Method for establishing underwater piston engine power system model

Abstract

The invention discloses a method for establishing a model of an underwater piston engine power system, which belongs to the field of underwater vehicle power systems, and includes: simplifying and drawing a three-dimensional model of a piston engine; establishing a grid model and setting boundary conditions; setting CFD software, selecting a calculation model to write a UDF, and simulating the working process of the piston engine; using MATLAB to process the data obtained by the simulation to obtain the indicated work of the piston engine; fitting multiple groups of indicated work values ​​to obtain a one-dimensional solution model for the indicated work of the piston engine; combining theoretical derivation with the one-dimensional solution model for the indicated work to obtain a piston engine power system model. Compared with the prior art, the invention has simple steps, no need for experiments, low cost, stability and reliability, and high precision.

Description

Technical Field

[0001] The invention belongs to the technical field of underwater vehicle power systems, and in particular relates to a method for establishing a model of an underwater piston engine power system. Background Art

[0002] Underwater vehicles play an increasingly important role in modern naval warfare, and the country needs a variety of underwater vehicles to meet different needs. The design of underwater engine power system is an important part of vehicle design. Its design involves multiple variable parameters and needs to be modified many times according to test results to achieve the expected results. However, experimental research is time-consuming and costly.

[0003] The underwater vehicle piston power system is mainly composed of a fuel storage tank, a fuel pump, a combustion chamber, a piston engine, a generator, a seawater pump, and a lubricating oil pump. The fuel enters the combustion chamber from the fuel storage tank by the fuel pump, and the combustion in the combustion chamber changes the combustion chamber pressure and then drives the piston engine to rotate and do work. The piston engine output torque drives the fuel pump, the generator, the seawater pump, and the lubricating oil pump to operate. The mathematical model of the underwater piston engine power system mainly includes three parts: the combustion chamber characteristic equation, the power system characteristic equation, and the piston engine output torque equation. The parameters of the combustion chamber characteristic equation and the power system characteristic equation are preset, and the indicated work value in the piston engine output torque equation needs to be solved by monitoring the piston engine output power through experiments and inversely deriving the pressure drop coefficient; however, some parameters when the model is established still need to be obtained through experiments, so the model is mostly used for theoretical research, and engineering applications are seldom, and the obtained model has low accuracy. Summary of the invention

[0004] The purpose of the present invention is to provide a method for establishing a model of an underwater piston engine power system, which can simplify the establishment process and obtain a high-precision model.

[0005] The technical solution adopted by the present invention is a method for establishing a model of an underwater piston engine power system, which is specifically implemented according to the following steps:

[0006] Step 1: Simplify the physical model of the piston engine, draw the three-dimensional geometric model of the piston engine, and perform meshing;

[0007] Step 2: Set the regional boundary conditions;

[0008] Step 3: Set the intake and exhaust grooves of the piston machine to rotate around the center of the inscribed circle of the arc surface. The bottom of the intake and exhaust grooves pass through the top of the cylinder successively. In addition, write a UDF to control the bottom of the cylinder to make reciprocating motion to simulate the piston motion. The reciprocating cycle is consistent with the rotation cycle of the intake and exhaust grooves.

[0009] Step 4: Select the k-omega-SST turbulence model under the pressure-velocity coupling basis for simulation and solution, set the pressure inlet pressure and temperature, the pressure outlet pressure and temperature, and the initial pressure and temperature inside the cylinder, monitor the changes of the internal volume V and the cylinder pressure P of the piston engine cylinder with time T, and obtain the PT diagram and VT diagram respectively; combine the two to draw the PV indicator diagram of the piston engine;

[0010] Step 5, fix the pressure outlet pressure to 2MPa, and the pressure inlet pressure starts from 3MPa and gradually increases at a rate of 1MPa each time; return to step 4, and simulate the piston engine working process under different working conditions; take the percentage of the peak pressure in the PV diagram exceeding the pressure inlet pressure under different working conditions as the over-expansion degree, and when the over-expansion degree reaches 3%, it is taken as the expansion criticality, and the ratio of the intake pressure to the exhaust pressure when the over-expansion criticality occurs is calculated;

[0011] Step 6: When the intake-exhaust pressure ratio is less than or equal to the critical intake-exhaust pressure ratio of overexpansion, it is an overexpansion condition; otherwise, it is a normal condition. Select 20 groups of overexpansion conditions and 20 groups of normal conditions respectively, return to step 4, calculate the area of ​​the corresponding PV diagram, take the intake pressure and exhaust pressure in the condition as independent variables, and the area value as the dependent variable, use a one-dimensional polynomial to fit the overexpansion condition and the normal condition respectively, and the obtained fitting formula is the one-dimensional solution model of the indicated work;

[0012] Step 7: Multiply the one-dimensional solution model of the indicated work of the piston engine by the coefficient C e The output torque equation of the piston engine is obtained, and the characteristic equation of the power system is solved according to the output torque equation of the piston engine. The speed is substituted into the complete gas state equation to obtain the characteristic equation of the combustion chamber. The combination of these three equations is the underwater piston engine power system model.

[0013] The present invention is also characterized in that:

[0014] Step 1: The method for simplifying the three-dimensional geometric model of the piston engine is as follows: only retain the three components of the air intake groove, the exhaust groove, and the cylinder. The air intake groove and the exhaust groove are simplified into open columns, and the cylinder is simplified into a cylinder. The top of the cylinder coincides with the bottom of the air intake groove and the exhaust groove.

[0015] The boundary conditions in step 2 are: set the top of the air inlet groove as the pressure inlet, the top of the exhaust groove as the pressure outlet, the bottom of the air inlet groove, the bottom of the exhaust groove, and the top of the cylinder as the interface, and the remaining surfaces are set as walls.

[0016] In step 3, write a UDF to control the reciprocating motion of the bottom of the cylinder to simulate the piston motion. The displacement equation of the reciprocating motion is:

[0017] Where S is the piston stroke, i is the number of working cycles of each cylinder per revolution of the piston engine, and ω is the speed of the piston engine.

[0018] In step 6, one-dimensional polynomials are used to fit the overexpansion condition and the normal condition respectively. The specific process is: the two sets of indicated work values ​​and the corresponding inlet pressure and exhaust pressure are imported into MATLAB respectively, and the cftool tool is used for fitting; the fitting formula is w = x1P c +x2P e +x3; where P c is the inlet pressure, P e is the exhaust port pressure, and w is the indicated work.

[0019] In step 7, multiply the one-dimensional solution model of the indicated work of the piston engine by the coefficient C e The output torque equation of the piston engine is obtained as follows:

[0020] M e =C e w;

[0021] in i is the number of working cycles of each cylinder per revolution of the piston machine, z is the number of cylinders of the piston machine, v is the cylinder volume of the piston machine, and η is the mechanical efficiency of the piston.

[0022] The specific equation of the power system characteristic equation in step 7 is:

[0023]

[0024] Among them, M e is the output torque of the piston engine, I is the moment of inertia of the piston engine, a n1 、a n2 、a n3 Determined by the power of the auxiliary machines in the piston engine power system.

[0025] The combustion chamber characteristic equation in step 7 is expressed as:

[0026]

[0027] Where R is the complete gas constant, R is the complete gas constant, T is the cylinder temperature, C em It is the positive combination factor of the piston engine's working fluid consumption per second.

[0028] The beneficial effects of the present invention are:

[0029] 1) The method for establishing a model of an underwater piston engine power system of the present invention has simple establishment steps, low cost, high model accuracy, and only requires a three-dimensional model of the underwater piston engine and an electronic computer to complete the entire process;

[0030] 2) The model established by the method of the present invention can complete simulation under different working conditions by simply modifying parameters;

[0031] 3) It is applicable to various types of piston engines, and different environmental media can be set through CFD. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A flow chart of a method for establishing a power system model of an underwater piston engine according to the present invention;

[0033] Figure 2 It is a simplified three-dimensional model of the piston engine in the present invention;

[0034] Figure 3 is the boundary condition of the model in the present invention;

[0035] Figure 4 This is the PV schematic diagram under the working conditions of intake pressure 20Mpa and exhaust pressure 2Mpa;

[0036] Figure 5 This is the structural diagram of the underwater piston engine power system. DETAILED DESCRIPTION

[0037] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] The method for establishing the underwater piston engine power system model of the present invention is as follows: Figure 1 As shown, the specific implementation steps are as follows:

[0039] Step 1: Simplify the physical model of the piston engine, draw the three-dimensional geometric model of the piston engine, and perform meshing;

[0040] The method for simplifying the three-dimensional geometric model of the piston engine is as follows: only retain the three components of the intake groove, exhaust groove, and cylinder; the intake groove and exhaust groove are simplified into open columns; the cylinder is simplified into a cylinder; and the top of the cylinder coincides with the bottom of the intake groove and exhaust groove.

[0041] Step 2, set the regional boundary conditions; the boundary conditions are: set the top of the air inlet groove as the pressure inlet, the top of the exhaust groove as the pressure outlet, the bottom of the air inlet groove, the bottom of the exhaust groove, and the top of the cylinder as the interface, and the remaining surfaces are set as walls.

[0042] Step 3: Set the intake and exhaust grooves of the piston machine to rotate around the center of the inscribed circle of the arc surface. The bottom of the intake and exhaust grooves pass through the top of the cylinder successively. In addition, write a UDF to control the bottom of the cylinder to make reciprocating motion to simulate the piston motion. The reciprocating cycle is consistent with the rotation cycle of the intake and exhaust grooves.

[0043] Write a UDF to control the reciprocating motion of the bottom of the cylinder to simulate the piston motion. The displacement equation of the reciprocating motion is:

[0044] Where S is the piston stroke, i is the number of working cycles of each cylinder per revolution of the piston engine, and ω is the speed of the piston engine.

[0045] Step 4, select the k-omega-SST turbulence model under the pressure-velocity coupling basis (a solution model in Fluent software, used to solve simulation calculations involving turbulence) for simulation solution, set the pressure inlet pressure and temperature, the pressure outlet pressure and temperature, and the initial pressure and temperature inside the cylinder, monitor the changes of the internal volume V and the cylinder pressure P of the piston engine cylinder with time T, and obtain the PT diagram and VT diagram respectively; combine the two to draw the PV indicator diagram of the piston engine;

[0046] Step 5, fix the pressure outlet pressure to 2MPa, and the pressure inlet pressure starts from 3MPa and gradually increases at a rate of 1MPa each time; return to step 4, and simulate the piston engine working process under different working conditions; take the percentage of the peak pressure in the PV diagram exceeding the pressure inlet pressure under different working conditions as the over-expansion degree, and when the over-expansion degree reaches 3%, it is taken as the expansion criticality, and the ratio of the intake pressure to the exhaust pressure when the over-expansion criticality occurs is calculated;

[0047] Step 6: When the intake-exhaust pressure ratio is less than or equal to the critical intake-exhaust pressure ratio of overexpansion, it is an overexpansion condition; otherwise, it is a normal condition. Select 20 groups of overexpansion conditions and 20 groups of normal conditions respectively, return to step 4, calculate the area of ​​the corresponding PV diagram, take the intake pressure and exhaust pressure in the condition as independent variables, and the area value as the dependent variable, use a one-dimensional polynomial to fit the overexpansion condition and the normal condition respectively, and the obtained fitting formula is the one-dimensional solution model of the indicated work;

[0048] One-dimensional polynomials are used to fit the overexpansion condition and the normal condition. The specific process is: the two sets of indicated work values ​​and the corresponding inlet pressure and exhaust pressure are imported into MATLAB, and the cftool tool is used for fitting; the fitting formula is w = x1P c +x2P e +x3; where P c is the inlet pressure, P e is the exhaust port pressure, and w is the indicated work.

[0049] Step 7: Multiply the one-dimensional solution model of the indicated work of the piston engine by the coefficient C e The output torque equation of the piston engine is obtained, and the characteristic equation of the power system is solved according to the output torque equation of the piston engine. The speed is substituted into the complete gas state equation to obtain the characteristic equation of the combustion chamber. The combination of these three equations is the underwater piston engine power system model.

[0050] Multiply the one-dimensional solution model of the indicated work of the piston engine by the coefficient C e The output torque equation of the piston engine is obtained as follows:

[0051] M e =C e w;

[0052] in i is the number of working cycles of each cylinder per revolution of the piston machine, z is the number of cylinders of the piston machine, v is the cylinder volume of the piston machine, and η is the mechanical efficiency of the piston.

[0053] The specific equation of the power system characteristic equation is:

[0054]

[0055] Among them, M e is the output torque of the piston engine, I is the moment of inertia of the piston engine, a n1 、a n2 、a n3 Determined by the power of the auxiliary machines in the piston engine power system.

[0056] The combustion chamber characteristic equation is expressed as:

[0057]

[0058] Where R is the complete gas constant, R is the complete gas constant, T is the cylinder temperature, C em It is the positive combination factor of the piston engine's working fluid consumption per second.

[0059] In the model building method of the present invention, the piston machine output torque equation is established by numerically simulating the piston machine working process, i.e., steps 1-5, instead of experiments, and then the underwater piston machine power system model is established in combination with theoretical derivation. Compared with the traditional method, it has the advantages of no need for experiments, low cost, stable and reliable, and high accuracy.

[0060] Example

[0061] The following is a specific description with reference to the accompanying drawings and a certain type of piston machine:

[0062] The underwater piston engine power system consists of Figure 2 As shown. The underwater piston engine power system model can be simplified into the following equations:

[0063] The characteristic equation of the power system:

[0064]

[0065] The output torque of the piston engine is:

[0066] M e =C e w

[0067] The one-dimensional model of indicated work obtained by substituting into the above text is:

[0068] M e =C e (x1P c +x2P e +x3)

[0069] The combustion chamber characteristic equation is:

[0070]

[0071] Where ω is the engine speed, q is the fuel pump displacement, M e is the output torque of the piston engine, and w is the indicated work. Among them, x1, x2, and x3 have been calculated above. The rest a n1 、a n2 、a n3 、a n4 , C e , α p0 , α p1 All of them can be calculated from the parameters set when the piston engine is designed.

[0072] The working part of the piston engine is mainly related to its valve mechanism and cylinder. The physical model of the piston engine is simplified, and the valve mechanism is simplified to the exhaust groove and the intake groove, and only one of the cylinders is processed. The simplified result is as follows Figure 3 shown.

[0073] Import the simplified piston engine 3D model into ICEM for meshing and set boundary conditions. The boundary conditions are set as follows: Figure 4 shown.

[0074] Import the divided grid into the CFD software, set the pressure inlet pressure to 20 MPa, the temperature to 1430 K; the pressure outlet pressure to 2 MPa, the temperature to 800 K; the initial pressure to 10 MPa; the piston engine speed to 2150 r / min; the iteration step size to 2.08×10 -5 s, monitor the changes of cylinder volume and cylinder pressure over time. The simulation piston machine only needs to rotate one circle, so the number of iteration steps is 1341.

[0075] After the simulation is completed, the monitored data is imported into MATLAB, and the PV indicator diagram of the piston engine is obtained by taking the change in the internal volume of the cylinder as the independent variable and the pressure in the cylinder as the dependent variable, as shown in Figure 5 Using the above formula, the area of ​​the PV indicator diagram is 670.47, that is, the indicated work of the piston engine under the conditions of intake pressure of 20Mpa and exhaust pressure of 2Mpa is 670.47J.

[0076] Modify the pressure inlet pressure and pressure outlet pressure in the initial conditions, draw the PV indicator diagram under different intake and exhaust pressure ratio conditions, and calculate the percentage of peak pressure exceeding the intake pressure. It is found that when the intake and exhaust pressure ratio is greater than 3.5, the over-expansion degree is less than 3%.

[0077] Taking the intake-exhaust pressure ratio of 3.5 as the dividing line, 20 operating conditions with intake-exhaust pressure ratios greater than and less than 3.5 were simulated respectively, and the corresponding indicated power values ​​were calculated.

[0078] Import the two sets of indicated work values ​​into MATLAB and use the cftool tool to fit. The fitting formula is w = x1P c +x2P e +x3; where P c is the inlet pressure, P e is the exhaust port pressure, and w is the indicated work. The one-dimensional calculation model of the indicated work of the piston engine is:

[0079] When the intake-exhaust pressure ratio is less than 3.5:

[0080] w=3.868×10 -5 P c -5.167×10 -5 P e -1.452

[0081] When the intake-exhaust pressure ratio is greater than or equal to 3.5:

[0082] w=3.885×10 -5 P c -5.257×10 -5 P e -1.197

[0083] The results obtained by using the one-dimensional calculation model of indicated work are compared with the simulation results. The average relative error is 0.08%, and the maximum relative error is 0.27%, which meets the accuracy requirements.

[0084] Through the above-mentioned method, the present invention provides a method for establishing a model of an underwater piston engine power system, which belongs to the field of underwater vehicle power systems, and includes: simplifying and drawing a three-dimensional model of a piston engine; establishing a grid model and setting boundary conditions; setting CFD software, selecting a calculation model to write a UDF, and simulating the working process of the piston engine; using MATLAB to process the data obtained by the simulation to obtain the indicated work of the piston engine; fitting multiple groups of indicated work values ​​to obtain a one-dimensional solution model for the indicated work of the piston engine; combining theoretical derivation with the one-dimensional solution model for the indicated work to obtain a piston engine power system model. Compared with the prior art, the present invention has simple steps, no need for experiments, low cost, stability and reliability, and high precision.

Claims

1. A method for establishing a power system model of an underwater piston engine, characterized in that: Follow the steps below to implement it: Step 1: Simplify the physical model of the piston engine, draw the three-dimensional geometric model of the piston engine, and perform meshing; Step 2: Set the regional boundary conditions; Step 3: Set the intake and exhaust grooves of the piston machine to rotate around the center of the inscribed circle of the arc surface. The bottom of the intake and exhaust grooves pass through the top of the cylinder successively. In addition, write a UDF to control the bottom of the cylinder to make reciprocating motion to simulate the piston motion. The reciprocating cycle is consistent with the rotation cycle of the intake and exhaust grooves. Step 4: Select the k-omega-SST turbulence model under the pressure-velocity coupling basis for simulation and solution, set the pressure inlet pressure and temperature, the pressure outlet pressure and temperature, and the initial pressure and temperature inside the cylinder, monitor the changes of the internal volume V and the cylinder pressure P of the piston engine cylinder with time T, and obtain the PT diagram and VT diagram respectively; combine the two to draw the PV indicator diagram of the piston engine; Step 5, fix the pressure outlet pressure to 2MPa, and the pressure inlet pressure starts from 3MPa and gradually increases at a rate of 1MPa each time; return to step 4, and simulate the piston engine working process under different working conditions; take the percentage of the peak pressure in the PV diagram exceeding the pressure inlet pressure under different working conditions as the over-expansion degree, and when the over-expansion degree reaches 3%, it is taken as the expansion criticality, and the ratio of the intake pressure to the exhaust pressure when the over-expansion criticality occurs is calculated; Step 6: When the intake-exhaust pressure ratio is less than or equal to the critical intake-exhaust pressure ratio of overexpansion, it is an overexpansion condition; otherwise, it is a normal condition. Select 20 groups of overexpansion conditions and 20 groups of normal conditions respectively, return to step 4, calculate the area of ​​the corresponding PV diagram, take the intake pressure and exhaust pressure in the condition as independent variables, and the area value as the dependent variable, use a one-dimensional polynomial to fit the overexpansion condition and the normal condition respectively, and the obtained fitting formula is the one-dimensional solution model of the indicated work; Step 7: Multiply the one-dimensional solution model of the indicated work of the piston engine by the coefficient C e The output torque equation of the piston engine is obtained, and the characteristic equation of the power system is solved according to the output torque equation of the piston engine. The speed is substituted into the complete gas state equation to obtain the characteristic equation of the combustion chamber. The combination of these three equations is the underwater piston engine power system model.

2. The method for establishing a power system model of an underwater piston engine according to claim 1, characterized in that: The method for simplifying the three-dimensional geometric model of the piston engine described in step 1 is: only retaining the three components of the air intake groove, the exhaust groove, and the cylinder, simplifying the air intake groove and the exhaust groove into open cylindrical bodies, simplifying the cylinder into a cylinder, and coinciding the top of the cylinder with the bottom of the air intake groove and the exhaust groove.

3. The method for establishing a power system model of an underwater piston engine according to claim 2, characterized in that: The boundary conditions described in step 2 are: setting the top of the air inlet groove as the pressure inlet, the top of the exhaust groove as the pressure outlet, the bottom of the air inlet groove, the bottom of the exhaust groove, and the top of the cylinder as the interface, and the remaining surfaces are set as walls.

4. The method for establishing a power system model of an underwater piston engine according to claim 1, characterized in that: In step 3, write a UDF to control the reciprocating motion of the bottom of the cylinder to simulate the piston motion. The displacement equation of the reciprocating motion is: Where S is the piston stroke, i is the number of working cycles of each cylinder per revolution of the piston engine, and ω is the speed of the piston engine.

5. The method for establishing a power system model of an underwater piston engine according to claim 1, characterized in that: In step 6, one-dimensional polynomials are used to fit the overexpansion condition and the normal condition respectively. The specific process is: two sets of indicated work values ​​and the corresponding inlet pressure and exhaust pressure are imported into MATLAB respectively, and the cftool tool is used for fitting; the fitting formula is w = x1P c +x2P e +x3; where P c is the inlet pressure, P e is the exhaust port pressure, and w is the indicated work.

6. The method for establishing a power system model of an underwater piston engine according to claim 1, characterized in that: In step 7, multiply the one-dimensional solution model of the indicated work of the piston engine by the coefficient C e The output torque equation of the piston engine is obtained as follows: M e =C e w; in i is the number of working cycles of each cylinder per revolution of the piston machine, z is the number of cylinders of the piston machine, v is the cylinder volume of the piston machine, and η is the mechanical efficiency of the piston.

7. The method for establishing a power system model of an underwater piston engine according to claim 1, characterized in that: The specific equation of the power system characteristic equation in step 7 is: Among them, M e is the output torque of the piston engine, I is the moment of inertia of the piston engine, a n1 、a n2 、a n3 Determined by the power of the auxiliary machines in the piston engine power system.

8. The method for establishing a power system model of an underwater piston engine according to claim 1, characterized in that: The combustion chamber characteristic equation in step 7 is expressed as: Where R is the complete gas constant, R is the complete gas constant, T is the cylinder temperature, C em It is the positive combination factor of the piston engine's working fluid consumption per second.

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