The Effect of Fuel-Injection Timing on In-cylinder Flow and Combustion Performance in a Spark-Ignition Direct-Injection (SIDI) Engine Using Particle Image Velocimetry (PIV)

This study applies particle image velocimetry (PIV) to an optical spark-ignition direct-injection engine in order to investigate the effects of fuel-injection on in-cylinder flow. Five injection timing combinations, each employing a stoichiometric 1:1 split ratio double-injection strategy, were analysed at an engine speed of 1200 RPM and an intake pressure of 100 kPa. Timings ranged from two injections in the intake stroke to two injections in the compression stroke, resulting in a variety of in-cylinder environments from well-mixed to highly turbulent. PIV images were acquired at a sampling frequency of 5 kHz on a selected swirl plane. The flow fields were decomposed into mean and fluctuating components via two spatial filtering approaches — one using a fixed 8 mm cut-off length, and the other using a mean flow speed scaled cut-off length which was tuned in order to match the turbulent kinetic energy (TKE) profile of a 300 Hz temporal filter. From engine performance tests, the in-cylinder pressure traces, indicated mean effective pressure (IMEP), and combustion phasing data showed very high sensitivity to injection timing variations. To explain the observed trend, correspondence between the measured flow and these performance parameters was evaluated. An expected global trend of increasing turbulence with retarded injection timing was clearly observed; however, relationships between TKE and burn rate were not as obvious as anticipated, suggesting that turbulence is not the predominant factor associated with injection timing variations which impacts engine performance. Stronger links were observed between bulk flow velocity and burn rate, particularly during the early stages of flame development. Injection timing was also found to have a significant impact on combustion stability, where it was observed that low-frequency flow fluctuation intensity revealed strong similarities with the coefficient of variance (CoV) of IMEP, suggesting that these fluctuations are a suitable measure of cycle-to-cycle variation — likely due to the influence of bulk flow on flame kernel development.     

Stereoscopic multi-planar PIV measurements of in-cylinder tumbling flow

The non-reacting flow field within the combustion chamber of a motored direct-injection spark-ignition engine with tumble intake port is measured. The three-dimensionality of the flow necessitates the measurement of all three velocity components via stereoscopic particle-image velocimetry in multiple planes. Phase-locked stereoscopic PIV is applied at 15 crank angles during the intake and compression strokes, showing the temporal evolution of the flow field. The flow fields are obtained within a set of 14 axial planes, covering nearly the complete cylinder volume. The stereoscopic PIV setup applied to engine in-cylinder flow and the arising problems and solutions are discussed in detail. The three-dimensional flow field is reconstructed and analyzed using vortex criteria. The tumble vortex is the dominant flow structure, and this vortex varies significantly regarding shape, strength, and position throughout the two strokes. The tumble vortex center moves clockwise through the combustion chamber. At first, the tumble has a c-shape which turns into an almost straight tube at the end of the compression. Small-scale structures are analyzed by the distribution of the turbulent kinetic energy. It is evident that the symmetry plane only represents the 3D flow field after 100 CAD. For earlier crank angles, both kinetic energy (KE) and turbulent kinetic energy (TKE) in the combustion chamber are well below the KE and TKE in the symmetry plane. This should be taken into account when the injection and breakup of the three-dimensional fuel jet are studied. The mean kinetic energy is conserved until late compression by the tumble motion. This conservation ensures through the excited air motion an enhancement of the initial air-fuel mixture which is of interest for direct-injection gasoline engines.     

Decomposition of Turbulent Velocity Fields in an SI Engine

In this study, the turbulence filter, the phase averaging and the proper orthogonal decomposition methods are used to decompose experimentally measured turbulent velocity fields in an SI engine. The radial and circumferential turbulent velocity fields were measured using hot wire anemometer under motored conditions at different engine configurations. The decomposed results of each technique are compared with each other. In addition, the obtained organized and turbulence motions and their energy spectra are examined. Finally, coherent structures of velocity fields and their activities are investigated. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Study of the Organized in-Cylinder Motion by a Dynamic Adaptive Scale-Resolving Turbulence Model

The feasibility of using a scale-adaptive turbulence model for the simulation of the unsteady flow in Internal Combustion (IC) Engines is investigated. The original theory of the Dynamic Length-Scale Resolution Model (DLRM), which includes an adaptive rescaling procedure for the modeled turbulent length and time scales, has been trimmed and applied to the simulation of the flow field of a geometrically simplified square-piston engine working under motored conditions. The flow field exhibits a strong tumbling motion, which is a major characteristic of modern turbo-charged, direct-injection gasoline engines. Multiple consecutive cycles have been computed and turbulent statistics, including the ensemble mean and the variance of the velocity field, have been compared with both experimental measurements and simulation results predicted by URANS and conventional LES with identical numerical setups.     

An investigation of forced structures in turbulent jet flows

In many cases, turbulence is superimposed on an unsteady organized motion of a mean flow. In the past, these turbulent flows have been studied by time or ensemble averaging methods and some decomposition techniques such as proper orthogonal decomposition (POD). In this study, a new decomposition technique called the turbulence filter will be used to decompose the forced turbulent jet flows. By using the turbulence filtering technique, the fluctuating (turbulent) part and the more organized (forced) part of the velocity field are analyzed. Within this context several experiments on organized turbulent jet flow have been carried out. In the experiments, variable frequency and amplitude oscillation are imposed on a 1D jet. An elliptical plate was used in order to obtain sinusoidal forcing. The axial distance, Reynolds number and the forcing frequency of the signal were varied. The multiple hot wires (six probes) were used to investigate the evolution of the signal along the radial distance. The obtained results of the turbulence filter are compared with those of phase-averaging and POD techniques. The eigenmodes of the data are also evaluated by using the POD method. Received: 31 July 1998/Accepted: 19 January 2000     

Experimental assessment of a nonlinear turbulent stress relation in a complex reciprocating engine flow

A nonlinear turbulent stress relationship, based on an explicit algebraic Reynolds stress closure, is compared against experimental data obtained in a swirl-supported, light-duty engine motored at constant speed. The model relationship is applied to measured mean velocity gradients and turbulence scales, and the predictions compared against the measured shear stress and normal stress anisotropy. Significant improvement over the linear stress relationship typically used in two-equation turbulence models is observed. Conditions under which the model predictions are poor are identified and the reasons for the poor performance discussed.     

Proper orthogonal decomposition analysis for cycle-to-cycle variations of engine flow. Effect of a control device in an inlet pipe

This paper aims to investigate cycle-to-cycle variations of non-reacting flow inside a motored single-cylinder transparent engine in order to judge the insertion amplitude of a control device able to displace linearly inside the inlet pipe. Three positions corresponding to three insertion amplitudes are implemented to modify the main aerodynamic properties from one cycle to the next. Numerous particle image velocimetry (PIV) two-dimensional velocity fields following cycle database are post-treated to discriminate specific contributions of the fluctuating flow. We performed a multiple snapshot proper orthogonal decomposition (POD) in the tumble plane of a pent roof SI engine. The analytical process consists of a triple decomposition for each instantaneous velocity field into three distinctive parts named mean part, coherent part and turbulent part. The 3rd- and 4th-centered statistical moments of the proper orthogonal decomposition (POD)-filtered velocity field as well as the probability density function of the PIV realizations proved that the POD extracts different behaviors of the flow. Especially, the cyclic variability is assumed to be contained essentially in the coherent part. Thus, the cycle-to-cycle variations of the engine flows might be provided from the corresponding POD temporal coefficients. It has been shown that the in-cylinder aerodynamic dispersions can be adapted and monitored by controlling the insertion depth of the control instrument inside the inlet pipe.     

Numerical simulation of expansion/compression effect on shock induced turbulent flow

The interaction of homogeneous and isotropic turbulence with a shock wave is observed by solving the Reynolds-averaged Navier–Stokes equations with the k–? turbulence model. All turbulent fluctuations are measured at the period of expansion in the turbulent field and during compression by the reflected shock on turbulent field, and it is observed that the longitudinal turbulent velocity fluctuation is enhanced more at the period of expansion due to incident shock wave movement far from the turbulent field. The amplification of the turbulent kinetic energy (TKE) level in the shock/turbulence interaction depends on the shock wave strength and the longitudinal velocity difference across the shock wave. On decreasing the longitudinal velocity difference across the shock, the turbulent kinetic energy (TKE) level is less amplified. The TKE level is amplified by the factor of 1.5–1.8 in the shock/turbulence interaction where the dissipation rate of TKE decreases in all cases of shock/turbulence interaction. After the shock/turbulence interaction, the turbulent dissipative-length scale is amplified slightly and the amplification of the length scales decreases when increasing the shock strength. To cite this article: M.A. Jinnah, K. Takayama, C. R. Mecanique 333 (2005).     

Constant temperature hot-wire anemometer practice in supersonic flows

The performance of a constant-temperature normal hotwire in a supersonic flow is critically examined. It is shown that this instrument is inherently unsuitable for measuring turbulent temperature correlations because of the highly non-linear response to temperature fluctuations, particularly at low overheat ratios. The instrument is therefore limited to measurements of mean and fluctuating mass-flow rates. Suitable calibration procedures. as well as the limits on spatial and temporal resolution are discussed. and corrections for mean stagnation temperature changes are suggested. The instrument was used to measure the mass-flow fluctuations in a zero pressure gradient Mach 2.9 turbulent boundary layer. A comparison with the available data suggests good agreement.     

Modeling droplet dispersion and interphase turbulent kinetic energy transfer using a new dual-timescale Langevin model

Dispersion of spray droplets and the modulation of turbulence in the ambient gas by the dispersing droplets are two coupled phenomena that are closely linked to the evolution of global spray characteristics, such as the spreading rate of the spray and the spray cone angle. Direct numerical simulations (DNS) of turbulent gas flows laden with sub-Kolmogorov size particles, in the absence of gravity, report that dispersion statistics and turbulent kinetic energy (TKE) evolve on different timescales. Furthermore, each timescale behaves differently with Stokes number, a non-dimensional flow parameter (defined in this context as the ratio of the particle response time to the Kolmogorov timescale of turbulence) that characterizes how quickly a particle responds to turbulent fluctuations in the carrier or gas phase. A new dual-timescale Langevin model (DLM) composed of two coupled Langevin equations for the fluctuating velocities, one for each phase, is proposed. This model possesses a unique feature that the implied TKE and velocity autocorrelation in each phase evolve on different timescales. Consequently, this model has the capability of simultaneously predicting the disparate Stokes number trends in the evolution of dispersion statistics, such as velocity autocorrelations, and TKE in each phase. Predictions of dispersion statistics and TKE from the new model show good agreement with published DNS of non-evaporating and evaporating droplet-laden turbulent flow.     

Large-eddy Simulation of Motored Flow in a Two-valve Piston Engine: POD Analysis and Cycle-to-cycle Variations

Large-eddy simulation (LES) has been performed for a single-cylinder, two-valve, four-stroke-cycle piston engine through 70 consecutive motored cycles. Initial comparisons of ensemble-averaged velocity fields have been made between LES and experiment, and proper orthogonal decomposition (POD) has been used to analyze the complex in-cylinder turbulent flows. Convergence of POD modes has been quantified, several POD variants have been explored, and sensitivity of results to analyzing different subsets of engine cycles has been studied. In general, it has been found that conclusions that were drawn earlier from POD analysis of a simplified non-compressing piston-cylinder assembly with a fixed valve carry over to the much more complex flow in this motored four-stroke-cycle engine. For the cases that have been examined, the first POD mode essentially corresponds to the ensemble-averaged mean velocity. The number of engine cycles required to extract converged POD modes increases with mode number, and varies with phase (piston position). There is little change in the lower-order phase-invariant POD modes when as few as 24 phases per cycle (30° between samples) are used, and complex 3-D time-dependent in-cylinder velocity fields through full engine cycles can be reconstructed using a relatively small number of POD modes. Quantification of cycle-to-cycle variations and insight into in-cylinder flow dynamics can be extracted through analysis of phase-invariant POD modes and coefficients.     

Effect of bubbles on turbulent kinetic energy transport in downward flow measured by time-resolved PTV

A time-resolved particle tracking velocimetry (PTV) system and a shape projection imaging system were used to investigate the turbulence modifications by bubbles in a downward bubbly flow. Two bubble sizes and three mean void fractions were tested at a Reynolds number of about 20,000. The strong modifications in the mean velocity, turbulent kinetic energy (TKE) budget, and velocity spectra are observed in the central region of the pipe that has a high local void fraction. In particular, kinetic energy production decreased, whereas the TKE dissipation rate increased. This suggests that the transfer of energy due to bubbles has a very large effect on the TKE budget. Moreover, velocity spectra reveal that the presence of bubbles modifies the length scales of turbulent eddies, which contain, transfer, and dissipate energy.     

Flow rate through a long channel with a rotating damper at the end

The experimental data on the flow rate (velocity) of a fluctuating air flow are presented on a wide fluctuation frequency range at a constant pressure difference between the channel entry and exit. The superimposed flow fluctuations were produced by periodic cut-off of the exit section by a rotating damper. A considerable dependence of the mean flow rate (velocity) on the wave structure of the flow is established. A flow rate minimum corresponds to resonance flow modes with a maximum relative amplitude of the flow velocity fluctuations.     

Occurrence of caustics for high-frequency acoustic waves propagating through turbulent fields

A numerical technique for simulating the propagation of high-frequency acoustic waves through turbulent fields is introduced. The technique involves two elements: the generation of a random isotropic scalar or vectorial field in terms of a superposition of discrete random Fourier modes; and the integration of the ray-trace equations of geometrical acoustics to describe the trajectories of rays and their distortion. For each realization we compute the ray trajectories and the evolution of the cross section of an elementary ray tube. We then accumulate statistics over an ensemble of realizations to estimate the probability of occurrence of the first caustic. Our results demonstrate that the position of caustics is governed by universal parameters related to the derivatives of the correlation function of the fluctuating components of the turbulent medium.Dedicated to Professor J.L. Lumley on the occasion of his 60th birthday.     

On The Validation of LES Applied to Internal Combustion Engine Flows: Part 1: Comprehensive Experimental Database

Improved understanding of in-cylinder flows requires knowledge from well-resolved experimental velocimetry measurements and flow simulation modeling. Engine simulations using large eddy simulations (LES) are making large progress and the need for well documented velocimetry measurements for model validation is high. This work presents velocimetry measurements from PIV, high-speed PIV, stereoscopic PIV, and tomographic PIV to extensively describe the in-cylinder flow field in a motored optical engine operating at 800 RPM. These measurements also establish a comprehensive database designed for LES model development and validation. Details of the engine, engine accessory components, and well-controlled boundary conditions and engine operation are presented. The first two statistical moments of the flow field are computed and show excellent agreement among the PIV database. Analysis of statistical moments based on limited sample size is presented and is important for modeling validation purposes. High-speed PIV resolved the instantaneous flow field throughout entire engine cycles (i.e. 719 consecutive crank-angles), while tomographic PIV images are further used to investigate the 3D flow field and identify regions of strong vortical structures identified by the Q-criterion. Principle velocity gradient components are computed and emphasize the need to resolve similar spatial scales between experimental and modeling efforts for suitable model validation.     

Improved atomization,collision and sub-grid scale momentum coupling models for transient vaporizing engine sprays

A computationally efficient spray model is presented for the simulation of transient vaporizing engine sprays. It is applied to simulate high-pressure fuel injections in a constant volume chamber and in mixture preparation experiments in a light-duty internal combustion engine. The model is based on the Lagrangian-Particle/Eulerian-Fluid approach, and an improved blob injection model is used that removes numerical dependency on the injected number of computational parcels. Atomization is modeled with the hybrid Kelvin–Helmholtz/Rayleigh–Taylor scheme, in combination with a drop drag model that includes Mach number and Knudsen number effects. A computationally efficient drop collision scheme is presented, tailored for large numbers of parcels, using a deterministic collision impact definition and kd-tree data search structure to perform radius-of-influence based, grid-independent collision probability estimations. A near-nozzle sub-grid scale flow-field representation is introduced to reduce numerical grid dependency, which uses a turbulent transient gas-jet model with a Stokes–Strouhal analogy assumption. An implicit coupling method was developed for the Arbitrary Lagrangian–Eulerian (ALE) turbulent flow solver. A multi-objective genetic algorithm was used to study the interactions of the various model constants, and to provide an optimal calibration. The optimal set showed similar values of the primary breakup constants as values used in the literature. However, different values were seen for the gas-jet model constants for accurate simulations of the initial spray transient. The results show that there is a direct correlation between the predicted initial liquid-phase transient and the global gas-phase jet penetration. Model validation was also performed in engine simulations with the same set of constants. The model captured mixture preparation well in all cases, proving its suitability for simulations of transient spray injection in engines.     

Experimental investigation of streamwise velocity fluctuation based on the Reynolds-number dependency in turbulent viscoelastic-fluid flow

This paper describes an experimental verification of energy supply mechanisms for the streamwise component of the turbulent kinetic energy (TKE) at different Reynolds numbers in viscoelastic-fluid flow. We investigated the characteristics of the streamwise turbulent velocity fluctuation by analyzing the production and turbulent diffusion terms in the TKE transport equation. In addition, we reported on the Reynolds-number dependency in a high Reynolds-number regime where direct numerical simulation cannot demonstrate changes in fluid properties. Based on the experimental verification, we proposed a conceptual model of the energy-exchange term between the TKE and the elastic energy, with focusing on the dependency of the fluid properties on the shear stress. This model is indirectly reflected in the streamwise TKE, the instantaneous velocity field, and the wave number relevant to energy-containing eddies. The main gain term of the TKE switches between the energy-exchange term and the production term dependently on the Reynolds number: as the Reynolds number exceeds the value which provides the maximum drag reduction rate, the production term becomes dominant and the magnitude of streamwise TKE becomes high compared to the water flow case.     

The development and application of a fast-response DC-coupled flame ionization current sensor

The principles of the design of a fast-response DC-coupled ionisation probe and associated electronic circuit are described for the measurement of the mean and fluctuating components of the ion concentration in the reaction zones of stationary and transient flames. The performance of the ionisation probe is evaluated by bench tests and with measurements of ion current mean and fluctuating levels in laminar and turbulent premixed flames and in the combustion chamber of a spark ignition engine. The results demonstrate the wide frequency bandwidth, good spatial resolution and electronic noise immunity of the ionisation probe, making it suitable for a host of combustion applications.     

Effect of Particle Clusters on Carrier Flow Turbulence: A Direct Numerical Simulation Study

Experiments indicate that particle clusters that form in fluidized–bed risers can enhance gas-phase velocity fluctuations. Direct numerical simulations (DNS) of turbulent flow past uniform and clustered configurations of fixed particle assemblies at the same solid volume fraction are performed to gain insight into particle clustering effects on gas-phase turbulence, and to guide model development. The DNS approach is based on a discrete-time, direct-forcing immersed boundary method (IBM) that imposes no-slip and no-penetration boundary conditions on each particle’s surface. Results are reported for mean flow Reynolds number Re _p ?=?50 and the ratio of the particle diameter d _p to Kolmogorov scale is 5.5. The DNS confirm experimental observations that the clustered configurations enhance the level of fluid-phase turbulent kinetic energy (TKE) more than the uniform configurations, and this increase is found to arise from a lower dissipation rate in the clustered particle configuration. The simulations also reveal that the particle-fluid interaction results in significantly anisotropic fluid-phase turbulence, the source of which is traced to the anisotropic nature of the interphase TKE transfer and dissipation tensors. This study indicates that when particles are larger than the Kolmogorov scale (d _p ?>?η), modeling the fluid-phase TKE alone may not be adequate to capture the underlying physics in multiphase turbulence because the Reynolds stress is anisotropic. It also shows that multiphase turbulence models should consider the effect of particle clustering in the dissipation model.     

Integral Length Scales and Time Scales of Turbulence in an Optical Spark-Ignition Engine

In-cylinder air flow structures are known to play a major role in mixture preparation and flame development in spark-ignition engines. In this paper both LDV and PIV measurements were undertaken in an optical spark-ignition at 1500 RPM 0.5 bar inlet plenum pressure. One of the primary PIV planes was vertical cutting through the centrally located spark plug (tumble plane) inside the pentroof at ignition timing. The other plane was horizontal inside the pentroof 1 mm below the spark plug LDV was conducted 1 mm below the spark plug on a line from inlet to exhaust but also on a lower line 14 mm below the spark plug. In-cylinder PIV data at specific crank angles in the intake and compression strokes were also analysed on the central tumble plane and on a horizontal plane 14 mm below the spark plug. The combination of both techniques allowed high spatial and temporal resolution as the two data sets complemented each other to provide details of mean flow and turbulence characteristics on different levels, aiming ultimately for quantification of integral time scales and length scales. LDV cycle-resolved analysis distinguished between the classic approach of using the time integral of the autocorrelation function to obtain the integral time scale and a high-frequency cut-off analysis to obtain high- and low-frequency fluctuations about an in-cycle mean.