CN102996280B - Cavity insulation titanium alloy piston and design method thereof - Google Patents

Abstract

The invention relates to a piston which uses titanium alloy as the piston material and uses a special cavity structure for heat insulation and a design method thereof, which is suitable for high-speed and high-strength diesel engines and belongs to the technical field of thermal energy and power engineering. The piston of the present invention specifically includes a piston upper part, a piston lower part, a piston inner chamber, a pin hole, a heat insulating cavity and a cooling oil chamber. The heat insulation cavity is located on the central axis of the upper part of the piston, between the bottom surface of the combustion chamber, the inner cavity of the piston and the cooling oil cavity, and matches the pin seat to form an optimal load-bearing structure for transmitting explosive pressure. The heat insulation cavity passes through Change the heat transfer route, hinder the heat transfer, and play the role of heat insulation, thereby greatly reducing the temperature of the top surface of the piston inner cavity, so as to solve the problem that the cooling oil is coked or even burned due to excessive temperature and cannot be cooled by oil injection. The titanium alloy piston based on this structure does not need to be cooled by spraying oil, but can only be cooled by splashing.

Description

A titanium alloy piston based on cavity heat insulation and its design method

technical field

The invention relates to a piston which uses titanium alloy as a piston material and uses a special cavity structure for heat insulation and a design method thereof, which is suitable for high-speed and high-strength diesel engines and belongs to the technical field of thermal energy and power engineering.

technical background

At present, diesel engines are developing toward high-strength and ultra-high-strength. The power per liter has been greatly increased, and the average speed of the piston has also been greatly increased. If the mass is not controlled, the inertial force will be large, which will definitely cause vibration and noise of the diesel engine. The problem is serious, and at the same time it affects the lubrication performance between the piston and the cylinder liner, and the wear on the cylinder liner will also be aggravated, so the quality of this high-speed moving part must be controlled and minimized. It is required that the piston can withstand higher mechanical and thermal loads under the premise of maintaining appropriate size and weight and ensuring reliability.

The aluminum alloy piston is light in weight and has good thermal conductivity. However, the inherent disadvantages of aluminum alloys such as low thermal strength, large thermal expansion coefficient, and poor wear resistance make the overall aluminum piston unable to meet the requirements of medium-speed diesel engines with a specific power greater than 0.3kW/cm, especially when burning heavy oil. Sex and lifespan are not ideal.

The all-steel piston has high strength, small expansion coefficient, and can withstand high mechanical and thermal loads. However, due to the high density of steel and poor thermal conductivity compared to aluminum alloy, the mass of the all-steel piston is too large, resulting in its running inertia. Large; poor thermal conductivity leads to high piston surface temperature.

Contents of the invention

The purpose of this invention is to solve the problem that the traditional piston cannot meet the requirements of the engine due to its overweight or strength, and to provide a high-speed and high-strength diesel engine with a higher thermal strength (compared to steel piston), more Titanium alloy pistons that are lighter (compared to aluminum pistons), more reliable, and have better thermal insulation properties.

The purpose of the present invention is achieved through the following technical solutions:

A titanium alloy piston based on cavity heat insulation includes an upper part of the piston, a lower part of the piston, a piston inner cavity, a pin hole, a heat insulating cavity and a cooling oil cavity.

The upper part of the piston and the lower part of the piston are connected together by friction welding, and the position of the weld seam is adjusted according to the curvature of the inner cavity of the piston, and the stress concentration area is avoided.

Three parallel ring grooves are processed on the outer surface of the upper part of the piston, which are respectively the first ring groove, the second ring groove and the third ring groove from top to bottom; the third ring groove is located at the junction of the upper part of the piston and the lower part of the piston, corresponding to The position of the friction welding surface, in order to reduce the weight of the piston, shorten the height of each ring land, so as to reduce the overall height of the piston. The top surface of the upper part of the piston is coated with a layer of high temperature resistant material.

The ring land below the third ring groove is processed with ring grooves to reduce the weight of the piston and at the same time relieve the thermal deformation constraints on the top of the piston and above the second ring groove.

The part below the annular groove forms the piston skirt; the part above the annular groove forms the piston head.

The cooling oil chamber is located between the combustion chamber and the three annular grooves on the piston head, and its top is higher than the upper edge of the first annular groove to reduce the temperature of the first annular groove; the volume of the cooling oil chamber meets the requirements of the friction welding process Under the requirements of the distance between it and the ring groove, the processing is the largest; and the average temperature of the first ring groove of the piston under normal working conditions is <260°C.

The inner cavity of the piston is located below the heat insulation cavity, and adopts the bionic structure of the tortoise shell, which is designed to be arched, so that the pin hole bears more uniform force; Volume, to reduce the mass of the piston, while improving the bearing strength.

The heat insulation cavity is located on the central axis of the piston head, between the bottom surface of the combustion chamber, the inner cavity of the piston and the cooling oil cavity, and matches the pin seat to form an optimal load-bearing structure for transmitting explosive pressure. The heat insulation cavity By changing the heat transfer route, hindering the heat transfer, and playing the role of heat insulation, thereby greatly reducing the temperature of the top surface of the piston inner cavity, so as to solve the problem that the cooling oil is coked or even burned due to excessive temperature and cannot be cooled by oil injection . A small hole is drilled in the lower part of the heat insulation cavity to release the pressure generated by the thermal expansion of the gas. The volume of the heat insulation cavity is manufactured to be the largest under the condition that the piston has a good force transmission path.

The center of the piston pin hole is located below the midpoint of the piston skirt. According to the characteristics of the titanium alloy piston, it is designed as a single profile, which effectively reduces the maximum stress of the pin hole and greatly improves the bearing capacity of the piston.

The piston removes part of the material on both sides of the piston skirt along the axial direction of the pin hole to shorten the length of the piston pin hole and reduce the moment on the pin hole, thereby reducing the uncoordinated deformation of the pin hole and the piston pin.

In the piston head, the wall thickness between the cooling oil chamber and the inner surfaces of the three annular grooves, the bottom surface of the combustion chamber, the cooling oil chamber and the heat insulation cavity is uniform, which effectively reduces the mass of the piston.

The piston material is titanium alloy. In order to reduce the adverse effect of poor wear resistance of titanium alloy and improve the wear condition of the titanium piston surface, wear-resistant multi-layer composite coatings are coated on the surface of each ring groove of the piston and the surface of the piston skirt.

A design method for a titanium alloy piston based on cavity heat insulation, comprising the steps of:

Step 1, perform structural topology optimization on the structural model of the traditional piston.

The traditional piston structure is used as the design space of topology optimization, the unit density of all solid elements of the piston is used as the design variable, the total compliance is taken as the design goal, and the volume percentage conversion is used as the constraint condition of topology optimization. The optimal material distribution path of the piston under mechanical load is calculated and analyzed from the perspective of the structural load-bearing frame.

Step 2, according to the optimal material distribution path obtained in step 1, combined with the functional requirements of the key parts of the piston, part of the material on both sides of the piston skirt along the axial direction of the piston pin is removed, and the piston is designed as a thin-walled structure, and a large-capacity cooling oil is designed The cavity and the inner cavity of the piston are used to obtain a structure that meets the requirements of light weight. The mass of the titanium alloy piston of this structure is less than that of the aluminum alloy piston of the same diameter.

The key parts include the first ring groove, the flange of the piston pin hole, the top surface of the piston inner cavity and the throat of the combustion chamber.

Step 3, combined with the high-speed, high-power-density engine performance and thermal boundary conditions in the system design requirements, perform finite element calculations on the structure obtained in step 2, and analyze the temperature, coupling stress and deformation of key parts of the piston. If the average temperature of the first ring groove of the piston is <260°C, the temperature of the top surface of the piston inner cavity is <300°C, and there is no obvious stress concentration on the flute edge of the piston pin hole, the piston structure design is completed, and the optimal piston structure form is obtained, proceed to step 7 ; If the temperature and strength do not meet the system design requirements, go to step 4.

Step 4, further consider the heat load problem, and design a large-capacity heat-insulation cavity in the head of the piston by taking advantage of the fact that the thermal conductivity of air is much smaller than that of metal. By changing the volume of the heat insulation cavity and the position of the piston head, the finite element analysis of the improved structure is carried out, and the optimal heat insulation cavity structure that meets the heat flow distribution, temperature and thermal stress of the piston is obtained.

Step 5. Change the volume and position of the cooling oil chamber, analyze the influence of different volumes and positions on the piston temperature and thermal stress, and determine the final size and position of the cooling oil chamber according to the average temperature requirement of the first ring groove.

In step 6, the piston pin hole is designed as a single profile, and the deformation curve of the piston pin hole flange is further analyzed to obtain the most excellent structure that meets the requirements of the piston's bearing capacity.

Step 4, step 5, and step 6 are performed simultaneously or sequentially, and the optimal piston structure is obtained by combining the analysis results of the above three steps, and then step 7 is performed.

Step 7: Process the titanium alloy piston according to the optimal piston structure obtained from the design, and carry out surface protection design, by coating the top surface of the piston with high-temperature resistant materials, and coating the ring grooves of the piston and the surface of the piston skirt with wear-resistant multilayer The method of multi-layer composite coating is used to improve the anti-ablation and friction and wear characteristics of the piston.

Through the above design method, a titanium alloy piston with a heat-insulating cavity that can withstand high temperature and high pressure is obtained.

A titanium alloy piston based on cavity heat insulation, its heat conduction process is:

The heat generated by the high-temperature gas is transferred from the bottom of the combustion chamber to the upper part of the piston. Among them, the heat transferred in the direction of the central axis of the piston in the heat-insulating cavity can be studied by one-dimensional steady-state heat conduction, and the heat conduction process can be simplified as a flat wall The one-dimensional, steady-state, heat conduction problem without an internal heat source in :

$q=-\mathrm{\&lambda;}\frac{\&PartialD;t}{\&PartialD;\mathrm{no}}\stackrel{\&Right\; Arrow;}{\mathrm{no}}=-\mathrm{\&lambda;}\frac{\mathrm{dt}}{\mathrm{dx}}=\mathrm{\&lambda;}\frac{t_{W1}-t_{W2}}{\mathrm{\&delta;}},$ W/ ^m2

In the above formula, q is the heat flux density downward along the central axis of the piston, λ is the thermal conductivity, t _W1 and t _W2 are the temperatures on both sides of the flat wall, and δ is the thickness of the flat wall.

Since there is air in the heat-insulating cavity, the thermal conductivity of the air is much smaller than that of the titanium alloy, the former is about 0.005 times that of the latter, and it can be seen from the above formula that the heat conduction of the heat-insulating cavity is much smaller than that of the titanium alloy of equal volume The heat conduction of the material hinders the downward transfer of heat along the piston axis, thereby reducing the temperature of the top surface of the piston cavity to <300°C. Therefore, the piston of the present invention does not need to be cooled by spraying oil, but only by splash cooling.

Beneficial effect

Compared with the prior art, the technical solution of the present invention has the following advantages:

The large-volume heat insulation cavity structure in the titanium alloy piston can play a good heat insulation effect, greatly reducing the temperature on the back side of the piston (the top surface of the inner cavity), and because most of the heat is blocked on the top surface of the piston, the piston The temperature of the top surface is very high, which is conducive to combustion and the evaporation of fuel on the wall; the titanium alloy piston of this structure does not need to be cooled by oil injection, only splash cooling is enough; the thermal expansion coefficient of titanium alloy is small, which can greatly reduce the gap between cylinders , so as to reduce air leakage, reduce noise and fuel consumption rate; the titanium alloy piston with this structure can meet the strict requirements of high power density diesel engine on the high temperature mechanical properties of the piston, and at the same time, the manufacturing process is good and easy to realize.

Description of drawings

Fig. 1 is a sectional view of a titanium alloy piston structure based on cavity heat insulation of the present invention;

Fig. 2 is a side view of a titanium alloy piston structure based on cavity heat insulation of the present invention;

Fig. 3 is a flow chart of the technical route of the embodiment in the specific implementation manner.

1-Piston upper part, 2-Piston lower part, 3-Combustion chamber, 4-Cooling oil chamber, 5-Heat insulation cavity, 6-Piston ring area, including 6(a)-First ring groove, 6(b)- The second ring groove, 6(c)-the third ring groove, 7(a), 7(b), 7(c), 7(d)-friction welding surface, 8-pin hole, 9-air hole, 10- Piston cavity, 11-annular groove, A-piston head, B-piston skirt.

Detailed ways

In order to better illustrate the purpose and advantages of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Titanium alloy has excellent high and low temperature performance, and can still maintain good mechanical properties at high temperatures. The working temperature can reach 600-650 ° C, and its heat resistance is much higher than that of aluminum alloy. The specific strength of titanium alloy below 500 ℃ is also significantly better than that of stainless steel and heat-strength steel; the specific strength of titanium alloy is high, and the density is much lower than that of steel. Using titanium alloy as the piston material can produce a piston with high unit strength, good rigidity and light weight, which meets the design requirements of light weight and high strength.

The piston structure of the present invention is shown in Figure 1 and Figure 2, including 1-piston upper part, 2-piston lower part, 3-combustion chamber, 4-cooling oil chamber, 5-insulation cavity, 6-piston ring area, including 6(a)-first ring groove, 6(b)-second ring groove, 6(c)-third ring groove, 7(a), 7(b), 7(c), 7(d)- Friction welding surface, 8-pin hole, 9-air hole, 10-piston inner cavity, 11-annular groove.

In this embodiment, the piston structure design is first carried out from the perspective of the structural load-bearing frame, and the optimization analysis software OptiStruct is used to calculate and analyze the optimal material distribution path of the piston under mechanical load, and a new structural form that meets the requirements of lightweight design is proposed; and Based on this new structure, combined with the traditional design experience, further considering the heat load problem, using the characteristics that the thermal conductivity of air is much smaller than that of metal, a large-capacity heat-insulating cavity is designed on the piston head, and the cooling oil cavity is optimized at the same time. The shape of the titanium alloy piston with a heat-insulating cavity is designed, so that the piston does not need to be cooled by spraying oil, but can withstand the harsh working environment of high temperature and high pressure only by splash cooling.

The technical route of this embodiment is shown in FIG. 3 .

The distances between the sides of the cooling oil chamber near the three annular grooves and the inner surfaces of the combustion chamber and the annular grooves are less than 5mm, and the cooling volume is greater than 75cm ³ . Between the cooling oil chamber and the inner surfaces of the three annular grooves, the bottom surface of the combustion chamber, the cooling oil chamber and the heat insulation cavity, the wall thickness is uniform, all less than 5mm.

The wall thickness between the piston inner cavity and the piston head is less than 3mm.

The wall thickness between the heat insulating cavity and the cooling oil cavity is less than 10mm, and the volume of the heat insulating cavity is greater than 25cm ³ , so as to ensure that the temperature of the top surface of the piston inner cavity is less than 300°C.

The top surface of the upper part of the piston is coated with TiAlN, and the surface of each ring groove and skirt is coated with multi-layer composite film.

The mass of the titanium alloy piston in this embodiment is less than 1.35 kg, which is lighter than an aluminum alloy piston with the same diameter, can effectively reduce the inertial force of the piston, and is suitable for high-speed engines.

The piston of this embodiment is used in high-speed, high-power-density engine operating conditions, and the temperature and coupling stress values of the key parts of the piston are obtained through simulation calculations, as shown in Table 1 and Table 2.

Table 1 Temperature of key parts of piston

key position Unit: ℃ combustion chamber center 668 combustion chamber throat 603 The average temperature of the first ring groove <230 Piston chamber top surface <195 pin hole flange <186

Table 2 Coupling stress of key parts of piston

key position Unit: MPa Pin hole <96 first ring groove <114 Second ring groove <95 The third ring groove (at the weld) <70

It can be seen from Table 1 and Table 2 that the average temperature of the first ring groove of the piston is <230°C, the temperature of the top surface of the inner cavity is <195°C, and the coupling stress of the edge of the pin hole is <96MPa.

The above results show that the heat insulation cavity has a good heat insulation effect. The titanium alloy piston of this embodiment has high strength, good heat insulation and high reliability, can withstand higher mechanical load and thermal load, and can meet the stringent requirements of high-speed and highly-strengthened engines.

Those of ordinary skill in the art, without departing from the spirit and scope of the present invention, can also make appropriate changes and adjustments to the specific shape and size, so all equivalent technical solutions also belong to the category of the present invention. The scope of patent protection is defined by the claims of this application.

Claims (5)

1. A titanium alloy piston based on cavity heat insulation, characterized in that: the piston material is titanium alloy; it includes the upper part of the piston, the lower part of the piston, the inner cavity of the piston, the pin hole of the piston, the heat insulation cavity and the cooling oil cavity; The upper part of the piston and the lower part of the piston are connected by friction welding, and the position of the weld seam is adjusted according to the curvature of the inner cavity of the piston, and the stress concentration area is avoided; The outer surface of the upper part of the piston is processed with three parallel ring grooves, which are respectively the first ring groove, the second ring groove and the third ring groove from top to bottom; the third ring groove is located at the junction of the upper part of the piston and the lower part of the piston, corresponding to friction The position of the welding surface; the upper surface of the piston is coated with a layer of high temperature resistant material; The ring land under the third ring groove is processed with an annular groove; the part below the annular groove forms the piston skirt, and the part above the annular groove forms the piston head; The cooling oil chamber is located between the combustion chamber and the three annular grooves on the piston head, and its top is higher than the upper edge of the first annular groove; The heat insulation cavity is located on the central axis of the piston head, between the bottom surface of the combustion chamber, the inner cavity of the piston and the cooling oil cavity, and matches the pin seat to form an optimal load-bearing structure for transmitting explosive pressure. The heat insulation cavity By changing the heat transfer route, hindering the heat transfer, and playing the role of heat insulation, thereby greatly reducing the temperature of the top surface of the piston inner cavity, so as to solve the problem that the cooling oil is coked or even burned due to excessive temperature and cannot be cooled by oil injection A small hole is drilled in the lower part of the heat insulation cavity to release the pressure generated by the thermal expansion of the gas; the volume of the heat insulation cavity is processed to the maximum under the condition that the piston has a good force transmission path; The inner cavity of the piston is located below the heat insulation cavity, and adopts a turtle shell bionic structure, forming an arch; The center of the piston pin hole is located below the midpoint of the piston skirt, and is designed as a single profile. 2. A titanium alloy piston based on cavity heat insulation according to claim 1, characterized in that: the volume of the cooling oil chamber is processed to the maximum under the requirement of the distance between the cooling oil chamber and the ring groove in the friction welding process, Make the average temperature of the first ring groove of the piston <260°C under normal working conditions. 3. A titanium alloy piston based on cavity heat insulation according to claim 1, characterized in that: in the piston head, the cooling oil chamber and the inner surfaces of the three annular grooves, the bottom surface of the combustion chamber and the cooling oil chamber And the wall thickness between the insulation cavity is uniform. 4. A titanium alloy piston based on cavity heat insulation according to claim 1, characterized in that: each ring groove of the piston and the surface of the piston skirt are coated with a wear-resistant multi-layer composite coating. 5. A method for designing a titanium alloy piston based on cavity heat insulation according to claim 1, characterized in that: comprising the steps of: Step 1, perform structural topology optimization on the structural model of the traditional piston; The traditional piston structure is used as the design space of topology optimization, the unit density of all solid elements of the piston is used as the design variable, the total compliance is taken as the design goal, and the volume percentage is converted as the constraint condition of topology optimization; from the perspective of the structural load-bearing frame Calculation and analysis to obtain the optimal material distribution path of the piston under mechanical load; Step 2, according to the optimal material distribution path obtained in step 1, combined with the functional requirements of the key parts of the piston, part of the material on both sides of the piston skirt along the axial direction of the piston pin is removed, and the piston is designed as a thin-walled structure, and a large-capacity cooling oil is designed The cavity and the inner cavity of the piston are obtained to obtain a structure that meets the requirements of light weight, and the mass of the titanium alloy piston of this structure is less than that of the aluminum alloy piston of the same diameter; The key parts include the first ring groove, the edge of the piston pin hole, the top surface of the piston inner cavity and the throat of the combustion chamber; Step 3, combined with the high speed, high power density engine performance and thermal boundary conditions required by the system design, perform finite element calculation on the structure obtained in step 2, and analyze the temperature, coupling stress and deformation of the key parts of the piston; if the piston first The average temperature of the ring groove is <260°C, the temperature of the top surface of the piston inner cavity is <300°C, and there is no obvious stress concentration on the edge of the piston pin hole. If the strength does not meet the system design requirements, go to step 4; Step 4, further considering the heat load problem, using the characteristics that the thermal conductivity of air is much smaller than that of metal, design a large-capacity heat-insulating cavity at the head of the piston; change the volume of the heat-insulating cavity, and the The location, the finite element analysis of the improved structural form is carried out, and the optimal heat-insulating cavity structure that satisfies the heat flow distribution, temperature and thermal stress of the piston is obtained; Step 5, change the volume and position of the cooling oil chamber, analyze the influence of different volumes and positions on the piston temperature and thermal stress, and determine the final size and position of the cooling oil chamber according to the average temperature requirement of the first ring groove; Step 6, design the piston pin hole as a single profile, further analyze the deformation curve of the piston pin hole flute edge, and obtain the most excellent structure that meets the requirements of the piston bearing capacity; The step 4, step 5, and step 6 are carried out simultaneously or sequentially, combined with the analysis results of the above three steps, the optimal piston structure is obtained, and step 7 is carried out; Step 7: Process the titanium alloy piston according to the optimal piston structure obtained from the design, and carry out surface protection design, by coating the top surface of the piston with high-temperature resistant materials, and coating the ring grooves of the piston and the surface of the piston skirt with wear-resistant multilayer The method of multi-layer composite coating is used to improve the anti-ablation and friction and wear characteristics of the piston; a titanium alloy piston with a heat-insulating cavity that can withstand high temperature and high pressure is obtained.