Development of a novel passive top–down uniflow scavenged two-stroke GDI engine

The design and performance characteristics of a novel top–down uniflow scavenged gasoline direct-injection two-stroke engine are presented. The novelty of the engine lies in the cylinder head that contains multiple check valves that control scavenging airflow into the cylinder from a supercharged air plenum. When the cylinder pressure drops below the intake plenum pressure during the expansion stroke, air flows into the cylinder through the check valves. During compression the cylinder pressure increases to a level above the intake plenum pressure and the check valves close preventing back-flow into the intake plenum. The engine head design provides asymmetrical intake valve timing without the use of poppet valves and the associated valve-train. In combination with an external Roots-type supercharger that supplies the plenum and exhaust ports at the bottom of the cylinder wall, the novel head provides top–down uniflow air scavenging. Motoring tests indicated that the check valves seal and the peak pressure is governed by the compression ratio. The only drawback observed is that valve closing is delayed as the engine speed increases. In order to investigate the valve dynamics, additional tests were performed in an optically-accessible cold flow test rig that enabled the direct measurement of valve opening and closing time under various conditions.     

Cycle resolved multi-planar flow measurements in a four-valve combustion engine

The non-reacting flow in a one-cylinder four-valve combustion engine is measured via cycle resolved two-component/two-dimensional (2C/2D) particle-image velocimetry (PIV). The three-dimensional structure of the velocity field is analyzed based on the flow field measured in eight planar planes within the cylinder for several crank angles during the intake and compression phase. Using the mean and statistical values of the single planes quasi three-dimensional flow fields are reconstructed for crank angles of 80°, 160°, and 240° atdc. This enables the detailed analysis of the spatial distribution of the large and small scale flow structures, e.g., by visualizing large vortical structures and the distribution of the turbulent kinetic energy. It was found that two ring vortices evolving beneath the inlet valves are the dominant large scale structures that seem to be of major concern for the mixing process in the cylinder of a four-valve combustion engine operated at 1500 rpm. Furthermore, the temporal evolution of the flow field within the symmetry plane of the cylinder, measured for crank angles between 40° and 320° atdc in steps of 20°, is discussed. The results give new insight into the complex three-dimensional flow in the combustion chamber of a one-cylinder four-valve combustion engine. That is, the tumble vortex only seems to be of secondary importance for the flow concerning the mixing process at 1500 rpm. This is an essential result for future work considering the fluid mechanics of fuel-air-interaction processes and mixing principles in combustion engines.     

Comparison Between Numerically Simulated and Experimentally Measured Flowfield Quantities Behind a Pulsejet

Pulsed combustion is receiving renewed interest as a potential route to higher performance in air breathing propulsion and ground based power generation systems. Pulsejets offer a simple experimental device with which to study unsteady combustion phenomena and validate simulations. Previous computational fluid dynamics (CFD) simulations focused primarily on pulsejet combustion and exhaust processes. This paper describes a new inlet sub-model which simulates the fluidic and mechanical operation of a valved pulsejet head. The governing equations for this sub-model are described. Sub-model validation is provided through comparisons of simulated and experimentally measured reed valve motion, and time averaged inlet mass flow rate. The updated pulsejet simulation, with the inlet sub-model implemented, is validated through comparison with experimentally measured combustion chamber pressure, inlet mass flow rate, operational frequency, and thrust. Additionally, the simulated pulsejet exhaust flowfield, which is dominated by a starting vortex ring, is compared with particle imaging velocimetry (PIV) measurements on the bases of velocity, vorticity, and vortex location. The results show good agreement between simulated and experimental data. The inlet sub-model is shown to be critical for the successful modeling of pulsejet operation. This sub-model correctly predicts both the inlet mass flow rate and its phase relationship with the combustion chamber pressure. As a result, the predicted pulsejet thrust agrees very well with experimental data.     

Investigation of boundary layers in internal combustion engines using a hybrid algorithm of high speed micro-PIV and PTV

Heat transfer properties vary locally and temporally in internal combustion engines due to variations in the boundary layer flow. In order to characterize the dynamics in the boundary layer, crank-angle resolved high-speed micro particle image velocimetry (μPIV) and particle tracking velocimetry (PTV) have been used for near-wall velocity measurements in a spark-ignition direct-injection single cylinder engine. A 527-nm dual cavity green Nd:YLF laser was used for velocity measurements near the cylinder head wall between the intake and exhaust valves in the tumble mean flow plane parallel to the cylinder axis. A long-distance microscope was used to obtain a spatial resolution of 45 μm. Flow fields were determined from 180 to 490 CAD in the compression and expansion strokes. The data show significant variation in the flow during the compression and expansion strokes and from cycle to cycle. Flow deceleration was observed during the end of the compression that continued during the expansion stroke until 400 CAD when the flow direction reverses. Sub-millimeter-sized vortical structures were observed within the boundary layer over extended periods of time.     

Flame/Wall Interactions: Laminar Study of Unburnt HC Formation

This work presents a numerical study dedicated to the formation of unburnt hydrocarbon. Two configurations: head-on quenching (HOQ) on a planar wall and in crevices, are considered. It is well known that they contribute for an important part to the sources of hydrocarbon (HC) emission in a combustion chamber. The aim of this work is to use laminar flame simulations (LFS) to understand how the unburnt HC are produced near walls in gasoline engine. A skeletal mechanism (29 species and 48 reactions) mimicking iso-octane combustion is used. In the HOQ configuration, the flame front propagates toward the cold wall where quenching occurs. The numerical procedure and the chemical scheme used in this study are first validated by comparisons with literature results for the 1D case. Several aspects of flame wall quenching such as oxidation of unburnt HC, wall heat flux, quench distances as well as HC families are investigated by varying parameters like wall temperature and equivalence ratio. In a second part, crevices are considered to study the impact of wall imperfections in combustion chambers. Configurations with different geometrical and thermodynamic properties are tested. It leads to a wide range of flame properties and HC production modes. When incomplete combustion occurs, total HC (fuel + HC) concentration can reach very high levels at the wall. When the crevice is not wide enough, the flame cannot propagate and the quantity of HC is smaller than in the case where the flame can propagate (but the fuel is not oxidizing). If the crevice is wide enough for the flame to propagate, HOQ occurs at the bottom of the crevice and HC accumulate in the corners. The computational results obtained in this work demonstrate the ability of LFS to reproduce incomplete combustion mechanisms that are responsible for a major part of HC production in gasoline engines.     

Study of the intake tumble motion by flow visualization and particle tracking velocimetry

The purpose of this work is to characterize the in-cylinder tumbling flow generated by an engine head during the induction process using flow visualization and particle tracking velocimetry (PTV). The study was carried out for a 4-valve engine head with shrouded intake valves in a special single cylinder transient water analog. This shrouded intake valve configuration was used to obtain a prototypical pure tumble flow suitable for fundamental combustion studies. The results revealed that the shrouded intake valves generate a strong, well-behaved tumble vortex on the axial plane between the cylinder head and the piston face. This vortex dominates the entire flow field and seems to be highly repeatable from cycle to cycle. The effect of engine speed on this tumbling flow was studied. An equivalent tumble ratio was defined and evaluated using the measured velocity fields at BDC (bottom dead center).List of symbols ABDC after bottom dead Center - ATDC after top dead center - BBDC before bottom dead center - BDC bottom dead center - BTDC before top dead center - dm mass of the volume element - M total angular momentum - PTV particle tracking velocimetry - r radial distance from the reference point - t total pulse duration - TDC top dead center - U instantaneous velocity - v velocity of the center point of the element - X streak length     

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.     

A semi‐implicit scheme for large Eddy simulation of piston engine flow and combustion

A semi‐implicit scheme is presented for large eddy simulation of turbulent reactive flow and combustion in reciprocating piston engines. First, the governing equations in a deforming coordinate system are formulated to accommodate the moving piston. The numerical scheme is made up of a fourth‐order central difference for the diffusion terms in the transport equations and a fifth‐order weighted essentially nonoscillatory (WENO) scheme for the convective terms. A second‐ order Adams–Bashforth scheme is used for time integration. For higher density ratios, it is combined with a predictor–corrector scheme. The numerical scheme is explicit for time integration of the transport equations, except for the continuity equation which is used together with the momentum equation to determine the pressure field and velocity field by using a Poisson equation for the pressure correction field. The scheme is aimed at the simulation of low Mach number flows typically found in piston engines. An efficient multigrid method that can handle high grid aspect ratio is presented for solving the pressure correction equation. The numerical scheme is evaluated on two test engines, a laboratory four‐stroke engine with rectangular‐shaped engine geometry where detailed velocity measurements are available, and a modified truck engine with practical cylinder geometry where lean ethanol/air mixture is combusted under a homogeneous charge compression ignition (HCCI) condition. Copyright © 2012 John Wiley & Sons, Ltd.     

进口润滑条件对活塞环-缸套摩擦副润滑性能的影响

目前内燃机活塞环-缸套摩擦副润滑分析中,活塞环与缸套之间的润滑状态一般假设为充分润滑或固定状况的贫油润滑,不是通过对实际润滑油膜形成情况的分析确定.本文中以一多缸四行程内燃机为研究对象,基于润滑油流量以及控制体体积变化方程,建立活塞环-缸套间润滑油的流动模型,进行了不同进口处润滑油膜供给量对活塞环-缸套摩擦副润滑特性的影响分析.结果表明:活塞环进口处的润滑条件对活塞环-缸套摩擦副的润滑性能有显著影响;进口处润滑油供给量增加,活塞环-缸套摩擦副的最小油膜厚度增加,最大油膜压力、微凸体作用力、摩擦力和功耗均相应减小;进口处供给油膜厚度较小的情况下,增加油膜供给厚度可以明显改善活塞环-缸套摩擦副的润滑性能.     

Dynamic effects of the power-conversion module in a reciprocating engine

The forced response characteristics of piston, connecting rod and their assembly, henceforth called power-conversion module, is studied subjecting a forced response model of such a module to combustion characteristics in order to investigate clattering noise characteristics brought with compression ignition excitation. Existing research either focused on the piston or the connecting rod solely. As demonstrated by the modal analysis of the whole power-conversion module, it is revealed that the natural frequencies of the entire module dominate the noise-characteristics of clattering noise even when using a linear model. A subsequent parametric study applying different combustion characteristics with different pressure rise rates, but similar peak pressures on the modal-model of the power-conversion module delivered novel insights into the root cause of clattering noise characteristics. Moreover, the approach delivers an amended understanding of disturbing noises occurring in knock control systems of internal combustion engines. The reason for empirically elaborated limits of the maximum cylinder pressure rise rate to achieve smooth engine acoustics, published first in the late 1920s, was revealed.     

An investigation into the interaction of closely-spaced starting jets

The transient, three-dimensional scavenging flow inside a novel two-stroke engine has been investigated both experimentally in a scaled water model as well as numerically using a commercial CFD code incorporating an unsteady Reynolds averaged Navier–Stokes (URANS) formulation. The scavenging flow consists of 16 round jets in close proximity of each other and the cylinder wall, developing from the top of the combustion chamber down towards the exhaust ports located along the wall at the bottom of the cylinder. Flow visualization of the scavenging flow was performed using a scaled fixed-piston water model and was used as a means of validating the URANS simulations themselves. The flow visualization experiments provided insight into the complex jet–jet and jet–wall interactions within the engine cylinder. These interactions were not as well predicted by the CFD simulations. In fact, the CFD simulations were found to significantly under-predict the turbulent mixing between the jets. This suggests that unsteady-RANS formulations are incapable of reproducing the large-scale and unsteady mixing structures associated with the vortex shedding between the closely-spaced jets.     

Comparative study of in-cylinder tumble flows in an internal combustion engine using different piston shapes—an insight using particle image velocimetry

This paper deals with the experimental investigations of the in-cylinder tumble flows in a single-cylinder engine with five different piston crown shapes at an engine speed of 1,000 rev/min., during suction and compression strokes under motoring conditions using particle image velocimetry. Two-dimensional in-cylinder tumble flow measurements and analysis are carried out in combustion space on a vertical plane passing through cylinder axis. Ensemble average velocity vectors are used to analyze the tumble flow structure. Tumble ratio and average turbulent kinetic energy are evaluated and used to characterize the tumble flows. From results, it is found that at end of compression, pentroof-offset-bowl piston shows about 41 and 103% improvement in tumble ratio and average turbulent kinetic energy respectively, compared to that of flat piston. The present study will be useful in understanding effect of piston crown shapes on nature of the in-cylinder fluid tumble flows under real engine conditions.     

Dependence of combustion dynamics in a gasoline engine upon the in-cylinder flow field, determined by high-speed PIV

We apply time-resolved high-speed particle image velocimetry (PIV) in an optically accessible gasoline engine to determine the effect of the in-cylinder flow field upon combustion dynamics. Our PIV setup involves solid particles as tracer, which enables also measurements at firing top dead center and during the combustion process itself. We analyze the flow field for the entire intake and compression phase, as well as the decay of a prominent large-scale tumble structure in the flow field. The data indicate significant cycle-to-cycle flow field variations, characterized by detection of kinetic energy and tumble center. Measurements in fired engine operation demonstrate the influence of the flow field on combustion dynamics. At stoichiometric operation, we find that variations in the kinetic energy of the flow field are a major cause of cycle-to-cycle variations. From simultaneous imaging of the combustion flame and PIV at lean operation, we find that the velocity distribution in the flow field induces a macroscopic motion of the flame kernel??which significantly effects the combustion process.     

Effects of injection timing on performance and droplet characteristics of a sixteen-valve four cylinder engine

 The performance and droplet characteristics of a sixteen-valve, four cylinder engine operating with combustion in one cylinder have been measured with part load, a speed of 1200 rpm and a stoichiometric mixture of gasoline and air. The indicated mean-effective cylinder pressure was found to be constant with initiation of injection from 150° to 630° of crank angle after top-dead-centre of intake and with a 10% reduction between 30° and 60° which coincided with maxima in the covariance in pressure and in the emissions of unburned hydrocarbon. There was also a tendency for performance to decline with injection after 660°. Measurements with laser- and phase-Doppler velocimeters showed that the number of droplets entering the cylinder was much reduced with injection at crank angles corresponding to closed inlet valves due to evaporation, and that the few large droplets which emerged did not survive until top-dead-centre of compression. In contrast, some of the many droplets associated with injection with the valves open survived to the crank angle of ignition and it is likely that these led to an inhomogeneous charge with poorer flame-front propagation responsible for reduction in performance. Received: 19 February 1996/Accepted: 8 October 1996     

关于吸气式高超声速推进技术研究的思考

回顾了吸气式高超声速推进技术的研究进展, 分析了超燃冲压发动机研制面临的关键科学问题, 并从不同角度探讨了增大超燃冲压发动机推力的可能方法.这些方法包括: 能够降低总压损失的高超声速来流压缩方法、生成三维涡流的超声速混合增强技术、碳氢燃料的预热喷射、可以控制燃烧过程的燃烧室设计优化方法、通过减小发动机流道湿面积来降低摩擦阻力和催化复合解离的燃气降低高温气体效应.考虑到等压热力学循环的热效率,还建议研究在高超声速推进系统中应用热效率高的爆轰过程, 并探讨了爆轰推进方法研究的进展与问题.吸气式高超声速推进技术是高超声速飞行器发展的关键技术, 认真思考和探索其发展方向是非常必要的.      

In-cylinder tumble flows and performance of a motorcycle engine with circular and elliptic intake ports

The temporal and spatial evolution processes of the flows in the cylinder of a four-valve, four-stroke, single cylinder, reciprocating motorcycle engine installed with the elliptic and circular intake ports were experimentally studied by using the particle image velocimetry (PIV). The engine was modified to fit the requirements of PIV measurement. The velocity fields measured by the PIV were analyzed and quantitatively presented as the tumble ratio and turbulence intensity. In the symmetry plane, both the circular and elliptic intake ports could initiate a vortex around the central region during the intake stroke. During the compression stroke, the central vortex created in the cylinder of the engine with the circular intake port disappeared, while that in the engine cylinder with the elliptic intake port further developed into the tumble motion. In the offset plane, weak vortical structures were initiated by the bluff-body effect of the intake valves during the intake stroke. The vortical structures induced by the elliptic intake port were more coherent than those generated by the circular intake port; besides, this feature extends to the compression stroke. The cycle-averaged tumble ratio and the turbulence intensity of the engine with the elliptic intake port were dramatically larger than those of the engine with the circular intake port. The measured engine performance was improved a lot by installing the elliptic intake port. The correlation between the flow features and the enhancement of the engine performance were argued and discussed.     

Flow in the coolant passages of an internal combustion engine cylinder head

Detailed measurements of the coolant flow field have been made in the water passages of the cylinder head of an internal combustion engine. They were obtained by casting a transparent acrylic model of the cylinder head and using a mixture of hydrocarbon fluids at a predetermined temperature and concentration which ensured that the refractive index of the fluid was identical to that of the acrylic. This arrangement allowed the use of laser Doppler velocimetry to measure local velocities throughout the cooling passages. The results show that the flow was unevenly distributed with around three quarters of the total passing through the exhaust-portside passages and larger coolant velocities close to the top of the head than close to the gas face. The results of this study aimed to assist the improvement of the specific cylinder head design.     

计及机体变形的内燃机主轴承弹性流体动力润滑分析

针对某四行程四缸内燃机曲轴主轴承,考虑机体变形因素的影响,进行了曲轴轴承弹性流体动力润滑分析.计算中采用动力学法进行曲轴轴承的润滑分析,采用变形矩阵法计算油膜压力作用下轴承表面的变形.结果表明,与不考虑机体变形影响相比,计入机体变形影响时轴承轴心轨迹基本没有改变,在1个工作循环中的大部分时间轴承最大油膜压力、最小油膜厚度、端泄流量和轴颈摩擦系数几乎没有变化,仅在某些时刻附近轴承最大油膜压力略有减少,轴承最小油膜厚度略有增加.因此在计算精度要求不是非常高的情况下,曲轴主轴承润滑分析可以不考虑轴承表面因油膜压力作用产生的弹性变形的影响.     

Optical investigation of the combustion behaviour inside the engine operating in HCCI mode and using alternative diesel fuel

In order to understand the effect of both the new homogeneous charge compression ignition (HCCI) combustion process and the use of biofuel, optical measurements were carried out into a transparent CR diesel engine. Rape seed methyl ester was used and tests with several injection pressures were performed. OH and HCO radical were detected and their evolutions were analyzed during the whole combustion. Moreover, soot concentration was measured by means the two colour pyrometry method. The reduction of particulate emission with biodiesel as compared to the diesel fuel was noted. Moreover, this effect resulted higher increasing the injection pressure. In the case of RME the oxidation of soot depends mainly from O₂ content of fuel and OH is responsible of the NO formation in the chamber as it was observed for NO_x exhaust emission. Moreover, it was investigated the evolution of HCO and CO into the cylinder. HCO was detected at the start of combustion. During the combustion, HCO oxidizes due to the increasing temperature and it produces CO. Both fuels have similar trend, the highest concentrations are detected for low injection pressure. This effect is more evident for the RME fuel.     

Measurements of the dynamic lift force acting on a circular cylinder in cross-flow and exposed to acoustic resonance

A piezoelectric transducer is developed to perform direct measurements of the dynamic lift force acting on a circular cylinder in cross-flow, in the presence and absence of acoustic resonance. Details of the force transducer design are presented in the paper. The dynamic lift force is measured for a single cylinder with two different diameters, D=12.7 and 15.8 mm. During the tests, the first transverse acoustic mode of the duct housing the cylinder is self-excited. The fluctuating pressure on the top wall of the duct is measured simultaneously with the dynamic lift force. In the absence of acoustic resonance, the measured dynamic lift coefficients agree favorably with those reported in the literature. However, when the acoustic resonance is initiated, the dynamic lift experiences a drastic increase in amplitude associated with abrupt changes in the phase between the lift force and the acoustic pressure. A methodology to extract the hydrodynamic lift component from the total lift measured during acoustic resonance is also proposed. The hydrodynamic lift force is then decomposed into in-phase and out-of-phase components, with respect to the resonant sound pressure. This decomposition procedure provides new insights into the nature of the aeroacoustic sources in the cylinder wake. The proposed methodology, together with the test results provide a general design approach to assess the increase in the dynamic fluid loading on bluff bodies in cross-flow due to the excitation of acoustic resonance.