Automotive pistons, like the central part of a car engine, play an extremely important role in engine start-up.The automobile piston is used to with stand the gas pressure, and the piston pin drives the crankshaft to rotat through the piston pin. The top of the piston is also a component of the combustion chamber. The general automotive pistons are divided into three categories: diesel pistons, gasoline engine pistons, and general-purpose pistons.
      In recent years, due to the continuous improvement of engine reinforcement, the engine speed, average effectiv pressure and average piston speed have been greatly improved compared with the previous ones, so the engine's thermal load and mechanical load have increased. The piston has also undergone great changes and improvements in design and manufacture due to its important role in the engine, so as to meet the high emission standards of the real society.
      I. Piston height.
      The height of the piston is gradually shortening, and the diameter of the piston is much larger than the height of the piston; the compression height is also shortened, and these changes on the piston shorten the height of the body and the whole machine, and improve the rigidity of the body and the whole machine. Reduced engine size,
weight,metal consumption and cost, and reduced reciprocating mass, inertial force and main bearing load. In addition, when the piston height is shortened, the weight of the piston group can also be reduced. The total weight of each pistongroup of the engine generally accounts for 1.2% and 3.0% of the total weight of the whole engine.
      Ⅱ.Number of Piston Rings.
      In recent years, in order to reduce friction work, improve mechanical efficiency, and reduce the height of the ring region to shorten the piston compression height and total height, the number of piston rings has been decreasing.From the 1930s 5, 6 ring grooves developed to 2, 3 ring grooves. It has been found that it is not necessary to have more than one compression ring, because once the upper compression ring fails, the other compression ring can not seal the gas of the combustion chamber so that it does not leak into the crankcase, nor can it control the oil to swell along the cylinder wall. Into the combustion chamber. Therefore, instead of increasing the
number of piston rings, it is better to take measures to improve the performance of the piston ring. A compression ring and two oil ring grooves are used to ensure the sealing gas and the control of the oil.

      III. Shore fire. 

      There is a tendency for the height of the fire shore to rise appropriately, especially for semi-separated combustor diesel engines in which the combustion chamber is arranged in the top of the piston. In order to prevent carbon deposits and gluing in the top ring, the temperature of the first ring groove is not required to be too high.
      Therefore,in addition to the ring groove, the fire shore has a tendency to lengthen, and the higher the degree of strengthening,the more obvious the trend is. .
      In order to reduce the strong heating of the first ring groove by the gas, on the piston of the diesel engine, a spira shallow groove is often driven around the fire shore,and the labyrinth pressure reducing principle is used to reduce the sag of the gas along the gap between the piston and the cylinder liner.
      Ⅳ.Piston top.
      Piston crown thickness generally tends to thin. It is mainly restricted by two contradictory factors: combustion pressureand decreasing temperature gradient. If the thickness is too thin, the mechanical load capacity is poor, which wil cause mechanical deformation; if the thickness is too thick, the temperature gradient is too large,causing excessive thermal stress and thermal deformation. Therefore, the piston top thickness varies with the engine's maximum combustion pressure, cylinder bore, oil cooling,and combustion chamber form.
      The inner wall of the piston top adopts a heat flow type, that is, a transition arc radius from the top of the piston to the side wall of the ring region is large,Some also add support ribs (generally 4), the purpose is to make the heat at the top of the piston more smoothly transfer to the ring zone and the pin seat (the support ribs also play an important role in strengthening the stiffness and strength of the piston), thereby reducing the first pass. The ring and the ring groove are heated, Improve the reliability of the key parts of the piston group.Piston pin seat. The pin seat is strengthened and the diameter of the piston pin is enlarged properly. The connecting part of the pin seat and the
ring area is the dangerous section area of stress concentration. Now in the internal combustion engine aluminum alloy piston the most stress position is in the pin seat, the fatigue crack often appears in the pin hole top, and along the piston pin seat longitudinal plane extension. A large circular arc transition is used in the region to reduce stress concentration and to increase the area under stress. Four tendons are usually used to connect the piston pin pedestal with the piston top in order to directly transfer a part of the load and reduce  the load on the above dangerous parts.
There is a tendency to enlarge the diameter of piston pin slightly, the purpose is also to increase the bearing area of pin and pin seat. The larger the cylinder diameter is, the higher the strengthening degree of the engine is, and the larger the ratio of piston pin diameter to cylinder diameter is.
      Ⅴ.Piston profile.
      The piston profile has a great influence on the performance and reliability of the engine, so it is an important research subject to determine the piston profile when designing a new engine. The friction of piston face cylinder wall, the noise caused by piston knock, and the ring gap have important effects on the formation of hydrocarbons in the cold oil engine and on the air utilization and compression ratio of the diesel engine, which directly affect the performance of the engine. The piston profile is closely related to the increase of fuel consumption and gas channeling caused by cylinder pulling, abrasion and skirt deformation, which directly affects the reliability of the engine. Piston profile from the original multi-section cylinder gradually evolved to the present ring zone multi-segment positive cone and tapered elliptical cone, the skirt gradually changed barrel surface elliptical cone.
      Skirt shape is generally a barrel face-oval. The axis of the barrel coincides with the axis of the piston. The circumference of the piston section perpendicular to the axis is elliptical, and the short axis is along the direction of the piston pin. Some piston skirts are made into multistage continuous elliptical cones, multistage continuous special circles,etc. The advantages of barrel skirt are uniform distribution of oil, anti-pull cylinder, anti-oil channeling, large oil film bearing capacity, reduced friction work and friction. However, the determination of the optimal profile is an extremely complex process that requires repeated tests, often over a long period of time. The traditional method is to analyze the temperature field and stiffness of the piston, use the finite element method and draw a piston profile according to the experience design, and then put the obtained test sample into the engine to carry on the piston wear mark test.Abserve the wear marks on the pistons and cylinder liners. After successive approximation, the best piston profile can be determined. However, due to the movement of the piston and the thermal deformation caused by various related factors, the piston may also be affected by the cylinder wall deformation caused by tightening torque of the cylinder head screw,uneven cooling, temperature distribution of the cylinder wall, and so on. And the influence of uneven combustion of each cylinder, as well as the characteristics of the piston itself (such as stiffness, the expansion rate of the automatically controlled thermal expansion piston varies with the piston section) make the determination of the profile more difficult.
      Now the composite material is coated on the smaller size of the test piston surface, so that piston skirt and cylinder wall clearance close to zero. Wear test was carried out at maximum engine power. The surface determined by the composite material left after the test is the final piston surface. With this method, the optimum piston profile can be determined by only one abrasion mark test. The development time is greatly saved, in addition, the wear strength can also be quantitatively analyzed from the wear marks of the worn part of the test piston.
      Ⅶ.the piston skirt structure.

      The piston skirt should not only bear the lateral thrust of the connecting rod, but also ensure the good direction of the piston, so there should be sufficient bearing area to form a sufficient thickness of lubricant film. Not because the gap is too large to knock the cylinder, causing noise and accelerated wear and not due to the gap is too small to occur cylinder. 

      The trend is to use thin-walled stiffeners to reduce weight and ensure adequate stiffness to prevent deformation.
The contact area between piston skirt and cylinder wall directly affects the friction loss of engine. However, when the contact area is small, the film thickness also decreases. When the film thickness is less than the root-mean-square value of the two contact surfaces, boundary lubrication will occur, resulting in an increase in friction work. Shortening
the piston skirt can reduce the contact surface and reduce the friction work, but it can increase the piston sloshing and the contact stress between the top and the bottom of the skirt.
      It also deteriorates the working performance of the piston.
      In order to solve these problems, British AE Group's Volvo has developed a method of machining a number of "protrusions" on the top and bottom of the piston, which can improve the fatigue life of the skirt. Because the skirt top very high side thrust produces the bending moment to be lower than the original structure. This design can reduce the contact area by more than 75%, keep the contact area and skirt rigidity unchanged in a wide range of load and temperature, reduce the tendency of cylinder pulling by improving lubrication, and increase the fatigue life by
reducing mechanical deformation. Turin, Italy, the company further developed a "X" skirt piston. That is to say, two guide bands are retained on each side of the thrust side of the skirt,which are connected with the piston pin seat and the piston top with a rib. The skirt area is reduced to 1 / 3 of the traditional piston, the weight is reduced by more than 20%, the friction loss of the engine is significantly reduced, the fuel consumption is reduced by 2%, and the output power is increased.
      Ⅷ. anti-expansion piston.
      It is difficult to control the clearance between piston skirt and cylinder liner because of the difference of working temperature in different parts of piston and the expansion characteristics of aluminum alloy piston and cast iron cylinder liner. The most effective skirt shape also fails to overcome the differential expansion rate between the piston and the cylinder.
      At the same time, the minimum clearance determined under the maximum engine load will also cause excessive clearance under partial load, resulting in cylinder knock noise and increased fuel consumption. In order to control the collocation clearance of piston skirt and make it neither too small to cause cylinder pulling, nor too big to cause noise and vibration, a pair of steel plates were symmetrically buried between the piston skirt and the pin seat, and the steel plates were properly extended along the skirt. The steel sheet, together with the surrounding material, forms a bimetallic sheet, which contracts vertically along the piston pin axis during thermal expansion. There is no heat insulation slot in the transition section between the ring groove and the skirt, so the heat flow can be transferred to the skirt part smoothly, so the temperature in the first ring groove area is lower than that in the slot  piston, and the strength is higher. Aluminum alloy pistons are widely used to control thermal expansion by inserting steel plates or rings in skirts.
      Ⅸ. the cooling of the piston.
      When the temperature of the top ring groove is above 220 ℃, the coke and glue may be formed, which will cause the piston ring to bond and pull the cylinder, and the strength of the aluminum alloy will decrease rapidly when the temperature is over 180 ℃, and the strength of the aluminum alloy will be reduced when the temperature is over 180 ℃. It may cause damage to the piston top support and the high load area of the piston pin bearing. The piston needs to be cooled when the average effective pressure in a given working condition is greater than 1.034 N / mm2.
Piston is often cast into steel cooling plate incense tube or welded with soluble salt core and electron beam to form afull circle cooling chamber.
      Ⅹ. several failure modes of the piston:
      1. If the wear of the piston exceeds the allowable value, the power and speed will be reduced, and the fuel consumption will increase to knock the cylinder.
      2. The verticality of the center of the pin hole and the center of the ellipse of the skirt is extremely poor. The torque is generated during use to deform the piston and produce a pull cylinder.
      3. poor thermal stability, due to long-term work at high temperatures, equal to continue to aging and stabilization treatment, piston expansion, the original 0.05mm clearance gap disappears, resulting
in seizure.
      4. the first ring groove wear, so that the piston ring loses its elasticity, no sealing effect. Summarizing the above, in thedesign of the piston,the change of the piston profile and the piston pin
seat, the thickening of the top of the piston,and the shortening of the piston height are gradually developing toward the "short" direction in order to improve the rigidity.
      The overall height of the engine is shortened and shortened. The main measures are to shorten the piston skirt and reduce the number of ring grooves. The latter can reduce the compression height and the engine itself is continuously strengthened. Therefore, the performance of the piston must be correspondingly enhanced to become "fat". Mainly means that the wall thickness of each part is increasing,and the R at the transitional fillet is also increasing.
Due to the high rotational speed of the engine, the piston must reduce the weight. In order to reduce the inertial force, although the wall thickness is increasing, the entire weight of the piston is continuously reduced, because the weight drop caused by the shortening of the entire piston height is greater than the weight gain due to the increase in wall thickness. Since the piston structure becomes "short", the surface area of the piston is reduced in proportion to the entire volume, which is disadvantageous for heat dissipation of the piston.
      In order to ensure that the piston is not melted and properly lubricated, in addition to some measures must be taken in design to reduce its thermal load; the piston must also be changed and improved interms of manufacturing, such as piston material, casting and machining, To meet its requirements for heat load. In addition, the surface of the piston is treated to improve oil storage and improve lubrication conditions.