An engine is a machine that converts chemical energy into mechanical energy, and its conversion process is actually a working cycle process. Simply put, it burns the fuel in the cylinder to generate kinetic energy, driving the piston in the engine cylinder to move back and forth, thereby driving the connecting rod connected to the piston and the crank connected to the connecting rod to move back and forth in a circular motion around the center of the crankshaft, and outputting power.
The engines are mainly divided into four-stroke engines and two-stroke engines.The working process of a four-stroke gasoline engine is a complex process, consisting of four-strokes: intake, compression, combustion expansion, and exhaust. The working principle of a two-stroke engine is to complete one working cycle within two strokes, that is, the crankshaft rotates once, and the engine performs one external work cycle.
For a four-stroke engine:
Intake stroke
At this point, the piston is driven by the crankshaft to move from top dead center to top dead center, while the intake valve opens and the exhaust valve closes. When the piston moves from top dead center to bottom dead center, the volume above the piston increases, and the gas pressure in the cylinder decreases, forming a certain degree of vacuum. Due to the opening of the intake valve, the cylinder is connected to the intake pipe, and the mixture is drawn into the cylinder. When the piston moves to the bottom dead center, the cylinder is filled with fresh mixture and exhaust gas that was not discharged from the previous working cycle.
Compression stroke
The piston moves from bottom dead center to top dead center, and the intake and exhaust valves close. The crankshaft rotates under the inertia of the flywheel, and the piston is pushed upwards by the connecting rod. The gas volume in the cylinder gradually decreases, and the gas is compressed. As a result, the pressure and temperature of the mixed gas in the cylinder increase.
Work stroke
At this point, the intake and exhaust valves are simultaneously closed, the spark plug ignites, and the mixture burns violently. The temperature and pressure inside the cylinder rise sharply, and the high-temperature and high-pressure gas drives the piston to move downwards, driving the crankshaft to rotate through the connecting rod. During the four working strokes of the engine, only this stroke can convert thermal energy into mechanical energy, so this stroke is also known as the working stroke.
Exhaust stroke
At this point, the exhaust valve opens, the piston moves from bottom dead center to top dead center, and the exhaust gas is discharged from the cylinder as the piston moves upwards. Due to the resistance of the exhaust system and the fact that the combustion chamber also occupies a certain volume, it is impossible to completely exhaust the exhaust gas at the end of the exhaust process. The remaining exhaust gas is called residual exhaust gas. Residual exhaust gas not only affects inflation, but also has adverse effects on combustion.
At the end of the exhaust stroke, the piston returns to top dead center. This completes a work cycle. Subsequently, driven by the inertia of the flywheel, the crankshaft continued to rotate and began the next cycle. So repeatedly, the engine keeps running.
For a two-stroke engine:
First stroke
The piston moves upwards from the bottom dead center. After the three air holes are closed simultaneously, the mixture entering the cylinder is compressed; When the intake port is exposed, the combustible mixture flows from the carburetor into the crankcase.
Second stroke
When the piston is compressed near the top dead center, the spark plug ignites the combustible mixture, and the gas expands to push the piston downward to do work. At this point, the air inlet is closed, and the combustible mixture sealed in the crankcase is compressed; When the piston approaches the bottom dead center, the exhaust hole opens, and exhaust gas rushes out; Then open the air exchange port, and the fresh combustible mixture that enters the cylinder is flushed into the cylinder and the exhaust gas is discharged, and the air exchange process is carried out.
