Valve clearance refers to the clearance when the valve is closed.

During the use of automobile engine, the wear or looseness of some parts of valve train will lead to the change of the original valve clearance. Therefore, when driving around 10000 km for maintenance, the valve clearance should be checked and adjusted to meet the technical specifications. When the valve clearance is too small, although the noise is low, due to the thermal expansion of the valve in the working process, the valve can not be closed normally, resulting in air leakage, overheating and ablation of the valve and valve seat. Especially for diesel engines, if the valve clearance is too small, it will also lead to insufficient cylinder compression pressure, thus reducing the engine power, and it is difficult to start in serious cases (diesel engines rely on compression ignition). At the same time, too small valve clearance will also lead to incomplete combustion of combustible mixture, which will obviously increase HC content in exhaust gas. Excessive valve clearance, too late valve opening and too early valve closing will not only produce noise, but also cause insufficient air intake and unclean exhaust. When the piston goes down, the mixture will continue to burn, which will overheat the engine (especially at the exhaust manifold), reduce the engine power and increase fuel consumption. Inspection and adjustment methods of valve clearance There are two common methods to check and adjust valve clearance: one is cylinder-by-cylinder adjustment method, that is, after determining the compression top dead center position of a cylinder piston according to the ignition sequence of the cylinder, the clearance between the intake valve and the exhaust valve of the cylinder can be adjusted; After adjustment, turn the crankshaft and gradually adjust the valve clearance of other cylinders according to this method. Second, the method of twice adjustment is adopted, that is, the crankshaft is rotated to make the piston of the first cylinder at the compression top dead center, and the flywheel mark is aligned with the inspection hole mark line (such as EQ6 100 engine). At this time, the valves of 1, 2, 4, 5, 8 and 9 can be adjusted (referring to the sequence of engine valves from front to back); Then turn the crankshaft once, so that the six-cylinder piston is at the top dead center of the compression stroke, and then adjust 3, 6, 7, 10 "plus two valves (i.e. 1 1, 12), which is actually memory adjustment. When adjusting, while tightening the adjusting screw, insert the thickness gauge between the valve stem end and the rocker arm and pull it back and forth. If you feel slight resistance, check it again until it is suitable. The cylinder-by-cylinder method requires the crank to rotate the crankshaft more times, and the inspection and adjustment time is longer. However, for the seriously worn engine, the cylinder-by-cylinder method is more accurate to check and adjust the valve clearance. Adjusting the valve clearance twice saves time and effort, but it is difficult for maintenance personnel to remember the different adjustable valve serial numbers of different models, which is more complicated. Matters needing attention in valve clearance inspection and adjustment 1, according to the specific requirements and regulations of automobile manufacturers on valve clearance adjustment. 2. Pay attention to the influence of temperature when adjusting: the temperature of valve rocker arm and valve stem will affect the valve clearance. Generally speaking, the valve clearance adjustment is smaller when the engine is hot than when it is cold. Some cars need to be adjusted when the engine is cold, and some cars can be adjusted both hot and cold, but the clearance value is different. 3. The valve clearance of each cylinder should be adjusted consistently to avoid unbalanced engine operation at work. 4. When adjusting the valve clearance, the adjusted valve should be completely closed, and then the adjusted clearance value is accurate. 5. Pay attention to check the working face of the rocker arm head before adjustment. When the engine is working, the arc-shaped working surface of the rocker arm head constantly collides and rubs with the end of the valve stem, especially in the case of poor lubrication, which will cause wear and tear out of the pit, and in serious cases, the end of the valve stem will get stuck in the pit, which will break the rocker arm. Therefore, it should be repaired or replaced with a new one according to the wear condition, so as not to affect the accuracy of its adjustment. Valve clearance adjustment is one of the items that must be completed when maintaining the engine, and it is also an important work content. Whether the adjustment is proper or not will directly affect the power and economy of the engine, which should attract the attention of drivers and maintenance personnel!