What is the reason for the engine still making noise after the car is turned off?
2 Answers
The reason for the engine still making noise after the car is turned off is due to an issue with the three-way catalytic converter. The three-way catalytic converter is a purification device in the car's exhaust system that converts harmful substances in the exhaust into carbon dioxide, water, and nitrogen. The operating temperature of the three-way catalytic converter is very high, nearly 1000 degrees Celsius. After the engine is turned off, the metal inside undergoes thermal expansion and contraction, which causes the noise. Turning off the engine is also referred to as extinguishing. When a reactor is operating at a significant reaction rate, if the temperature at the feed inlet is reduced to the lower limit of the multiple steady-state region, the reaction rate suddenly drops significantly, and the reaction basically stops. This phenomenon is called extinguishing, and the corresponding inlet temperature is called the extinction point.
I've been driving for many years, and it's quite common for the engine to keep making noise after turning off the car, usually a normal phenomenon. For example, after long-distance driving in summer, the engine temperature is high, and the cooling fan will automatically run for a few minutes to dissipate heat, preventing overheating damage. Turbocharged cars may hear slight noises from the turbo cooling down. The exhaust system's thermal expansion and contraction can also produce cracking sounds, which is normal as metal parts contract. However, if the sound is abnormal, like grinding or squeaking, you should be cautious—it could indicate loose belts, bearing wear, or water pump failure. I recommend observing for a few minutes after turning off the engine; if the noise disappears, it's fine. If it persists, take it to a repair shop. Regular maintenance should include checking the cooling system and belt condition to prevent small issues from becoming major problems.
