A simulation method for skirt lubrication characteristics considering thermal deformation of piston and cylinder liner

Abstract

The application aims to provide a skirt lubrication characteristic simulation method considering piston and cylinder liner thermal deformation, comprising the following steps: setting piston second-order motion variable initial value, starting crank angle cycle; combining piston profile to calculate oil film thickness; calculating oil film pressure and micro-convex body contact pressure, using Gauss-Legendre integral to obtain oil film bearing capacity and micro-convex body contact force; calculating the force and torque of connecting rod to pin and other forces and torques at the current time through dynamic equation, solving the dynamic equation; judging whether the crank angle meets the cycle termination condition; judging whether the current working cycle meets the convergence condition, if not, updating the second-order motion variable initial value for the next calculation cycle, if yes, outputting the result. The application overcomes the problems of distorted calculation result and unstable solution under special conditions such as near explosion pressure and small minimum oil film value. It provides certain support for actual diesel engine piston skirt lubrication prediction and optimization design.

Description

Technical Field

[0001] The present invention relates to a simulation method for internal combustion engines, specifically a simulation method for the lubrication characteristics of internal combustion engine friction pairs. Background Technology

[0002] Among various power machinery, diesel engines are widely used due to their advantages such as high thermal efficiency, large torque, good economy, wide power range, rapid start-up, simple maintenance, safe operation, and long service life. They are mainly used in heavy-duty trucks, large buses, construction machinery, tanks, ships, generator sets, etc., and play an important role in national economic and defense construction. As one of the most important components of a diesel engine, the lubrication condition between the piston and the cylinder liner is crucial to ensuring the engine's power and reliability.

[0003] During diesel engine operation, the piston not only bears mechanical loads such as cylinder pressure, inertial force, lateral thrust, and friction, but also thermal loads from the high-temperature combustion gases. This leads to thermal deformation of the piston and cylinder liner, and changes in the lubrication state of the piston skirt-cylinder liner friction pair. Due to the relatively low combustion pressure of the research subjects, a significant portion of past studies either did not consider the thermal deformation of the piston and cylinder liner or only considered the piston's thermal deformation. However, in recent years, with the development of diesel engines towards higher power density and higher performance, the mechanical and thermal loads of diesel engines have further increased, resulting in severe thermal deformation of the piston and cylinder liner. This significantly impacts the clearance of the friction pair and the kinetic oil film. Under these more demanding operating conditions, even minor design flaws in the piston skirt profile can significantly increase piston and cylinder liner wear, and may even lead to cylinder scoring. Therefore, considering the thermal deformation of the piston and cylinder liner and making more accurate calculations of the piston skirt lubrication characteristics is essential.

[0004] When considering the impact of piston and cylinder liner thermal deformation on skirt lubrication calculations, using the deformation calculation results for each node as input would result in a massive amount of data processing and a long calculation time. To reduce calculation costs and design verification time, it is necessary to appropriately simplify the thermal deformation data. Based on the research results of Li Chuang et al. in the article "Deformation Analysis of Piston Skirt of a Diesel Engine" and Bi Yuhua et al. in the article "Research on the Influence of Coolant Flow Uniformity on Cylinder Liner Thermal Deformation", the radial deformation on the piston axis section is elliptical, while the radial deformation of the cylinder liner is uneven and pea-shaped. Different simplification methods need to be adopted for the piston and cylinder liner.

[0005] In summary, it is essential to consider the thermal deformation of the piston and cylinder liner in order to accurately calculate the lubrication characteristics of the piston skirt. At the same time, it is also important to simplify and optimize the calculation method to balance accuracy and calculation cost. Summary of the Invention

[0006] The purpose of this invention is to provide a simulation method for piston skirt lubrication characteristics that takes into account the thermal deformation of the piston and cylinder liner, which can more accurately and efficiently predict the contact and lubrication state of the piston skirt, thereby significantly improving the design accuracy and efficiency of the piston skirt.

[0007] The objective of this invention is achieved as follows:

[0008] This invention provides a simulation method for skirt lubrication characteristics considering the thermal deformation of the piston and cylinder liner, characterized by:

[0009] (1) Set the initial value of the second-order motion variable of the piston and start the crankshaft rotation cycle;

[0010] (2) The thermal deformation of the piston and cylinder liner is processed, and the oil film thickness is calculated in combination with the piston profile;

[0011] (3) Calculate the oil film pressure and micro-protrusion contact pressure using the hybrid lubrication model, and obtain the oil film bearing capacity, micro-protrusion contact force and torque using the Gauss-Legend integral;

[0012] (4) Based on the known gas force in the cylinder and piston ring friction at the current moment, the force and torque of the connecting rod on the pin, as well as other forces and torques at the current moment, are calculated through the dynamic equation. The dynamic equation is solved by the variable step size fourth-order Runge-Kutta method. The second-order motion variable values ​​at the next moment are obtained from the forces and torques at the current moment.

[0013] (5) Increase both the current time and the crankshaft angle by one step, and determine whether the crankshaft angle meets the loop termination condition. If it does not meet the condition, update the variable value at the current time and continue the calculation. If it does meet the condition, proceed to the next step.

[0014] (6) Determine whether the current working cycle meets the convergence condition. If not, update the initial value of the second-order motion variable and proceed to the next working cycle. If it meets the condition, output the result after the calculation is completed.

[0015] The present invention may also include:

[0016] 1. Step one specifically involves: setting the lateral displacement x of the piston at the center of the piston pin. p lateral piston velocity at the center of piston pin Piston swing angle β and piston oscillation speed Initial value.

[0017] 2. Step two specifically involves: simplifying the piston profile by using an elliptical surface with continuously varying major and minor axes along the piston's central axis, taking into account piston thermal deformation; and simplifying the cylinder liner inner surface by using four 1 / 4 elliptical surfaces with continuously varying major and minor axes along the cylinder liner's central axis, taking into account cylinder liner thermal deformation. The specific method for calculating the oil film thickness is as follows:

[0018] Determine the curvature Rp at the piston profile convex point, the cylinder liner radius R, and the axial profile f after thermal deformation on the piston thrust side and non-thrust side. 1,0° (y) and f 1,90° (y), the axial deformation curves of the cylinder liner at circumferential angles of 0°, 90°, 180° and 270°. 1,0° (y), d 1,90° (y), d 1,180° (y) and d 1,270° (y), the shrinkage of the skirt relative to the convex point and the deformation of a certain part of the cylinder liner relative to the cold state are calculated as follows:

[0019]

[0020] Where y p This is the axial displacement of the piston skirt tip from the top dead center.

[0021] Calculate the change in oil film thickness on the thrust surface caused by the second-order piston motion:

[0022]

[0023] Where a is the axial position of the piston center of gravity relative to the top of the piston skirt, and b is the axial position of the piston pin center relative to the piston center of gravity.

[0024] The oil film thickness between the skirt and the cylinder liner is:

[0025]

[0026] 3. Step three specifically involves:

[0027] The oil film pressure is calculated using the average Reynolds equation after Vogenpohl transformation. The equation is as follows:

[0028]

[0029] Among them, oil film pressure φ x φ y φ c φ s Where σ is the flow factor, U is the piston axial velocity, and σ is the combined roughness of the piston skirt and cylinder liner friction pair;

[0030] The equation for calculating the contact pressure of the micro-asperities is as follows:

[0031]

[0032] Where E′ is the combined elastic modulus of the piston skirt and cylinder liner, η is the peak density, β is the peak radius of curvature, and H is the film thickness ratio (h / σ).

[0033] The oil film bearing capacity, micro-protrusion contact force, and torque are obtained by using Gauss-Legend integration.

[0034] 4. Step four is as follows:

[0035] Based on the known in-cylinder gas force, piston ring friction force, and forces and torques related to the oil film and micro-protrusions at the current moment, the force exerted by the connecting rod on the piston pin is obtained. Then, the frictional torque between the connecting rod and piston pin is obtained from the friction coefficient between the piston pin and the connecting rod. Finally, the lateral acceleration of the piston pin at the current moment is calculated using the established piston skirt dynamics equations. and piston oscillation acceleration The dynamic equation is:

[0036]

[0037]

[0038]

[0039] in, M pin =μ pin F L μ is the coefficient of friction; subscript f represents the force and torque related to friction; subscript c represents the force and torque related to micro-protrusion contact; subscript j represents the force and torque related to corner contact; subscript h represents the force and torque related to oil film hydrodynamic lubrication; G represents the force and torque related to in-cylinder gas; L represents the force and torque related to the connecting rod acting on the piston pin; pin represents the force and torque related to the piston pin; I p Let be the moment of inertia of the piston about its center of gravity;

[0040] Based on the current moment's pin lateral motion acceleration and oscillation acceleration The lateral velocity of the pin at the next moment is calculated using the fourth-order Runge-Kutta method with variable step size. and oscillation speed Simultaneously, based on the lateral movement velocity of the pin calculated from the previous moment... and oscillation speed The pin lateral displacement x at the next moment is calculated using the variable step-size fourth-order Runge-Kutta method. p And the swing angle β.

[0041] The advantages of this invention are as follows: It considers the impact of piston skirt and cylinder liner thermal deformation on skirt lubrication during stable operation of an actual diesel engine, adds force factors such as piston pin friction torque and the effect of piston rings on the piston, and proposes a more comprehensive and scientific method for calculating skirt lubrication characteristics. It utilizes optimization algorithms such as the average Reynolds equation after Vogenpohl transformation and the variable-step-size fourth-order Runge-Kutta method, overcoming the problems of calculation result distortion and unstable solutions under special conditions such as near-explosion pressure and small minimum oil film values, using the same computational resources. This provides certain support for the prediction and optimization design of piston skirt lubrication in actual diesel engines. Attached Figure Description

[0042] Figure 1 This is a flowchart of the present invention;

[0043] Figure 2 This is a schematic diagram showing the variation of oil film thickness along the axial direction.

[0044] Figure 3 This is a schematic diagram showing the variation of oil film thickness along the circumferential direction.

[0045] Figure 4 This is a schematic diagram of the forces acting on the piston. Detailed Implementation

[0046] The invention will now be described in more detail with reference to the accompanying drawings:

[0047] Combination Figure 1-4 The present invention includes the following steps:

[0048] Step 1: Set the initial value of the piston's second-order motion variable and start the crankshaft angle cycle.

[0049] Set the lateral displacement x of the piston at the center of the piston pin. p lateral piston velocity at the center of piston pin Piston swing angle β and piston oscillation speed The initial value is usually set to 0.

[0050] Step 2: Process the thermal deformation of the piston and cylinder liner, and calculate the oil film thickness based on the piston profile.

[0051] The piston profile is simplified by using an elliptical surface whose major and minor axes continuously change along the piston's central axis, taking into account the piston's thermal deformation. Similarly, the inner surface of the cylinder liner is simplified by using four quarter-elliptical surfaces whose major and minor axes continuously change along the cylinder liner's central axis, taking into account the cylinder liner's thermal deformation. The specific method for calculating the oil film thickness is as follows:

[0052] First, determine the curvature Rp at the piston profile convex point, the cylinder liner radius R, and the axial profile f after thermal deformation on the piston thrust side and non-thrust side. 1,0° (y) and f 1,90°(y), the axial deformation curves of the cylinder liner at circumferential angles of 0°, 90°, 180° and 270°. 1,0° (y), d 1,90° (y), d 1,180° (y) and d 1,270° (y), the shrinkage at a certain point on the skirt relative to the convex point and the deformation at a certain point on the cylinder liner relative to the cold state are calculated as follows:

[0053]

[0054] Where y p This represents the axial displacement of the piston skirt tip from the top dead center.

[0055] Secondly, calculate the change in oil film thickness on the thrust surface caused by the second-order motion of the piston:

[0056]

[0057] Where a is the axial position of the piston's center of gravity relative to the top of the piston skirt, and b is the axial position of the piston pin center relative to the piston's center of gravity.

[0058] Finally, the oil film thickness between the skirt and the cylinder liner at a certain point was obtained as follows:

[0059]

[0060] Step 3: Calculate the oil film pressure and micro-protrusion contact pressure using a hybrid lubrication model, and then use the Gauss-Legend integral to obtain the oil film bearing capacity, micro-protrusion contact force, and torque.

[0061] First, the oil film pressure is calculated using the average Reynolds equation after Vogenpohl transformation. The equation is as follows:

[0062]

[0063] Among them, oil film pressure φ x φ y φ c φ s σ is the flow factor, U is the piston axial velocity, and σ is the combined roughness of the piston skirt and cylinder liner friction pair.

[0064] Secondly, the contact pressure of the micro-assurance is calculated using the following equation:

[0065]

[0066] Where E′ is the combined elastic modulus of the piston skirt and cylinder liner, η is the peak density, β is the peak radius of curvature, and H is the film thickness ratio (h / σ).

[0067] Finally, the oil film bearing capacity, micro-protrusion contact force, and torque are obtained by using the Gauss-Legend integral.

[0068] In step three, the average Reynolds equation after Vogenpohl transformation is applied to the mixed lubrication model to calculate the oil film pressure, which corrects the problem of severe distortion in the calculation results of the classical average Reynolds equation when the minimum oil film thickness is very small, when the mesh is not very fine.

[0069] Step 4: Based on the known gas force in the cylinder and piston ring friction at the current moment, calculate the force and torque of the connecting rod on the pin, as well as other forces and torques, at the current moment through the dynamic equation. Solve the dynamic equation using the variable step size fourth-order Runge-Kutta method, and obtain the second-order motion variable values ​​at the next moment from the forces and torques at the current moment.

[0070] First, based on the known in-cylinder gas force and piston ring friction at the current moment, as well as the forces and torques related to the oil film and micro-protrusions calculated in the previous step, the force exerted by the connecting rod on the piston pin is obtained. Then, the frictional torque between the connecting rod and piston pin is obtained from the friction coefficient between the piston pin and the connecting rod. Finally, the lateral acceleration of the piston pin at the current moment is calculated using the established piston skirt dynamics equations. and piston oscillation acceleration The dynamic equation is:

[0071]

[0072]

[0073]

[0074] in, M pin =μ pin F L μ is the coefficient of friction; subscript f represents the force and torque related to friction; subscript c represents the force and torque related to micro-protrusion contact; subscript j represents the force and torque related to corner contact; subscript h represents the force and torque related to oil film hydrodynamic lubrication; G represents the force and torque related to in-cylinder gas; L represents the force and torque related to the connecting rod acting on the piston pin; pin represents the force and torque related to the piston pin; I p Let be the moment of inertia of the piston about its center of gravity.

[0075] Secondly, based on the current moment's lateral acceleration of the pin and oscillation acceleration The lateral velocity of the pin at the next moment is calculated using the fourth-order Runge-Kutta method with variable step size. and oscillation speed Simultaneously, based on the lateral movement velocity of the pin calculated from the previous moment... and oscillation speed The pin lateral displacement x at the next moment is calculated using the variable step-size fourth-order Runge-Kutta method. p And the swing angle β.

[0076] In step four, the dynamic equations take into account the frictional torque between the piston pin and the piston, the lateral frictional force of the piston rings on the piston, and other forces and torques. The dynamic equations are solved using the variable step-size fourth-order Runge-Kutta method, and the calculation step size near the burst pressure is automatically refined.

[0077] Step 5: Increase both the current time and crankshaft angle by one step, and determine whether the crankshaft angle meets the loop termination condition. If it does not meet the condition, update the variable value at the current time and continue the calculation. If it does meet the condition, proceed to the next step.

[0078] Step 6: Determine whether the current working cycle meets the convergence condition. If not, update the initial value of the second-order motion variable and proceed to the next calculation cycle. If it meets the condition, complete the calculation and output the result.

[0079] Using the lateral displacement of the piston pin and the piston swing angle as convergence criteria, when the convergence condition is not met, the lateral displacement of the pin, the swing angle, the lateral velocity of the pin, and the swing velocity at the maximum crankshaft angle will be calculated as the initial values ​​for the next calculation cycle.

Claims

1. A simulation method for skirt lubrication characteristics considering the thermal deformation of the piston and cylinder liner, characterized by: (1) Set the initial value of the second-order motion variable of the piston and start the crankshaft rotation cycle; (2) The thermal deformation of the piston and cylinder liner is processed, and the oil film thickness is calculated in combination with the piston profile; (3) Calculate the oil film pressure and micro-protrusion contact pressure using the hybrid lubrication model, and obtain the oil film bearing capacity, micro-protrusion contact force and torque using the Gauss-Legend integral; (4) Based on the known gas force in the cylinder and piston ring friction at the current moment, the force and torque of the connecting rod on the pin, as well as other forces and torques at the current moment, are calculated through the dynamic equation. The dynamic equation is solved by the variable step size fourth-order Runge-Kutta method. The second-order motion variable values ​​at the next moment are obtained from the forces and torques at the current moment. (5) Increase both the current time and the crankshaft angle by one step, and determine whether the crankshaft angle meets the loop termination condition. If it does not meet the condition, update the variable value at the current time and continue the calculation. If it does meet the condition, proceed to the next step. (6) Determine whether the current working cycle meets the convergence condition. If not, update the initial value of the second-order motion variable and proceed to the next working cycle. If it meets the condition, output the result after the calculation is completed.

2. The simulation method for skirt lubrication characteristics considering the thermal deformation of the piston and cylinder liner according to claim 1, characterized in that: Step one specifically involves: setting the lateral displacement x of the piston at the center of the piston pin. p lateral piston velocity at the center of piston pin Piston swing angle β and piston oscillation speed Initial value.

3. The simulation method for skirt lubrication characteristics considering the thermal deformation of the piston and cylinder liner according to claim 1, characterized in that: Step two specifically involves: simplifying the piston profile by using an elliptical surface whose major and minor axes continuously change along the piston's central axis, taking into account the piston's thermal deformation; and simplifying the cylinder liner inner surface by using four 1 / 4 elliptical surfaces whose major and minor axes continuously change along the cylinder liner's central axis, taking into account the cylinder liner's thermal deformation. The specific method for calculating the oil film thickness is as follows: Determine the curvature Rp at the piston profile convex point, the cylinder liner radius R, and the axial profile f after thermal deformation on the piston thrust side and non-thrust side. 1,0° (y) and f 1,90° (y), the axial deformation curves of the cylinder liner at circumferential angles of 0°, 90°, 180° and 270°. 1,0° (y), d 1,90° (y), d 1,180° (y) and d 1,270° (y), the shrinkage of the skirt relative to the convex point and the deformation of a certain part of the cylinder liner relative to the cold state are calculated as follows: Where y p This is the axial displacement of the piston skirt tip from the top dead center. Calculate the change in oil film thickness on the thrust surface caused by the second-order piston motion: Where a is the axial position of the piston center of gravity relative to the top of the piston skirt, and b is the axial position of the piston pin center relative to the piston center of gravity. The oil film thickness between the skirt and the cylinder liner is:

4. The simulation method for skirt lubrication characteristics considering the thermal deformation of the piston and cylinder liner according to claim 1, characterized in that: Step three specifically involves: The oil film pressure is calculated using the average Reynolds equation after Vogenpohl transformation. The equation is as follows: Among them, oil film pressure φ x φ y φ c φ s Where σ is the flow factor, U is the piston axial velocity, and σ is the combined roughness of the piston skirt and cylinder liner friction pair; The equation for calculating the contact pressure of the micro-asperities is as follows: Where E′ is the combined elastic modulus of the piston skirt and cylinder liner, η is the peak density, β is the peak radius of curvature, and H is the film thickness ratio (h / σ). The oil film bearing capacity, micro-protrusion contact force, and torque are obtained by using Gauss-Legend integration.

5. The simulation method for skirt lubrication characteristics considering the thermal deformation of the piston and cylinder liner according to claim 1, characterized in that: Step four is as follows: Based on the known in-cylinder gas force, piston ring friction force, and forces and torques related to the oil film and micro-protrusions at the current moment, the force exerted by the connecting rod on the piston pin is obtained. Then, the frictional torque between the connecting rod and piston pin is obtained from the friction coefficient between the piston pin and the connecting rod. Finally, the lateral acceleration of the piston pin at the current moment is calculated using the established piston skirt dynamics equations. and piston oscillation acceleration The dynamic equation is: in, M pin =μ pin F L μ is the coefficient of friction; subscript f represents the force and torque related to friction; subscript c represents the force and torque related to micro-protrusion contact; subscript j represents the force and torque related to corner contact; subscript h represents the force and torque related to oil film hydrodynamic lubrication; G represents the force and torque related to in-cylinder gas; L represents the force and torque related to the connecting rod acting on the piston pin; pin represents the force and torque related to the piston pin; I p Let be the moment of inertia of the piston about its center of gravity; Based on the current moment's pin lateral motion acceleration and oscillation acceleration The lateral velocity of the pin at the next moment is calculated using the fourth-order Runge-Kutta method with variable step size. and oscillation speed Simultaneously, based on the lateral movement velocity of the pin calculated from the previous moment... and oscillation speed The pin lateral displacement x at the next moment is calculated using the variable step-size fourth-order Runge-Kutta method. p And the swing angle β.

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