Gasoline Generator Principle

A working cycle consists of four piston strokes (the piston stroke is the distance between the top dead center and the bottom dead center): intake stroke, compression stroke, expansion stroke (work stroke) and exhaust stroke.

1. Air intake stroke

In this process, the engine's intake valve opens and the exhaust valve closes. As the piston moves from the top dead center to the bottom dead center, the volume of the cylinder above the piston increases, so that the pressure in the cylinder will be below the atmospheric pressure, that is, a vacuum suction force is caused in the cylinder, so that the air is sucked into the cylinder through the intake pipe and the intake valve, and the fuel injector sprays atomized gasoline and mixes it with the air completely. At the end of the intake, the gas pressure in the cylinder is about 0.075-0.09MPa. At this time, the temperature of the combustible mixture in the cylinder has risen to 370-400K.

2. Compression stroke

In order to make the combustible mixture that is sucked into the cylinder burn rapidly, so as to produce a larger pressure, so that the engine can emit more power, the combustible mixture must be compressed before combustion, so that its volume is reduced, the density is increased, and the temperature is increased, that is, a compression process is required. In this process, the intake and exhaust valves are all closed, and the crankshaft pushes the piston to move one stroke from the bottom dead center to the top dead center, that is, the compression stroke. At this time, the pressure of the mixture will increase to 0.6-1.2MPa, and the temperature can reach 600-700K.

There is a very important concept in this trip, which is the compression ratio. The so-called compression ratio is the ratio of the maximum volume of gas in the cylinder before compression to the minimum volume after compression. Generally, the larger the compression ratio, the higher the pressure and temperature of the mixture at the end of compression, and the faster the combustion rate, so the greater the power emitted by the engine and the better the economy. The compression ratio of the average sedan is between 8 and 10, but now the latest Polo has reached a high compression ratio of 10.5, so its torque performance is relatively good. However, when the compression ratio is too large, not only can the combustion situation not be further improved, but abnormal combustion phenomena such as burst ignition and surface ignition will occur.

Burst ignition is an abnormal combustion caused by the spontaneous combustion of a terminal combustible mixture far from the ignition center in the combustion chamber due to high gas pressure and temperature. During a burst of ignition, the flame propagates outward at an extremely high rate, and even before the gas has time to expand, the temperature and pressure rise sharply, forming a pressure wave that proppels forward at the speed of sound. When this pressure wave hits the wall of the combustion chamber, it makes a sharp knocking sound. At the same time, it will also cause a series of adverse consequences such as engine overheating, power loss, and fuel consumption increase. Severe burst fire can even cause damage to the valve burnout, the bearing pad cracking, and the spark plug insulator being broken down.

In addition to flash ignition, engines with excessively high compression ratios may also face another problem: surface ignition. This is another abnormal combustion (also known as hot spot fire or pre-ignition) caused by the ignition of the mixture between the red-hot surface in the cylinder and the red-hot area (e.g., exhaust valve head, spark plug electrode, carbon deposit). Surface ignition is also accompanied by a strong knocking sound (which is dull), and the high pressure generated will increase the load on the engine and reduce the life of the engine.

3. Expansion stroke

During this process, the intake and exhaust valves remain closed. When the piston approaches the top dead center, the spark plug emits an electric spark that ignites the compressed flammable mixture. When the combustible mixture is burned, a large amount of heat energy is released, and the pressure and temperature of the gas increase rapidly. The maximum pressure it can achieve is 3-5MPa, and the corresponding temperature is up to 2200-2800K. The high-temperature and high-pressure gas pushes the piston to move from the top dead center to the bottom dead center, rotates the crank through the connecting rod and outputs mechanical energy, in addition to maintaining the engine itself to continue to run, the rest is used to do external work. During the movement of the piston, the cylinder volume increases, the gas pressure and temperature drop rapidly, and at the end of this stroke, the pressure drops to 0.3-0.5MPa and the temperature is 1300-1600K.

4. Exhaust stroke

When the expansion stroke (work stroke) is near the end, the exhaust valve is opened, and the pressure of the exhaust gas is used to exhaust freely, and when the piston reaches the bottom dead center and then moves to the top dead center, the exhaust gas is forcibly discharged into the atmosphere, which is the exhaust stroke. In this stroke, the pressure in the cylinder is slightly higher than the atmospheric pressure, about 0.105-0.115MPa. When the piston reaches the vicinity of the top dead center, the exhaust stroke ends, at which point the exhaust gas temperature is about 900-1200 K.

With this, we have already described a working cycle of the engine, during which the piston moves four strokes back and forth between the top and bottom dead center, and the crankshaft rotates for two weeks accordingly.

A gasoline generator is formed by the gasoline engine with an alternator.