Method for improving stability of fuel gas pressure of engine

Abstract

The invention discloses a method for improving engine gas pressure stability, which comprises the following steps: calculating energy change and friction loss in a combustion process according to a first law of thermodynamics and an ideal gas state equation, and establishing a combustion thermodynamic model and a differential equation of gas pressure changing along with a crank angle; basic parameters of an engine are set, an improved Euler formula is used for solving the differential equation, and a fuel gas pressure curve generated when a crank rotates within one period is obtained; and the change range of the target parameters is set, and the influence of different parameter values on the gas pressure curve is analyzed, so that key parameters are optimized, the engine performance is improved, and the gas pressure stability is improved.

Description

Technical Field

[0001] The present invention relates to the field of predicting engine gas pressure in the field of thermodynamics, and particularly to a method for improving the stability of engine gas pressure based on an improved Euler formula. Background Art

[0002] The solution and analysis of gas pressure curves are widely used in internal combustion engines, gas turbines, aerospace and other fields, such as automotive engines, marine engines, aircraft engines, and micro gas turbines. Gas pressure is a key factor affecting the power performance, combustion efficiency, and emission characteristics of engines. Its variation law with the crankshaft angle directly determines the output power and working stability of the engine. In actual working conditions, the gas pressure curve is affected by various factors such as cylinder structure, combustion process, heat loss, and friction. Especially under high load, high speed, and low emission requirements, its variation law is more complex. The change of gas pressure not only affects the thermal efficiency of the engine but also may cause vibration and noise problems, thus affecting the service life and overall performance of the engine. Therefore, in order to optimize engine performance, it is necessary to deeply study the variation law of gas pressure with engine parameters and establish an efficient and accurate solution method to provide technical support for engine design, performance optimization, and fault diagnosis.

[0003] Accordingly, the literature (Shadloo M S, Poultangari R, Abdollahzadeh Jamalabadi M Y, et al. A new and efficient mechanism for spark ignition engines. J Energy Conversion and Management, 2015, 96: 418 - 429) studied the variation of gas pressure in the engine and its influence on the working process. The results showed that by adjusting parameters such as compression ratio, ignition timing, and intake air volume to optimize the gas pressure curve, the thermal efficiency, power output of the engine can be improved, and emissions can be reduced. Especially under high load and high speed, optimizing the combustion process has a significant effect on improving gas pressure.

[0004] It should be noted that many scholars' research on gas pressure curves is based on establishing and solving thermodynamic models, but the solution method has low computational efficiency, cannot guarantee the continuity and efficiency of the solution under high-precision requirements, and there is little research on the multi-parameter analysis method for gas pressure curves. Summary of the Invention

[0005] The present invention provides a method for improving the stability of engine gas pressure based on the improved Euler formula. In view of problems such as the solution of the engine thermodynamic model and parameter analysis, based on the improved Euler formula, the gas pressure in the cylinder is dynamically solved to predict the influence of the main engine parameters on the gas pressure curve and engine performance, thereby improving the stability of the engine. See the following description for details:

[0006] A method for improving the stability of engine gas pressure, the method comprising:

[0007] According to the first law of thermodynamics and the ideal gas state equation, calculate the energy change and friction loss during the combustion process, and establish a combustion thermodynamic model and a differential equation of the gas pressure changing with the crank angle;

[0008] Set the basic engine parameters, use the improved Euler formula to solve the differential equation, and obtain the gas pressure curve within one cycle of crank rotation;

[0009] Set the variation range of the target parameters, analyze the influence of different parameter values on the gas pressure curve, optimize the key parameters to improve the engine performance and enhance the stability of the gas pressure.

[0010] Wherein, the differential equation of the gas pressure changing with the crank angle is:

[0011]

[0012] In the formula, P is the gas pressure, θ is the crank angle, k is the specific heat ratio, V is the gas volume, Q in is the total input heat, x b is the fuel combustion rate, h cg is the convective heat transfer coefficient, A h is the heat transfer area, Ω is the engine speed, T g is the gas temperature, T w is the average cylinder wall temperature, μ is the dynamic viscosity of the oil, U p is the piston linear velocity, L skirt is the piston body length, L ring is the piston ring length, C is the tolerance between the piston and the cylinder wall, ε is the oil film thickness between the piston ring and the cylinder block.

[0013] Wherein, the improved Euler formula is:

[0014]

[0015] In the formula, f() is a function composed of the crank angle θ and the gas pressure P, P0 is the initial value of the gas pressure, and h is the calculation step size.

[0016] The beneficial effects of the technical solution provided by the present invention are:

[0017] 1. The present invention transforms the differential equation solved by the thermodynamic model into numerical calculation, reduces the complexity of the calculation process, solves the problem of no solution existing in the traditional method, greatly improves the calculation speed, and the method is simple and efficient.

[0018] 2. The present invention can improve the calculation accuracy by reducing the calculation step size, and maintain a high calculation efficiency under high-precision requirements, accurately predict the gas pressure, and ensure the continuity and reliability of the solution.

[0019] 3. The present invention has characteristics such as universality, accuracy and novelty. At the same time, the principle is clear and the calculation is simple, which is convenient to carry out in practice. More importantly, it can better cope with the dynamic changes of the engine working state, provide a new reference idea and method for realizing more accurate engine structure design and fault diagnosis, and thus improve the working stability of the engine. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a graph showing the change of gas pressure with the crank angle obtained by solving through the improved Euler formula;

[0021] Figure 2 It is a graph showing the change of gas pressure when the cylinder inner diameter D = 6.00×10 -2 ~10.00×10 -2 m;

[0022] Figure 3 It is a graph showing the change of gas pressure when the piston stroke H = 1.00×10 -2 ~7.00×10 -2 m;

[0023] Figure 4 It is a graph showing the change of gas pressure when the compression clearance h c =1.00×10 -2 ~1.60×10 -2 m;

[0024] Figure 5 It is a graph showing the change of gas pressure when the average cylinder wall temperature T w =3.50×10 2 ~4.50×10 2 K. DETAILED DESCRIPTION OF THE INVENTION

[0025] To make the objectives, technical solutions and advantages of the present invention clearer, the following further describes the embodiments of the present invention in detail.

[0026] By studying the variation law of gas pressure, the engine structure design can be optimized, the combustion parameters can be reasonably adjusted, the gas pressure distribution can be made uniform, extreme pressure peaks can be avoided, and the operation stability of the engine can be improved. In addition, the mapping relationship between the structural parameters and the gas pressure can predict the fatigue life of key components, guide the material selection and structural strengthening design, reduce the risk of fracture or deformation caused by stress concentration from the root, reduce component damage, and improve the reliability and service life. This research not only provides a theoretical basis for engine design, but also lays a data foundation for the development of real-time fault diagnosis and predictive maintenance systems, ultimately achieving the coordinated improvement of the power system in terms of high efficiency, reliability, and long life.

[0027] An embodiment of the present invention provides a method for improving the gas pressure stability of an engine based on the improved Euler formula. According to the first law of thermodynamics and the ideal gas state equation, the energy change and friction loss during the combustion process are calculated, a combustion thermodynamics model and a differential equation of the gas pressure varying with the crank angle are established, the basic parameters of the engine are set, and the improved Euler formula is used to solve the differential equation to obtain the gas pressure curve within one cycle of crank rotation. Under the condition that other parameters remain unchanged, the variation range of the target parameters is set, and the influence of different parameter values on the gas pressure curve is analyzed to optimize the key parameters and improve the engine performance. This method is simple, fast, and efficient, and can be used to solve the gas pressure curve of the engine, as well as to optimize the engine structure design and fault diagnosis, improving the adaptability and stability of the engine.

[0028] The embodiment of the present invention is carried out according to the following steps:

[0029] (1) Calculate the piston displacement equation according to the spatial position relationship between the crank and the connecting rod;

[0030] x(θ)=(l + R)-(l sinα + R cosθ) (1)

[0031] In the formula, x(θ) is the piston displacement, l is the length of the connecting rod, R is the length of the crank, α is the connecting rod angle, and θ is the crank angle.

[0032] (2) According to the first law of thermodynamics and the ideal gas state equation, combined with the energy changes during the combustion process and the friction loss process in the engine, construct a combustion thermodynamics model;

[0033] According to the first law of thermodynamics:

[0034] δQ - δW = dU (2)

[0035] In the formula, δQ is the change in the amount of heat, δW is the change in the work done by the environment on the system, and U is the internal energy of the system.

[0036] Based on the concepts of energy and work of an internal combustion engine, Equation (2) can be rewritten as:

[0037] (δQ in -δQ loss )-(PdV - δW irrev ) = dU (3)

[0038] Where δQ in is the heat input, δQ loss is the heat loss, P is the gas pressure, δW irrev is the change in irreversible work, V is the gas volume, and its expression is as follows:

[0039]

[0040] Where V c is the clearance volume, satisfying V c = πD 2 h c / 4, where h c is the compression clearance and D is the cylinder diameter.

[0041] When the specific heat capacity is constant, the change in internal energy of an ideal gas is:

[0042] dU = mC v dT g (5)

[0043] Where m is the gas mass, C v is the specific heat capacity at constant volume, and T g is the gas temperature.

[0044] According to the ideal gas state equation:

[0045] PdV + VdP = mR g dT g (6)

[0046] Where R g is the ideal gas constant.

[0047] Combining Equation (5) and Equation (6), we can obtain:

[0048]

[0049] By considering the influence of friction on the piston body and piston rings to determine the irreversible work, we have:

[0050]

[0051] Where L skirt is the length of the piston body, L ring is the length of the piston rings, μ is the dynamic viscosity of the oil, C is the tolerance between the piston and the cylinder wall, Up U(θ) is the piston linear velocity, and ε is the oil film thickness between the piston ring and the cylinder block, which can be approximated by a trigonometric function, i.e., ε(θ) = A + B|sin(θ)|, where A and B are tolerance coefficients. The piston linear velocity U p (θ) satisfies:

[0052]

[0053] In the formula, t is the time, and Ω is the engine speed, is the angular velocity of the connecting rod, which satisfies:

[0054]

[0055] The total heat generated during fuel combustion is:

[0056] Q in = m f LHV (10)

[0057] In the formula, Q in is the total input heat, m f is the fuel mass, and LHV is the lower heating value of the fuel.

[0058] The relationship between the heat input and the crank angle is:

[0059] δQ in (θ) = Q in dx b (11)

[0060] In the formula, x b is a function of the fuel combustion rate, and its expression is as follows:

[0061]

[0062] In the formula, θ s is the crank angle at the start of combustion, and △θ is the entire combustion range.

[0063] The heat loss can be expressed as:

[0064]

[0065] In the formula, h cg is the convective heat transfer coefficient, T w is the average temperature of the cylinder wall, and A h is the heat transfer area. The heat transfer area A h and the convective heat transfer coefficient h cg respectively satisfy:

[0066]

[0067] hcg = 3.26 D -0.2 P 0.8 T g -0.55 w 0.8 (15)

[0068] Wherein, w is the average velocity of the combustion gas, and the calculation method is as follows:

[0069]

[0070] In the formula, P r , V r , T r are respectively the pressure, volume and temperature of the gas inhaled into the cylinder after closing the gas valve, and P m is the average in-cylinder pressure based on the crank angle change in the non-combustion state. During the gas compression process, when combustion does not occur, C1 and C2 are constantly 2.28 and 0; during the gas expansion process, when combustion occurs, these two items are taken as 2.28 and 0.0324 respectively.

[0071] Considering the specific heat ratio k(T g ) = C p (T g ) / C v (T g ), and C v (T g ) = C p (T g ) - R g , the change of the specific heat at constant pressure C p can be calculated using the following relational formula (applicable temperature range 300 - 3500 K):

[0072]

[0073] (3) Establish a differential equation for the change of gas pressure with the crank angle

[0074] Substitute equations (7), (8), (11), (13) into equation (3), and rearrange both sides of the equation according to the gas pressure P, we can get:

[0075]

[0076] Differentiate both sides of the equation with respect to the crank angle θ, we have:

[0077]

[0078] (4) Set the basic parameters of the engine

[0079] Take the numerical values of each parameter as shown in Table 1:

[0080] Table 1 Basic parameters of the engine

[0081]

[0082]

[0083] (5) Numerically solve using the improved Euler formula

[0084] Transform the differential equation in Equation (20) into a numerical calculation problem through the improved Euler formula, that is:

[0085]

[0086] In the formula, f() is a function composed of the crank angle θ and the gas pressure P, and P0 is the initial value of the gas pressure. Taking the step size h = π / 180, the expression of the gas pressure P is:

[0087]

[0088] (6) Plot the curve of gas pressure P varying with the crank angle

[0089] Equation (22) can transform the solution process of the thermodynamic model into a numerical calculation. Taking the crank angle -π / 2 ≤ θ ≤ π / 2, the starting position is the bottom dead center of the piston, and P(0) = 9.34×10 5 Pa. Figure 1 It shows the variation of the engine gas pressure curve with the crank angle.

[0090] (7) Solve the gas pressure curves under different values of the key parameters and analyze their effects on the engine performance.

[0091] Figure 2 It shows the variation law of the gas pressure when the cylinder bore D = 6.00×10 -2 ~10.00×10 -2 m. In the shown range, as the cylinder bore increases, the change rate of the gas pressure decreases near the minimum value and increases near the maximum value, and the crank angle corresponding to the maximum pressure shifts significantly to the left, thus affecting the opening and closing times of the valves, while the crank angle corresponding to the minimum pressure remains unchanged. In addition, the range of the gas pressure also gradually increases with the cylinder bore. When the cylinder bore is too large, the minimum value of the gas pressure will be too low, which will lead to hazards such as incomplete combustion.

[0092] Figure 3 It shows the case when the piston stroke H = 1.00×10 -2 ~7.00×10 -2The variation law of gas pressure at m. Similar to the cylinder bore, as the piston stroke increases, the rate of change of gas pressure decreases near the minimum value and increases near the maximum value, and the range also shows an increasing trend. The difference is that the crank angle when the gas pressure reaches the maximum value is not affected by the piston stroke, and the change range of the range is larger. In addition, too short a stroke makes the range of gas pressure too small, which is not conducive to combustion.

[0093] Figure 4 For the compression clearance h c = 1.00×10 -2 ~1.60×10 -2 The variation law of gas pressure at m. In the figure, as the compression clearance increases, both the maximum and minimum values of gas pressure show a decreasing trend. However, since the cylinder clearance mainly affects the combustion situation when the piston is at the top dead center, the maximum pressure is more affected. Similarly, the crank angle when the pressure reaches the extreme value is not affected by the value of h c value. It can be seen that when it increases to a certain value, the range and maximum value of gas pressure will be too small at the same time, seriously affecting the combustion efficiency and output power.

[0094] Figure 5 For the average cylinder wall temperature T w = 3.50×10 2 ~4.50×10 2 The variation law of gas pressure at K. As the average cylinder wall temperature increases, the extreme values of gas pressure show an increasing trend, and the combustion efficiency increases at this time.

[0095] In summary, the embodiment of the present invention provides a method for analyzing the gas pressure of an engine based on the improved Euler formula. According to the first law of thermodynamics and the ideal gas state equation, calculate the energy change and friction loss during the combustion process, establish a combustion thermodynamics model and a differential equation for the variation of gas pressure with the crank angle, set the basic parameters of the engine, use the improved Euler formula to solve the differential equation, obtain the gas pressure curve within one cycle of crank rotation, and under the condition that other parameters remain unchanged, set the change range of the target parameters, and analyze the influence of different parameter values on the gas pressure curve to optimize the key parameters and improve the engine performance. This method is simple, fast, and efficient, can be used to solve the gas pressure curve of the engine, and can also be used to optimize the engine structure design and fault diagnosis, improve the adaptability and stability of the engine, and has strong engineering application value.

[0096] Those skilled in the art can understand that the drawings are only schematic diagrams of a preferred embodiment, and the serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages and disadvantages of the embodiments.

[0097] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for improving the gas pressure stability of an engine, characterized in that, The method includes: According to the first law of thermodynamics and the ideal gas state equation, calculate the energy change and friction loss during the combustion process, and establish a combustion thermodynamics model and a differential equation for the change of gas pressure with the crank angle; Set the basic parameters of the engine, and use the improved Euler formula to solve the differential equation to obtain the gas pressure curve within one cycle of crank rotation; Set the variation range of the target parameters, analyze the influence of different parameter values on the gas pressure curve, and optimize the key parameters to improve the engine performance and enhance the gas pressure stability.

2. A method for improving the stability of the gas pressure of an engine according to claim 1, characterized in that, The differential equation for the change of gas pressure with the crank angle is: Where P is the gas pressure, θ is the crank angle, k is the specific heat ratio, V is the gas volume, Q in is the total input heat, x b is the fuel combustion rate, h cg is the convective heat transfer coefficient, A h is the heat transfer area, Ω is the engine speed, T g is the gas temperature, T w is the average cylinder wall temperature, μ is the dynamic viscosity of oil, U p is the piston linear velocity, L skirt is the piston body length, L ring is the piston ring length, C is the tolerance between the piston and the cylinder wall, and ε is the oil film thickness between the piston ring and the cylinder block.

3. A method for improving the stability of the gas pressure of an engine according to claim 1, characterized in that, The improved Euler formula is: In the formula, f() is a function composed of the crank angle θ and the gas pressure P, P0 is the initial value of the gas pressure, and h is the calculation step size.

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