Piston is one of the most important parts in the engine. Because it works under conditions of high temperature, high pressure, corrosion, friction, high speed motion, etc., it requires high material performance. The piston material of internal combustion engine uses cast iron, steel and aluminum alloy. In the development process of internal combustion engines, people continue to put forward higher requirements for its economy, power, emissions, etc., so that the requirements for internal combustion engine piston materials are correspondingly improved, mainly focusing on heat resistance, wear resistance, and friction reduction. , Corrosion resistance and light weight. Traditional piston materials can basically meet these conditions, but with the development of the power system, also determines the development direction of the piston. As the internal combustion engine needs to develop more power and higher thermal efficiency, the piston material is required to have better high-temperature mechanical properties and lighter mass, to meet the development requirements of modern internal combustion engines. In addition to optimizing the chemical composition of traditional piston materials and improving the manufacturing process to improve mechanical properties, developing new piston materials to adapt to different working conditions and operating environments is a new development trend. With the deepening of material research, ceramic materials have been developed piston.
Ceramic pistons are generally used in diesel engines.
Using ceramic materials to replace precious metals in swirl chamber diesel engines can further reduce cooling devices, so the overall cost is expected to be reduced. In the direct injection diesel engine, the high temperature resistance of ceramic materials is used to insert a ceramic block on the top of the piston, and the thermal efficiency, noise and emissions are all improved. The size and shape of the insert in the ceramic piston should be selected appropriately, otherwise due to the difference in the thermal expansion coefficient of the material, stress will be generated on the ceramic block, which will affect the service life of the piston; in addition, the ceramic fiber piston made of silicon nitride ceramic material, Because of its good wear resistance, it can prevent aluminum alloy pistons from "cold knocking and hot pulling" due to their large thermal expansion coefficient.
Good high temperature strength
Silicon nitride and silicon carbide can maintain high strength at high temperature of 1373K, while the strength of high-temperature nickel alloy can only maintain 1123K. Generally speaking, when the temperature exceeds 1173K, the high-temperature strength advantage of ceramics is shown. Therefore, ceramic materials are first used to manufacture combustion chamber parts that work at high temperatures for a long time.
Low thermal conductivity
Ceramic materials have low thermal conductivity, and are often used to make combustion chamber parts such as pistons, cylinder liners and cylinder head bottom plates, and to insulate the combustion chamber. In ceramic non-cooling engines, even the independent cooling system of the engine is cancelled to prevent the loss of heat energy in the cylinder.
The density of silicon carbide and silicon nitride is about 10% higher than aluminum, and less than cast iron 55%. The combination of low density and high temperature strength makes ceramics not only suitable for manufacturing reciprocating parts such as valve mechanisms, ceramic pistons and piston pins, but also suitable for manufacturing Rotating moving parts such as turbocharger turbines. Reduced weight of moving parts can bring benefits such as friction reduction, energy saving, faster response and reduced vibration.
High elastic modulus
The greater the elastic modulus of the material, the smaller the deformation of the part. Some high-strength ceramics have a greater modulus of elasticity than steel, and are used to make valves, which can reduce the buckling of the valve disc and the bounce when seated, and reduce noise and vibration.
Good friction and wear performance
The hardness of high-temperature structural ceramics is much higher than that of metals, and the wear resistance at high temperatures is better than that of metals, and especially better than cemented carbide. Metal and ceramic pair sliding. Under lubricated conditions, not only the wear of ceramics is extremely small, but the wear of metal is also smaller than when metals are paired. Most of the parts of the valve mechanism slide at high speed under high load and insufficient lubrication conditions. It is very suitable for ceramic manufacturing to reduce wear. The main disadvantage of ceramic parts in terms of performance is that they are prone to internal cracks during manufacturing. The internal cracks may propagate under stress and cause brittle failure of parts. In addition, temperature sudden changes caused by changes in engine operating conditions, surface damage caused by wear and foreign objects, etc., may affect the reliability and service life of the parts. By applying ceramic coating, the temperature in the combustion chamber of the engine can be improved, and the working efficiency of the engine can be improved.
Ceramics have a variety of excellent characteristics. When used in automobiles, they can effectively reduce the weight of the vehicle, increase the thermal efficiency of the engine, reduce fuel consumption, reduce exhaust pollution, increase the life of wearing parts, and improve the intelligent functions of the car. Therefore, the development and application of ceramic parts in automobiles has very broad prospects.