Everyone must have seen a tractor. Do you know the working principle of a tractor? It doesn't matter if you don't understand. Let's briefly introduce it to you:

1. Compression stroke. During compression, the piston moves from BDC to TDC. This stroke has two functions: one is to raise the temperature of the air to prepare for the self ignition of the fuel; the other is to create conditions for the expansion of the gas.

When the piston goes up and the intake valve is closed, the air in the cylinder is compressed. As the volume becomes smaller, the air pressure and temperature will continue to rise. The pressure and humidity at the compression end point are related to the degree of air compression, that is, to the compression ratio.

2. Intake stroke. Its task is to fill the cylinder with fresh air. When the intake stroke begins, the piston is at top dead center and some exhaust gas is left in the combustion chamber in the cylinder. When the crankshaft rotates the elbow, the connecting rod moves the piston from the top dead center to the bottom dead center. At the same time, the transmission mechanism connected with the crankshaft is used to open the intake valve.

With the downward movement of the piston, the volume above the piston in the cylinder gradually increases: the air pressure in the cylinder is lower than that in the intake pipe, so the outside air is constantly filled into the cylinder.

3. Combustion expansion stroke. At the beginning of this stroke, most of the fuel injected into the combustion chamber is burned. A large amount of heat is released during combustion, so the pressure and temperature of the gas rise sharply. The piston moves downward under the action of high temperature and high pressure gas, and makes the crankshaft rotate through the connecting rod to do external work. Therefore, this stroke is also called work or working stroke.

4. Exhaust stroke. The function of the exhaust stroke is to discharge the expanded exhaust gas in order to fill with fresh air and prepare for the intake of the next cycle. When the piston moves near the BDC in the working stroke, the exhaust valve opens, and the piston moves from BDC to TDC driven by the crankshaft and connecting rod, and exhausts the exhaust gas out of the cylinder.