Structural characteristics of crank connecting rod mechanism of diesel engine

Crank connecting rod mechanism is the diesel engine to achieve the work cycle, complete the energy conversion mechanism, is the diesel combustion released by the heat energy into mechanical energy, and output power. Crank connecting rod mechanism is composed of piston connecting rod group and crankshaft flywheel group.

1.Piston connecting rod

(1)Piston.The piston made of aluminum alloy is composed of top, leakproof part, pin seat hole and skirt part. The top is a component of the combustion chamber, with flat roof, convex roof and concave roof and other forms. The leakage prevention part is provided with upper and lower ring grooves, which are respectively used to install the gas ring and oil ring of the piston ring. The oil ring groove has many small oil holes through which the oil scraped off the oil ring can return to the sump. The pin seat hole is used to install the piston pin, and the oil splashed by the oil hole lubricates the contact surface between the pin and the pin hole, and the inner groove of the hole is used to install the piston pin retaining ring. The skirt acts as a guide and bears side pressure. When working, due to more metal in the direction of the pin hole, the diameter changes greatly after thermal expansion, while the diameter changes little in the direction of the vertical pin hole due to the small thermal expansion, under the action of lateral pressure. In order to make the piston skirt work close to the circle, the skirt is often made into an oval. The piston skirt of some gasoline engines also has a slanting expansion groove to ensure that there is room for full expansion after being heated. When installing, pay attention to the direction, so that the side with expansion groove is facing the side with lower lateral pressure.

The failure forms of piston include wear, ablation, scratch and crack. Wear is the most common failure forms, are available when appraisal method of observation check whether there is any scratch, crack, and on piston ring groove edge collapse, etc., and then check ring grooves, pin hole, and separately with the measuring tool wear skirt and deformation conditions, when the piston and cylinder, pin hole and fitting clearance of piston pin, piston ring groove and can continue to use when does not exceed allowable values. If the piston appears one of the following conditions should be scrapped: crack, ring groove wear seriously or edge collapse, pin hole wear beyond the limit size, surface has serious scratches or diameter wear to the limit.

(2)Piston ring. The piston ring is divided into gas ring and oil ring. The gas ring is used to seal the cylinder, so that the high-pressure gas in the cylinder does not leak into the crankcase, and can disperse the heat from the upper part of the piston through the cylinder wall. The oil ring lubricates the cylinder wall and wipes the oil back to the crankcase. In order to prevent the piston ring heat expansion and stuck, the piston incision should have a certain "opening gap", its size and cylinder diameter, generally 0. 25 ~0. 8 mm. If the opening gap is too small, it will accelerate the wear of the ring and the cylinder wall, and even make the ring stuck, broken and scratch the cylinder wall; The opening gap is too large, the sealing performance deteriorates, resulting in air leakage and lubricating oil channelling. There should also be a certain clearance between the piston ring and the ring groove in the height direction, known as the "edge clearance", generally 0. 04 ~ 0. 15 mm. If the edge clearance is too small, the piston ring will be stuck in the ring groove, lose elasticity, and destroy the sealing and scraping effect; If the edge clearance is too large, it will cause serious ring and ring groove collision and aggravation of wear, and produce serious "pump oil action", so that the lubricating oil into the combustion chamber.

(3) Piston pin. The piston pin connects the piston to the connecting rod, transferring the gas pressure from the piston to the connecting rod. In order to prevent the piston pin from scratching the cylinder wall by axial movement, a clamping spring is arranged in the ring groove of the two pin holes of the piston. In the cold state, the outer diameter of the piston pin is slightly larger than the aperture of the piston pin seat, forming interference fit (interference is 0. 001 ~0. 01 mm). Piston pin and connecting rod bushing fit clearance is generally 0. 015 ~0. 045 mm, limit clearance is 0.12 mm.

(4)Connecting rod.The function of the connecting rod is to connect the piston and the crankshaft, and make the reciprocating motion of the piston and the rotational motion of the crankshaft mutually transform. A connecting rod bushing is pressed into the small head hole of the connecting rod, which is connected to the piston by the piston pin. Oil holes are drilled on the top of the small head and the bushing, and the inner surface of the bushing is provided with a cloth oil groove, which is generally lubricated by the splash of oil in the crankcase. When installing bushings, pay attention to the oil hole.

Connecting rod body section is i-shaped to meet the requirements of good strength, high stiffness and light weight. If the stiffness of the connecting rod is not enough, it will lead to bending of the connecting rod, resulting in partial wear of the piston and cylinder, gas leakage and oil channeling of the piston ring. The connecting rod head is connected with the connecting rod journal of the crankshaft, and the bearing bush is arranged in the hole. Connecting rod big head has two ways to separate, one is bisector, its separate surface and connecting rod center line vertical; The other is the oblique fraction, which separates the surface from the center line of the connecting rod at an Angle of 45°. In order to ensure the correct installation of the two parts of the big head of the connecting rod, there is a matching mark on the same side of the rod body and the connecting rod cover (some also hit the cylinder number on the other side). Therefore, disassembly should be put in pairs, installation should not be replaced. The two parts of the big end of the connecting rod are fastened together by two bolts, which cannot be substituted.

2.Crankshaft flywheel set

(1)Crankshaft. Crankshaft is one of the most important parts of diesel engine, all the power of diesel engine is output through it. In engine work, the crankshaft must bear the centrifugal force of rotating mass, the gas pressure of periodic change and reciprocating inertia force, so that the crankshaft produces complex alternating stress such as bending, torsion, shear, tension and pressure. The sliding friction of crankshaft journals under high pressure occurs at a large relative speed, which is easy to cause wear, heating and burning loss. In order to ensure the reliable work of the crankshaft, the crankshaft is required to have enough stiffness and strength, the working surface should be wear-resistant and good lubrication, and try to make it smooth operation, reduce vibration. The crankshaft consists of a front axle, a main shaft journal, a connecting rod journal, a crank and a tail. The front end of the crankshaft is equipped with a crankshaft timing gear, and the tail is equipped with a flywheel. The main shaft neck supports and rotates in the main bearing of the body, and is the supporting part of the whole crankshaft. An oil channel leading to the connecting rod journal is provided on the spindle journal. The crank is the part that connects the main journal to the journal of the connecting rod. The crank is equipped with a balance block to balance the unbalanced force and torque of the crankshaft and reduce the load of the main bearing and the vibration of the diesel engine. The crankshaft axial clearance is adjusted by the main bearing cover gasket. Clearance is too large, the crankshaft will produce axial channeling and damage the work of crank connecting rod mechanism; Too small clearance will increase friction loss.

(2) Flywheel. Flywheel function is mainly to work in the stroke of a part of the energy storage, to overcome the auxiliary stroke resistance, so that the crankshaft rotation uniform, and improve the engine to overcome short-term overload capacity; It also has the function of making the engine easy to start, for vertical engines, it is also the active part of the clutch.

Diesel engine crank connecting rod mechanism each relative movement of the parts have a strict relationship, the use of maintenance process should focus on the inspection of the main parts of the coordination, if necessary, can be adjusted and repaired.