Key points of the two-stroke working cycle for aviation piston engines

The key points of the two-stroke working cycle of an aviation piston engine can be summarized as follows:

Overview of the two-stroke working cycle

The two-stroke working cycle is one of the basic operating forms of internal combustion engines. Compared with four-stroke engines, two-stroke engines only need two strokes to complete the four movements of intake, compression, power generation and exhaust within one working cycle. This means that each piston stroke involves two operations, enabling a two-stroke engine to do more work within the same period of time and theoretically provide higher power output.

Key points of the two-stroke working cycle

Structural characteristics

The structure of a two-stroke engine is relatively simple and usually includes parts such as the cylinder head, cylinder, piston, and piston rings. The intake port and the exhaust port are respectively located at the left and right ends of the cylinder. The intake port is at the lower part of the cylinder, and the exhaust port is at the upper part of the cylinder (at two-thirds of the full stroke of the piston when working). The ventilation port is slightly lower than the exhaust port.

Stroke description

First stroke (upward stroke) :

The piston moves upward from the bottom dead center (BDC).

During this process, the piston successively completes the intake, compression and ventilation (partial) operations. Specifically, when the piston moves upward, it first closes the intake and exhaust ports, compressing the mixture in the cylinder. Meanwhile, as the piston rises, the connection between the carburetor and the crankcase opens, and fresh mixture is drawn into the crankcase.

When the piston approaches the top dead center, the spark plug ignites and ignites the mixture in the cylinder.

Second stroke (downward stroke) :

The piston moves downward from the top dead center.

During this process, the piston successively completes the operations of power generation, exhaust and ventilation (completion). Specifically, when the high-temperature and high-pressure gas generated by the combustion of the mixed gas pushes the piston downward, work is completed.

When the piston descends to a certain position, the exhaust port opens first, discharging almost all the exhaust gas.

Subsequently, the air intake hole opens, and the compressed fresh mixture in the crankcase enters the cylinder through the air intake hole, while the remaining exhaust gas in the cylinder is discharged, completing the air exchange process.

Crankshaft rotation

A two-stroke engine requires the crankshaft to rotate twice to complete one working cycle. Each stroke corresponds to a half full rotation of the crankshaft.

Lubrication method

Two-stroke engines usually rely on additional injection oil or premixed lubricating oil to lubricate engine components. These lubricating substances are usually mixed with fuel and used together.

The advantages and disadvantages of the two-stroke working cycle

Advantages

It has a simple structure, few moving parts and a relatively low manufacturing cost.

Each revolution of the crankshaft generates work. Under the same power, the engine is smaller in size and lighter in weight.

It is suitable for occasions where the engine is exposed and frequently swings significantly, such as motorcycle engines.

Disadvantage:

The exhaust pollution is relatively high, and the waste gas may contain unburned fuel, causing air pollution.

Due to the lubrication method, it is prone to cause relatively serious exhaust gas pollution.

Its durability is relatively low and not as long-lasting as that of a four-stroke engine.

Application field

Two-stroke engines are mainly applied in smaller or larger engine sets, such as large ships, model aircraft engines, some external high-power low-displacement motorcycles, go-karts, as well as lawn mowers, chainsaws and other garden machines. In the field of unmanned aerial vehicles (UAVs), the application of two-stroke engines is relatively rare, but there are also some small UAVs or special-purpose UAVs that use two-stroke engines as their power source.

In summary, the two-stroke working cycle of aviation piston engines has advantages such as simple structure and high power density, but it also has disadvantages such as high exhaust pollution and low durability. In specific application scenarios, such as those requiring high power density and lightness, two-stroke engines have certain advantages.