1：What is it?
The compression ratio is the ratio between the maximum volume of the cylinder at its bottom dead center and the minimum volume at its top dead center.
2：Why is gas compressed?
Gas compression serves several purposes: increasing pressure can increase the density of the gas, reducing the distance between molecules. This brings fuel molecules and oxygen molecules closer together, resulting in faster combustion. Temperature, on the other hand, increases the speed of molecular motion, facilitating easier interaction between fuel and oxygen molecules and making the fuel-air mixture easier to ignite. Additionally, a smaller combustion chamber allows for quicker completion of the combustion process, speeding up the overall combustion and improving performance.
3：What are some misconceptions?
While modern gasoline engines are designed with higher compression ratios, this is achieved under the premise of advancing technology and the availability of higher-octane gasoline grades. It is a gradual process rather than an immediate change. In earlier engine technologies, cylinders couldn't withstand excessive pressure, and even if the engine allowed for it, higher-octane fuels were not widely available. For example, engines from the 1980s and 1990s had compression ratios of around 7, allowing them to run on 75-octane fuel or even lower. Conversely, introducing a low-compression engine like the Cherokee that requires fuel below 85 octane would result in abnormal operation. Moreover, for diesel engines, higher compression ratios inevitably lead to higher pressures, testing the durability of engine components.
Therefore, in general, engines with higher compression ratios offer better performance and fuel efficiency for the same displacement. However, there is a limit to everything, and purely increasing the compression ratio for the sake of high performance while maintaining normal operation becomes exponentially challenging. Additionally, the quality of fuel in the domestic market is currently not up to par, limiting the widespread adoption of some excellent engines in the country.
