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What is the problem of the difficulty of starting the chain saw heat engine?

Updated:25-10-2021
You may have encountered some problems when using a chain saw, but you don't know the cause. Today, I will mainly talk to you about the difficulty of starting the heat engine of the chain saw. In fact, one of the root causes of the problem that the hot engine cannot be started is that the engine is overheated. So what are the factors that cause the engine to heat up?

 

 

1. The mixing ratio is too thin
The mixing ratio is the mixing ratio of gasoline and engine oil volume. First of all, talk about the role of oil in the engine, lubrication, sealing, heat conduction, cleaning, and corrosion protection. These 5 functions are interrelated. If the lubrication is not good, dry friction will emit more heat, and in severe cases, it will cause the piston to melt and wear (commonly known as cylinder pull); if the sealing is not good, it will blow-by into the crankcase, resulting in combustible mixing The air becomes thinner; the heat conduction is not good, and the heat cannot be dissipated in time; the effect of cleaning and anti-corrosion will be greatly reduced. Here, another thing that needs to be explained is the quality of the engine oil. Two-stroke engines have very high requirements for engine oil, which is difficult to achieve for general engine oils. The requirements for it are: high flash point, low freezing point, easy to mix (dissolve) and fast closing ( 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 must not be used in two-stroke engines. If you cannot find special two-stroke engine oil for a while, you can use No. 10 car oil, which is steam engine oil. This oil can be used all year round in North China and Northwest China. , Used in the northeast in summer, in the south in spring, autumn and winter, and No. 15 car oil in summer. Remember! ! Never use diesel oil.

2.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 starting, 15 to 1 at maximum power, and 16 to 1 in order to save fuel when running at a constant speed for a long time. After the carburetor is adjusted, the throttle (also called throttle valve, commonly called throttle) controls the size of the throat area to adjust. If there is a problem with the carburetor design, the air intake is too large and the oil intake is insufficient, which is what we often call "oil thin". The combustion speed is fast, the engine speed is high, and the work is weak. What we can see is that when the fuel tank is used up and the throttle is not moving, the engine speed suddenly increases and then stalls. This is a temporary phenomenon in which 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.

3.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, divided by the volume of the combustion chamber, and it is equal to the theoretical compression ratio. The actual compression ratio is the working volume after the exhaust port is completely closed, plus the combustion chamber volume, and then divided by the combustion chamber volume. The actual compression ratio of a two-stroke engine should be between 6.5 and 7.3. If it is too small, the power is insufficient, and if it is too large, overheating and even knocking will occur. The compression ratio is determined by the manufacturer, and dealers and users can only make fine adjustments if they are very proficient. In the formula, V is the engine displacement, Pe is the average effective pressure on the top of the piston at the time of explosion, N is the number of engine revolutions, and 75×6=450 is a constant. It can be seen in the formula that the constant is constant. Then, 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 when the displacement and the number of revolutions remain unchanged, that is, increase the compression ratio, but if the compression ratio is too large, within a few minutes, Even if the power is a little bit higher in about 20 minutes, long-term work will cause the engine to overheat, and the power will drop instead, and the hot engine will not start.

4.The exhaust area is insufficient
The size of the exhaust port area is related to the displacement, that is to say, it is related to the working area corresponding to the displacement. The area of ​​the exhaust port occupies about 5%-5.5% of the working area (empirical data). If it is too small, the exhaust will not be smooth, the engine will overheat, and if it is too large, it will cause insufficient cylinder strength and affect the position of the piston ring. People’s Congresses who have ridden a motorcycle (two-stroke) have this experience. After a period of time, the engine will overheat and become weak. Just clean the top of the piston, the combustion chamber, and the coke deposits in the exhaust port. , You can restore the original working condition. This phenomenon is: coke deposits cause the volume of the combustion chamber to decrease, the compression ratio increases, the thermal conductivity becomes worse, the exhaust port becomes smaller, and the exhaust is not smooth, which causes the engine to overheat and reduce the power. Shanghai Youtuo Industrial Co., Ltd. provides chain saw maintenance and integrated garden machinery services. You can rest assured to buy Crep chain saws to ensure that you can rest assured.

 

5.Exhaust too late
The cylinder structure of a two-stroke engine is more complicated than that of a four-stroke engine. Air intake, scavenging, and exhaust are all on the cylinder wall (asymmetric intake air inlet is on the crankcase). Various air ports must not only ensure the work needs, but also ensure the strength of the cylinder block and the position of the piston ring. The amount of stay. The positions of intake, scavenging, and exhaust are very important, that is, the intake and exhaust phases are reasonably arranged. It is determined based on the top and bottom dead center of the piston and the crank angle, and is also related to the engine S/D (S-stroke , D─Cylinder diameter) When the S/D value is about 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, the smaller the number, the higher the number of revolutions, and the higher the number of revolutions, the shorter the absolute exhaust time. Therefore, it is necessary to turn on early. If the turn on time is too early, the engine power will be insufficient. If it is too late, the heat will stay for a long time, 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 fans are opened on the fan casing and provided by the impeller). Here, it is necessary to talk about the function of the flywheel. We know that the working cycle of the engine is the four strokes of suction, compression, explosion, and exhaust. Only the explosion stroke is the only one that does work and emits power, while the other three strokes are all. It consumes power. In order to ensure the continuous operation of the engine, it is necessary to store the energy of the explosion stroke and release it during other power-consuming strokes. Therefore, the first function of the flywheel is to store energy, the second is to cool the cylinder, and the third is to generate electricity, which is the inner (outer) rotor of the magneto. The spark required), and the fourth is the link (or output power connector) when starting. The air volume 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, and it is also related to the space area of ​​the air inlet screen. If the flywheel is designed well, the space area of ​​the air inlet hood is too small , Or there is debris blocking the mesh cover or blockage 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 at present)

7.The heat dissipation area of ​​the cylinder blades is not enough
For every air-cooled gasoline engine, its heat dissipation area is basically fixed according to its displacement and power. It is easier to use the following formula to find the approximate value: Ff=C,S,D(Ps)/vh In the c㎡ formula, Ff is the total heat dissipation area required, S is the stroke, D is the cylinder diameter, 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 each index of a two-stroke air-cooled small engine changes, then its heat dissipation area must change accordingly, or forced air The cold air volume increases accordingly. If only the engine displacement or compression ratio is changed, and other changes are not made, the engine will also be overheated.

8. Insufficient air intake area
Similar to scavenging, if the intake port is too small, the crankcase will be undercharged. When the piston is descending, the airflow into the scavenging channel is not strong, and the ability to drive exhaust gas is reduced. The exhaust gas mixture), the combustion speed is fast, the power drops, and the engine is overheated. The opening angle of the intake port, that is, the intake phase, is related to the number of revolutions of the engine. It is less than 6000 revolutions, which is 52˚-55˚ before and after top dead center, and is greater than 6000 revolutions, which is 55˚-58˚ before and after top dead center. Because the engine revolutions are high and the absolute intake time is short, the intake phase of the engine with high revolutions needs to be advanced. However, it is not that the earlier the better, because it is symmetrical air intake, the air intake is early, and it is bound to be closed late, which will cause severe carburetor back injection, but even if it is opened in advance, if the air intake area is too small, it still cannot reach the engine The demand will also cause overheating, so the area of ​​the air inlet is related to the working area corresponding to the displacement like scavenging and exhaust. The area of ​​the air inlet accounts for about 4.5% of the working area (experience ratio). Requirements: When the piston is at the top dead center, the upper edge of the air inlet overlaps with the lower edge of the piston. When the piston is at the bottom dead center, the top of the piston and the upper edge of the air inlet must not leak.

9. 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 beginning of ignition to complete combustion. This process requires a certain amount of time to make the piston burn completely after reaching the top dead center, and push the piston down with the greatest explosive force, which can exert the greatest power. At idling speed, the number of revolutions is slow, and the ignition advance angle can lag a little bit. At high speed, the number of revolutions is fast, and the ignition advance angle must be more advanced. At present, there are two types of magneto ignition devices on the market, one is the inductive type, referred to as TCI, and the other is the capacitive discharge type, referred to as CDI. The TCI ignition advance angle is 25˚-28˚. Within this angle, the idle speed and High speed can be taken care of, but it is not the best state, while 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˚. Regardless of the ignition device, the ignition timing is controlled by the keyway position on the crankshaft and flywheel. The difference is that the TCI ignition angle cannot be moved, while the CDI automatically advances as the engine speed increases. If the position of the crankshaft and the keyway is not well controlled, it will cause the ignition advance angle to be too early or too late. Too early, the rebound is strong, after starting, it will cause knocking, resulting in damage to the parts, the engine overheating; too late, the mixed gas is not completely burned out of the cylinder, forming a secondary combustion in the muffler, commonly known as "The engine fires". Both sides of the combustion (cylinder and muffler) generate heat on both sides, causing the engine to overheat, and the power is seriously insufficient. This kind of phenomenon rarely occurs in the design. If there is a failure, it is due to assembly quality problems and after a period of use, the nut of the pressing flywheel will be loosened, causing the rolling key and damaging the parts. Therefore, there is a "maintenance" requirement in the manual. .

10. Insufficient scavenging area
In a two-stroke engine, the cycle of intake, compression, explosion, and exhaust is completed by the crankshaft rotating one circle and the piston in the cylinder one up and down two strokes, so it is called a two-stroke engine. After the explosion, the piston goes down and the exhaust is opened. When the air port is at a certain level, the scavenging port is also opened, and scavenging is performed to drive the exhaust gas after combustion. When the piston is at the bottom dead center position, the exhaust port is fully opened and the scavenging port has the largest opening. When the piston moves up, the combustible mixture in the cylinder begins to compress, but the scavenging port and exhaust port are not closed. Part of the mixture escapes from the exhaust port and is discharged into the atmosphere, causing pollution, and some enters the crankcase from the scavenging duct. In order to reduce the escape of mixed gas, some manufacturers did not measure accurately during imitation, and opened the scavenging port relatively low, resulting in insufficient opening of the scavenging port when the piston was at the bottom dead center. Insufficient scavenging area) Insufficient scavenging volume, unable to fully fill the cylinder, excessive residual exhaust gas, mixing with the fresh combustible mixture, resulting in the actual air-fuel ratio, the mixture ratio is too lean, and the engine overheats. So how high the scavenging port is appropriate depends on the scavenging phase, which 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 18˚-20˚, the specific sweep difference size varies with the stroke S and should be calculated. The empirical calculation formula for the height of the scavenging port: h sweep = (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 crankcase compression ratio is too small
Crankcase compression ratio refers to the ratio of the maximum and minimum volumes of the crankcase (both include the scavenge volume). The situation that occurs when the crankcase compression ratio is too small has been discussed above, so I won't repeat it here.

 

12. Gasoline (fuel) octane number is low
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 at a higher temperature. For combustion, it must be burned at a predetermined time to make the engine work normally. To achieve this goal, it is necessary to add an antiknock agent in the gasoline. In the past, tetraethyl lead was added. According to the different proportions, the gasoline is divided into No. 66, No. 73, and No. 80. With the development of science and technology and environmental protection requirements, the use of leaded gasoline is not allowed. Now, isooctane and n-heptane are added as antiknock agents. The labels are No. 90, No. 93, and No. 97 (there are also other labels, which are used less). The gasoline of which label is used is determined according to the compression ratio of the engine. The higher the compression ratio, the higher the gasoline label is required. The purpose is to prevent the temperature at the end of compression from causing the combustible mixture to ignite spontaneously. If the combustion speed is faster, the temperature will rise a little, and the engine with a larger compression ratio will have a higher temperature at the end of compression than an engine with a smaller compression ratio. Engines with a compression ratio of 8 or less can use No. 90 gasoline, but do not buy gasoline from a local oil refinery. Use lead antiknock agent or less antiknock agent. Otherwise it will cause overheating and damage the machine.

13. The spark plug has a low calorific value
There are many types of spark plugs. In garden machinery, spark plugs are mostly 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 side of the hexagon, and the Arabic numerals at the back are the calorific value of the spark plug. The calorific value of the spark plug is low, medium and high respectively expressed in Arabic numerals. 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 the cold type spark plug, and the low calorific value is the hot type. Spark plug. The selection of spark plugs is also determined by the compression ratio of the engine. Engines with larger compression ratios use high-heat value (cold type) spark plugs, and engines with low compression ratios use low-heat value (hot type) spark plugs. If the compression ratio of a two-stroke engine is greater than 6, use a spark plug with a calorific value of 7; then, if the compression ratio is greater than 7, use a spark plug with a calorific value of 8. At present, the compression ratio of forced-air-cooled two-stroke engines, without special cooling methods, will cause overheating if the compression ratio is greater than 7.5. 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 explodes once every revolution, while the four-stroke engine explodes once every two revolutions. Theoretically, the heat is half that of the two-stroke engine, so a spark plug with a lower calorific value is used. Spark plug thread diameter The thread pitch must be consistent with the cylinder in order to be installed firmly and reliably without damaging the cylinder. The length of the thread must be the same as that of the cylinder. Carbon deposits will occur on the threaded out thread. When the spark plug is removed, 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 in the threaded hole of the cylinder. The fresh combustible mixture is not easy to sweep and cooling is difficult. At the same time, the residual exhaust gas gathers in the deep socket of the threaded hole. When the spark plug is ignited, it is not easy to burn. The hot engine is difficult to start. The spark plug has a low calorific value. It is easy to breakdown and ablate when used in a high compression ratio, that is, the spark plug is burnt. Their common phenomenon is that the engine is difficult to start when the engine is hot. You can start immediately after changing a spark plug. If the spark plug is not broken, wait until the engine is cold It can be started to some extent. If all indicators of the engine are designed reasonably and the spark plug with low calorific value is used, although it will not cause the engine to overheat, it will make it difficult to start the hot engine.
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