Why should the valve clearance be adjusted? How to adjust?

When the engine is cold and the valve is closed, the clearance between the valve and the transmission part is called valve clearance. \x0d\ Excessive clearance: After the intake valve and exhaust valve are opened late, the intake and exhaust time is shortened, the valve opening height is reduced, the normal valve timing is changed, and the engine power is reduced due to insufficient intake and unclean exhaust. In addition, the impact of valve mechanism parts increases and the wear accelerates. Too small clearance: after the engine works, the parts are heated and expanded, pushing open the valve, which makes the valve not close tightly, causing air leakage, reducing power, causing serious carbon deposition or burns on the sealing surface of the valve, and even the valve hits the piston. The valve mechanism with hydraulic tappet does not need to leave valve clearance. \x0d\ First of all, we should know that the gap between the valve rocker arm and the valve (that is, the valve gap) exists because the intake valve and the exhaust valve are installed at the top of the combustion chamber, which is also the place with the highest temperature. In order to leave room for expansion, there must be a gap. As for the size of the valve clearance \x0d\[ 1], the intake valve clearance is usually 0.2 ~ 0 due to the different designs of different manufacturers. After long-term action and wear, the clearance between the rocker arm of the engine valve and this valve will become larger and larger, so only the clearance of the valve foot can be adjusted. However, not all cars need to adjust the valve foot clearance. Some cars have automatic oil pressure adjustment, so there is no need to adjust the valve foot clearance. \x0d\( 1) Remove the valve cover. \x0d\ Remove the fixing screw of the bonnet, and carefully remove the bonnet, taking care not to damage the bonnet gasket. Wipe off the oil stains on the valve and rocker shaft with a rag to facilitate the valve adjustment operation. \x0d\(2) Find the top dead center of one cylinder compression. \x0d\ Turn the crankshaft or pry the flywheel with a hand crank, so that the first cylinder is at the compression top dead center position. When viewed from the front of the engine, the timing pit of the crankshaft pulley is aligned with the timing mark. On some large vehicles, the inspection hole 1-6 cylinder line of the flywheel housing is aligned with the timing mark of the flywheel housing. For example: Dongfeng EQ6 100- 1 engine and flywheel 1-6 cylinder should be marked with the steel ball of the flywheel housing. At this time, from the valve point of view, all valves of a cylinder should be in the closed state. If the valve of a cylinder is not completely closed, it means that the piston of a cylinder is at the bottom dead center position. You should turn the crankshaft for another 360 degrees so that the first cylinder is at the compression top dead center position. \x0d\(3) Method for determining that each cylinder is at compression top dead center. \x0d\ According to the principle of engine structure, we know that when each cylinder is at compression top dead center, the valve of the cylinder is closed. Therefore, you can open the distributor cover, determine the position of the high-pressure branch pipeline of each cylinder, and turn the crankshaft. When the distributor points to the high-pressure branch pipeline position of the cylinder, the cylinder is at the top dead center position of the compression stroke at the moment when the contact is disconnected. In this way, the position of compression top dead center of each cylinder can be accurately determined, and the valve can be adjusted conveniently. \x0d\(4) Measure the valve clearance. \x0d\ Valve clearance is divided into cold car value and hot car value, so you should measure it under the condition of meeting the requirements of car use. Valve Clearance \x0d\ Select a plug gauge that meets the specifications and insert it between the valve stem and the valve rocker arm (or cam). Gently pull the plug gauge. If there is slight resistance, the clearance is correct. In order to determine whether the gap is within the specified range, it is generally measured by the range limit value (such as the gap range value is between 0.29 mm and 0.35 mm). First, insert a 0.29 mm feeler gauge into the valve clearance. At this time, it should be normal if the feeler gauge can pass. Then insert the 0.35mm feeler gauge into the valve clearance, and the feeler gauge should not be inserted, indicating that the clearance is within the given clearance range. If the 0.29mm plug gauge cannot be inserted into the gap, the gap is too small; If the 0.35mm plug gauge can pass through the insertion gap, the gap is too large. If any of the above items do not meet the requirements, it means that the valve clearance is abnormal and the clearance must be adjusted. \x0d\(5) Adjust the valve clearance \x0d\ 1) Adjust the valve clearance. First, loosen the fixing nut of the valve adjusting screw, insert the plug gauge with a specified thickness into the valve gap, pull the plug gauge with one hand and turn the adjusting screw with the other hand until the plug gauge is slightly resisted. After correct adjustment, insert the plug gauge into the center of the valve clearance, keep the adjusting screw still, and tighten the fixing nut to lock the adjusting screw. After locking the screw, re-measure the valve clearance with a plug gauge, because you may have inadvertently moved the adjusting screw during locking, thus changing the valve clearance. If the valve clearance changes, it should be readjusted until it is correct. 2) twice adjustment method. According to the structural principle of valve train, we know that the arrangement of intake valve and exhaust valve is regular. According to the ignition sequence and the arrangement sequence of intake and exhaust valves, the clearance of four (four-cylinder engine) or six (six-cylinder engine) valves can be checked and adjusted; Then turn the crankshaft once so that four or six cylinders are at the compression top dead center position, and then adjust the other four or five or six valves. 3) Cylinder by cylinder adjustment method. Because the order of engine valves is different, it is difficult to remember the order of intake valves and exhaust valves. The throttle clearance can also be adjusted cylinder by cylinder according to the ignition sequence or injection sequence of the engine. In order to accurately adjust the valve clearance, you can use the direction of the distributor burner to adjust the intake and exhaust valve clearance of the cylinder one by one. \x0d\ Edit the adjustment method of automobile valve clearance in this paragraph \x0d\ Method (1) \x0d\ valve clearance \x0d\ 1. Draw a line every 4mm in the radial direction on the valve working surface with a soft pencil, so that the matched valve contacts the valve seat, and rotate the valve 1/8 ~ 1. If some lines are not broken, or the contact position is wrong, it means that the seal is not tight or not ideal and needs to be polished or repaired again. 2. After patting the valve several times on the mating seat, check the working faces of the valve and the valve seat, and a bright and complete halo should appear. The halo position on the valve should be below the center of the working cone, so it is considered that the sealing requirements have been met. 3. Check with the valve seal tester with barometer that the valve group parts are in the assembled state. Press the cylinder of the testing machine tightly at the position of the valve head, so that the joint surface between the end face of the container and the cylinder head (or cylinder block) can maintain a good seal. Then pinch the rubber ball and inflate the air container to make it have an air pressure of 0.6 ~ 0.7 MPa. If the barometer reading does not drop within half a minute, it means that the combination between the valve and the seat is well sealed. The principle of checking and adjusting the valve clearance should be carried out when the valve is completely closed and the valve tappet is in the lowest position. The overhead valve shall measure the clearance between the end face of the valve stem and the rocker arm, and the side valve shall measure the clearance between the end face of the valve stem and the tappet. There are two ways to check and adjust. \x0d\ Check the adjustment method \x0d\ 1, cylinder by cylinder adjustment method. First, find the compression end point of a cylinder, adjust the gap between the intake valve and the exhaust valve of the cylinder, and then crank the crankshaft one by one according to the ignition order. 2. Twice adjustment method. Taking a six-cylinder engine working in 1, 5, 3, 6, 2 and 4 ignition sequence as an example, the explanation is as follows: ① First, put the piston of a cylinder at the compression end, then the intake and exhaust valves of this cylinder will be adjustable. (2) According to the principle of "two in three rows". That is, the intake valve of the second cylinder and the exhaust valve of the third cylinder must be completely closed at this time, and the adjustment can also be checked. ③ Only one pair of valves can be adjusted at a time for two cylinders with connecting rod shaft diameters in the same plane, so the exhaust valve of the fifth cylinder and the intake valve of the fourth cylinder can also be checked and adjusted. ④ When the piston of the sixth cylinder is at the end of compression, the remaining valves that have not been checked and adjusted must be completely closed. Therefore, all valves of the engine can be checked and adjusted by rotating the crankshaft twice. \x0d\ Method (2) \x0d\ (1) marking method. Use a soft pencil to draw a line on the ground valve working surface every 8mm or so, then put the matching valve on the valve seat and rotate it 1/4 times. If all the drawn lines are cut off, the sealing performance is good. If some lines are not cut, explain. (2) Pressurization method: inject 50 ml kerosene from the inlet and outlet pipes respectively, and then apply 20 ~ 30 kPa air pressure to see if kerosene seeps from the valve. If there is oil oozing, take it out and grind it. (3) Coloring method: coat a blue film on the working surface of the valve, and when the valve naturally presses the valve seat, rotate the valve relative to the valve seat. At this time, if the valve sealing surface 360. Gong blue appears, indicating that the valve is concentric, otherwise the valve should be replaced. Excessive valve clearance will make the valve open late and close early. Therefore, the opening time is too short, and the combustible mixture can not be fully inhaled during the intake process. So that that normal pow of the engine cannot be exerted. In the exhaust process, the exhaust gas can not be fully discharged, which is easy to overheat the engine. In addition, when the engine is working, it will also produce valve knocking sound, which will affect the service life of the parts. The valve clearance is too small, which leads to the early opening and late closing of the valve and makes the cylinder unable to work normally. With the increase of engine temperature, valves and valve seats will leak due to poor sealing. At the same time, it may also make the valve carbon deposit and even burn the valve. The general methods of inspection and adjustment are as follows: ① Preheat the engine to make the cooling water temperature reach 80℃-90℃. ② Open the timing mark inspection holes on the clutch housing and cylinder head cover. ③ Confirm that the cylinder head bolts have been tightened to the specified torque. ④ Turn the crankshaft so that the "0" engraved line on the flywheel is aligned with the marking line on the clutch housing, and confirm that the cambered surface of the rocker arm of the intake and exhaust valve of the first cylinder is in contact with the base circle of the camshaft cam, that is, the piston of the first cylinder is at the compression top dead center (if the rocker arm is in contact with the cam, the crankshaft should rotate 360), and the valve is in the closed position. ⑤ Loosen the lock nut 2 of the adjusting screw 1 and turn the adjusting screw with a screwdriver to make the gap a between the lower end face of the screw and the upper end face of the valve stem 3 reach the specified value (determined by the thickness of the thickness gauge). Keep the screwdriver still, tighten the lock nut to the specified torque, and then insert the thickness gauge into the gap A for re-inspection so that the gap between the intake valve and the exhaust valve of the first cylinder can be adjusted. ⑥ Then turn the crankshaft clockwise (as viewed from the front of the engine). For a 4-cylinder engine, the valve clearance of the next ignition cylinder can be adjusted in the order of 1-3-4-2 every time it rotates 180. For a 3-cylinder engine, it can be adjusted according to the ignition sequence of 1-2-3 every 240 (the rotation angle of crankshaft can be converted by the number of teeth of flywheel ring gear).