In-cylinder Charge Motion Research Articles

A universally applicable 0D turbulence model based on the physical analysis of fundamental tumble behaviors in spark-ignition engines

Predicting turbulence in a zero-dimensional (0D) simulation is an undeniably challenging task due to the complexity of in-cylinder charge motion. In that respect, a physics-based understanding of in-cylinder flow phenomena is of vital importance for establishing high-fidelity 0D turbulence models under diverse engine conditions. The proposed 0D model is hence built upon the kinetic energy analyses of tumble, evidenced by three-dimensional computational fluid dynamics. Specifically, the overall 0D turbulence model consists of an intake model, a spinning up model, and a turbulence production model. The major difference between this model and the existing 0D turbulence models is that this model is grounded on a kinetic energy perspective of tumble, as opposed to an angular momentum perspective of tumble. That is, the behaviors of tumble, such as spinning up and vortex breakdown, are interpreted and modeled based on the changes in the kinetic energy of tumble. This enables the proposed model to secure wider applicability at various engine conditions, which were otherwise difficult to achieve. Therefore, the 0D simulation in this study was able to predict turbulent intensities at the conditions, differing in valve strategy, engine geometry, and engine operation, without changing any validation constants. Furthermore, along with the validation points used in this study, better predictions of turbulent intensity were achieved using the proposed model compared to the existing state-of-the-art model.

Experimental and numerical investigations of charge motion and combustion in lean-burn natural gas engines

It is important to understand the lean-burn combustion process of large-bore natural gas engines and influences on it in order to improve next generations of gas engines to meet the increasing requirements for high efficiencies and low emissions. The investigations in this study focus on the ignition and early flame propagation phase using optical experiments on a single-cylinder research engine, since both phases highly influence the subsequent main-combustion. Scavenged prechambers with different operating conditions and a tangential and radial nozzle alignment as well as unscavenged prechamber and direct spark plug ignition are compared. Beside the phenomenology of the ignition system itself, the interaction of the main-chamber charge motion and the ignition system is important to understand. Therefore, different valve timings (conventional timings and Miller cycle) as well as a low and high turbulence setup are subject of the study. Numerical simulations of the cold flow are used to understand the charge motion and mixture formation in the prechamber as well as in the main-chamber. The experiments depict an enhancement of the first flame propagation phase using a scavenged prechamber due to hot turbulent jets emerging from the nozzles. Furthermore, an influence of the nozzle geometry and the boundary conditions on the jet development and on the early flame propagation is observed. It is seen by the optical measurements that cycle-to-cycle variations can originate from the hot turbulent jets and its influence on the ignition of the main-chamber charge. Further, the optical measurements show that a low in-cylinder swirl and turbulence due to Miller cycle has an impact on the interaction between the in-cylinder charge motion and the jets. A comparison between high turbulence and low turbulence in-cylinder flows on the combustion is carried out. It is shown that the scavenged prechamber can compensate the lack of turbulence in the main-chamber. Hence, the scavenged prechamber enhances the flame propagation due to induced turbulence in the main-chamber and larger flame front surfaces generated by penetrating flame torches.

Development of a Highly Flexible Variable Valvetrain System for Combustion System Investigations

<div class="section abstract"><div class="htmlview paragraph">For the optimization and extension of operating range of future combustion systems aiming at ultra-low emissions and high efficiency, the control of gas exchange plays a major role. Important parameters for optimization are e.g. volumetric efficiency, residual gas control, in-cylinder charge motion and precise control of the level of homogeneity or inhomogeneity of the charge as required by the particular combustion mode. In addition, advanced operating modes such as Miller or Atkinson cycle or gasoline compression ignition demand a high degree of variability in cam timing. A highly flexible variable valvetrain has been designed for the investigation and development of such new combustion processes. This novel valvetrain is based on a mechanically fully variable actuation concept using two independently rotating cam disks per valve. By this arrangement, new degrees of freedom in the design of the valve opening curve arise. For a single cylinder research engine using a four-valve cylinder head, this valvetrain has been designed in a way so that all four valves can be controlled independently, offering the possibility of e.g. inlet valve phasing combined with a second event on the exhaust. The new valvetrain has been manufactured and tested on a mechanical component test bench. In a “backward” design approach, the valve lift curves and the timing variation range required for the generation of particular variations of in-cylinder charge motion were defined by 3D CFD simulation. On this basis, the layout of the valvetrain components for realization in the research engine was defined. The new valvetrain will be employed for the development of highly efficient low-emission combustion systems on the test bench.</div></div>

High-Performance Computing and Analysis-Led Development of High Efficiency Dilute Opposed Piston Gasoline Engine

Pinnacle is developing a multicylinder 1.2 L gasoline engine for automotive applications using high-performance computing (HPC) and analysis methods. Pinnacle and Oak Ridge National Laboratory executed large-scale multidimensional combustion analyses at the Oak Ridge Leadership Computing Facility to thoroughly explore the design space. These HPC-led investigations show high fuel efficiency (∼46% gross indicated efficiency) may be achieved by operating with extremely high charge dilution levels of exhaust gas recirculation (EGR) at a light load key drive cycle condition (2000 RPM, 3 bar brake mean effective pressure (BMEP)), while simultaneously attaining high levels of fuel conversion efficiency and low NOx emissions. In this extremely dilute environment, the flame propagation event is supported by turbulence and bulk in-cylinder charge motion brought about by modulation of inlet port flow. This arrangement produces a load and speed adjustable amalgamation of swirl and counter-rotating tumble which provides the turbulence required to support stable low-temperature combustion. At higher load conditions, the engine may operate at more traditional combustion modes to generate competitive power. In this paper, the numerical results from these HPC simulations are presented. Further HPC simulations and test validations are underway and will be reported in future publications.

Evaluation of the Predictive Capabilities of a Phenomenological Combustion Model for Natural Gas SI Engine

Abstract The current study evaluates the predictive capabilities of a new phenomenological combustion model, available as a part of the GT-Suite software package. It is comprised of two main sub-models: 0D model of in-cylinder flow and turbulence, and turbulent SI combustion model. The 0D in-cylinder flow model (EngCylFlow) uses a combined K-k-ε kinetic energy cascade approach to predict the evolution of the in-cylinder charge motion and turbulence, where K and k are the mean and turbulent kinetic energies, and ε is the turbulent dissipation rate. The subsequent turbulent combustion model (EngCylCombSITurb) gives the in-cylinder burn rate; based on the calculation of flame speeds and flame kernel development. This phenomenological approach reduces significantly the overall computational effort compared to the 3D-CFD, thus allowing the computation of full engine operating map and the vehicle driving cycles. Model was calibrated using a full map measurement from a turbocharged natural gas SI engine, with swirl intake ports. Sensitivity studies on different calibration methods, and laminar flame speed sub-models were conducted. Validation process for both the calibration and sensitivity studies was concerning the in-cylinder pressure traces and burn rates for several engine operation points achieving good overall results.

CFD study and experimental investigation of piston geometry induced in-cylinder charge motion on LPG fuelled lean burn spark ignition engine

The performance and emission characteristics of LPG fuelled SI engine is determined by efficient combustion. This depends on the in-cylinder motion of intake homogenous charge generated by combustion chamber geometry. The inherent, unsteady, turbulent motion of the mixture helps in flame propagation which in turn enhances the combustion rate. Compression ratio, charge squish velocity and turbulent kinetic energy are responsible for stable combustion and laminar flame propagation. The in-cylinder charge motion is generally described by swirl, squish, tumble and turbulence which have a vital role on air-fuel mixing and combustion. The piston geometry helps in aiding in-cylinder motion for reducing fuel consumption and enhancing the combustion parameters. In particular, piston squish area in combustion chamber geometry has a major influence in charge motion inside the combustion chamber. The effect of piston squish area on charge motion was studied by CFD using STAR-CD with 25%, 30%, 35% and 40% squish areas to optimize piston crown geometry. Experiments were also conducted with above squish area pistons at a compression ratio of 10:1 to validate the CFD study. It was found that 30% piston squish area improved the performance, combustion characteristics and reduced the emission with LPG fuelled lean burn SI engine.

Investigation of an IC Engine Intake Flow Based on Highly Resolved LES and PIV

To reduce emissions and fuel consumption, the current generation of gasoline engines uses technologies such as direct injection, downsizing and supercharging. All of them require a strong vortical in-cylinder charge motion, usually described as “tumble”, to improve fuel-air mixing and enhance flame propagation. The tumble development strongly depends on the flow field during the intake stroke. This flow field is dominated by the intake jet, which has to be captured well in the simulation. This work investigates the intake jet on a steady-state flow bench, especially in the vicinity of the intake valve. At first, the general flow dynamics of the intake jet for three different valve lifts and three different mass flows were investigated experimentally. For the smallest valve lift (3 mm), flow-field measurements using Particle Image Velocimetry (PIV) show that the orientation of the intake jet significantly depends on the air flow rate, attaching to the pent roof for low flow rates. This phenomenon is less pronounced for higher valve lifts. An intermediate valve lift and flow rate were chosen for further investigations by scale-resolving simulations. Three different meshes (coarse, medium and fine) and two turbulence models (Sigma and Detached Eddy Simulation-Shear Stress Transport (DES-SST)) are applied to consider their effect on the numerical results. An ad-hoc post-processing methodology based on the ensemble-averaged velocity field is presented capturing the jet centerline’s mean velocity and velocity fluctuations as well as its orientation, curvature and penetration depth. The simulation results are compared to each other as well as to measurements by PIV.

DigitalAir™ Camless FVVA System - Part 2, Gasoline Engine Performance Opportunities

The paper describes a completely new approach to fully variable valve actuation (FVVA), which allows almost unlimited continuously variable control of intake and exhaust valve opening and closing events, and duration without the use of a camshaft. DigitalAir replaces conventional poppet valves with horizontally actuated valves located directly above the combustion deck of the cylinder head, which open and close a number of slots connecting the cylinder with the intake and exhaust ports, Figure 1. The stroke of the valves to provide the full flow area is approximately 25% of the stroke of the equivalent poppet valve, thus allowing direct electrical actuation with very low power consumption. This design arrangement also avoids the risk of poppet valve to piston collision, or the need for cut-outs in the piston crown, since the valves do not open into the cylinder. The paper will present analytical and experimental data which confirms that the proposed FVVA system can meet the basic performance requirements of modern GDI engines with respect to breathing characteristics across the speed range, throttleless operation at and above idle, opening and closing event optimization, cylinder deactivation, control of residual gas fraction / scavenging and exhaust thermal management. Analytical results were developed using GT-POWER Cycle Simulation and CONVERGE computational fluid dynamics (CFD). Cycle simulation was used to study the system level performance, such as full load capability and transient response, and in particular to quantify the fuel consumption benefits of throttleless operation. CFD was used to better understand the opportunities for in-cylinder charge motion - tumble, swirl and turbulence. JP SCOPE Inc. has been running experimental engines with DigitalAir for several years and has successfully completed performance and durability tests. The mechanical and thermal design of the cylinder head, and the design of the actuator will be covered in Part 1 of this paper [1].

In-Cylinder Flow Correlations Between Steady Flow Bench and Motored Engine Using Computational Fluid Dynamics

Intake port flow performance plays a substantial role in determining the volumetric efficiency and in-cylinder charge motion of a spark-ignited engine. Steady-state flow bench and motored engine flow computational fluid dynamics (CFD) simulations were carried out to bridge these two approaches for the evaluation of port flow and charge motion (such as discharge coefficient, swirl/tumble ratios (SR/TR)). The intake port polar velocity profile and polar physical clearance profile were generated to evaluate the port performance based on local flow velocity and physical clearance in the valve-seat region. The measured data were taken from standard steady-state flow bench tests of an intake port for validation of CFD simulations. It was reconfirmed that the predicted discharge coefficients and swirl/tumble index (SI/TI) of steady flow bench simulations have a good correlation with those of motored engine flow simulations. Polar velocity profile is strongly affected by polar physical clearance profile. The polar velocity inhomogeneity factor (IHF) correlates well with the port discharge coefficient, swirl/tumble index. Useful information can be extracted from local polar physical clearance and velocity, which can help for intake port design.

Development of MGE 1.8T combustion system assisted by numerical simulation

This paper describes the CFD assisted combustion system development for a 1.8 L turbo GDI engine which has been developed by SAIC Motor. The combustion system is a direct injected turbocharged system employing centre-mounted multi-hole injectors. The in-cylinder charge motion, fuel injection, air/fuel mixing and combustion processes in the combustion system were evaluated. Proposed intake port options were numerically investigated and compared. Impingement of fuel droplets on piston crown, and wetting on the walls of combustion chamber and the subsequent impacts on mixture formation and oil dilution were investigated by both CFD tool and engine test approaches. Spray targeting was optimised to minimise oil dilution, and multiple fuel injection strategies were investigated to understand its impact on injection, mixture formation and combustion process. The concentrations of pollutants including CO, NOx, HC and soot were simulated. The present simulation results agree well with the measurement data from engine testing, including cylinder gas pressure.

Effect of implementing large-scale charge motion, reducing hydraulic flow of the injector and increasing injection pressure on particle emissions of a GDI engine at WOT and boosted operation

Gasoline direct injection (GDI) shows advantages compared with port fuel injection (PFI) regarding efficiency and specific power. Due to stricter regulations for fuel consumption (via the regulation of carbon dioxide), GDI engines are becoming increasingly favourable compared with PFI engines. Therefore the share of GDI engines, especially in combination with turbocharging, is increasing in most of the markets with CO2 regulations. Challenging for GDI engines is the mixture formation process due to the short time between fuel injection and the start of combustion. Thus, the injector needs to provide a fine fuel atomization in a considerably short time. The generated spray pattern thereby interacts with the in-cylinder charge motion to generate an appropriate air–fuel mixture. Because of this challenging mixture formation process, the formation of soot in local fuel-rich areas is possible. Thus GDI engines emit more particles compared with PFI engines and need special attention on the mixture formation process. To understand the reasons for the increased particle number (PN) emissions, a project concerning the cause of particle emissions was started at Institute of Internal Combustion Engines (IFKM) in Karlsruhe in 2011. During the project, different causes for PN emissions were identified. This article discusses the possible reasons for particle emissions under high engine load and low engine speed and shows some possible solutions to reduce the emission of particles. Discussed possible solutions to enhance the mixture formation process are the generation of a large-scale charge motion (tumble and swirl), the reduction of the hydraulic flow of the multi-hole, solenoid-activated injector and an increase of the rail pressure up to 50 MPa. The reduction of the hydraulic flow and the increase of the injection pressure lead to smaller average droplets and thus to a faster evaporation. An implementation of a large-scale charge motion enhances the mixture formation process and leads to a reduction of the emitted PN concentration at high engine load. This is shown for wide open throttle (WOT) as well as for boosted operation. The reduction of the hydraulic flow of the injector by reducing the bore hole diameter at constant number of holes and spray targeting of the injector leads to smaller droplets. By increasing the injection pressure, the injection duration as well as the average droplet size can be reduced and leads to a better homogenization and a further reduced PN concentration.

Validation of turbulent flame speed models for methane–air-mixtures at high pressure gas engine conditions

To describe the premixed combustion of natural gas in internal combustion engines, the flamelet approach is often assumed to be appropriate. It postulates a thin flame front which is wrinkled by turbulence but locally spreads like a laminar flame. Numerous correlations for the macroscopic turbulent flame speed to the local laminar flame speed exist. In this study seven correlations are validated at engine conditions, i.e., at high pressure (up to 7.0MPa) and high turbulence (turbulent Reynolds number up to 4.4·104), with the help of measurements from a single cylinder research gas engine. The stoichiometry, the exhaust gas recirculation rate, and the engine speed are varied. Simulations of the detailed reaction kinetics with the diffusive species and energy transport are performed to calculate the laminar flame speed. The flame propagation is described by a zonal cylinder model on the basis of the measured in-cylinder pressure traces, to determine the turbulent flame speed with respect to the thermal expansion of the reacting gas. Additionally, Computational Fluid Dynamics (CFD) simulations of the in-cylinder charge motion are performed to describe the turbulent flow field. It is found that four flame speed correlations including a pressure-dependent term predict flame speed ratios in quantitative accordance to the measurements. This is noteworthy, as these correlations are mostly developed for lower turbulence and at considerable lower pressure. It supports the hypothesis that flamelet-like combustion is still found above the Klimov–Williams criterion.

Benefits of the Electromagnetic Actuated Valve Train in Gasoline Engine Application

Engines equipped with camless valve trains will have greater space for performance optimization. In this paper, based on the electromagnetic actuated valve train system, one-dimensional and three-dimensional simulation models on engine thermodynamic cycle and in-cylinder charge motion are established, respectively. With the application of early-intake-valve closing (EIVC) strategy，unthrottled load control and Miller cycle are obtained and their effects on engine pumping losses and NOx emission are researched. The variation of engine in-cylinder charge movement intensity， pumping losses and volumetric efficiency are also revealed at variable intake valve actuation (VIVA). In addition, a particular variation trend of engine pumping losses, increasing in proportion to the engine load, is detected with the application of unthrottled load control. Finally, Genetic Algorithm (GA) is adopted to optimize engine performance with the EAVT system. Compared to engine with a camshaft valve train, the electromagnetic actuated valve train (EAVT)-driven engine shows a significant improvement on fuel consumption, power performance and NOx emission.

PIV examination of spray-enhanced swirl flow for combustion stabilization in a spray-guided stratified-charge direct-injection spark-ignition engine

Practical implementation of spray-guided stratified-charge direct-injection spark-ignition engines can be inhibited by combustion instability, in particular the occurrence of misfire and partial burns. Performance testing in an all-metal spray-guided stratified-charge direct-injection spark-ignition engine shows that increasing the engine speed from 1000 to 2000 r/min can cause a deterioration of the combustion stability for operation without intake-generated swirl. Introducing swirl to the in-cylinder air charge motion maintains combustion stability while the speed is increased. To gain understanding how swirl reduces cycle-to-cycle variability of the flow, two-dimensional Particle Image Velocimetry (PIV) measurements were made in a horizontal swirl plane near the top of the piston bowl and in a central vertical tumble plane. Tests with and without injection were conducted at 1000 and 2000 r/min for operation both with and without swirl. The results demonstrate that the swirl creates flow patterns in each cycle that are more similar to the ensemble-averaged cycle, and with decreased variability. Furthermore, the fuel injection causes a redistribution of angular momentum resulting from spray–swirl interaction. The gas-phase swirl flow is redistributed by the spray to create a very repeatable vortex with enhanced angular momentum close to the spray centerline. This decreases the cycle-to-cycle variability of the flow patterns. Quantified changes in the stability of the flow patterns with swirl and engine speed are consistent with the combustion-instability trends.

In-Cylinder Flow and Fuel Spray Interactions in a Stratified Spray-Guided Gasoline Engine Investigated by High-Speed Laser Imaging Techniques

Spray-guided direct injection spark-ignition engines operated in stratified charge mode have a high potential for improved fuel economy. As fuel is injected late in the compression stroke mixture preparation is crucial for reliable ignition. Multiple injections per cycle have proven to increase the overall combustion stability. Nevertheless cycle-to-cycle variations (ccv) are observed whose origin is not well understood. Strong impact of in-cylinder flows and spray-induced turbulence of preceding injections upon subsequent spray development and mixture formation is one possible reason for ccv. In this work mutual interactions of in-cylinder charge motion and sprays from multiple injections were investigated. Time resolved particle image velocimetry (PIV) and Mie scattering of fuel droplets at 16 kHz was used to simultaneously measure the temporal evolution of in-cylinder flow fields and spray formation. The data revealed significant spray-induced vortices perturbing the tumble flow. Sprays from subsequent injections were disturbed and showed greatly enhanced ccv compared to the first injection. A distinct upwards fluid flow impinging the cylinder head at the injector’s location (termed funnel flow) was identified as primary origin of spray deformation for second and third injections.

Effects of intake flow and coolant temperature on the spatial fuel distribution in a direct-injection gasoline engine by PLIF technique

The spatial fuel distributions of the homogeneous and stratified charge of a high pressure 6-hole injector were examined in a single cylinder optical direct injection spark ignition (DISI) engine. The effects of in-cylinder charge motion, fuel injection pressure and coolant temperature were investigated using a planar laser induced fluorescence (PLIF) technique. It was found that in the case of homogeneous charge mode, early injection in the intake stroke generated similar fuel distributions at the crank angle of 12° BTDC regardless of the in-cylinder air motion at the coolant temperature of 90°C. In the case of stratified charge mode, the in-cylinder tumble flow played more effective role in mixture preparation than the swirl flow during the compression stroke; and the increase of the coolant temperature improved fuel evaporation; but the increase of the fuel supplying pressure could not change the pattern of the fuel vapour distribution against the expectation.

The Effect of Intake Valve Deactivation on Lean Stratified Charge Combustion at an Idling Condition of a Spark Ignition Direct Injection Engine

A combined experimental and analytical study was carried out to understand the improvement in combustion performance of a four-valve spark ignition direct injection (SIDI) wall-guided engine operating at lean, stratified idle with enhanced in-cylinder charge motion by deactivating one of the two intake valves. A fully warmed-up engine was operated at low speed, light load by injecting the fuel from a pressure-swirl injector during the compression stroke to produce a stratified fuel cloud surrounding the spark plug at the time of ignition. Steady state flow-bench measurements and computational fluid dynamics (CFD) calculations showed that valve deactivation primarily increased the in-cylinder swirl intensity as compared with opening both intake valves. Engine dynamometer measurements showed an increase in charge motion led to improved combustion stability, increased combustion efficiency, lower fuel consumption, and higher dilution tolerance. A CFD study was conducted using in-house models of spray and combustion to simulate the engine operating with and without valve deactivation. The computations demonstrated that the improved combustion was primarily driven by higher laminar flame speeds through enhanced mixing of internal residual gases, better containment of the fuel cloud within the piston bowl, and higher postflame diffusion burn rates during the initial, main, and late stages of the combustion process, respectively.

Numerical investigation of the effect of swirl flow in-homogeneity and stability on diesel engine combustion and emissions

The present study is aimed at numerically investigating the effect of in-cylinder charge motion on mixture preparation, combustion and emission formation in a high-speed direct-injection diesel engine. Previous investigations have shown that different valve-lift strategies nominally lead to similar in-cylinder filling and global swirl levels. However, significant differences in engine-out emissions, especially soot emission, give rise to the assumption that the flow structure and local differences of the swirl motion distribution have a noticeable effect on emission behaviour. In this work, different swirl generation strategies applying different intake valve actuation schemes are numerically investigated by applying transient in-cylinder computational fluid dynamic simulations using both the Reynolds-averaged Navier–Stokes model and the multi-cycle large-eddy simulation approach. Two operating points within the operating range of current diesel passenger cars during federal test procedure 75 and new European driving cycles are simulated. The injection and combustion simulations of different valve strategies show that an in-homogeneity in the in-cylinder flow structure leads to a significant increase in soot emissions, and agree with the observed trends of corresponding experimental investigations.

6공 연료분사기를 장착한 DISI 엔진 내 균질급기의 연료증기 분포 특성

The spatial fuel vapor distribution of the homogeneous charge by a 6-hole injector was examined in a optically accessed single cylinder direct injection spark ignition(DISI) engine. The effects of in-cylinder charge motion, and fuel injection pressure, and coolant temperature were investigated using a planar LIF (Laser Induced Fluorescence) technique. It was confirmed that the in-cylinder tumble flow played a little more effective role in the spatial fuel vapor distribution than the swirl flow during the compression stroke at 10 mm and 2 mm planes under cylinder head gasket and the increased fuel injection pressure activated spatial distributions of the fuel vapor. In additions, richer mixtures were concentrated around the cylinder wall by the increase of the coolant temperature.

Establishing New Correlations Between In-Cylinder Charge Motion and Combustion Process in Gasoline Engines Through a Numerical DOE

Establishing New Correlations Between In-Cylinder Charge Motion and Combustion Process in Gasoline Engines Through a Numerical DOE