Intake Swirl Research Articles

Computational analysis of piston shape effects on in-cylinder gas flow, fuel-charge mixing, and combustion characteristics in a two-stroke rod-less spark ignition opposed-pistons engine

Compared with the traditional in-cylinder direct-injection spark ignition engine, the side-injection and side-spark-ignition characteristics of the two-stroke opposed-piston engine increase the ignition kernel offset and flame propagation distance. Increasing the flame propagation speed can to some extent solve the drawbacks caused by the non-central arrangement of spark plugs. The combustion chamber structure plays a crucial role in gas flow, fuel-charge mixing, and combustion characteristics. Therefore, three pistons were designed and comparatively analyzed in this study. The results show that: The pancake piston is beneficial to maintaining the intake swirl strength due to its simple and smooth spherical arc structure. The swirl strength of the pit and pit-guided piston decreases obviously, and the tumble strength can be maintained well. Compared to pancake and pit-guided pistons, the average TKE for the pit piston increased by approximately 25%, with a more concentrated distribution at the spark timing. The pancake piston exhibits the best scavenging performance, reducing the residual exhaust gas ratio by 2.1% and fresh air loss by 3.3% to the pit piston. A stable ignition core can be formed at the spark timing, but significant differences are observed in the flame propagation process for three pistons. Compared to the pit-guided piston, the pit piston has a 0.3% decrease in the indicated thermal efficiency, but a 13.1% decrease in combustion duration, which reduces knock tendency.

Cyclic Analysis of In-Cylinder Vortex Interactions Based on Data-Driven Detection and Characterization Framework

Abstract The complex vortex flow interactions are critical to affect the fuel–air mixing and combustion stability in direct-injection engine. However, due to the strong cyclic variations inside engine, the multiscale swirl flow characteristics with cyclic details are difficult to be sufficiently revealed. Therefore, a vortex detection and characterization framework, including physical and data-driven methods, is implemented to elucidate the cyclic vortex interaction process. In this study, a high-speed time-resolved particle image velocimetry is applied to record the spatiotemporal flow behavior under three different swirl ratio conditions. First, the presence of vortex motion is detected at each crank angle for each engine cycle. Results show that the vortex interaction processes under different swirl ratio conditions exhibit distinctive characteristics. The presence of multiple vortices and their interactions are found to trigger dramatic changes and variations in swirl flow behavior. Then, the individual-cycle analysis of the vortex interaction effects on flow characteristics is conducted. The vortex characteristics including vortex location, strength, and size are examined with cyclic detail using data-driven unsupervised clustering. Results indicate that the vortex merging is the main source inducing the vortex characteristics variations. Furthermore, the occurrence and duration of the vortex merging process are found to be closely related to the intake swirl ratio and valve lift profile. Increased swirl ratio and valve lift cause vortex to merge earlier and reduce the merging duration. This finding provides a potential idea to alleviate the cyclic variation issue by controlling the vortex merging process.

Effects of multitype intake structures on combustion performance of different opposed-piston engines

The uniflow scavenging opposed-piston (USOP) engine has more potential for performance improvement than the conventional engine. The combustion performance of the USOP engine is greatly affected by the uniflow scavenging process. However, the uniflow scavenging process fluctuates greatly. In addition, the opposed free piston linear generator (OFPLG) and the opposed-piston two-stroke (OP2S) belonging to the USOP engine have different piston motion laws, so the influence of intake structure on different USOP engines should be explored. This study established and verified a CFD simulation model and selected three optimization directions (intake swirl, scavenging performance, and exhaust gas backflow inhibition). The effects of intake wall thickness, intake port shape, intake port offset, and the number of intake ports were investigated by an orthogonal test. Last, three optimum schemes that respond to the three optimization directions were used to compare the combustion performance of different USOP engines. Results showed that the intake wall thickness, intake port shape, and intake port offset significantly affect the combustion performance. Improving the intake swirl and scavenging performance are effective methods to optimize comprehensive performance. Intake swirl regulation improves combustion performance by accelerating the uniform distribution of oil spray. Scavenging performance optimization improves combustion performance by increasing the in-cylinder fresh air mass. Intake wall thickness and intake port offset can simultaneously improve the intake swirl and scavenging performance.

Investigation on the showerhead film cooling of the turbine vane considering combustor swirling outflow

The mutual interference between the combustor and turbine of modern aero-engines has gradually increased with recent advancements, and turbine cooling design considering the flow characteristics at the combustor exit has not been investigated thoroughly. More importantly, no effective solution has been proposed for the negative impact of swirling inflow on vane cooling. This paper presents a comprehensive study on vane leading edge cooling, where the region is most susceptible to the swirling inflow. The film cooling effectiveness (η) of the showerhead film cooling is measured by PSP (pressure-sensitive-paint) technology, and flow characteristics are obtained by numerical simulation. Based on the in-depth analysis, a new film hole layout is proposed to suppress the negative impact of the swirl intake condition on film cooling. The results show that the swirling inflow changes the vane pressure distribution, leading to a significant radial deflection of the film. This causes the film to concentrate in the partial area of the vane, thereby increasing the film distribution inhomogeneity. The mass flow ratio increase only restrains the negative influence of swirling inflow on the cooling effect to a certain extent under the high swirl intensity intake condition (SN = 0.45). At SN = 0.45, an efficient showerhead cooling scheme is desired, and we propose a new cooling scheme to this end. According to the pressure distribution on the leading edge, the new film hole layout scheme can exhibit excellent inhibitory effect on the radial deflection of the film. Therefore, the new cooling scheme can increase the film coverage area and improve the uniformity of film distribution. The increase in surface-averaged film cooling effectiveness (ηsur-ave) is up to 23.1%. Furthermore, the new cooling scheme reduces the relative standard deviation of film cooling effectiveness by up to 24.1%. Thus, the proposed scheme highlights the influence of the combustor outflow on the showerhead film cooling, which can provide a reference for vane cooling design.

A Numerical Investigation on the Effects of Intake Swirl and Mixture Stratification on Combustion Characteristics in a Natural-Gas/Diesel Dual-Fuel Marine Engine

A Numerical Investigation on the Effects of Intake Swirl and Mixture Stratification on Combustion Characteristics in a Natural-Gas/Diesel Dual-Fuel Marine Engine

Concept of rapid and controllable combustion for high power-density diesel engines

Over recent years, the demand to abate global carbon emissions has attracted significant interest and resulted in more stringent policies and regulations. For diesel engines, the fundamental solution to addressing carbon emissions entails optimizing the thermal efficiency. However, for high power-density marine diesel engines, large amount of injection mass companied with little intake swirl makes further optimization of fuel consumption quite difficult. To address this issue, a concept for rapid and controllable combustion is proposed herein. The concept is a theory-based method to design and optimize the combustion system in a forward direction from theory to control method. The concept of rapid combustion relies on the optimization of the combustion process based on ideal Sabathe-Miller cycle to get the optimal heat release rate. The concept of controllable combustion was conducted by theoretically relating the in-cylinder spray evolution with heat release rate so as to determine the controllable parameters of combustion system. The concept proposed was validated by the computational fluid dynamics simulation as well as a high power-density diesel engine with a cylinder power of 450 kW and displacement of 18.9L which has been optimized by traditional benchmark and design of experiments method. The results showed that, the optimization method suggested is reliable to define the combustion system to achieve thermal efficiency as high as possible under the boundary constrains. Moreover, without excessive requirement for turbocharger capacity and injection system, both the fuel consumption and NOx emission decrease under all tested loads by using the proposed method, when compared to the results of traditional optimization method. For the commonly-used 100 % and 75 % load, the fuel consumption respectively drops from 199.02 to 196.5 and 199.75 to 197.15 g/kWh, and NOx emission is reduced by 22 % and 12.6 %, which is relatively remarkable for high power-density marine diesel engines. Therefore, it offers the advantage of significantly reducing the workload when designing and optimizing the combustion of diesel engines.

Investigations on combustion system optimization of a heavy-duty natural gas engine

Nowadays, the carbon-free policies and the crisis of crude oil make natural gas get increasing attention. However, spark-ignited natural gas engines continually suffer from problems of thermal efficiency and NOx emissions. Optimizing intake and combustion systems are effective ways to improve combustion and emission performance, but comprehensive work involving both intake and combustion systems is still lacking, especially for heavy-duty commercial engines. In this work, both intake and combustion systems of spark-ignited heavy-duty natural gas engines were investigated using muti-dimensional numerical simulations. Two intake ports and four combustion chambers were considered. An optimized chemical model was employed to accelerate the computation efficiency of combustion processes. The optimal combination of intake and combustion systems was obtained, with impressive thermal efficiency and acceptable emission performance. The results show that the mixed-flow intake port performs better than the swirl intake port for the natural gas engine with premixed combustion mode, manifesting increased in-cylinder tumble ratio and turbulent kinetic energy. Meanwhile, the eccentric hemispherical combustion chamber (EHCC) combined with mixed-flow intake ports presents the best optimal combustion performance, exhibiting an effective thermal efficiency beyond 41.53%. However, the EHCC scheme shows an increased NOx emission due to fast combustion speed and high combustion temperature while negligible CO and CH4 emissions. Despite this, natural gas engines equipped with three-way catalysis after-treatment can still meet emission regulations under stoichiometric operating conditions.

Flow Characteristics of a Dual-Intake Port Diesel Engine with Guide Vanes

The intake port of a CA4DD diesel engine was investigated by a computational fluid dynamics (CFD) numerical simulation to enhance the intake swirl and improve the flow characteristics in the engine. The uniform design method was applied to study the influence of guide vanes with different parameters on the intake process at various valve lifts. Nine guide vane models were established and compared to the base model using CATIA™ 3D CAD software. Numerical simulations were conduct with Xflow software based on the lattice Boltzmann method (LBM). The distributions of the streamlines, vorticity, velocity and turbulent intensity in each cylinder were simulated and analyzed. The results show that the influence of the guide vanes on the swirl ratio was greater than 37%, and the flow coefficient was less than 5% compared to the base model. Scheme 5, H7.5-L50-θ20 (guide vane height of 7.5 mm, length of 50 mm and angle of 20°), provided good performance. The flow characteristics of the optimal guide vane model were verified through a steady flow test. When the guide vane aspect ratio and angle were within the ranges of 3.5-6.9 and 12.2°-20.2°, respectively, the swirl ratio had the best effect at maximum valve lift. This study provides a theoretical basis for improving the performance of dual-intake diesel engines.

Cold emission optimization of a diesel- and alternative fuel-driven CI engine

This paper deals with the emission optimization of a compression ignition (CI) engine during cold ambient operation. Hence, in the present study, the effect of different injector nozzle geometries and pilot injection strategies, but also the influence of intake swirl, rail pressure, exhaust gas recirculation (EGR) as well as EGR cooling on the emission behavior during cold run are investigated. Therefore, test bed experiments under steady-state cold conditions are conducted on a state-of-the-art CI single cylinder research engine (SCRE) with approximately 0.5 l swept volume representing the typical passenger car (PC) cylinder size. The cold charge air temperature of down to −8 ^{circ }hbox { C} and a low engine coolant and lube oil temperature represent a cold run close to reality. For emulating the higher friction of a typical 4-cylinder PC engine during cold run, the indicated mean effective pressure (IMEP) is increased according to a specifically developed equation and the turbocharger main equation is solved permanently to adjust the gas exchange loss. To take account of a potential future tightening of emission legislation, in addition to limited exhaust gas emissions, non-limited emissions such as carbonyls are measured as well. Since alternative fuels are able to make a significant contribution to the defossilisation of transportation, an oxygen-containing fuel, consisting of 100 % renewable blend components (HVO, ethers and alcohols) and fulfilling the EN 590 legislation is investigated under the same cold conditions in addition to the research on conventional diesel fuel.

Analysis of intake swirl in a compression ignition engine at different intake valve lifts

In the present work, analysis of intake swirl is carried out on a single cylinder, direct injection diesel engine to determine the effect of swirl ratio on engine performance. A two dimensional CAD model was built to predict the pressure, temperature and velocity variation of air entering the cylinder during the suction and compression strokes of a cycle and the swirl ratio was calculated. Later, a three-dimensional model of the cylinder and a valve was made to carry out simulations at different valve lifts to identify and obtain the optimized swirl. Here, the maximum swirl ratio was obtained at 1 mm of valve lift whereas minimum at 6 mm of valve lift. The simulated results are validated with experimental data and good agreement was found between them. In addition to this, guide vanes were introduced in the intake manifold increasing the overall swirl ratio and hence leading to better engine performance.

Effects of intake swirl on the fuel/air mixing and combustion performance in a lateral swirl combustion system for direct injection diesel engines

To study how intake swirl affects the combustion performance of the lateral swirl combustion system (LSCS), experiments under different swirl ratios were carried out in a single-cylinder diesel engine to optimize the circumferential injection angle (CIA) and test the eventual combustion performance. Then, simulation under different swirl ratios was conducted to reveal the influence mechanisms of intake swirl on the fuel/air mixing characteristics of the LSCS.Experimental results show that, as the swirl ratio (SR) was increased from 0 to 1.0, the optimal CIA increased 2.5° in the opposite direction to that of the intake swirl. Besides, the LSCS obtained better combustion performance with SR = 0 than with SR = 1.0, especially at high loads and low excess air coefficients. Simulation results indicate that, generating intake swirl weakens the wall-flow-guided and interferential interactions of the LSCS, thereby deteriorating the in-cylinder air utilization and fuel/air mixing uniformity, especially at high loads and low excess air coefficients, in which the interactions between the LSCS chamber and spray jets is strong due to the enhanced spray penetration ability. Therefore, the LSCS without intake swirl obtains the optimal fuel/air mixing and combustion performance in DI diesel engines.

Numerical optimization of a diesel combustion system to reduce soot mass and particle number density

In view of the recent emission legislations imposing a limit on the soot particle number density, a numerical optimization has been presented for a constant speed heavy-duty diesel engine to reduce soot particle number density while ensuring no penalty on NOx emissions. Closed cycle combustion simulations have been performed at three loads with the sectional soot model coupled to gas phase kinetics. The combustion model has been tuned at each load to ensure a good agreement for in-cylinder pressure, heat release rate and exhaust emissions. The validated combustion model has been used to study the effect of various parameters like intake swirl, start of injection, number of nozzle holes, spray angle and piston bowl shapes. Combustion simulations have been performed for every case of a full factorial design of experiments with 486 cases. Change in plume dynamics due to each parameter has been discussed in detail with the help of in-cylinder soot contours. A reentrant bowl with a higher swirl, lower number of nozzle holes and a deeper spray angle has been found to be the most promising option as it reduces the exhaust soot mass, number density with no penalty on NOx and specific fuel consumption.

Influence of fuel injection and intake port on combustion characteristics of controllable intake swirl diesel engine

In this study, the synergy effects of fuel injection system, intake system and combustion system on the combustion characteristics of the controllable intake swirl diesel engine were investigated. A research platform consisting of CFD numerical simulation and experiment was established in this study. In ways of combination of numerical simulation and experiment based on fuel injection test bed, intake steady flow test bed and single cylinder engine test, the prediction accuracy was improved. As such, the complex fuel spray and combustion phenomenon can be theoretically explained from the views of fluid dynamics and heat transfer, thus leading to further improvement of the engine combustion system. The optimal designs of fuel injectors were proposed and the corresponding fuel injection rate was derived which was used as initial input parameters for CFD numerical studies. Meanwhile by optimization of structure of intake port volute radius and the inlet and outlet section dimensions of spiral intake port, the swirl ratio is increased by 16.5% while the flow coefficient is increased by 4%. Finally, with the optimized fuel injection and air intake system, the effective power was found increased by 3.5% compared to the original diesel engine while the BSFC is reduced by 1.8 g/kW·h. NO emission is slightly increased while soot emission is reduced compared to original diesel engine combustion.

Study on dynamic characteristics of intake system and combustion of controllable intake swirl diesel engine

This paper investigated the intake flow field of a controllable intake swirl diesel engine using computational fluid dynamics (CFD) methodology. It can be observed that the variation of the intake swirl with the opening of the intake baffle shows the two-stage characteristics, while the baffle opening angle of 48° is as a cut-off point. Through the comprehensive analysis of the influence of valve lift and baffle opening angle on flow coefficient, it is concluded that the influence of valve lift on flow coefficient is more sensitive. A mathematical method is used to fit the formula which can calculate the key characteristics of the controllable intake swirl diesel engine. Meanwhile the influence of the swirl ratio on combustion characteristics is investigated. It can be concluded that the swirl ratio has greater influence on the power performance of the diesel engine. When the swirl ratio increases from 0.4 to 1.2, the power performance is increased by 5.79%, and the fuel consumption was also improved. Finally, according to the actual structure of the intake system of diesel engine, a steady-state intake flow test bed was established to verify the accuracy of CFD study of the intake system.

Experimental and numerical study on the influence of intake swirl on fuel spray and in-cylinder combustion characteristics on large bore diesel engine

In the present study, the numerical simulation of the performance of fuel injection and spray atomization on diesel engine combustion was conducted, which was validated against fuel injection test, spray test and single cylinder diesel engine test. Thus, the effect of the intake swirl on the fuel spray and diesel engine combustion performance was explored. Quantitative analysis of the characteristics of fuel-air mixing and combustion characteristics under different swirl ratio was given. It was found that the spray atomization and fuel-air mixing begins from the front of fuel beam which has deflection due to the presence of the intake swirl. The function of swirl is to accelerate the mixing and diffusion speed of fuel, thus promoting the combustion process in cylinder. Meanwhile, the NO and Soot emissions were quite sensitive to the swirl ratio, which leads higher NO but lower Soot as the swirl ratio is increased. In addition, the performance of varied swirl ratio on spray penetration and spray angle was well captured by numerical studies in the paper. Consequently, this study reveals the influence of different swirl ratio on the fuel spray performance, output power and fuel consumption, thus exploring the influence of inlet swirl of the controllable swirl diesel engine on the fuel atomization behavior as well as dynamic performance, and verifying the accuracy of the simulation results.

Effect of intake swirl on combustion performance in an unthrottled spark ignition engine

A Fully Hydraulic Variable Valve System is described in this article which can achieve continuous variation in valve lift, duration, and timing. The system was installed in a four-cylinder port fuel injection spark ignition engine and achieved unthrottled load control through early intake valve closing. The in-cylinder pressure measured experimentally showed that pumping losses of the unthrottled spark ignition engine at 2000 r/min and 0.189 MPa brake mean effective pressure was reduced by 85.4% compared with the throttled spark ignition engine. However, its slow and unstable combustion reduced the indicated thermal efficiency. Compared with the throttled spark ignition engine, the amount of residual exhaust flowing back into the intake port was greatly reduced at the early stage of the intake process. Consequently, it negatively influenced fuel evaporation and fuel–air mixing processes in the intake port of the port fuel injection spark ignition engine and decreased the flow of in-cylinder gases, which resulted in a low combustion rate. A new centrosymmetric helical valve is proposed in this article to improve the fuel–air mixing and combustion rate of the unthrottled spark ignition engine. The experiments demonstrate that the helical valve can generate a strong intake swirl at small intake valve lift. It helps to increase combustion rate and lower cycle-to-cycle variation, which improves indicated thermal efficiency and fuel economy of the unthrottled spark ignition engine at low load.

Experimental research on pumping losses and combustion performance in an unthrottled spark ignition engine

A novel fully hydraulic variable valve system is described in this paper, which achieves continuous variations in maximum valve lift, valve opening duration, and the timing of valve closing. The load of the unthrottled spark ignition engine with fully hydraulic variable valve system is controlled by using an early intake valve closing rather than the conventional throttle valve. The experiments were carried out on BJ486EQ spark ignition engine with fully hydraulic variable valve system. Pumping losses of the throttled and unthrottled spark ignition engines at low-to-medium loads are compared and the reason of it decreasing significantly in the unthrottled spark igntion engine is analyzed. The combustion characteristic parameters, such as cyclic variation, CA50, and heat release rate, were analyzed. The primary reasons for the lower combustion rate in the unthrottled spark ignition engines are discussed. In order to improve the evaporation of fuel and mix with air in an unthrottled spark ignition engine, the in-cylinder swirl is organized with a helical intake valve, which can generate a strong intake swirl at low intake valve lifts. The effects of the intake swirl on combustion performance are investigated. Compared with the throttled spark ignition engine, the brake specific fuel consumption of the improved unthrottled spark ignition engine is reduced by 4.1% to 11.2%.

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.

Effect of intake swirl on the performance of single cylinder direct injection diesel engine

In the present work, the effect of inlet manifold geometry and swirl intensity on the direct injection (DI) diesel engine performance was investigated experimentally. Modifications in inlet manifold geometry have been suggested to achieve optimized swirl for the better mixing of fuel with air. The intake swirl intensities of modified cylinder head were measured in swirl test rig at different valve lifts. Later, the overall performance of 435 CC DI diesel engine was measured using modified cylinder head. In addition, the performance of engine was compared for both modified and old cylinder head. For same operating conditions, the brake power and brake specific fuel consumption was improved by 6% and 7% respectively with modified cylinder head compared to old cylinder head. The maximum brake power of 9 HP was achieved for modified cylinder head. The results revealed that the intake swirl has great influence on engine performance.

Investigation of Cycle-to-Cycle Variation of In-Cylinder Engine Swirl Flow Fields Using Quadruple Proper Orthogonal Decomposition

It has been observed that the swirl characteristics of in-cylinder air flow in a spark ignition direct injection (SIDI) engine affect the fuel spray dispersion and flame propagation speed, impacting the fuel mixture formation and combustion process under high swirl conditions. In addition, the cycle-to-cycle variations (CCVs) of swirl flow often degrade the air–fuel mixing and combustion quality in the cylinder. In this study, the 2D flow structure along a swirl plane at 30 mm below the injector tip was recorded using high-speed particle image velocimetry (PIV) in a four-valve optical SIDI engine under high swirl condition. Quadruple proper orthogonal decomposition (POD) was used to investigate the cycle-to-cycle variations of 200 consecutive cycles. The flow fields were analyzed by dividing the swirl plane into four zones along the measured swirl plane according to the positions of intake and exhaust valves in the cylinder head. Experimental results revealed that the coefficient of variation (COV) of the quadruple POD mode coefficients could be used to estimate the cycle-to-cycle variations at a specific crank angle. The dominant structure was represented by the first POD mode in which its kinetic energy could be correlated with the motions of the intake valves. Moreover, higher order flow variations were closely related to the flow stability at different zones. In summary, quadruple POD provides another meaningful way to understand the intake swirl impact on the cycle-to-cycle variations of the in-cylinder flow characteristics in SIDI engine.