You may have encountered some problems when using a chain saw, but you don’t know the reason. Today, I will mainly talk about the difficulty of starting the chain saw heat engine. In fact, there is a problem that the heat engine cannot be started, and the fundamental reason is that the engine is overheated. So what are the causes of engine heat?
1. The mixing ratio is too thin
The mixing ratio is the mixing ratio of the volume of gasoline and engine oil. First, talk about the role of engine oil in the engine, lubrication, sealing, heat conduction, cleaning, anti-corrosion. These five functions are interrelated. If the lubrication is not good, the dry friction will emit more heat, and in severe cases, it will cause the piston to dissolve and wear (commonly known as the pulling cylinder); if the seal is not good, it will blow into the crankcase and cause flammable mixing. The gas becomes thinner; the heat conduction is not good, and the heat cannot be radiated in time; the effect of cleaning and antisepsis will also be greatly reduced. Here, it should be noted that the quality of the engine oil is very high. The two-stroke engine has high requirements for engine oil. It is difficult to achieve general engine oil. The requirements for it are: high flash point, low freezing point, easy to mix (dissolve) and gather quickly ( Good stickiness). If the requirements are not met, the same mixing ratio will also cause the engine to overheat. It should also be noted that four-stroke engine oil should never be used for two-stroke engines. If you cannot find a special two-stroke engine oil for a while, you can use No. 10 vehicle oil, that is, steam engine oil, which can be used all year round in North China, Northwest China It is used in northeast China in summer, spring, autumn and winter in south, and motor oil No. 15 in summer. Remember! ! Never use diesel oil.
Second, the air-fuel ratio is too lean
The air-fuel ratio is the ratio of air to fuel. The air-fuel ratio required by the engine is 13 to 1 at start-up and 15 to 1 at maximum power. It is 16 to 1 to save fuel when operating at a constant speed for a long time. After the carburetor is adjusted, the size of the throat area is controlled by a throttle (also called a throttle valve, commonly known as a throttle). If there is a problem with the design of the carburetor, the air intake is too large, but the oil intake is insufficient, which is what we often call 'oil thin'. The burning speed is fast, the engine speed is high, and it seems to be weak at work. The phenomenon we can see is that when the fuel tank runs out of fuel, the engine speed increases suddenly when the throttle does not move, and then the engine is turned off. This is a temporary phenomenon where the air-fuel ratio is too lean. If the air-fuel ratio is too lean to work for a long time, it will cause insufficient engine power and overheating.
Third, the compression ratio is too large
The compression ratio is the working volume of the engine (also known as displacement) plus the volume of the combustion chamber, and then divided by the volume of the combustion chamber to equal the theoretical compression ratio. The actual compression ratio is the working volume after the exhaust port is completely closed, plus the volume of the combustion chamber, and then divided by the volume of the combustion chamber. The actual compression ratio of two-stroke engines should be between 6.5-7.3. If it is too small, the power is insufficient, and if it is too large, it will cause overheating and even knock. The size of the compression ratio is determined by the manufacturer, and dealers and users can only make fine adjustments under the very familiar circumstances. In the formula, V is the engine displacement, Pe is the average effective pressure on the top of the piston at the time of the explosion, N is the engine speed, and 75×6=450 is a constant. It can be seen in the formula that the constant is constant, then, to increase the power of the engine: 1. Increase the displacement, 2. Increase the effective pressure, (the greater the compression ratio, the greater the pressure after the explosion) 3. Increase the rotation number. At present, the manufacturer can only increase the effective pressure on the top of the piston to increase the power of the engine without changing the displacement and the number of revolutions, that is, to increase the compression ratio. Even about 20 minutes, the power is a bit larger, but long-term work will cause the engine to overheat, but the power will decrease, and the heat engine will not start.
4. Insufficient exhaust area
The size of the exhaust area is related to the displacement, that is to say, the working area corresponding to the displacement. The area of ??the exhaust port accounts for about 5%-5.5% of the working area (experience data). If it is too small, the exhaust will be poor, and the engine will be overheated. If it is too large, it will cause insufficient cylinder strength and affect the position of the piston ring counterpart. People’s congresses who ride motorcycles (two-stroke) have such experience. After a period of time, the engine will overheat and become weak. Just clean the carbon deposits on the top of the piston and the combustion chamber, and the exhaust port. , You can restore the original working conditions. This phenomenon is caused by: coke deposits reduce the volume of the combustion chamber, increase the compression ratio, deteriorate the thermal conductivity, and reduce the exhaust port, resulting in poor exhaust, overheating the engine, and reducing power.
5. Exhaust too late
The cylinder structure of a two-stroke engine is more complicated than that of a four-stroke engine. The intake air, scavenging air, and exhaust air are all on the cylinder wall of the cylinder (the intake port of the asymmetric intake air is on the crankcase). Various air ports must ensure the work needs, as well as the strength of the cylinder block and the position of the piston ring The amount of retention. The position of intake, scavenging, and exhaust is very important, that is, the phase of intake and exhaust is arranged reasonably. It is determined based on the top and bottom dead center of the piston and the crank angle, and it is related to the engine S/D (S-stroke , D─cylinder diameter) when the S/D value is around 0.8, the exhaust phase is 100°─105° after top dead center, when the S/D value is 0.9─1.0, the exhaust phase is 103°─108 after top dead center ° The S/D value basically determines the number of revolutions of the engine. If the value is small, the number of revolutions is high, and when the number of revolutions is high, the absolute exhaust time is short. Therefore, it is necessary to start early. If the start time is too early, the engine power is insufficient. If it is too late, the heat retention time will be long, which will cause the engine to overheat.
6. Insufficient cooling air volume
The cooling air of the two-stroke forced-air-cooled engine is provided by the blades on the flywheel (a considerable part of the fan is opened in the fan shell and provided by the impeller). Here, it is necessary to talk about the role of the flywheel. We know that the working cycle of the engine is four strokes of suction, compression, explosion, and exhaust. The only one that actually works and emits power is the explosion stroke, while the other three strokes are all Is power consuming. In order to ensure the continuous operation of the engine, the energy of the explosion stroke must be stored and released during other power-consuming strokes. So the first function of the flywheel is to store energy, the second is to cool the cylinder, and the third is to generate electricity, that is, the inner (outer) rotor of the magneto Required spark), the fourth is the link (or output power connection) at start-up. The amount of air required to cool the cylinder is related to the size of the flywheel, the number of blades, the size of the blades and the wind pressure angle, as well as the space area of ??the air inlet grille. If the flywheel design is better, the space area of ??the air inlet cover is too small Or, there is debris blocking the mesh cover or clogging between the cylinder blades during work, which will cause insufficient cooling air volume and cause the engine to overheat. (This is a problem that needs to be solved urgently)
7. The cooling area of ??the cylinder blade is not enough
For each type of air-cooled gasoline engine, its heat dissipation area, based on its displacement and power, is basically fixed. It is relatively simple to use the following formula to obtain the approximate value: Ff=C,S,D(Ps)/ vh In the formula of c㎡, Ff is the total required heat dissipation area, S is the stroke, D is the cylinder bore, Ps is the effective power (metric horsepower) Vh is the cylinder volume (liter), and the natural air-cooled small two-stroke engine C=3.4-3.8, Forced air-cooled small two-stroke engine C=2.7-3.3, as can be seen from the formula, if there is a change in each index of the two-stroke air-cooled small engine, then its heat dissipation area will have a corresponding change, or forced air The amount of cold air is increased accordingly. If only the engine displacement or compression ratio is changed, and the other is not changed accordingly, it will also cause the engine to overheat.
8. Insufficient intake area
In the same way as scavenging, if the intake port is too small, the crankcase will be undercharged. When the piston is down, the air flow into the scavenging channel is not strong, and the ability to drive exhaust gas is reduced, resulting in a fresh combustible mixture in the cylinder. The exhaust gas is mixed), the burning speed is fast, the power is reduced, and the engine is overheated. The opening angle of the air inlet, that is, the intake phase, is related to the number of engine revolutions. It is less than 6000 revolutions, which is 52?-55? before and after top dead center, and greater than 6000 revolutions, which is 55?-58? before and after top dead center. Because the engine speed is high and the absolute intake time is short, the intake phase of the engine with high speed needs to be advanced. However, it is not the earlier the better, because the intake is symmetrical, the intake is early, and it will inevitably be closed late, which will cause the carburetor to reverse injection. However, despite the early opening, if the intake area is too small, it still cannot reach the engine’s Demand will also cause overheating, so the intake area is related to the working area corresponding to the displacement like scavenging and exhaust. The intake area accounts for about 4.5% of the working area (experience ratio). Requirement: When the piston is at top dead center, the upper edge of the air inlet overlaps with the lower edge of the piston. When the piston is at bottom dead center, the top of the piston and the upper edge of the air inlet must not leak.
Nine, the ignition angle is wrong
Regardless of the two or four stroke engine, there is an ignition advance angle. The reason is that there is a process from the start of ignition to complete combustion. This process requires a certain time to allow the piston to completely burn after reaching the top dead center, and to push the piston downward with the greatest explosive force, it can exert the maximum power. At idle speed, the rotation speed is slow, and the ignition advance angle can lag a little. At high speed, the rotation speed is fast, and the ignition advance angle must be advanced much. At present, there are two types of magneto ignition devices on the market, one is inductive type, referred to as TCI, and the other is capacitive discharge type, referred to as CDI, TCI ignition advance angle is 25?-28?, within this angle, idle speed and High speed can be taken into account, but it is not the best state. CDI is different. When starting, the ignition angle is small and does not rebound. It fires at about 450 revolutions and the advance angle is about 14?. At 7000 revolutions, the ignition advance angle is automatically advanced Up to about 30?. No matter what kind of ignition device, the ignition timing is controlled by the position of the keyway on the crankshaft and flywheel. The difference is that the TCI ignition angle is immovable, and the CDI is automatically advanced as the engine speed increases. If the crankshaft and keyway positions are not well controlled, it will cause the ignition advance angle to be too early or too late. Prematurely, the rebound is strong at the start, and it will cause knocking after the start. The result is damage to the parts and the engine is overheated; when it is too late, the mixture is not completely burned, it is discharged outside the cylinder, and secondary combustion is formed in the muffler. 'Engine cannon'. Combustion on both sides (cylinder and muffler) produces heat on both sides, which overheats the engine and severely lacks power. This phenomenon rarely occurs in the design. If it fails, it is due to assembly quality problems and after a period of use, the nut pressing the flywheel will loosen and cause the rolling keys to damage the machine parts. Therefore, there is a 'maintenance' requirement in the manual.
10. Insufficient scavenging area
The two-stroke engine's intake, compression, explosion, and exhaust cycle work. The crankshaft rotates once, and the piston completes two strokes in the cylinder, so it is called a two-stroke engine. After the explosion, the piston goes down and opens the exhaust. When the air port is to a certain extent, the scavenging port is opened to scavenge and expel the exhaust gas after combustion. When the piston is at the bottom dead center position, the exhaust port is fully opened, and the opening of the scavenging port is the largest. When the piston goes up, the combustible mixture in the cylinder starts to compress, but the scavenging port and the exhaust port are not closed. Some of the mixture escapes from the exhaust port and is discharged into the atmosphere, causing pollution, and some enters the crankcase from the scavenging channel. In order to reduce the escape of the mixed gas, some manufacturers did not measure accurately during the imitation, and opened the scavenging port relatively low, resulting in insufficient opening of the scavenging port when the piston is at the bottom dead center. (One is late, the second is Insufficient scavenging area) The amount of scavenging is insufficient, the cylinder cannot be fully filled, there is more residual exhaust gas, and it is mixed with the fresh combustible mixed gas phase, resulting in the fact that the air-fuel ratio and the mixing ratio are too lean, causing the engine to overheat. So how high is the opening of the scavenging port? It depends on the phase of the scavenging and is also related to S/D. When S/D is less than 0.8, the scavenging phase is 120?-122? after top dead center, and when S/D is 0.8-1, the scavenging phase is 122?-124? after top dead center, that is, the scavenging phase is behind In the exhaust phase of 18?-20?, the specific sweep difference size varies with the stroke S and should be calculated. The empirical calculation formula of the scavenging port height: hscan=(0.17－0.23)S, S-stroke. When the piston is at the bottom dead center, the maximum area of ??the scavenging port is about 3.5% of the working area (experience ratio).
11. The compression ratio of the crankcase is too small
The crankcase compression ratio refers to the ratio of the maximum and minimum volume of the crankcase (both including the volume of the scavenging airway). The situation where the compression ratio of the crankcase is too small has been discussed above, and will not be repeated here.
12. Low octane number of gasoline (fuel)
90% isooctane and 10% n-heptane are No. 90 gasoline. Gasoline is flammable. High temperature and sparks will cause combustion, but in the engine, the temperature at the end of compression is relatively high, and it cannot be produced as soon as a higher temperature Combustion, it must be allowed to burn according to a predetermined time, in order to make the engine work properly, to achieve this goal, it is necessary to add antiknock agent in gasoline. In the past, tetraethyl lead was added. According to different proportions, gasoline is divided into No. 66, No. 73, No. 80. With the development of science and technology and environmental protection requirements, the use of leaded gasoline is not allowed. Isooctane and n-heptane are now added as antiknock agents. The labels are No. 90, No. 93, No. 97 (there are also other codes, which are less used). What type of gasoline is used is determined according to the compression ratio of the engine , The greater the compression ratio, the higher the label of the gasoline is required. The purpose is to prevent the temperature at the end of compression from causing the combustible mixture to spontaneously ignite. That is to say, the gasoline with a high label contains a large proportion of antiknock agents and the combustion rate is slow. If the combustion speed is faster, the temperature will increase a little. An engine with a large compression ratio will have a higher temperature than the end of compression with a small compression ratio. For engines with a compression ratio of 8 or less, use No. 90 gasoline, but do not buy gasoline from the local oil refinery. Use leaded antiknock agents or add less antiknock agents. Otherwise it will cause overheating and damage the machine.
12. The calorific value of the spark plug is low
There are many types of spark plugs. In garden machinery, spark plugs are mostly used in L-type, M-type, and E-type. These are the first letters of the spark plug model, indicating the installation size, including spark plug thread diameter, pitch, The length of the thread and the size of the opposite sides of the hexagon, and the Arabic numerals behind are the calorific value of the spark plug. Example: E7RTC
The first letter E indicates that the thread diameter is m14, the thread pitch is 1.25 mm, the thread length is 12.7 mm, and the size of the six sides is 20.8 mm (the socket wrench is 21 mm);
The second number 7 indicates that the calorific value is 7 (high calorific value is a cold spark plug);
The third letter R indicates that the spark plug has resistance;
The fourth letter T indicates that the center electrode is a protruding type;
The fifth letter C indicates that the center electrode is a copper-nickel alloy.
The calorific value of spark plugs is divided into Arabic numerals for low, medium, and high. The larger the number, the higher the calorific value, and the colder the spark plug (meaning faster heat dissipation). In other words, the high calorific value is a cold spark plug, and the low calorific value is a hot type. Spark plug. The selection of spark plugs is also determined according to the compression ratio of the engine. High calorific value (cold type) spark plugs for engines with large compression ratios and low calorific value (hot type) spark plugs for engines with small compression ratios. For a two-stroke engine with a compression ratio greater than 6, a spark plug with a calorific value of 7 is used; then, for a compression ratio greater than 7, a spark plug with a calorific value of 8 is used. At present, the compression ratio of a forced air-cooled two-stroke engine will cause overheating if the compression ratio is greater than 7.5 without the use of special cooling methods. In the case of a four-stroke engine with a compression ratio of 7, a spark plug with a calorific value of 6 is used, and so on. The reason is that the two-stroke engine bursts once every revolution, and the four-stroke engine bursts once every two revolutions. In theory, the heat is half less than the two-stroke engine, so a spark plug with a lower calorific value is used. The diameter and pitch of the spark plug thread must be consistent with the cylinder to install it firmly and reliably, without damaging the cylinder. The length of the thread should also be consistent with the cylinder. Dangerous, carbon deposits will be generated on the growing threads. When removing the spark plug, the carbon deposits will easily fall into the cylinder, which may cause the cylinder to be pulled. If the thread is too short, the center electrode of the spark plug will shrink into the threaded hole of the cylinder, the fresh combustible mixture is not easy to sweep, and the cooling is difficult. At the same time, the residual exhaust gas collects in the deep socket of the threaded hole. The heat engine is difficult to start. The spark plug has a low calorific value, and it is easy to break down and ablate when used in a high compression ratio, that is, the spark plug is burned. Their common phenomenon is that it is difficult to start the heat engine. It can be started immediately by changing a spark plug. It can also be started to a certain extent. If all indexes of the engine are designed reasonably, the use of spark plugs with low heating value will not cause the engine to overheat, but will cause the heat engine to be difficult to start.
In short, the problem of 'the chain saw heat engine cannot be started, and it can be started after a while' is generally not a single fault, but is caused by a combination of factors. There are manufacturing aspects, and some will not be adjusted by the user. The opening of the main oil needle is too small, resulting in a lean air-fuel ratio. There are no regular maintenance, too much carbon. When this kind of failure occurs, first consider the oil quality, mixing ratio, adjust the carburetor, clean the debris between the air inlet cover and the cylinder blades, and the heating value of the spark plug. If the air intake is blocked too much and obstructs the air intake, consider replacing the conventional starter. Second, consider the actual compression ratio, scavenging phase and exhaust phase. Of course, these all require inspection methods, which are more difficult to implement in the hands of dealers and users, but it is feasible to add a cylinder gasket to reduce the compression ratio, exhaust in advance, scavenge in advance, and increase the area of ??scavenging. If it still does not work , Question feedback to the manufacturer.
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