High compression ratios are not used in spark-ignition engines because they can cause irregular combustion, such as pre-ignition and knocking, which can reduce efficiency and damage the engine. Pre-ignition is when the fuel-air mixture ignites before the spark plug fires, and knocking is when the mixture detonates unevenly and creates pressure waves that can harm the engine components. These phenomena are more likely to occur when the compression ratio is high because the end-gas temperature and pressure are higher, which increases the reactivity of the mixture. To avoid pre-ignition and knocking, spark-ignition engines need to use high-octane fuels that have higher resistance to auto-ignition. However, high-octane fuels are more expensive and less available than low-octane fuels, which limits the use of high compression ratios in spark-ignition engines. Some possible ways to enable high compression ratios in spark-ignition engines are to use high-pressure gasoline direct injection, which can enhance the air-fuel mixing and flame speed, or to use fuels with high octane sensitivity, which can reduce the end-gas reactivity.