If the skirt of a
New Car piston is round when it is cold, it will become an ellipse when it is working, making the circumferential clearance between the New Car piston and the cylinder unequal, causing the
Fuel Car piston to get stuck in the cylinder, and the engine cannot work normally. Therefore, the piston skirt is made into an ellipse in advance during processing. The long axis direction of the ellipse is perpendicular to the pin seat, and the short axis direction is along the pin seat direction. In this way, the New Car piston approaches a perfect circle when working.
The temperature of the New Car piston along the height direction is very uneven. The temperature of the New Car piston is high at the top and low at the bottom, and the expansion volume is also large at the top and small at the bottom. In order to make the upper and lower diameters of the New Car piston equal when working, that is, cylindrical, the Fuel Car piston must be made into a stepped or conical shape with a small top and a large bottom in advance.
In order to reduce the heat received by the piston skirt, a horizontal heat insulation groove is usually opened on the skirt, and in order to compensate for the deformation of the skirt after heating, a longitudinal expansion groove is opened on the skirt. The shape of the groove is "T" or "Π" groove. The horizontal groove is usually opened below the lowest ring groove, on both sides of the pin seat on the upper edge of the skirt (some are opened in the oil ring groove) to reduce the heat transfer from the head to the skirt, so it is called a heat insulation groove.
The vertical groove will make the skirt have a certain elasticity, so that the gap between the New Car piston and the cylinder is as small as possible during assembly, and it has a compensatory effect in the hot state, so as not to cause the New Car piston to get stuck in the cylinder, so the vertical groove is called an expansion groove. After the vertical groove is opened on the skirt, the rigidity of the grooved side will be reduced. When assembling, it should be located on the side with less side pressure in the power stroke. The piston of a diesel engine New Car is subjected to large forces, and the skirt is generally not grooved.
