Maintenance methods for two-stroke and four-stroke brush-cutting machines

Maintenance methods for two-stroke and four-stroke brush-cutting machines

The maintenance of the engine mainly involves aspects such as lubrication, inspection and adjustment. 

For two-stroke engines, it is necessary to ensure the normal operation of the oil pump and the unobstructed oil passage, and to adjust the proportion of oil in the mixture. Regularly remove the carbon deposits from the spark plugs, combustion chambers, exhaust ports, scavenging ports and exhaust pipes. 

For four-stroke engines, the oil level in the gearbox should be regularly checked to maintain the normal oil level; ensure the normal operation of the oil pump and the smoothness of the oil passage; and replace the engine oil regularly. When necessary, check the tightness of the chain and the valve clearance, and make adjustments. 

Two-stroke engine: 

1. First stroke: The piston moves upward from the bottom dead center. After all three air holes are closed, the mixture entering the cylinder is compressed; when the intake hole is exposed, the combustible mixture flows into the crankcase. 

2. Second stroke: When the piston reaches the upper dead center, the spark plug ignites the combustible mixture, causing the gas to expand and push the piston downward to perform work. At this time, the intake hole is closed, and the combustible mixture sealed in the crankcase is compressed; when the piston approaches the lower dead center, the exhaust hole opens, and the exhaust gas is expelled; then the intake hole opens, and the pre-pressed combustible mixture rushes into the cylinder to expel the exhaust gas and complete the ventilation process. 

Two-stroke engine: 

Advantages: Two-stroke engines have no intake and exhaust valves, no complex valve train or lubrication system, and the cooling system is generally air-cooled. Therefore, in terms of structure, they are simpler than four-stroke engines, lighter in weight, have lower manufacturing costs, lower failure rates, and are easier to maintain.

Disadvantages: In the intake and exhaust processes of a two-stroke engine, only 130° to 150° of the crankshaft rotation is occupied. The intake and exhaust processes occur almost simultaneously. Utilizing fresh air to sweep away exhaust gas inevitably results in some fresh mixed gas flowing out of the exhaust port along with the exhaust gas, and the exhaust gas is not easy to be completely removed. Therefore, the ventilation quality of a two-stroke internal combustion engine is poor, the fuel efficiency is not high, and the fuel consumption of the entire vehicle is relatively high. 

Four-stroke engine 

Advantages: Due to the independent operation of intake and exhaust in a four-stroke engine, the efficiency of intake and exhaust is high, combustion is thorough, and fuel consumption is reduced. Due to the intake and exhaust methods, the stroke of the piston can be designed to be longer than that of a two-stroke engine. At medium and low speeds, the output is much greater than that of a two-stroke engine, allowing for the use of a larger paddle to generate greater pulling force. Moreover, in both light and heavy load conditions, the output can be consistent. 

Disadvantage: A four-stroke engine performs one cycle of work every two rotations. Therefore, engines of the same displacement have less power than two-stroke engines. According to the technical parameters provided by the manufacturers and the general consensus, it is believed that the power of a four-stroke engine is equivalent to 60% of that of a two-stroke engine. 

The differences between the two engines 

The differences in the composition of combustible mixtures 

The two-stroke engine is composed of a combustible mixture of gasoline, engine oil and air. The four-stroke engine is a combustible mixture of gasoline and air. 

2. Differences in the valve train system 

In a two-stroke engine, the mixture entering the crankcase is under a certain pressure. Through the movement of the piston, the opening and closing of the intake port and the scavenging port are controlled, completing the scavenging and exhaust processes. 

In a four-stroke engine, the rotation of the crankshaft is transmitted to the camshaft via a timing chain. As the camshaft rotates, the intake and exhaust cams push the rocker arms to swing, and under the action of the valve springs, the intake and exhaust valves open and close at the predetermined times. 

3. Differences in Lubrication Methods 

Two-stroke engines generally adopt either mixed lubrication or separate lubrication methods. Regarding the separate lubrication method, the crankshaft and connecting rod assembly, cylinder, piston assembly, etc. of the crankcase are all lubricated by the mist-like mixture of engine oil, gasoline and air. The lubricating oil in the engine plays roles such as lubrication, cooling, sealing and cleaning, and it is related to the wear of parts and their service life. 

The piston connecting rod mechanism of a four-stroke engine adopts the splash lubrication method of the crankshaft; while the camshaft, rocker arms, valves, etc. adopt the pressure lubrication method.