Slap Forces Research Articles

The Piston Slap Force Reconstruction of Diesel Engine Using WOA-VMD and Deconvolution.

In a diesel engine, piston slap commonly occurs concurrently with fuel combustion and serves as the main source of excitation. Although combustion pressure can be measured using sensors, determining the slap force is difficult without conducting tests. In this study, we propose a method to identify the slap force of the piston to solve this difficult problem. The traditional VMD algorithm easily receives noise interference, which affects the value of parameter combination [k, α] and thus affects the extraction accuracy of the algorithm. First, we obtain the transfer function between the incentive and vibration response through percussion tests. Secondly, a variational modal decomposition method based on whale algorithm optimization is used to separate the slap response from the surface acceleration of the block. Finally, we calculated the slap force using the deconvolution method. Deconvolution is a typical inverse problem of mathematics, often prone to ill-conditioning, and the singular value decomposition and regularization method is used to overcome this flaw and improve accuracy. The proposed method provides an important means to evaluate the angular distribution of the slap force, identify the shock positions on the piston liner, and determine the peak value of the waveform which helps us analyze the vibration characteristics of the piston and optimize the structural design of the engine.

Cylinder Liner Velocity Calculation under Dynamic Condition in the Pursuit of Liner Cavitation Investigation of an Internal Combustion Engine

<div>An analytical method for nonlinear three-dimensional (3D) multi-body flexible dynamic time-domain analysis for a single-cylinder internal combustion (IC) engine consisting of piston, connecting rod, crank pin, and liner is developed. This piston is modeled as a 3D piston that collides with the liner as in a real engine. The goal is to investigate the piston slap force and subsequent liner vibration. Liner vibrational velocity is directly responsible for pressure fluctuations in the coolant region resulting in bubble formation and subsequent collapse. If the bubble collapse is closer to the liner surface, cavitation erosion in the liner might occur. The mechanism of liner cavitation is briefly explained, which would take a full computational fluid dynamics (CFD) model to develop, which is out of scope for the present work. However, as a first step, the present method focused on a comprehensive and accurate estimation of the highest inward and outward liner velocities, which are directly related to the bubble formation and collapse, respectively. Sensitivity of liner velocity to different engine-operating conditions (warm and hot, with highest skirt temperatures of 178 and 130°C), piston pin bore offsets (thrust side, anti-thrust side directions in the amounts of 0.6 mm, and the nominal no offset case), and liner thicknesses are determined. Piston thermal growth is considered as part of the analysis resulting in interference condition between piston skirt and liner under the hot operating condition and low minimum clearance under the warm condition. Correlation of liner velocity contour plots with real engine liner cavitation erosion is presented. Analytical model showed a maximum liner inward velocity of 55 mm/s with no piston pin offset under nominal engine-operating configuration. A correlation has been found between location of this highest liner velocity and location of the actual cavitation erosion in the field.</div>

A study on vibration prediction of diesel engine based on virtual material modeling method of joint surfaces

The vibration prediction model of diesel engine is generally established by finite element method (FEM). In FEM, the components are firstly represented by mesh models, which will be connected to develop a combined model through joint surfaces. The processing method of joint surfaces is key to the accuracy of the combined model, for joint surfaces play an important role in transmitting force and vibration. For this purpose, an equivalent model of transversely isotropic virtual material is proposed to connect the mesh models of the components in this paper. In this model, the different connection characteristics of both normal and tangential are taken into consideration. In addition, the calculation models of the virtual material layer parameters, such as elastic modulus, shear modulus, Poisson’s ratio, thickness, density, and damping coefficient, are established in theory. At the same time, the modal test is introduced to acquire the natural frequency and damping ratio for identifying the unknown parameters of the virtual material layer, by using the BP neural network algorithm. After that, the complete finite element model of a diesel engine assembly can be established, and the vibration prediction is made in time domain, where cylinder gas pressures, piston slap forces, main bearing forces, and valve train forces are taken into consideration comprehensively. Furthermore, the experiment of the engine vibration response is carried out as well. The vibration acceleration level of engine feet and the vibration severity of engine block are adopted to evaluate the structure vibration response of the diesel engine. The results show that there is a good consistency between the prediction results and the experiment results in frequency domain. The consistency of the vibration acceleration level in 1/3 octave is up to 8 kHz, and that of the vibration severity is up to 1 kHz. And the results verify the accuracy of the vibration prediction model of the diesel engine, which is based on transversely isotropic virtual material modeling method of joint surfaces.

Investigation of Cavitation Bubble Dynamics Considering Pressure Fluctuation Induced by Slap Forces

Cavitation erosion is induced by the penetrating pressure from implosion of cavitation bubbles nearby solid boundary. The bubble evolution and the subsequent collapse pressure are especially important to evaluate the erosion degradation of solid boundary materials. The bubble dynamics equation taking into account the influence of distance between bubble and solid boundary is formulated to investigate the effect of boundary wall on bubble evolution process. The pressure fluctuation induced by slapping forces is adopted to evaluate the bubble dynamic characteristics. Negative pressure period which reflects the effect of vibration velocity and gap clearance also has large influence on bubble dynamics. The effects of standoff distance, initial radius and negative pressure period on bubble evolution and collapsing shock pressure are discussed. Maximum bubble radius increases with standoff distance and initial radius, while shock pressure increases with distance and decreases with bubble initial radius, and both of them increase with negative pressure period.

Study of the Piston Secondary Movement on the Tribological Performance of a Single Cylinder Low-Displacement Diesel Engine

The present study aims to analyze the secondary movement of the piston considering the deformations present in the piston skirt, the hydrodynamic lubrication, and the effects of the clearances in the connecting rod bearings. The analysis of the piston movement is performed by developing a mathematical model, which was used to evaluate the dynamic characteristics of the piston movement, the slap force on the piston skirt, the effect of the secondary piston movement on the connecting rod, and the influence of clearances in the connecting rod bearings and in the piston. For the study, the geometric of the crankshaft-connecting rod–piston system of a single-cylinder diesel engine is taken as a reference. The deformation model of the piston was carried out by means of a symmetric finite element model (FEM), which was integrated into the mathematical model of the piston. MATLAB® software (The MathWorks Inc., Natick, MA, USA) is used for the development of model simulations. The obtained results show that during the combustion cycle, there are six changes of direction in the secondary movement of the piston with lateral and angular velocities that can reach a magnitude of 0.13 m/s and 4 rad/s. The lateral and angular movement of the piston during its travel causes the appearance of impacts on the piston skirt with the cylinder liner, which produces an increase of approximately 500 N in the hydrodynamic forces in the connecting rod bearings. The force analysis shows that the range of the maximum magnitudes of these forces is between 1900 N and 3480 N. The increase in clearance between the cylinder liner and the piston skirt (Cpc) causes a greater lateral displacement and an increase in the angle of inclination of the piston. Analysis of the change in connecting rod bearing clearance shows that there are critical values in relation to clearance Cpc. The model presented allows us to analyze the different characteristics of the secondary movement of the piston, which involve the interaction between the piston skirt and the cylinder liner. Additionally, the influence of this movement on the connecting rod bearings is considered. The foregoing can be used as an analysis tool for the study of designs and/or modifications in the engine in such a way that greater durability of the components, reductions in acoustic emissions, and reduction in friction losses are achieved.

Analysis of water coolant pressure fluctuation induced by piston slap

Liner cavitation is caused by water pressure fluctuation in water coolant passage. When the negative pressure falls below the saturated vapor pressure, the impulsive pressure following the implosion of cavitation bubbles causes cavitation erosion on wet cylinder liner surface. Prediction of coolant pressure fluctuation is crucial to relieve liner cavitation erosion. Vibration of internal combustion engine especially liner vibration is thought of as the primary cause of liner cavitation. Piston slap is deemed as the major factor of engine vibration. The present work establishes a coupled system between entire engine structure and water coolant passage acoustic field considering their vibration characteristics by finite element method and boundary element method. Mechanism of piston slap is proposed and the resulting slap forces are applied on cylinder to predict water coolant pressure fluctuation. The influence of sound speed variation of acoustic field on pressure fluctuation is discussed. The contributions of natural modes of engine and water acoustic field to largest negative pressure are analyzed.

Piston dynamics analysis considering skirt-liner dynamic clearance

Piston slap force is one of the main sources of mechanical engine noise. To obtain a more accurate determination of the piston slap force and minimize piston slap noise, a new simulation model that considers dynamic clearance of the piston skirt and liner has been established in this study. The skirt-liner dynamic clearance is caused mainly by the elastic deformation and thermal deformation of the piston. Comparing three different models, it was found that the impact of dynamic clearance on the piston slap force is reflected mainly in the medium-high frequency, which is the sensitive frequency of the slap noise. Therefore, it is necessary to consider dynamic clearance in the simulation model. In addition, the mechanisms behind the dynamic clearance effect on fluid lubrication, piston dynamics, and piston slap noise were observed. In particular, three typical structural parameters in piston dynamics (the piston pin offset, piston-liner clearance, and piston skirt profile) were studied. The analyses provide guidance for the design of low-noise engines.

Investigation of liner cavitation considering coupling effect between engine vibration and acoustic field of water coolant passage

The engine vibration gives rise to pressure fluctuation of water coolant; therefore, the coupled system between engine structure and acoustic field of water coolant passage needs to be established to calculate the response of engine vibration and water coolant pressure fluctuation. A rectangular tank is used to evaluate water pressure induced by structural impact vibration. The dynamic characteristics of structural-acoustic coupled system are discussed by changing water depth. Considering the resonance effect, the relationship between natural frequencies of structure and acoustic field is examined by variation of sound speed. The equation of motion of engine structure coupled with acoustic field of water coolant passage is established to calculate coolant pressure fluctuation during engine working condition. The effect of piston slap forces on pressure fluctuation is evaluated. The influence of acoustic characteristics of water coolant passage on pressure fluctuation is discussed by varying sound speed.

Piston slap induced pressure fluctuation in the water coolant passage of an internal combustion engine

Liner cavitation is caused by water pressure fluctuation in the water coolant passage (WCP). When the negative pressure falls below the saturated vapor pressure, the impulsive pressure following the implosion of cavitation bubbles causes cavitation erosion of the wet cylinder liner surface. The present work establishes a numerical model for structural-acoustic coupling between the crankcase and the acoustic field in the WCP considering their dynamic characteristics. The coupling effect is evaluated through mutual interaction terms that are calculated from the mode shapes of the acoustic field and of the crankcase vibration on the boundary. Water pressure fluctuations in the WCP under the action of piston slap forces are predicted and the contributions of the uncoupled mode shapes of the crankcase and the acoustic field to the pressure waveform are analyzed. The influence of sound speed variations on the water pressure response is discussed, as well as the pressure on the thrust sides of the four cylinders.

Impact of the Piston Secondary Motion on its Slap Force

A theoretical model is developed for the analysis of piston secondary motion. Based on this model, the slap force of a specific L6 diesel engine was compared when considering different boundary conditions, such as lubricating oil on cylinder liner, surface roughness, deformation of cylinder liner and piston skirt. It is concluded that it is necessary to consider the secondary motion of piston in the analysis of the inner excitation for an internal combustion engine. A more comprehensive consideration of the boundary condition (i.e., more close to the actual condition) will lead to a smaller maximum slap force, and among all boundary conditions considered in this paper, the structural deformation of the piston skirt and cylinder liner is the most influential factor. The theoretical model developed and findings obtained in this study will benefit the future analysis and design of advanced internal combustion engine structures.

FEM Analysis on Block Vibration of Diesel Engine

In order to evaluate the vibration history of the engine block and predict its surface vibration status, a FEM model on 4102BZQ Diesel Engine block was constructed. Based on the cylinder pressure curve of 4102BZQ, some important exciting forces applied on the block were calculated. ANSYS software was used to compute and analyze dynamic response of the block. Simultaneously the node displacement history of the block model was obtained. The results of calculation and analysis showed that the vibration of the block skirts was more intense and the skirt was the main factor to influence the reliability of the engine. Cylinder head bolt force and piston slap force had significant effect on the top area of the block, and the surface vibration of the skirt was strongly influenced by the crank shaft load.

Vibration Response and Noise Radiation of Engine Block Coupled with the Rotating Crankshaft and Gear Train

This paper presents the theoretical procedure to predict the vibratory response and radiated noise of the engine block coupled with the rotating crankshaft and gear train shafts which drives the fuel injection pump and valve system. The exciting forces acting on the engine block and shaft system are combustion pressure, inertia forces of the moving parts, piston slap forces, fuel injection pressure and valve driving force and torque. Theoretical procedures consist of the following four steps; (1) Dynamic characteristics of the engine block and shafts are determined separately by Finite Element Method or experimental modal analysis, (2) Normal mode expansion technique is employed to derive the equation of motion of the total system in which rotating shafts with gear train are combined to the engine block by the oil film and contact stiffness, (3) The time histories of the vibratory response of the engine block and rotating shafts are calculated by the numerical integration technique, (4) Engine noise radiated from the engine block surface is evaluated using the Boundary Element Method. This method is applied to estimate the effect of the backlash of the gear train on the engine block vibration and radiated noise.

クランク軸・歯車軸系とエンジンブロックの連成振動応答及び騒音放射予測法

This paper presents the theoretical procedure to predict the vibratory response and radiated noise of the engine block coupled with the rotating crankshaft and gear train shafts which drives the fuel injection pump and valve system. The exciting forces acting on the engine block and shaft system are combustion pressure, inertia forces of the moving parts, piston slap forces, fuel injection pressure and valve driving force and torque. Theoretical procedures consist of the following four steps; (1) Dynamic characteristics of the engine block and shafts are determined separately by Finite Element Method or experimental modal analysis, (2) Normal mode expansion technique is employed to derive the equation of motion of the total system in which rotating shafts with gear train are combined to the engine block by the oil film and contact stiffness, (3) The time histories of the vibratory response of the engine block and rotating shafts are calculated by the numerical integration technique, (4) Engine noise radiated from the engine block surface is evaluated using the Boundary Element Method. This method is applied to estimate the effect of the backlash of the gear train on the engine block vibration and radiated noise.

376 クランク軸・歯車列とエンジンブロックの連成振動応答及び放射騒音の研究 : クランク軸捩り振動の影響

As the small-sized high speed diesel engine used for construction machinery and generator is operated under continuous heavy load condition, a gear train is employed to drive the fuel injection pump and valve train. When a loading torque acting on the crankshaft and gear train is small at the idling condition, the gear tooth separation and impact are caused by the fluctuating torque and they increase the engine noise level. Particularly when the torsional vibration of the crankshaft occurs and its rotational amplitude is magnified at a specified rotational speed, the gear impact is intensified and it leads to increase of the engine noise. Author has developed the theoretical procedure using modal analysis technique to predict the vibratory response and radiated noise of the engine block coupled with the rotating crankshaft and gear train shafts which drives the fuel injection pump and valve system. The exciting forces acting on the engine block and shaft system are combustion pressure, inertia forces of the moving parts, piston slap force, fuel injection pressure and valve driving force and torque. This method is applied to reveal the effect of the torsional vibration of crankshaft on the gear impact force and engine noise of the four-stroke four-cylinder engine. The gear train is equipped at the pulley side of the crankshaft and the torsional vibration mode of the crankshaft is enlarged at the pulley side than flywheel side. Countermeasure to reduce vibration and noise by changing the location of the gear train from the pulley side to the flywheel side is evaluated and its result indicates the prospects which can reduce the radiated noise from the engine block.

Approach to minimization of piston slap force for noise reduction – investigation of piston slap force by numerical simulation

Impact force caused by piston slap is one of the major mechanical noise sources in a diesel engine. The optimization of piston pin offset for impact force reduction has been reported by many researchers. However, if a modification has large effect at idling, high-speed noise tends to increase and vice versa. It was very difficult to reduce piston slap noise at all engine speeds. In this paper, the mechanism of the piston slap was investigated precisely utilizing numerical simulation. Simultaneous noise reduction in the conditions of both idling and high speed was achieved successfully by optimizing the piston's center of gravity as well as piston pin offset. Furthermore, engine sound quality was also significantly improved.

Experiments on the transmission paths and dynamic behavior of engine structure vibration. II. Motoring tests

In order to gain a better understanding of the transmission behavior of the excitation forces in the engine structure, such as the gas forces, the inertia forces, the piston slap forces, and the valve seating impact forces, a series of motoring tests was carried out on a four-cylinder (bore: 86 mm, stroke: 84 mm), in-line, water-cooled high-speed diesel engine. The motoring tests were carried out by changing the combinations of the excitation forces, and at different engine speeds. Under the motoring tests, it could be seen that even the cylinder block bottom sides could be steadily excited by the valve seating impact forces. The transmission behavior of each of the excitation forces can be identified distinctly by the particular frequency components in the power spectral density diagrams of the engine structure responses. Finally, new types of transfer functions were proposed in order to identify the relationship between the kinetc energy of the impulsive excitation forces (such as the piston slap forces and the valve seating impact forces) and the induced engine structure responses. These new transfer functions could be determined under motoring conditions and can greatly overcome the difficulties in estimating the engine structure responses.