Difference between a two-stroke engine and a four-stroke engine

1) Differences in working principles
Whether it is a two stroke engine or a four stroke engine, it must go through four working processes: intake (scavenging), compression, combustion expansion, and exhaust in order to complete a working cycle. The difference is:
1. In a four stroke engine, for every two revolutions (720 degrees) of the crankshaft, the piston moves back and forth twice, and the engine completes one working cycle, that is, one working cycle every four strokes.
In a two-stroke engine, for each revolution of the crankshaft (360 degrees), the piston moves back and forth once, and the engine completes one working cycle, that is, one working cycle is completed every two strokes.
2. Two stroke engines and four stroke engines only open and close their intake and exhaust valves or intake, exhaust, and scavenging ports once after completing each working cycle, but their opening and closing time periods are different.
2) Characteristics of engine performance
1. For every revolution of the crankshaft in a two-stroke engine, there is one power stroke. Therefore, under the same factors such as speed and intake conditions, theoretically speaking, the power generated by a two-stroke engine should be equal to twice the power generated by a four stroke engine with the same working volume. However, due to incomplete exhaust emissions from a two-stroke engine, and the additional exhaust generated by closing the scavenging port before the exhaust port, in reality, a two-stroke engine cannot be equal to twice the size of a four-stroke engine, but rather 1.5 to 1.7 times.
2. Due to the ventilation of a two-stroke engine, a portion of the combustible mixture is discharged along with the exhaust gas, resulting in a high consumption of fuel and lubricating oil.
3. Due to the short and incomplete ventilation time of a two-stroke engine, there is a large amount of residual exhaust gas in the cylinder. The low speed misfire rate is high, and the combustion situation is poor. In addition, some of the combustible mixture is discharged with the exhaust gas during the ventilation process without participating in combustion. Therefore, the emission pollution is severe, and the HC value in the pollutants is much higher than that of a four-stroke engine.
4. Due to the high frequency of power stroke in a two-stroke engine, it operates more smoothly.
5. Due to the frequent power strokes of a two-stroke engine, which requires one combustion per revolution, the heating of various components of the engine is much higher than that of a four-stroke engine, especially the piston.
3) Differences in overall layout
1. The four stroke engine has a complex valve type valve train, which is controlled by the camshaft to open and close the valves at a fixed time to complete the intake and exhaust processes. However, the two-stroke engine does not have a dedicated valve distribution mechanism. It uses a piston to control the opening and closing of the exhaust and scavenging ports to complete the scavenging and exhaust processes.
2. The scavenging and exhaust of a two-stroke engine are carried out near the bottom dead center of the piston, and the air ports are located at the lower end of the cylinder. The bottom dead center is symmetrically arranged. The valve train of a four stroke engine is located on the cylinder head (or on the side of the cylinder block).
3. Two stroke engines generally use mixed lubrication or separate lubrication, while four stroke engines generally use a combination of pressure lubrication and splash lubrication.
4) Differences in component structure
1. Most two-stroke engines use crankcase scavenging, so there is an intake window on the crankcase that serves as a channel for the mixture to enter the crankcase. Its opening and closing are commonly controlled by reed valves, rotary valves, or piston valves. As the crankcase contains the mixture from the carburetor, it is required that the crankcase be airtight. There are pressure oil passages or oil pipes in the crankcase of a four stroke engine.
2. Two stroke engines are equipped with exhaust ports and scavenging ports at the lower part of the cylinder block, and some also have intake ports.
3. The cylinder head structure of a two-stroke engine is simple, without intake and exhaust ports, valve distribution mechanisms, and no lubricating oil passages. The cylinder head of a four stroke engine is a very complex component. There are intake and exhaust ports, air passages, lubricating oil channels, and valves, valve guides, valve springs, rocker arms, rocker arm shafts, concave wheel shafts, and their driving mechanisms installed on them. There are also installation holes for carburetors (or intake pipes) and exhaust pipe silencers.
4. The piston rings of a two-stroke engine only have gas rings and no oil rings. The piston rings of a four stroke engine have both gas and oil rings. A positioning pin is pressed into the groove of the piston in a two-stroke engine to restrict the rotation of the gas ring, in order to prevent the surface of the cylinder body from being damaged or even the ring from breaking when the ring opening is turned to the gas port position.
5. The piston skirt of a two-stroke engine is longer than that of a four-stroke engine, and there are windows or notches in the piston skirt of a two-stroke engine to form an intake or scavenging passage with the cylinder block.