Analysis and prevention of engine knock of the hot

2022-08-01
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Analysis and prevention of motorcycle engine knock

knock, as the name suggests, is explosive combustion, also known as sudden explosion and combustion. The normal operation of the motorcycle engine depends on the normal combustion of the combustible mixture, that is, the mixture is ignited by the spark plug under high pressure to form a flame center, which spreads continuously around the combustion chamber at a certain rate, and all the mixture is burned in a very short time. Generally, the combustion process of the engine refers to the process from ignition to piston expansion for work, which lasts about a few thousandths of a second. If an engine with a speed of 13500r/min needs 4.44m/s to rotate its crankshaft for one revolution, while the combustion process needs 0.88m/s, which is equivalent to 63 ° crankshaft angle. It can be seen that combustion takes up a lot of time. The spark plug ignites and burns at a crankshaft angle of about 30 ° -40 ° before the piston reaches the top dead center (ignition starts from here), and the combustion beam at the crankshaft angle after the piston runs to the top dead center. During this period, the piston only moves 1mm-2mm to the bottom dead center. We know that when the combustible mixture is normally burned in the cylinder, the cylinder pressure is uniform, and the engine sound is clear and soft. However, due to the influence of many factors, the unburned gas in the cylinder "spontaneously combusts" before the flame front reaches, resulting in deflagration. The gas produced by the local high temperature and pressure rise has great impact and directly acts on the crankshaft connecting rod mechanism. At this time, the engine has abnormal sound and sends out obvious metal knocking sound. At the same time, it is accompanied by power decline, overheating and severe shaking of the whole machine, which seriously affects the normal play of the engine power

there are many reasons for engine knock, but no matter which reason, from the perspective of mechanism, they all have such characteristics:

first, the combustion of combustible mixture is not synchronous. As the flame front advances at a normal propagation speed, the unburned mixture at the last burning position is further thermally squeezed and radiated, which accelerates the early oxidation reaction, and releases part of the energy to preheat the unburned gas, making its temperature rise continuously. When the ignition point of the fuel is reached, before the normal flame arrives, a new flame center is formed at the most suitable ignition position of the unburned mixture, and the mixture spontaneously ignites, thus changing the propagation speed of the flame

II. The local temperature and pressure of the unburned mixture rise sharply, resulting in the rapid combustion of the unburned mixer, which greatly improves the flame propagation speed, usually in the range of 400m/s ~ 600m/s. When the detonation is serious, it is as high as 1500m/s. while the flame propagation speed of normal combustion is only 20m/s ~ 50m/s.

III. due to the rapid spontaneous combustion of the unburned mixture, the pressure in the cylinder rises sharply, resulting in a strong pressure shock wave, It causes strong vibration of cylinder wall and combustion chamber surface. It makes noise, and its vibration frequency can reach 2500Hz ~ 4500hz. Therefore, the knock sound is sharp and clear, which can be clearly distinguished from the working sound of the engine

once the engine produces knock, it will cause great harm. It will cause the engine power to drop and the fuel consumption to increase sharply. In serious cases, it will also deform and damage the engine cylinder wall, piston, piston ring, valve, connecting rod and its connecting rod bearing and other moving parts. Detonation is usually caused by the following points:

(1) excessive carbon deposition. There is too much carbon deposition in the engine combustion chamber, and its volume is relatively small, resulting in a corresponding increase in compression ratio. The heat storage and non thermal conductivity of the carbon deposition make the combustible mixture burn in advance due to the scorching heat. At the same time, it will reduce the eddy current intensity of the mixture at the end of compression, prolong the combustion time, and increase the tendency of spontaneous combustion, so it is very easy to cause detonation

(2) engine overheating. When the engine is in high-power, overload condition or low gear high-speed continuous driving for a long time, especially in summer, the external high temperature and poor heat dissipation may easily cause the engine to overheat. When the overheating is serious, the combustible mixture will be preheated when it enters the combustion chamber, causing the temperature of local mixture to be too high, reaching the temperature of the ignition point in advance, and spontaneous combustion before the ignition point, thus causing detonation

(3) improper use of fuel. The lower the grade of gasoline, the worse its antiknock performance. The octane number of gasoline stored for too long or poorly sealed will naturally decrease, making the antiknock performance worse. If misused, it is easy to cause incomplete combustion of the mixture. The first part of the mixture will expand and compress the remaining unburned mixture to achieve spontaneous combustion, and then all of the mixture will catch fire and produce high-speed explosive combustion

(4) the calorific value of spark plug is lower than the ground. The calorific value of the spark plug indicates that the proportion of the spark plug scattered in the engine combustion chamber to the exports of emerging countries will also be greatly increased. During the working process of the engine, the temperature of the spark plug changes at any time. The main factor affecting the temperature of the spark plug is the mixed air volume burned in the combustion chamber per second, that is, the engine load. When it is reflected on the motorcycle, it is the speed and load of the vehicle. When the vehicle speed and on-board increase, the amount of mixed gas burned per unit time increases correspondingly, and the amount of heat absorbed by the spark plug also increases. Under the same vehicle speed and on-board conditions, the spark plug with high calorific value has fast heat dissipation and low temperature; The spark plug with low heating value has slow heat dissipation and high temperature. Some users are afraid to burn the mixed oil (two-stroke vehicle) to contaminate the spark plug, so they choose the spark plug with low calorific value to achieve better purification effect. However, after the engine works continuously, especially for long-distance vehicles in summer, when the temperature at the end of the spark plug reaches high temperature, its electrode and insulation will form a heat source to make the mixture ignite in advance. At this time, even if the spark plug does not jump, the mixture can be ignited to produce detonation

(5) ignition time is too early. Because the crankshaft timing key shears in the keyway of the flywheel rotor or the trigger signal changes greatly, the ignition time is advanced; Another reason is that the electronic igniter (CDI) works abnormally, resulting in early ignition and eventually engine deflagration

(6) engine air leakage. The self tightening spring at the oil seal lip of the crankshaft of the two-stroke engine falls off or fails; The oil seal rubber becomes rigid due to aging, so that the self tightening force of the spring cannot play a sealing role; The oil seal of the engine is scratched or bruised during repair; The carburetor adapter (intake pipe or intermediate gasket) is not screwed, so the stress concentration is less harmful to the metal materials with large plasticity, etc. the air leakage caused by these components will dilute the mixture, destroy the normal combustion, and easily lead to early combustion and detonation

to prevent the motorcycle engine from knocking, it is necessary to use and maintain the vehicle reasonably and scientifically within the rated technical data range, and remember the following precautions:

(1) when maintaining the vehicle and disassembling the engine, do not change the thickness and specification of the cylinder gasket at will

(2) the fuel with appropriate octane number shall be selected according to the compression ratio of each model. The engine with high compression ratio shall not use the fuel with low octane number, and attention shall be paid to avoid oxidation and deterioration of the fuel

(3) spark plugs with the same calorific value shall be selected according to the requirements of each model manual. Pay attention to the color of the spark plug skirt and the tightness of the spark plug during use. Generally, when the external surface of the insulation of the spark plug is brownish yellow, there is no carbon deposit and oil stain, and the spark plug electrode is brownish brown, it indicates that the calorific value of the spark plug is appropriate. If there is serious carbon deposit and oil stain on the insulator surface or the insulator surface is gray white, the jaw shall be cleaned frequently, or even there is molten bead, it indicates that the calorific value of the spark plug is low, and the spark plug with higher calorific value shall be replaced and screwed in place. If the spark plug is loose, it is very easy to burn and cause spontaneous combustion

(4) regularly clean the carbon deposit in the combustion chamber and clean the heat sink to ensure that the engine has good heat dissipation performance and avoid overheating. When cleaning the carbon deposit, be careful not to damage the surface of the combustion chamber. Be aware that even if there is a small burr and flanging in the combustion chamber, it will form a hot spot and cause deflagration

(5) after repairing the crankcase of the two-stroke vehicle, the crankshaft oil seal must be installed in place to keep the crankcase chamber well sealed. The oil seal shall be carefully selected. In particular, the groove size of the self tightening spring of the oil seal must be larger than that of the self tightening spring to prevent the spring from slipping during installation. When installing the self tightening spring, turn 1-2 turns against the rotation direction of the spring, and then turn back to the original position along the direction after slightly tightening with force, so that the spring after tightening moves forward 1-2 turns to increase the connecting force. When closing the box, a thin layer of engine oil shall be coated on the outer diameter of the left and right shafts of the crank to prevent the spring from falling off during use

when detonation occurs, especially when the ignition switch is turned off and the ignition cannot be shut down, do not drag the high-voltage line. Turn off the fuel tank switch and the ignition switch respectively, and then turn the throttle to the maximum or use the brake without reducing the gear to shut down the vehicle

although the causes of detonation are complex and harmful, it is not difficult to prevent and eliminate detonation as long as we drive and maintain the vehicle carefully

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