Engine is a machine that converts chemical energy into mechanical energy. Its conversion process is actually the process of working cycle. In short, it is to generate kinetic energy by burning the fuel in the cylinder, which help to drive the reciprocating motion of the piston in the cylinder, thus driving crank which is connected with connecting rod and piston to make a circular reciprocating motion around the center of the crankshaft, and then to transmit power.
The working process of four stroke gasoline engine is a complex process, which consists of four strokes: intake, compression, combustion and expansion, and exhaust.
Intake stroke
At this time, the piston is driven by the crankshaft from the top dead center(TDC) down to the top dead center(BDC). At the same time, the intake valve is opened and the exhaust valve is closed. When the piston moves from TDC to BDC, the volume above the piston increases, and the gas pressure in the cylinder decreases, forming a certain degree of vacuum. Because the intake valve is opened, the cylinder is connected with the intake pipe, and the mixture is drawn into the cylinder. When the piston moves to the BDC, the cylinder is filled with fresh mixture and the exhaust gas not discharged from the previous working cycle.
Compression stroke
The piston moves from BDC to TDC and the intake and exhaust valves close. The crankshaft rotates under the action of inertia force such as flywheel, and the piston moves upward through the connecting rod. The volume of gas in the cylinder decreases gradually, the gas is compressed, and the pressure and temperature of the mixture in the cylinder increase.
Expansion stroke
At this time, the intake and exhaust valves are closed at the same time, the spark plug ignites, the mixture burns violently, the temperature and pressure in the cylinder rise sharply, the high-temperature and high-pressure gas pushes the piston to move downward, and the connecting rod drives the crankshaft to rotate. In the four strokes of the engine, only this stroke can realize the conversion of heat energy into mechanical energy. Therefore, this stroke is also called the working stroke.
Exhaust stroke
At this time, the exhaust valve is opened, and the piston moves from the BDC to the TDC. The exhaust gas is discharged from the cylinder with the piston going up. Because the exhaust system has resistance and the combustion chamber also has a certain volume, it is impossible to exhaust the exhaust gas completely at the end of exhaust, which is called residual exhaust gas. The residual exhaust gas not only affects the inflation, but also has a bad effect on combustion.
At the end of the exhaust stroke, the piston returns to TDC. That also means a work cycle completed. Then, the crankshaft continues to rotate under the inertia of the flywheel to start the next cycle. Thus the engine keeps running.
