1, meaning explanation
The minimum working volume of the cylinder, that is, the total volume above the piston when the piston is at the top dead center, is called the combustion chamber volume, which is expressed by Vc; When the piston is at the bottom dead center, the total volume above the piston, that is, the maximum volume of the cylinder, is called the total volume of the cylinder and is expressed by VA.
That is ε=Va/Vc.
The compression ratio indicates the degree to which the gas in the cylinder is compressed when the piston moves from the bottom dead center to the top dead center. Compression ratio is one of the important parameters of engine. Due to the limitation of knock, the compression ratio of modern automobile engines and gasoline engines is generally 8 ~ 1 1. Diesel engines have no knock limit, and the compression ratio is generally 12 ~ 22.
2. Performance indicators
Compression ratio has a great relationship with engine performance. As we all know, when a gasoline engine is running, it usually inhales a mixture of gasoline and air. In the process of compression, the piston goes up, which not only squeezes the mixture to reduce its volume, but also produces two phenomena: vortex and turbulence. When the gas in the closed container is compressed, the pressure increases with the increase of temperature. If the compression of the engine is relatively high, the cylinder pressure and temperature generated during compression will also increase relatively, and the gasoline molecules in the mixture can be vaporized more completely and the particles can be finer. In addition, the vortex and turbulence effects mentioned just now and the sealing effect obtained by high compression ratio will make the mixture burn at the moment when the spark plug jumps out of the spark in the next movement, releasing the maximum explosion energy to become the power output of the engine. On the contrary, if the combustion time is prolonged, the energy will consume and increase the temperature of the engine, instead of participating in the power output of the engine, so we can know that the engine with high compression ratio means that it can have greater power output.
3. Working temperature
The working temperature is closely related to the change of compression ratio. Everyone knows the close relationship between compression ratio and combustion temperature.
However, the operation of the engine has a suitable and normal working temperature range, and the cooling system of the engine must help the whole engine work in a suitable temperature range. Otherwise, whether the working temperature is too high or too low, the engine will not be able to exert its real efficiency, and worse, the cylinder and piston may be stuck and unable to work. This fault is called cylinder pulling, so the requirements and functions of the cooling system are self-evident. Generally speaking, the current working temperature of automobile engines is designed to be between 80- 1 10℃. At this suitable normal working temperature, the working efficiency of the engine can reach the ideal percentage of the original design.
When the working temperature is too high, the mixed gas entering the combustion chamber of the cylinder absorbs too much heat, which may cause spontaneous combustion and pre-combustion, thus causing knocking, making the engine weak and damaging mechanical parts. On the other hand, if the temperature is too low, the mixture will vaporize badly and the combustion effect will become worse. Non-vaporized gasoline will condense in every corner of the cylinder wall, forming carbon deposit or adhering to the oil ring. When the oil film is scraped off by the sealing ring, the exhaust gas in the engine cylinder will enter the oil tank at the bottom of the engine, polluting the engine oil and affecting its lubricity, sealing, adhesion, fluidity and other properties. From this point of view, the relationship between compression ratio and cooling system is very important.