Re-inspection of Diesel Engine Piston Components

1.Reinspection after piston assembly

After installing the piston components of all cylinders according to the above method, the following work can be carried out.

Measurement of cylinder compression clearance

(1) The cylinder compression clearance value represents the compression volume, also known as the compression chamber height. Each model of diesel engine must have its compression chamber height value, which must be maintained at its specified value during final assembly to ensure that the compression volume of each cylinder remains consistent.

(2) The height value of the compression chamber can be calculated from the assembly dimension chain of fixed and moving parts.

(3) In actual measurement, the following methods have been adopted. Install all moving parts inside the machine, but the adjustment shims may not be installed temporarily. Rotate the crankshaft so that the piston is at the top dead center position. Use a special measuring tool to measure two dimensions a and b. The difference between a and b is the required compression clearance value c specified in the design. The value of c can also be directly measured using a flat ruler and a steel ruler. Based on the measurement results of each cylinder, select and match the adjusting shims for each cylinder to ensure that their c values comply with the specified values in the drawings, in order to maintain a basically consistent compression clearance value for each cylinder, and thus have a compression ratio and compression volume that are close to each other.

2. Alignment of moving parts

After all moving parts are installed in the machine, the assembly clearance of each part should be adjusted within the specified range. All connecting bolts should be tightened or hydraulically tightened according to the requirements. At this time, without installing the piston ring, the piston should be calibrated in the cylinder, and the final installation coordination measurement inspection should be conducted to determine whether the relative position between the piston and the cylinder, as well as between the slider and the guide plate, is correct.

(1) Process methods for calibration

① Rotate the crankshaft to move the piston to the near top dead center position. On the outer surface of the piston rod, divide it into two mutually perpendicular sections. Use a frame level to check whether the centerline of the piston rod is vertical. If there is any deviation, use a wooden wedge to insert it between the piston head and cylinder liner, adjust its left and right front and rear positions, and ensure that the centerline of the piston rod is vertical.

② Measure the clearance around the piston and cylinder according to the required position. Generally, only four areas perpendicular to each other along the longitudinal and transverse directions of the diesel engine, as well as the upper and lower outer circles along the piston skirt, should be measured. At the same time, the clearance between the slider and the working plane of the guide plate, as well as the clearance with the side baffle, should be measured. Fill in the data from these two parts separately in the table for comparison.

③ Rotate the crankshaft again to move the piston to the near bottom dead center position, repeat operations 1 and 2, and make a second measurement record. Based on the data obtained from the top and bottom dead center measurements, necessary adjustments and adjustments can be made to achieve satisfactory alignment quality.

At present, the calibration of diesel engine pistons in the cylinder has widely adopted the one-time measurement calibration method. Simply make the position of the piston in the middle of the cylinder, that is, the position of the crank pin neck in the left and right horizontal positions, and follow the above calibration steps. After adjustment or repair, the calibration work can be completed.

(2) Requirements for piston alignment in the cylinder

① When the piston is at the top and bottom dead center positions without a piston ring, the minimum clearance between the piston skirt and the cylinder bore should not be less than 30% of the total clearance;

② The inclination of the piston in the cylinder should not be too large at any position along the longitudinal direction of the diesel engine. The piston of a diesel engine is allowed to tilt in the cylinder, and should not exceed 0.10mm per meter. If it exceeds this value, efforts should be made to repair it;

③ The piston is allowed to deviate parallel to one side along the longitudinal direction of the diesel engine in the cylinder. When the piston and connecting rod are pried towards the other side, their offset should be able to immediately transfer to the other side. If it cannot be transferred or quickly rebounded, the cause should be checked and eliminated.