to improve the engine thermal efficiency, thereby improving fuel economy and emissions. In this study, emissions and fuel consumption of a 125 c.c. commercially available motorcycle was investigated. In addition to modifying the intake and valve angles and maintaining the intake air volume, a software for computational fluid dynamics (CFD) was used for steady-state and transient analysis of the in-cylinder flow field, tumble ratio and turbulence kinetic energy (TKE), and the results were compared with the data of the experimental platform. The burn duration of the engine, indicated mean effective pressure (IMEP), indicated specific total hydrocarbons (ISHC), nitrogen oxide (ISNOx) and carbon monoxide (ISCO) were measured on the bench. The transient predicted results show that the tumble ratio is a maximum increase of 27.82% at 656 crank angle (CA) degrees near the top dead center (TDC) on the compression stroke. In the ignition area within 5°–20° before top dead center (BTDC), average of the turbulence kinetic energy (TKE) was 120.37 m2/s2, distributed hot area was wide and close to the ignition position. In the steady-state measurement, it was verified that the tumble ratio was significantly improved by modifying inlet, the same trend as the analysis result. In the bench test for full load measurement, the average of IMEP increased by 10.92%, indicated specific fuel consumption (ISFC) improved by 11.7%, burn duration increased by 25.27%, ISCO improved by 43.41%, and ISHC improved by 64.33%. For portion load measurement, the average of ISFC improved by 2.16%, burn duration increased by 24.84%, ISCO improved by 43.44%, ISHC improved by 17.7% and ISNO improved by 12.89%. Based on the results, the tumble and TKE were improved by optimizing the angle between the intake valve and the intake port. The ISFC of model B was significantly improved under the two states. The ignition delay time was longer, and the overall burn duration was much improved. When it came to emissions, the ISCO and ISHC were significantly improved, and the ISNO was much higher at full load but lower at partial load due to the engine temperature and engine loads. At partial loads, the high tumble did benefit the combustion, fuel economy and emissions of a single-cylinder motorcycle engine.

Effect of spark ignition location on the turbulent jet ignition characteristics in a lean burning natural gas engine

The Turbulent Jet Ignition is an effective concept to achieve stable lean burning for natural gas engines due to the multiple ignition sources, high ignition energy, and fast combustion rate. A variation of the ignition location has a non-negligible effect on the ignition performance of the TJI system. The present work aims to provide more details on this effect by numerical simulations. Both factors of the additional fuel supply to the pre-chamber and the in-cylinder flow field are taken into consideration in this study. A numerical model is built based on a lean burning natural gas engine and validated by experimental results. Five different spark ignition sources are equally arranged on the vertical axis of the pre-chamber, with different distances from the connecting orifices. Simulations are carried out under the same initial and boundary conditions except for the location of the ignition source. Combustion pressure, in-cylinder flow field, fuel mass fraction distribution, and heat release rate are analyzed to study the in-cylinder ignition and combustion process. The results show that a rotational flow and a non-uniform fuel distribution are formed in the pre-chamber during the compression stroke. The turbulent jet characteristics are significantly influenced by the coupling of two factors: the combustion rate inside the pre-chamber as well as the flame propagation distance from the ignition source to the connecting orifices. Rapid combustion rate and shorter flame propagation distance both lead to the earlier ejection of cold jets and hot jets. Among five ignition sources, the one located closest to the connecting orifices generates earlier hot jets with the highest mean velocity. The jets are more effective to ignite the lean mixture and could decrease the combustion duration of the main chamber.

Effects of various discharge strategies on ignition and combustion of lean natural gas mixture under the static and turbulent conditions

Diluted combustion can promote thermal efficiency and decrease NOx emissions for natural gas engines, but also presents high cycle-by-cycle variations. Enhancing the in-cylinder flow field is beneficial for the combustion stability, but brings challenges for the stable ignition. Therefore, advanced ignition strategies are critical for extending the diluted limit. In this work, by using the constant volume combustion vessel, rapid compression and expansion machine, and multi-cylinder natural gas engine, the lean methane/air or natural gas/air mixtures under the static and turbulent conditions are established to compare various discharge strategies. The comparisons include: (1) different discharge energies, (2) power and duration enhancements with similar discharge energies, (3) the continuous discharge and multi-strike. For the static condition, once the flame kernel can be successfully established, enhancing discharge power or duration has few effects on ignition. The continuous discharge and multi-strike show no clear differences. At the lean limit, the multi-strike shows better performance than the continuous discharge. For the turbulent condition, increasing discharge energy can promote the initial flame propagation. When the flow velocity around the spark plug increases to some extent, enhancing the discharge duration shows better performance than enhancing the discharge power. Moreover, the continuous discharge is better for the ignition processes than the multi-strike. For the multi-cylinder engine tests, compared to the single strike of one coil, the continuous discharge and multi-strike extend the lean limit from the excess air ratio of 1.51 to 1.61. The multi-strike is more beneficial for the combustion phase stability than the continuous discharge.

A Comprehensive Numerical Analysis of the Scavenging Process in a Uniflow Two-Stroke Diesel Engine for General Aviation

The scavenging process of two-stroke engines plays a fundamental role in cylinder flow patterns and in the overall engine performance. In this work, 3D CFD simulations of the scavenging in a uniflow, two-stroke, compression ignition engine for general aviation, named GF56, have been performed by using a 3D finite-volume FANS equations solver with k-ϵ closure. The GF56 engine consists of six cylinders, separated into two quasi-symmetric banks. Both the right and the left banks, together with the corresponding cylinders, are carefully analyzed. Charging and trapping efficiencies are computed as a function of the delivery ratio for different mass flow rates entering into the plenum, and the influence of the exhaust pressure and of the cylinder’s location in the bank are analyzed. The results show that the fresh air trapped during the scavenging process is quite similar for each cylinder of the right bank and it is about 92% of the in-cylinder mass. The cylinder’s location in the bank by itself slightly affects the scavenging performance, whereas the pressure profile at the outlet section has a major role. The design of the intake ports is fundamental for establishing the in-cylinder flow field and a new ports configuration is proposed to enhance the swirl ratio and, consequently, the scavenging performance with high delivery ratios.

On the use of particle image velocimetry (PIV) data for the validation of Reynolds averaged Navier-Stokes (RANS) simulations during the intake process of a spark ignition direct injection (SIDI) engine

Computational fluid dynamics (CFD) simulations of the in-cylinder flow field are widely used in the design of internal combustion engines (ICEs) and must be validated against experimental measurements to enable a robust predictive capability. Such validation is complicated by the presence of both large-scale cycle-to-cycle variations and small-scale turbulent fluctuations in experimental measurements of in-cylinder flow fields. Reynolds averaged Navier-Stokes (RANS) simulations provide overall flow structures with acceptable accuracy and affordable computational cost for widespread industrial applications. Due to the nature of averaging physical parameters in RANS, its validation against experimental results obtained by particle image velocimetry (PIV) requires consideration of how best to average or filter the measured turbulent flows. In this paper, PIV measurements on the cross-tumble plane were recorded every five crank angle degrees for [Formula: see text] cycles during the intake process of a motored, optically accessible spark ignition direct injection (SIDI) engine. Several methods including ensemble averaging, speed-based averaging and low-order proper orthogonal decomposition (POD) reconstruction were applied to remove the fluctuations from experimental PIV vector fields and thus enable comparison to RANS simulations. Quantitative comparison metrics were used to evaluate the performances of each method in representing the intake jet. Recommendations are made on how to provide a fair validation between measured data and simulation results in highly fluctuating flow fields such as the engine intake jet.

Numerical Investigation of the Turbulent Flame Propagation in Dual Fuel Engines by Means of Large Eddy Simulation

Dual fuel combustion depicts a possible alternative to reduce emissions from large engines and is characterized by injecting a small amount of diesel fuel into a lean natural gas–air mixture. Thereby, the presence of autoignition, diffusive and premixed combustion determine the high complexity of this process. In this work, an Extended Coherent Flame Model was adapted to consider the effect of natural gas on the ignition delay time. This model was afterward utilized to simulate 25 consecutive engine cycles employing LES. In this framework, the ensemble-average flow field was compared to a RANS solution to assess the advantages of LES in terms of the prediction of the in-cylinder flow field. A detailed investigation of the heat release characteristic showed that natural gas already highly contributes to the heat release at the beginning of combustion. Furthermore, a methodology to investigate the turbulent combustion regimes was utilized. It could be ascertained that the combustion mainly occurs in the regime of thin reaction zones. Possible triggers of cycle-to-cycle variations were determined in the velocity fluctuations in the cylinder axis direction and the flame formation in the gaps between the spray plume. The findings support the understanding of dual fuel combustion and serve as a basis for developing future combustion models.